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CAMERAS AND IMAGING IN ASTROPHOTOGRAPHY
There are many options available to the astrophotographer that cover a range of budgets and sophistication. Things can be exceedingly simple – a normal camera on a static tripod or mount – or they can quickly become more sophisticated, using dedicated CCD cameras with a motorized filter wheel and autoguiders.
Without the digital sensor, modern astrophotography would be a very different experience. Understanding the limitations of a digital sensor is pretty significant and your choice of camera has a large impact on system choices and budget. Their various properties play a pivotal role in the end result and these properties are mentioned throughout the article.
Why CMOS image sensors.
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Using your existing digital camera (often fitted with a CMOS sensor) with a motorized mount is a great place to start and for some, this solution can meet their needs for a while. There are several excellent books that concentrate on conventional camera-based astrophotography. At one time, this was limited to digital SLRs, but increasingly this can also be with one of the many mirror-less cameras that have interchangeable lenses, either using a telephoto lens or attached to the rear of a telescope using a simple adaptor.
Conventional Digital Cameras.
It makes sense to kick off astrophotography using your existing digital camera. All the main camera brands have their devotees and critics and although I might be shot for saying so, there is not much to choose between them. In astrophotography however, there are a few specialist requirements that favor a few brands. These are: remote operation and good quality long exposures.
Remote Operation.
SLR and mirror-less cameras are both capable for use in astrophotography. Focusing is an unavoidable requirement in any setup. Even with 10x image magnification on its LCD screen, it is very difficult to judge the precise focus on any star, even with a focusing aid such as a Bahtinov Mask.
With the right software, however, and tethered operation over a USB connection, things become much tidier; imaging software takes repeated images whilst you adjust the focus and plots the FWHM or HFD (half flux diameter) for each exposure. The best focus is achieved when the FWHM is at its lowest value. Although several camera brands supply general software utilities for remote capture, it is the third-party apps and full astrophotography programs that have the tools to assess focus accuracy.
At present, the best-supported SLR brand by astronomy programs is Canon EOS. The modern cameras operate via their USB connections but not all support long exposures (greater than 30 seconds) directly. Those models and the older pre-USB models can be triggered with a modified electric cable release and an interface circuit to a PC. Long exposures consume batteries and several will be needed for a full nights imaging. You can the Canon EOS 6D Mark II in our recommended products here.
The process of changing batteries can disturb the imaging alignment and for uninterrupted operation, a battery adaptor powered from 12 Volts is a better option. The battery adaptors supplied by the OEMs or third-party retailers comprise a battery adaptor and a small DC power supply, powered by mains. With a little ingenuity a small variable DC-DC converter module can be housed in a small plastic box and adjusted to provide the exact same DC voltage at the camera end, from a 12-volt power source. This is a safe and reliable alternative and can share the local 12-volt DC source.
Image Quality.
Camera shake from a SLR mirror and shutter is not an issue during long exposures, as any vibration fades in milliseconds. There are a few settings on a camera, however, that you need to take care of to extract the best quality. All digital cameras have a long-exposure noise-reduction option, which takes and subtracts a single dark frame exposure from the image.
This is not sufficient for high-quality astrophotography and should be disabled. It is also really important to use unmolested RAW files in the highest possible bit depth and without any interference from any in-camera processing. For best results, images are stored in the camera’s native RAW file format and to reduce thermal noise, any toasty “Live View” option is turned off whenever possible.
RAW files are not always what they seem; keen-eyed amateurs have noted that some older Nikon cameras process RAW files and mistakenly treat faint stars as hot-pixels and remove them. Additionally RAW files are not quite as unadulterated we are lead to believe. All the cameras I have tried have to some extent manipulated the RAW data in camera. This undisclosed manipulation is often detected in weird dark current results and require special attention during image calibration prior to stacking.
Image processing, especially with deep space images, severely distorts the tonal range to show faint detail and requires the highest tonal resolution it can muster. Most digital cameras have 12- or 14-bit resolution in their sensor electronics. These produce 212 (4,096) to 214 (16,384) light levels for red, green and blue light, stored in a 16-bit file format.
Image processing on the combined image files averages between exposures and creates a higher tonal resolution that ideally uses 32-bits per channel. JPEG files on the other hand have just 8-bits per channel resolution and this is insufficient to withstand extreme image processing without displaying abrupt tone changes (posterization). Choosing a RAW file format also bypasses any in-camera high-ISO noise reduction modes that generally mess things up.
What is Image Noise?
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In-camera noise reduction typically blurs the image to reduce the apparent noise but in doing so, destroys fine detail. For a static subject there are far more effective means to reduce shadow noise and astrophotographers use multiple exposures combined with statistical techniques to reduce image noise in dim images.
It is easy to imagine the benefits of remote operation. In addition to the welcome convenience during a long exposure sequence there are sometimes less obvious benefits to imaging quality too. A third, lesser-known benefit of remote operation occurs during the conversion from the separate RAW sensor element values to a RGB color image. Camera RAW files require processing (de-Bayering) to create a color image from the individually filtered sensor values.
