Introduction to Astrophotography

Astrophotography is the most complex aspect of amateur astronomy, both in terms of technique and tools, but also in terms of the knowledge necessary to produce good astrophotographs. With almost any telescope one can achieve different types of astrophotography, but producing a good image requires an optimized system. In this article we will detail the main issues and questions that everyone who tries taking astrophotos will face. For further details please see the few books published on the topic in the international literature. The success of each astronomer is dependent on both the technique employed and the knowledge to use the technology and to do image processing, but especially on the patience and time invested in uncovering the secrets of astrophotography.

1. Is it possible to take pictures through a telescope?

2. Can I view on a computer monitor the image seen through the telescope?

3. What are the types of astrophotography?

4. What telescope, camera and accessories are needed to take astrophotographs?

5. What is focal ratio and why is it important for astrophotography?

6. CCD Camera with monochrome or color sensor?


1. Is it possible to take pictures through a telescope?

The short answer is Yes. Pictures, both terrestrial and astronomical, can be taken through almost any telescope. But their quality varies according to the type of telescope used, if it is optimized for photography or not, mount type used, the type of camera or ccd camera used, the shooting method, method of processing raw images. Below we detail each of these issues, plus many more.

2. Can I view on a computer monitor the image seen through the telescope?

To see on the screen the image produced by the telescope, a few accessories are required. First of all, the mount of the telescope must be motorized in order to automatically compensate for the Earth’s rotation and thus to hold objects in the visual field of the eyepiece. To capture the image produced by the telescope, one needs a digital camera which can show the captured images in real time. To this respect, several manufacturers produced video cameras, web or with dedicated ccd that can be placed at the telescope’s focal point and connect to a computer. This way the image produced by the telescope is captured by the camera and transmitted to the computer where it can be viewed on the monitor. For observing the planets and the Moon, a better webcam could be enough, or a more effective dedicated ccd camera such as the Celestron NexImage camera, the Titan from ATIK, or the DMB/DBK/DFK from the Imaging Source. In order to observe deepsky objects, one requires a dedicated ccd camera for astrophotography that can achieve long exposures up to several minutes, such as ccd cameras from ATIK, SBIG, Starlight Xpress or other specialized manufacturers. Regular or DSLR cameras are not suitable for this type of astrophotography.

3. What types of astrophotography are there?

There are several ways to do astrophotography.

a) the simplest form of astrophotography is through the lens of a photo camera lens. The camera can be placed on a tripod or riding on the telescope (piggyback), and the desired celestial object is exposed through he camera lens using the functions offered by the camera. You can use any kind of cameras for this purpose. In the case of slr or dslr cameras, the lenses are changeable, so in order to obtain a wide visual field, a small focal ratio lens can be used, and in order to get a narrow field of view, a larger focal ratio lens can be used. The main problems with this technique are:

i) to capture most deepsky objects on sensor, the device must be able to do long exposures, between a few seconds and several minutes, depending on the magnitude of the object photographed. Not all cameras can do long exposures, generally long exposures being implemented only in SLR or DSLR cameras and in premium compact cameras.

ii) If the exposure exceeds 15-20 seconds, it’s necessary to rotate the camera for the stars to appear motionless. To this end, having the camera piggybacked on the telescope allows to use the telescope to guide the camera during exposure. The operator, using an eyepiece with illuminated reticle, will look through the telescope and will correct the mount’s tracking errors by keeping the aim centered on a star that serves as the guide.

iii) The final quality depends on the quality of the camera’s lens. In this sense, a lens with a good aberration correction (chromatic, astigmatism, distortions) will produce better image than a lens with lower correction.

b) Photography by projection through the telescope eyepiece

Through this technique, the camera is mounted in front of the eyepiece via an adapter for afocal photography (such as a tele-extender, a Microstage universal adapter, or another afocal projection system). The photo is done either through the camera lens (afocal projection through the eyepiece), or the image/film is exposed directly, the captured image resulting from the eyepiece (eyepiece projection).

The quality of the final image is a combination of the quality of the lens of the telescope, the image quality produced by the eyepiece, and by the camera lens. For long exposures it is necessary that the mount is motorized and guided. The major advantage of photography through eyepiece projection is that you can get very high magnification, by varying the distance between the eyepiece and the camera’s chip/film, useful especially for Planetary and Lunar photography. Also, in the case of the afocal projection through the eyepiece, there’s no need to remove the camera lens, which means one can use compact cameras whose lenses are not interchangeable.

c) Photography in the focal point of the telescope

This is the classic meaning of astrophotography. The film/digital camera or ccd camera is mounted without any intermediate optical element (except possibly a Barlow lens for focal reduction or field correction) at the telescope’s focal point. The telescope acts as a telephoto lens with a very long focal length (the focal length of the telescope with or without corrective optics) and a focal ratio particular to the telescope. This technique produces the best image, because the intermediate optical elements that are potential sources of aberrations are removed, so that the only remaining source of aberration is the telescope itself. If the telescope is optimized for photography, the various aberrations are reduced to a minimum, allowing to obtain the best images of all the techniques of astrophotography.

This technique can only be used with slr or dslr cameras, or with ccd cameras dedicated to astrophotography.

4. What telescope, camera and accessories are needed to take astrophotographs?

a) Telescope: refractor, cassegrain, or newtonian reflector

Astrophotography can be done in principle with any telescope. For best results it is necessary, however, that the telescope meets some minimal requirements: the quality of the mechanical parts of the telescope must be good and sturdy, without any plastic parts (that can bend or flex); the optical elements to be at least limited by diffraction; the lens/mirror cell needs to be made of metal; and the alignment of the optical elements to be correct and held in good alignment during astrophotography sessions. For yet better images, it is also important for the telescope to be optimized for photography and to be climatized to the surrounding environment. All good telescopes are capable of astrophotography, but some are specifically designed for astrophotography producing a wide field that is aberration-free.

Generally the most used telescopes for astrophotography are the apochromatic refractors and the Cassegrain-type telescopes (especially Schmidt-Cassegrain are preferred by amateurs, Ritchey-Remali, Dall-Kirham and not so much the Maksutov-Cassegrains), Newtonian reflectors being less used.

Refractors produce the most contrasting and sharp images of all the optical telescope designs, and the alignment of the optical elements holds very well, these being the main reasons why these scopes are preferred in astrophotography. Current apochromatic refractors produce an excellent image that’s free from chromatic aberrations (depending on the optical design of each particular refractor), and work with most ccd cameras and photo cameras. For refractors there are available a series of focal reducing and field correcting lenses that help produce a better image. Although refractors are available in small apertures, generally models used in Astrophotography come with diameters from 66mm to 160 mm, these relatively small apertures are sufficient for most of the objects photographed by amateur, as the image sensors in photo or ccd cameras and in long exposures they capture much more light than the human eye. An apochromatic refractor with lens diameter of between 80 mm and 120 mm is the choice of most astrophotography beginners, these scopes being produced in large quantities by many manufacturers, having weights up to 10 kg in general, so they don’t require a very large and very expensive mount for their installation. Refractors with lenses of 130 mm and larger have greater capacity to gather light, have better resolutions and generally are produced by manufacturers in smaller amounts and at a level of optical and mechanical quality that approaches perfection, so that their value increases nearly exponentially with every centimeter of additional aperture.

Schmidt-Cassegrain telescopes are popular in astrophotography because of their design flexibility, because of their superior aperture, and because they are mass-produced with excellent optical quality, which makes their price/cm to be less than the apochromatic refractors of equivalent apertures. If refractors used generally for astrophotography have apertures between 66mm and 150mm, Schmidt-Cassegrain telescopes offer apertures between 150 mm and 400 mm, with more resolution and light-gathering power than a refractor. There are many accessories designed for Schmidt-Cassegrain telescopes that optimize their potential for photography (such as focal reducers/plane correctors, guide adapters, systems for holding the cameras for both focal and afocal photography, the Hyperstar photography system in the main focus of the mirror at a f/2 focal ratio). Because of the optical design of the telescope, which focuses through the movement of the main mirror, it is possible to focus the image with the majority of photo and ccd cameras, and to add various adaptors in the path of light, without the telescope becoming unable to focus. The new Schmidt-Cassegrain plain telescopes optimized for photography (such as Edge HD series from Celestron) produce much better results than the classic design at the edge of the visual field, introducing their optical design in the range of astrographs capable of exceptional performance and to be used in scientific research.

Ritchey-Chretien and corrected Dall-Kirham telescopes are widely used by astronomical observatories due to the larger size of the corrected field produced by these optical designs. Made with apertures in general between 250 mm and up to a few meters in diameter, they offer a resolution and a light-gathering capacity greater than existing commercial telescopes. Until recently, however, their price was the main obstacle in their use by amateurs. In recent years, however, the new models of Ritchey-Chretien telescopes made in China, as well as the CDK telescopes produced by PlaneWave, have made these optical designs accessible to amateurs.

Maksutov-Cassegrain telescopes are less used in astrophotography mainly because of their reduced visual field (focal ratios from f/10 and upwards) that require higher shutter speeds to achieve the same level of detail, of the mechanical design implemented on most of these optical tubes which requires a longer time for that telescope to reach thermal equilibrium with the environment, and because of their high price once they exceed 200mm in aperture diameter. However once climatized, Maksutov-Cassegrain telescopes produce images far more contrasting than Schmidt-Cassegrain telescopes and with better aberration correction in the field of view. Existing commercial models with apertures between 90mm and 180mm are able to provide both excellent planetary images due to their very good contrast, and also images of small deepsky objects.

Newtonian reflectors are a popular design for visual observation, but are less than adequate for photography. And it’s not because of a problem in their optical design, but due to the fact that the majority of Newtonian reflectors on the market are optimized for visual observation, which determines certain build characteristics that don’t make these telescopes suitable for photography. Another element against the Newtonian reflector type is their size and weight once their aperture goes beyond 200mm. Equatorial mounts that are able to firmly sustain a Newtonian reflector of more than 250 mm in diameter and with a long tube are rather few and expensive. The positions that the optical tube can find itself in when pointed toward the various parts of the sky are considered by many astronomers to be quite uncomfortable. However, it’s possible to achieve great shots with a Newtonian reflector if the astronomer arrives to the proper combination of telescope, mount and ccd camera, or uses a Newtonian reflector optimized for astrophotography (quite rare nowadays, but there is a possibility for a seriously passionate astrophotographer to custom order one from those few producers specialized in astrographs or from one of the few telescope mirror polishers). The coma specific to Newtonian reflectors can be corrected easily by means of a coma corrector such as the the MPCC from Baader Planetarium, making it possible to take pictures with dslr and ccd cameras with larger sensors. Especially for planetary and lunar observations, the Newtonian reflectors are used with much success by astrophotographers. In this case, because of the small size of the sensors on the cameras, the optical aberrations at the edge of the field of view (especially coma) are cropped out and don’t make it to the final image.

Astrographs are not a different type of optical design, but scopes of different designs (refractos, Newtonian reflectors, cassegrain) optimized for astrophotography.

b) Mount

The basic element of an astrophotography system is the mount. If with regard to the design of the telescope one can make choices according to needs, preferences and budget, in regard to the mount the options are much restricted.