The standard RAW converters used by photographers and the standard RAW converters apply some averaging (interpolation) between adjacent sensor elements for pictorial smoothness. (Some cameras additionally have an anti-alias filter, a sort of mild diffuser, in front of the sensor that accomplishes the same thing.) In general photography, an antialias filter trades resolution for smoothness and is most needed to depict straight lines without jagged edges or strange colored banding.
While this is important for normal pictures it is all but irrelevant to astrophotography as there are no straight lines. Several astrophotography programs use their own optimized algorithms (especially for star-fields) that preserve and later convert the individual RGB information into a 16- or 32-bit RGB FITS or TIFF image file. If you have the choice, choose remote tethered image capture, using an astrophotography program rather than store on the memory card. Photography and astrophotography have different visual needs and the specialist capture programs are optimized for the purpose.
Color Sensitivity.
The individual sensor elements (some texts refer to these as photosites) in a CMOS or CCD have a broad color sensitivity that extends from ultraviolet (UV), through visible and includes infrared (IR) light. Video camcorder “night-shot” modes make good use of the extended infrared sensitivity but this is not a desirable feature in either general photography or astronomy. Infrared light will focus to a different point through any refractive optic and even color corrected compound elements will refract visible and infrared light to a different extent.
The outcome is a blurred image. (Mirrors reflect all light by the same amount but any glass-based correction optic may introduce the problem just before the sensor.) The answer is to block IR (and UV) light from the sensor. The filters used in a color sensor’s Bayer array are not sufficient and an additional IR blocking filter is required to stop infrared light reaching the sensor.
In general, the efficiency of a photographic camera is less than that of a filtered monochrome CCD and in some models, the added IR filter progressively reduces the light intensity of deep red light even more. (The primary color of emission nebula is a deep red from ionized hydrogen (Ha) at 656 nm and an even deeper red from ionized sulfur (SII) at 672 nm.) In these instances, a longer exposure is required to detect the faint glow, with the added risk of higher image noise and over-exposed stars in the region.
The standard Canon EOS camera bodies currently have the best third-party software support but at the same time their IR filters reduce the intensity of these important wavelengths by 80%, To their credit, Canon marketed two unfiltered bodies specifically for astrophotography (the EOS 20Da and 60Da). These cameras are not cheap and many use a consumer model or a used body and the services of a third-party company to remove or replace the infrared blocking filter. This modification improves the deep red sensitivity and depending on whether the IR filter is simply removed or replaced, may affect color balance and autofocus.
Light Pollution.
A camera’s color sensor is also sensitive to visible light-pollution. Although light-pollution filters block the principal street-lamp colors and reduce the background light intensity, they also reduce star intensity at the same time. They require color correction to remove their characteristic blue or green color cast. These filters mount in the camera lens throat (Canon EOS) or on the end of a T-thread to telescope adaptor (1.25- or 2-inch).
Light pollution filters can perhaps be more accurately described as nebula filters, since they pass the specific nebulae emission wavelengths, which thankfully are not the same as those found in urban light pollution. The result is that image contrast is improved and requires less manipulation to tease out faint details. Whilst on the subject of light pollution, the intense reflected light from a full Moon can wreck a nights imaging. No light pollution filter can remove this broad-band sky illumination.
Dedicated CCD cameras.
In the beginning, SLR cameras used both CCD and CMOS sensors in equal measure. Although CCD sensors have less noise, CMOS sensors have become more commonplace since they are cheaper to produce, use less power and through gradual development have narrowed the gap to a CCD’s noise performance.
High-quality dedicated astrophotography cameras still use CCDs exclusively, with the exception of some small format cameras designed for autoguiding and planetary imaging. These two applications only require short exposures and in practice are more tolerant to image noise. The uptake of camera sensors into astrophotography is quite slow. Most CCDs are made by Sony and Kodak and come in a range of sizes and resolutions.
The larger ones often require a physical shutter and have interlaced rather than progressive readouts. These require a little more care; moving shutters can disturb dust in the sensor housing and interlaced outputs may need a processing adjustment to ensure alternate lines have the same intensity. Dedicated CCD cameras represent a substantial investment.
There is no way of softening the blow but a used one is about the same price as a new semi-professional SLR. These are hand-made niche products without economies of scale. Having said that, each year more astronomy CCD cameras are launched and with better sensor performance for the same outlay.
Model turnover is much slower than that of consumer cameras and dedicated CCD cameras hold their value for longer. Once you have a CCD, there are only really three reasons for upgrading: better noise, bigger area or more pixels. The latter two will be practically capped by the telescope’s imaging field of view, the resolution limit of the system and seeing conditions.
Light Pollution.
Dedicated cameras come in color and monochrome versions. The “one-shot” color cameras use the familiar color Bayer array and share some of the limitations of their photographic cousins and as a consequence are equally susceptible to light pollution. The monochrome versions have a significant advantage over one-shot and color sensors, since the user has complete control over filtration. With a monochrome sensor, a single filter is placed over the entire sensor, one at a time and all the sensor elements contribute to each exposure.