In the picture above: Celestron CGE PRO mount with a periodical under error 9 arcsecunde, along with a Schmidt Cassegrain telescope C11 Edge HD is an ideal solution, easily accessible.
A Celestron CGE PRO mount with a periodic error under 9 arcseconds, along with a C11 Edge HD Schmidt Cassegrain telescope is an ideal solution, easily accessible.

A proper mount for astrophotography must be able to sustain the telescope stably, without vibrations, and to allow it to track closely the movement of objects/stars on the night sky. The level of precision required is 1-2 arc seconds, depending on the quality of the atmosphere, the mount must be able to maintain the stars in position on the camera sensor with an accuracy of a few pixels. The atmosphere in most parts of Europe limits resolution to about 2 arcseconds, but there are areas with better sky where 0.8-1 arcseconds can be reached. Few mounts are capable of tracking both accurately and stably enough for purposes of astrophotography. The more stable they are, they heavier they get; and the more accurate they are, the more expensive they will be.

The 10 Micron GM 2000 Professional Equatorial Mount can sustain large astronomical telescopes
The 10 Micron GM 2000 Professional Equatorial Mount can sustain large astronomical telescopes

Each mount manufacturer grades the ability to sustain a telescope in the form of a number of pounds of equipment that can be supported by the respective mount. This number usually pertains to visual observation. For photo stability requirements are more strict, a general rule to calculate the maximum capacity of the mounts being to divide by 2 the number given by the manufacturer. High-end mounts are manufactured with smaller tolerance ranges, and the total maximum weight suitable for astrophotography will be closer to the maximum total weight suitable for visual observation. The mounts currently considered suitable for astrophotography have a periodic error on their mechanics of under 7 arcseconds, which can be further reduced by software, as for instance the GM2000 (in the picture), GM4000, Vixen AXD, all AstroPhysics, Paramount ME mounts.

However this doesn’t mean one can’t do astrophotography with lower quality mounts with less accuracy, but in their case the loss of quality must be compensated by more aggressive guidance, own personal tolerance to errors, and readiness to seek solutions to problems specific to each mount model and each individual piece. Although most experienced astrophotographers use optimized mounts, the majority of pictures made by amateur astrophotographers  are taken using weaker mounts, such as the various mounts from Celestron, Vixen, Takahashi, Losmandy and Sky-Watcher. The mounts that come standard with starter telescopes (EQ1, EQ2, CG2, CG3, NexStar SLT, Dobson mounts, other small mounts and altazimuth tripods) are not suitable for astrophotography, or perhaps for no more than lunar and planetary photography using lightweight cameras, and only if the mount is motorized at least on the RA axis.

Equatorial mounts are preferred for astrophotography, because, once polarly aligned, they can track the night objects’ movement on just a single axis, thus eliminating the field rotation produced by the two-axis tracking done by altazimuth mounts. Altazimuth mounts (generally altazimuth fork types) can assist in taking shortly exposed images up to 30 seconds without field rotation effects, but for long-term exposure it’s necessary to use a derotator or put them in equatorial position using an equatorial wedge, which complicates a lot the mount assembly and can cause balance issues of the telescope on the mount.

Modern equatorial mounts suitable for sky photography are equipped with a system of tracking motors that can compensate precisely for the Earth’s movement of rotation; they also have under-sidereal tracking speeds, can be connected to the computer, and accept commands for error correction from the user (automatically through the computer or manually). The accuracy of these motors, combined with the precision of the mount’s mechanical elements, determine the general accuracy of the mount and tracking. There are many different manufacturers of tracking systems for mounts, but these days it is recommended to use a mount with goto tracking, as generally speaking, these systems are more accurate than the classic ones still in use, and can be connected to a computer to receive commands through Planetarium-type programs or for automated guiding.

Below are some equatorial mounts adequate for astrophotography with recommendations for the total weight of equipment that can be installed on stably, both for visual observation and for astrophotography. The levels of quality and precision are not similar, smaller and less expensive mounts being less accurate than the larger higher-end mounts.

Mount type
maximumum weight viewing use / astrophotography use
Examples of equipment that can be sustained stably for astrophotography
Celestron CG4 Omni with kit SynScan GOTO,Sky-Watcher EQ3-2 SynScan GOTO 5-6 kg / 2-3 kg
  • refractors with apertures between 66mm and 70mm
  • Schmidt-Cassegrain <102mm,
    Maksutov-Casssegrain <102mm
  • newtonian reflectors with apertures of maximum 130mm f/5
Celestron CG5- GT,Sky-Watcher EQ5 SynScan GOTO,Vixen GP2 8-10 kg / 5-6 kg - refractors with apertures between 80mm and 120mm

- Schmidt-Cassegrain 127mm – 203mm,
Maksutov-Casssegrain 102mm – 150mm

- newtonian reflectors with apertures of maximum 150mm

Sky-Watcher HEQ5 PRO,Vixen GPD2 12-15 kg / 8-10 kg - refractors with apertures between 80mm and 120mm- Schmidt-Cassegrain with apertures between 127mm and 203mm- newtonian reflectors with apertures of maximum 200mm
Celestron CGEM,Sky-Watcher EQ6 PRO,Losmandy GM8 15 kg / 10-12 kg - refractors with apertures between 80mm and 130mm- Schmidt-Cassegrain, Dall-Kirham, Ritchey-Chretien with apertures between 127mm and 280mm- newtonian reflectors with apertures of maximum 254mm
Celestron CGEM DX,Astrophysics Mach1GTO,Vixen AXD 20kg / <15 kg - refractors with apertures between 80mm and 150mm- Schmidt-Cassegrain, Dall-Kirham, Ritchey-Chretien with apertures between 127mm and 300mm
Celestron CGE,Vixen Atlux,Losmandy GM11 30 kg/ <20 kg - refractors with apertures between 80mm and 150mm- Schmidt-Cassegrain, Dall-Kirham, Ritchey-Chretien with apertures between 127mm and 300mm
AstrophysicsAP 900GTOLosmandy Titan 32 kg / 20-25 kg - refractors with apertures between 80mm and 160mm- Schmidt-Cassegrain, CDK, Dall-Kirham, Ritchey-Chretien with apertures of maximum 350mm
Celestron CGE PRO 45 kg / 30-35 kg - refractors with apertures between 80mm and 180mm- Schmidt-Cassegrain, CDK, Dall-Kirham, Ritchey-Chretien with apertures of maximum 400mm
10 Micron GM 2000,Vixen Gaiax 50 kg / <40 kg - refractors with apertures between 80mm and 200mm- Schmidt-Cassegrain, CDK, Dall-Kirham, Ritchey-Chretien with aperturi of maximum 400mm
AstrophysicsAP 1200GTO 64 kg / <50 kg - refractors with apertures between 80mm and 200mm- Schmidt-Cassegrain, CDK, Dall-Kirham, Ritchey-Chretien with apertures of maximum 500mm
10 Micron GM4000,Astrophysics AP3600 GTO 150 kg / <120 kg - refractors with apertures between 80mm and 350mm- Schmidt-Cassegrain, CDK, Dall-Kirham, Ritchey-Chretien with apertures of maximum 700mm

In evaluating the total weight of the telescope that can be mounted on a mount, one must bear in mind that besides the main telescope the mount will have to sustain the weights of: a guiding looking glass, various adapters, rings, clamps and photo camera, which all contribute to the final weight of the assembly. The main telescope weight will always be less than the above values precisely for the reason of leaving room for installation of the other necessary accessories, without exceeding the total weight limit specified for the mount.

Existing mounts are not perfect due to the errors admitted in the manufacturing process of mechanical parts and motors, so any mount, no matter how well aligned polarly, will have tracking errors that accumulate during long exposures. To reduce errors, the mount must be assisted by a manual or automatic guiding system. Also, one can use the periodic error reduction function implemented in the software of goto mounts.

c) Accessories

Guiding system: to reduce tracking errors in equatorial mounts (caused by both the mechanical imperfections of the mount, but

Atik Titan CCD Camera - can be used for deepsky photography with long exposure times, for planetary photography and as guide camera thanks to the ST4 port which allows connection directly to a GOTO equatorial mount
Atik Titan CCD Camera – can be used for deepsky photography with long exposure times, for planetary photography and as guide camera thanks to the ST4 port which allows connection directly to a GOTO equatorial mount

especially by the less than precise polar alignment) and to prevent the stars to be transformed from spots of light into lines of light, in the case of exposures longer than 15-20 seconds, the mounts must be necessarily guided. The guidance system is comprised of: i) a looking glass mounted parallel to the main telescope or an adapter for radial guidance, ii) an eyepiece with illuminated reticle through which the astrophotographer can watch for the star chosen as a guide to not go out of the center of the reticle – in the case of manual guidance, or iii) an automated guidance system constituted either by a third-party autoguider such as the LVI Smartguider or SBIG SG-4, or by a computer-controlled ccd camera (such as the NexImage,

Autoguider Smartguider - a third-party autoguider that has a control unit and a ccd camera able to automatically control any mount
Autoguider Smartguider – a third-party autoguider that has a control unit and a ccd camera able to automatically control any mount

DMK/DBK, SBIG, or any astrophotography-dedicated ccd camera) and an autoguidance software program that can automatically send control commands to the mount depending on the image captured by the ccd camera (such as CCD Soft, MaximDl, PHD Guiding, GuideDog, Envisage, or other proprietary software).

 

Telescope prepared for astrophotography almost ready to begin session -  An Equinox 120 refractor is installed on a HEQ5 SynScan PRO mount. Parallel to the refractor a second Scopos 66 ED is installed via guiding rings and used as guiding looking glass. The guiding camera is inserted into the focuser of the Scopos scope and sends images to the controller of the Smartguider autoguider which is conencted in turn to the HEQ5 mount to transmit correction commands based on the motion of the guide star viewed by the camera. Missing from the image is the ccd camera that will take the actual shots, and which is going to be installed on the focuser of the Equinox 120 tube.
Telescope prepared for astrophotography almost ready to begin session -
An Equinox 120 refractor is installed on a HEQ5 SynScan PRO mount. Parallel to the refractor a second Scopos 66 ED is installed via guiding rings and used as guiding looking glass. The guiding camera is inserted into the focuser of the Scopos scope and sends images to the controller of the Smartguider autoguider which is conencted in turn to the HEQ5 mount to transmit correction commands based on the motion of the guide star viewed by the camera. Missing from the image is the ccd camera that will take the actual shots, and which is going to be installed on the focuser of the Equinox 120 tube.

Adapters: For mounting the telescopes on the mount, and the cameras on the telescope, a series of adaptors are required, depending on the telescope, camera, and accessories to be installed.

Filters: filters are absolutely necessary when shooting with dedicated ccd cameras to both produce color images, and to isolate certain wave lengths emitted by the celestial objects to be photographed. Ccd cameras with monochrome sensors, popular among astrophotographers due to their great sensitivity, require the use of a set of LRGB filters to produce color images.
efw2
The sky object is photographed through each of the four filters, and by the composition of the 4 monochrome images we obtain the final color image. Also, for photographing in the narrow band lengths, to isolate only the main emissions of certain objects, special filters designed for astrophotography are used (h-alpha, h-beta, SII, OIII). For ease of changing the filters, they are mounted on a filter wheel with which one can change the filter by simply rotating a disc or automatically by a computer; the wheel is attached in front of the ccd camera (see the picture to the right). For planetary photography, colored filters allow the selective increase in contrast on certain wave lengths, but also to obtain a color image when using monochrome ccd cameras. Also, a UV/IR-cut filter is absolutely necessary in planetary photography with any ccd camera or webcam.