How Dose Light Pollution Affect Astrophotography.
Video by Ricky Ryan Ray
The designs of the individual filters (RGB, luminance and narrowband) maximize the contrast between the deep space objects and the sky background. The specialist dichroic red, green and blue filters are subtly different to their microscopic cousins in a Bayer array. The spectrums of the red and green filters do not overlap and effectively block the annoying yellow sodium street lamp glow that is a major proportion of light pollution is.
In addition to the normal RGB filters, there are a number of specialist narrowband filters, precisely tuned to an ionized gas wavelength. These pass only the desirable light emissions from nebulae but block light pollution, so much so that “narrowband” imaging is very popular with city-bound astrophotographers. The Hubble Space Telescope uses Hα, OIII and SII dichroic filters, the “Hubble Palette”. This produces fantastic false-color images, by assigning the separate exposures to red, green and blue channels in a conventional color image.
Color Sensitivity.
Dedicated cameras are the epitome of less-is-more; without the infrared blocking filter, they have a better deep-red sensitivity than their SLR counterparts, limited to the sensor itself. They still need a UV and IR blocking filter (often called a luminance filter or “L”) but the astronomy screw-in ones do not impair the deep red visible wavelengths.
The decision between one-shot color and monochrome is really one of priorities: Monochrome sensors offers more versatility but with the extra expense of a separate filters or a filter wheel and require a longer time to take separate exposures to make up a color image. Arguably, there is better resolution and color depth using a monochrome sensor through separate filters, since all sensor elements contribute in all exposures.
A common technique is to combine or “bin” red, green and blue filters to color a simple high-resolution monochrome image, taken through a plain UV/IR blocking filter. The human eye’s color resolution is lower than its monochrome and the brain is fooled by a high-resolution image with a low-resolution color wash.
Thermal Noise.
Many dedicated CCD sensors employ an electric cooling system by mounting the sensor chip onto a Peltier cooler. This special heat sink has a sandwich construction and effectively pumps heat from one side to the other when a voltage is applied across it. Alternating n- and p-type semiconductors are the “meat” and have the unique property of creating a thermal gradient between the opposite surfaces.
A single “Peltier” cooler can reduce a sensor temperature by about 25°C, with immediate benefits to thermal noise. Some cameras have a 2-stage cooler that can keep a sensor about 40°C lower than ambient temperature. The practical limit to cooling is around the -20°C mark. At this temperature, in all but the darkest sites, sky noise is dominant in the image and there is no observable benefit from a further sensor cooling.
There is a downside too from over-cooling; extreme cooling may cause ice to form on the cold sensor surface and for that reason most CCD modules have a desiccant system to reduce the chance of condensation in the sensor chamber. Desiccants can saturate over time and higher-end camera models seal the sensor cavity and fill it with dry argon gas or have replaceable desiccants.
Cameras for Planetary Imaging.
Apart from taking still images of very dim objects, another popular pursuit for good cameras is planetary imaging. The closer planets are considerably brighter than deep space objects and present a number of unique imaging challenges. Planetary imaging is done at high magnifications, usually with a special teleconverter (Barlow lens) on the end of a telescope of a focal length of about 1,000 mm or longer. Even at this long focal length the image is physically very small and will fit onto a small sensor.
Since these only span the central portion of the telescope’s field of view, there is no need for a field-flattener. There is a requirement for a considerable focus extension though. This is conveniently accomplished by screwing together several extension tubes, which either fit directly to the focuser draw-tube or as a series of T2 extension tubes. At high magnifications astronomical seeing is obvious and the trick is to take a few thousand frames at 7-60 frames per second and then align and combine the best of them.
The difference between a single frame and the final result is nothing short of miraculous. These challenges suit a sensitive CCD video camera. These output a video file via USB or FireWire. Many start with a modified webcam and upgrade later. This is an inexpensive way to start and the modification is simple; a threaded adaptor replaces the lens, which allows the camera to fit into an eyepiece holder and hold an IR blocking filter.
There are several after-market adaptations, including support for long exposures and cooling. Several companies either dedicated to astrophotography or as adaptations of security products, market alternative CCD designs, with better resolution, lower noise and full control over video exposure, frame rate and video format. These do give better results than webcams and with more convenience. A low cost webcam is remarkably good, however, and a lot of fun.
Related questions.
How do you shoot a good starry night?
To photograph the stars in the sky as pinpoints of light, start with as wide an f/stop as your lens allows, and shutter speed of about 20 seconds. Any more time than that and the stars will begin to blur. Increase the ISO as needed for a good exposure.
How do I capture the moon with my phone?
Switch your camera to Manual mode and your lens to manual focus. Your exact exposure will vary according to the conditions, but in manual exposure mode start with ISO800, a shutter speed of 1/250 sec and an aperture of f/5.6. Adjust the ISO or aperture until you can see detail clearly in the surface.