Focusers: For getting the best possible focus it takes a solid focuser capable of supporting the weight of the ccd/photo camera, with a fine course which allows a fine-tuning of the focus and that maintains its focused position during photo sessions without sliding/deflecting under the weight of the camera. Few of the mass produced telescopes have focusers smooth enough to be able to adjust the focus/sharpness precisely, generally only premium instruments being equipped with focusers suitable for astrophotography. High-end telescopes and some models optimized for photography come from manufacturers with focusers solid and fine enough for astrophotography. The rack and pinion focusers that come with most cheaper telescopes, because of the coarser steps of the cogwheel, do not allow a fine adjustment such as in the Crayford focusers. More expensive telescopes and high-end ones are fitted with Crayford-type focusers with one or two speeds. Crayford focusers also come in different levels of quality, the best ones featuring a dual focusing system (coarse and microfocusing), solid build, and smooth motion of the focuser’s tube. The recent flood of two-speed Crayford focusers produced in China and installed on more expensive telescopes of Chinese origin made the astrophotographers’ activity easier, but can’t provide total relief from this point of view, because their quality is still poor compared to good focusers: the course is not smooth enough, can not sustain cameras heavier than 4-500 grams without slipping under the weight of the camera, very rapid wear of the mechanism and the appearance of gear slops. In this area, the price is a good indicator of a focuser’s quality.
steeltrack-focuser
Premium focusers such as models from Feather Touch, Moonlite, Steeltrack (image on the left), JMI, Hyperion, or automatic ones from Optec, FLI, JMI or Hedrick stand out through their solidity and accuracy and the ability to automatically adjust through the computer for the best focus position; in the case of automatic focusers, this capability reduces and simplifies a critical operation for obtaining a good image. The solution to the issues of rack and pinion focusers as well as of lower-quality Crayfords lies either in changing the telescope’s focuser with a better one if the telescope allows it, or in the addition of an external manual Crayford focuser, or of an electronic or automated focusing system. Despite the fact that catadioptric telescopes (SCT, MCT, RC, CDK) have solid systems for smooth focusing, most astrophotographers resort to external Crayford or automated focusers for simpler and finer focus tuning.

d) Photo camera or CCD dedicated camera

A photo camera or a CCD dedicated camera is the device that captures all of the photons gathered by the telescope and turns them into an image visible to the human eye, which can be subjected to subsequent processing. Basically you can use any camera with film or digital in existence today that can be mounted via different adapters on the telescope’s focuser. But for best results it is advisable to use a dslr or slr, or a cooled ccd camera dedicated to astrophotography. Because film cameras are a lot less used today, we will only discuss about digital cameras.

Dslr cameras are an easy way to capture images as the current chips used in these cameras are highly sensitive and perform well during long exposures. Most astrophotographers who use Dslr cameras choose either a Canon or Nikon due to their more astrophotography-friendly characteristics (more sensitive sensors with very little noise, the possibility of mirror lockup in the upper position, saving of images in uncompressed raw format). The main shortcomings of Dslr cameras are: i) many of the celestial objects emit strongly in wavelengths at both ends of the visible spectrum (ultraviolet-infrared), but the sensors in dslr cameras are optimized for daytime photography in the visual spectrum of wavelengths. Thus their sensitivity at the ends of the spectrum
canon-eos-7d-astronomy
is very poor compared to the dedicated monochrome ccd cameras’ sensors, requiring much longer exposures to capture the same amount of information in these wavelengths. The filter installed by manufacturers in front of the sensor also blocks a portion of the infrared and ultraviolet spectrum, reducing sensitivity. Many astrophotographers resort to replacing the filter on the chip with a broader spectrum filter, but this change affects the color balance in daytime photography. ii) The camera’s sensor is not cooled so that the images have a very high total noise compared with a cooled chip. iii) The quantum efficiency of dslr chips used in cameras is extremely low, so that most of the photons that reach the chip are not transformed into electrons and thus do not make it in the final image; iv) The sensors in dslr cameras are color chips, and the Bayer matrix through which the chromatic information is obtained reduces the amount of light that reaches the sensor, filtering each photodiode to capture a single color. Thus the color chip’s sensitivity is much lower than that of a monochrome chip, requiring higher exposure times for gathering the same amount of light.

Atik4000
Cooled ccd camera Atik 4000

Cooled ccd cameras dedicated to astrophotography eliminate the main problems of dslr cameras. Through controlled cooling of the camera’s chip, noise is reduced to a minimum. In case of subsequent photo sessions on the same object, the temperature control on the ccd sensor allows the taking of photos under similar technical conditions, technique used especially in comparative quantitative measurements.

The quantum efficiency of the chips used in these cameras is superior to that of the chips in dslr cameras or webcams, such that most photons that arrive on the chip are transformed into electrons and make it in the end-image. The chips in current ccd cameras cover a wide range of physical dimensions, pixel sizes, types of illumination, and sensitivity spectrums, so that one can choose a camera based on the reolution and focal distance of the telescope and type of photography the camera will be used for. One can also choose between color and monochrome versions of the same chip, the majority of astrophotographers preferring cameras with monochrome chip and separate colored filters carousel for obtaining colored images, due to the increased sensitivity of monochrome chips and their better performance in narrow band photography. Not having their own lesn, these cameras are mounted directly in the telescope’s focuser, eliminating thus any distortion due to intermediate optical elements. Among the established producers of dedicated ccd cameras we mention ATIK (image above), SBIG (picture below), Apogee, FLI, Starlight Xpress, QSI, QHY, Trifid, but there are others as well.

Cooled ccd camera SBIG STL 11000
Cooled ccd camera SBIG STL 11000

Generally, the differences between good ccd cameras and ccd cameras with lower performance can be found on the level of chips used (chips with fewer defects, more efficient chips and with less noise, a wider range of chips deployed, with different sizes for different photographic applications), the level of optimization of the camera electronics to the chip used, longevity of the camera, efficiency of chip cooling, the software developed for the control of the camera, compatibility with other control software and image capturing software etc). A good ccd camera can be used for many years, with the same performance, so the best advice is to choose quality over saving some cash, to choose established manufacturers who actively developed and optimized several generations of cameras and can provide support for their products.

neximage
NexImage ccd camera for Moon and planets photography

In the case of planetary and lunar photography, the features of the ccd cameras are not that critical, the planets and the Moon being quite bright objects that don’t need long exposure times. Sensor size is not important in the case of planetary photography, as the apparent sizes of planets are small and do not come to occupy the entire surface of the smaller ccd chips except in very rare cases (shooting with large telescopes with extremely long focal distances under excellent atmospheric conditions). Thus for planetary photography the most widely used ccd cameras have chips with low resolutions of 640 x 480 pixels, but for getting a larger visual field, cameras with greater resolutions of 1024X767 or 1280 x 960 pixels are used.

More important than the size of the chip is its sensitivity, so that chips used in planetary ccd dedicated cameras (Sony chips in general) need to be very sensitive and effective. In the case of this type of photography the most used dedicated ccd cameras are the NexImage cameras from Celestron  (left image), the old series of successful ToU Cam webcams from Philips, now out of production (Vesta 740K, 804K, 900NC), TITAN cameras from Atik (see picture above), and especially the DMK/DBK ccd cameras from Imaging Source (image to the right). In the last years the cameras from Imaging Source came to be the choice of many astrophotographers experienced in planetary and lunar photography thanks to these cameras’ superior performance.

cameraccd_dmk_imagingsource
Camera for Planetary and Lunar photography DMK/DBK Imaging Source

They represent a perfect combination between highly sensitive chips and resilient electronics, industrial grade builds, superior to consumer cameras.

Satisfactory results can also be obtained with compact photo cameras by the method of afocal projection through the eyepiece, but in this case the image is affected by the quality of the camera lens and by the quality of the image produced by the eyepiece. Compact cameras are not designed for planetary photography, but for terrestrial photography during the day, so that the results obtained with compact cameras don’t have the same high level of detail and quality as the ones obtained with dedicated planetary ccd cameras.

e) programs for capturing and processing of the images

Ccd cameras dedicated for astrophotography require a permanent connection to a computer, as they don’t have storage for the captured images. Specialized software (such as CCD Soft, MaximDl, Nebulosity, or others proprietary to camera manufacturers) controls all the components of a ccd camera, allowing to take pictures with set exposure times and at various chip temperatures, controls various models of filter carousels, automatic focusers and the mounts that telescopes are installed on. Dslr type cameras save images on their card, the number of images that can be stored being limited by the card’s capacity.

Once exposure settings and the calibration frames are completed with a ccd, dslr or a webcam, for getting the final image the individual pictures have to be processed. Briefly, there are 3 steps in image processing: a) calibration of frames (extraction of darks, flats and biases, frames alignment), b) overlapping/stacking of frames, and c) processing of the image thus obtained for the emphasizing of certain details, adjustment of contrast, brightness, color, etc. All these processing stages are done using specialized software for image processing, or astrophotography programs like CCDSoft, MaximDl, DeepskyStacker, AstroArt, Iris, Nebulosity, PixInsight, Registax, and others, or general programs such as Adobe Photoshop, GIMP, PaintShop Pro, or any other more advanced image processing program. The transition from raw images produced by the ccd camera to the finished image requires good knowledge of the techniques of astrophotography and computer image processing. Getting a good set of exposures is just a premise for getting a good final image, since processing techniques can highlight details in the final image that were barely visible in the raw images. Also the processing can reduce certain flaws of the camera (mainly noise, defective pixels, dust on the sensor).

5. What is focal ratio and why is it important for astrophotography?

Focal ratio is the ratio between the focal length of the telescope and the diameter of the lens, both expressed in millimeters. Thus, a telescope with a diameter of 200 mm for the main mirror and a focal length of 1000 mm has a focal ratio f/5. Or, knowing the telescope’s focal ratio and the diameter of the main mirror, we can find the focal length of the mirror. The term focal ratio is very familiar to photographers, photo lenses being identified by their focal ratio. In the case of cameras, a lens with a shorter focal ratio will produce brighter images on film or sensor, allowing for shorter exposures when photographing dimly lit objects. The same goes for telescopes. Telescopes with lower focal ratios will produce brighter images on chip, reducing the exposure time needed to capture less bright objects. Telescopes with larger focal ratios need longer exposure times in order to obtain a similar level of detail compared to a telescope with an objective which is identical, but with a smaller focal ratio.

That’s the reason why for deepsky photography telescopes with smaller focal ratios are preferrred, or whose focal ratio can be lowered with focal reducers. In that sense, astrographs are made with small focal ratios to reduce the exposure time required to obtain a certain level of detail. However, deepsky photography can be achieved just as well with telescopes with longer focal ratios, but the exposure time needed to obtain the same level of detail will grow. Generally focal ratios between f/5 to f/7 are considered adequate for deepsky photography. Focal ratios shorter than f/5 are found only on special instruments optimized for astrophotography (certain refractors, reflectors, schmidt cameras, or schmidt-cassegrain telescopes whose secondary mirror was replaced with a Fastar or HyperStar system).

For Planetary and Lunar photography, the focal ratio that’s needed in order to capture detail is relatively large, focal ratios between f/20 and f/50 being currently in use by astrophotographers. The planets are objects of small angular size, and a high magnification power is needed to capture details, which translates into long focal distances and large focal ratios. Being very bright objects, exposure times are generally less than 1/10 seconds, and telescopes with large focal ratios can be used successfully (either native or by use of a barlow lens to increase focal length).

Focal ratio however, does not have the same effect on the visual observation. A telescope with a small focal ratio will not produce brighter images than a telescope with a higher focal ratio. The image seen through a telescope with a diameter of 200 mm and f/5 focal ratio, and that seen through a telescope with a diameter of 200 mm and f/8 focal ratiowill be identical if the magnification powers used are the same. In the case of visual observation, the brightness of an object seen through the telescope is dependant on the size of the telescope’s objective and the magnification power used. That way, the larger the diameter of the telescope’s objective, the brighter the resulted images are, because the telescope captures more light. The more light captured by the scope’s objective, the more the image produced by it can be magnified. And greater magnification power results in a less brighter image.

6. Ccd camera with monochrome or color chip?

Choosing between cooled ccd cameras with monochrome and color chip is an eternal dilemma for beginner amateur astrophotographers due to lack of experience and knowledge about ccd chips. The easiest way to answer this question is to state that the most experienced astrophotographers use ccd cameras with monochrome chip. What is the difference between cameras with monochrome and chips and those with color chips? The answer lies in the way they are made.

Color chips are actually monochrome chips, the sensor pixels being covered with a filter in the form of a Bayer matrix which contains alternative areas of red, green and blue-GRGB filters. The Bayer matrix allows getting a color image instantly, with a single exposure, sacrificing the spatial resolution of the chip and the native pixel sensitivity to the entire visible light spectrum/ultraviolet/infrared. The raw data from the sensor and subsequently processed by the camera are in the form of a black and white image with pattern shapes. The pattern is due to the Bayer matrix on the sensor. Each pixel is sensitive to only one color, of the filter superposed on it. To convert this raw image into color image, each pixel is assigned a value corresponding to the filter above it. The camera software interpolates for each pixel the values of the missing colors by analyzing the adjacent pixels and by estimating the missing values, a process called debayering or simply color conversion. There are many algorithms for converting to color, and every algorithm is a compromise.

The Bayer Matrix
The Bayer Matrix

Dslr cameras effect the debayering process in the internal software of the camera, producing color images. On some models there is the option to capture raw images, the debayering being done by the user through the use of various computer programs. Astronomy ccd cameras with color chips do not work the debayering process internally, but this is done later on through image capture software and processing.

Monochrome chip ccd cameras produce black and white images. Generating a color image using monochrome chips takes place after a process similar  to the bayer matrix principle, except that the entire camera chip is exposed successively to light filtered through red, green and blue filters, plus a luminance filter. Thus, filters are placed one at a time in front of the ccd sensor, an image being taken through each filter. To make it easy to change the filters, they are mounted in a filter wheel which is attached in front of the camera and which allows the manual or computer-controlled change of the filters. The raw images thus obtained are monochrome, but the information captured in each reflects the light spectrum of each filter. After the completion of the 4 images through the 4 filters, they are processed by means of image processing software such as Adobe Photoshop, CCDSoft, MaximDL, AstroArt, Nebulosity, etc to get the color image. The four images are juxtaposed and each image is colored automatically or manually to the color of the filter through which it has been exposed. This way, a final color image is obtained.

Advantages of ccd cameras with monochrome chip over ccd cameras with color chip:

-Images in higher resolution: because there is no bayer matrix in the monochrome chips, all the pixels of the chip are exposed to each light channel. In the case of color chips, 50% of the pixels are exposed to green light, 25% of the pixels are exposed to red light, and 25% of the pixels are exposed to blue light. Color chips need the debayering of raw images in order to get the colored image, a process which interpolates the values of missing color channels for each pixel of the chip. For this, the camera software or an image processing program applies an algorithm that analyzes all the adjacent pixels to make a guess as to what values these pixels should have if they were exposed to the other color channels. It generates such approximate values for each pixel. The raw spatial resolution and the amount of information contained in the image is lower than in the case of a monochrome chip without bayer matrix, fully exposed to each color channel. Depending on the debayering algorithm used, resolution is at least 30% lower than of an image obtained with a monochrome chip, and also various artifacts appear in the image, that further reduce the quality of the final image.

-Higher pixel sensitivity and a larger amount of captured information: because there is no bayer matrix, a monochrome chip is exposed in its entirety to the entire light spectrum, and each pixel captures the photons of light that reach it, until the pixel’s photon well is filled. Through the interplay of a color filter, the entire ccd chip captures all the photons emitted in the light spectrum of the respective color that reach the chip. Thus the chip works at maximum capacity, using all the pixels it’s made of to capture photons on its entire surface for each color channel. In the case of color chips, because of the bayer matrix, not all pixels captures the entire quantity of light: 50% of the pixels capture photons in the spectrum of green light, but not in other areas of the spectrum, 25% of the pixels capture light in the red spectrum, but they are insensitive to the rest of the spectrum, and 25% of the pixels only capture blue light and are insensitive to the rest of the spectrum. Furthermore, it was found that the filters that make up the bayer matrix have a smaller rate of light transmission than astronomy ccd filters, whereby the sensitivity of the chip is further reduced than in the case of astronomy ccd filters. Due to the limited spectrum of light transmitted by the filters in the bayer matrix in the visible light spectrum, and the smaller amount of photons reaching the pixels, color chips are almost insensitive to the spectrum of the light located at the ends of the visible spectrum, namely, the h-alpha, h-beta, infrared, ultraviolet wavelengths, which are very important in astronomy, as many celestial bodies have strong light emission in these wavelengths. Monochrome chips are sensitive to these wavelengths. This increased sensitivity of the monochrome chip is very important for astronomy, because the light that comes from the celestial objects is very poor. Due to its higher sensitivity and its use of the entire available surface, the monochrome chip allow to get the same level of detail in less time, with fewer exposures than color chips. Or looking from another angle, a monochrome chip can capture more light/photons at the same time than a color chip, so more details are visible. The most important color channel is luminance, which contains the black/white information from the entire visible spectrum of light. For obtaining colors, or the chrominance channels with a monochrome chip, one can assign shorter times for exposures through the RGB filters, or use binning to reduce exposure time.

-The possibility of using narrow-band filters: Besides getting color images in the visible light spectrum by the use of LRGB filters, color chip cameras allow the use of narrow-band filters, centered only on certain well isolated wavelengths in which some celestial bodies emit strongly. Thus, highly used are the h-alpha, h-beta, OIII, S2 filters, especially for photographing nebulae that emit strongly in these wavelengths. The images obtained by the filters can be superimposed with images obtained through the LRGB filters in order to obtain a final image with more details. One can also take composite pictures in false colors, obtained by the superimposition of images produced by only 3 or 4 narrow band filters, in a color palette similar to that produced by the Hubble space telescope. The advantage of this technique rests in the fact that these wavelengths are less blocked by the light pollution produced by lighting systems, which in cities flood the sky making it impossible to view the majority of sky objects. Narrow band photography can be made in the cities, without interference from light pollution. For amateur astronomers who can’t go out into light pollution-free areas, narrow band photography is the only option to take images of deepsky objects.

-The possibility of pixel binning: monochrome chip cameras can be binned  2 x 2, 3 x 3, 4 x 4, to obtain larger pixels (4, 9, 16 times larger), more sensitive, with improved signal-to-noise ratio. Binning takes place at the level of reading the photo-sites, which are combined and read in such a way as to create larger equivalent photo-sites. The total capacity of the photon well of new superpixels is also greater, making it possible to collect a charge greater than one pixel. Pixel binning helps adjusting the optimum pixel size for different telescopes and to reduce download times of images to the computer, a useful operation in the stage of telescope focusing. Color ccd chips cannot be binned due to the bayer matrix.

-Are better suited to scientific research: all ccd cameras used for scientific research have monochrome chips. For scientific research color is not important, whereas more important is the possibility of isolating different segments of the spectrum of light by using special filters in front of the camera chip that only let pass the desired wavelengths. It is also important that the chip performs at full capacity to gather the maximum amount of photons from celestial objects/stars of little brightness in the minimum amount of time. Using the chip at its entire resolution, without the artifacts introduced by the bayer matrix, allows recording all the unspoiled information coming from the region of sky under study, which is very important for instance in photometry, astrometry, or the study of asteroids. An example in that sense are the very weak stars: when using color chips, in to correctly identify the position and color of a star, its image must spread over at least 3 adjacent pixels; in the case of monochrome chips, it is enough that it only touches 1 pixel.

Advantages of color ccd chip cameras over monochrome ccd chip cameras:

-Simpler procedure for capturing and processing of images: thanks to the bayer matrix in color chips, the capture of the luminance and chrominance channels takes place in one exposure. In the case of monochrome chips, it’s necessary to carry out separate exposures through at least 3 different filters. In image processing it is no longer necessary to overlap the color channels, but debayering is required.

-Rapidity in getting pictures of objects that change rapidly: In the case of photography of the planets and the Sun, the color ccd chips are preferred by many astrophotographers because these objects go through rapid transformations of their surface and atmosphere. For example, the rotation of planet Jupiter becomes visible in pictures after about 2 minutes of exposure. So in order to “freeze” the image, a photo session lasts for a maximum of 2 minutes, enough time for a color ccd camera to obtain up to 7,000 frames. However, some astrophotographers choose to utilize monochrome ccd cameras together with a filter wheel with 4 LRGB filters, allotting 15-20 seconds to the exposure through each filter, enough time for getting up to 1000 frames through every filter, and a total of about 4,000 final frames. The lower number of total frames obtained from exposure through separate color filters is compensated by the better quality of these frames.

-Lower equipment costs: In the case of color chip cameras it’s no longer necessary to purchase any LRGB filters or a wheel for them.

Overall, ccd cameras with monochrome chip produce images of a better quality, are more sensitive and have better resolution. Although most beginners in the field of astrophotography are attracted to color chip ccd cameras, most of them, after a short period of time, become aware of the limitations of the color chip, and make the transition to monochrome ccd cameras. Photography with monochrome chip cameras is not more difficult, obtaining an image takes just as much time as in the case of a color camera. Moreover, for many applications getting a color image is pointless. There are many amateur astronomers who perform personal or scientific research using amateur telescopes and ccd cameras, such as studies on variable stars, astrometry, spectrometry, search for asteroids, situations where monochrome chips only offer advantages due to their flexibility. Even for aesthetic astrophotographs, monochrome chips add to the increased quality of image and details, the possibility of using narrow-band filters. For amateur astronomers who carry photography in cities this possibility is a life saver, because otherwise they would find it impossible to shoot through the light pollution that almost completely blocks the visible light spectrum coming from stars. Narrow band photography is the only form of astrophotography that yields good results from cities.

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