Telescopes and Equipment

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Here I describe my astrophoto setup, my equipment, and imaging techniques. Links within this part are external and point to various manufacturers. You may notice my telescopes and most of my equipment are not new, but trusty old friends. This is the way I like it: I try to master all of my equipment and reach the best performance it can offer before moving on, and this takes years. Plus it helps saving cost in an expensive technical hobby.

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For taking most of the astrophotos found in this gallery I used one of three different telescopes:

JSO 4.9" f/3.8 Wright-Newtonian and guidescope in astrophotographic use

For wide-field imaging the JSO LS-12D, a (nominally) 125/475 mm Wright Väisälä Camera (shown in the photo below the Telementor refractor, which was used as guidescope). This rather exotic telescope is designed as astro-camera, has a Newtonian focus, and was produced by Japan Special Optics Co., a company which ceased to exist around 1996. I got my sample used from a long chain of predecessors. At a measured focal length of 469 mm and a focal ratio of f/3.8, it is very well suited for imaging large nebulae and Milky-Way structure, the results often have Schmidtcamera-like quality. Its drawback are an inadequate drawtube and a mild degree of off-axis astigmatism. The latter can actually be used to adjust the focal plane with precision.

9.5" f/4.9 Newtonian in astrophotographic use

For medium focal length imaging a 9.5" Newtonian with a nominal aperture/focal length of 250/1200 mm; actually measured as 242/1180 mm at an optical bench. Its optics were purchased from Orion Optics UK via Teleskop-Service, while the shiny aluminium tube assembly was custom-made by a mechatronics expert. The secondary mirror has a small diameter of 3.5". Its drawback are a reduced serviceablity of the main mirror because it can't be removed from the lower end and some faults in the main mirror coating which formed over time.
I use two different coma correctors with this telescope:

TMB 4.1" f/6.2 APO and guidescope in astrophotographic use

For medium-to-wide field imaging the TMB 105 (shown in the photo mounted side-by-side with the guidescope), which is a 105/650 mm triplet apochromatic refractor with optics designed by Thomas M. Back (USA), manifactured by LZOS (Russia) and sold for Europe by APM Telescopes in Germany. My version of this refractor has a heavy German-made tube. For astrophotography, a flatfield corrector is mandatory, I use the flatfield corrector which is optionally sold with this telescope. It is designed for medium film format, so I use a custom-made adapter for my CCD and digital cameras. I have measured the refractor's effective focal length with flatfield corrector as 699 mm at a focal ratio of f/6.7, which I find a bit slow for astrophotography. Also, the focuser has no gear reduction for fine focusing.

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My primary mount for astrophotography is an OTE-150 (2002 version), which is a mid-weight German Equatorial Mount produced in Germany. It may not look attractive, but it is rigid, and its drives run quite smoothly with a built-in mechanical (!) periodic error correction. This mount handles the small telescopes well, even under windy conditions; better than the VIXEN GP-DX , which is still in use as secondary mount for astrophotography with small telescopes and for visual observations. However, the 9.5" Newtonian plus guidescope, camera, autoguider and other accessories pushes the OTE-150 mount to its limits. I therefore switched to guiding via finderscope, which eliminates the need for a separate guidescope and saves weight.

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The OTE-150 is mounted on a massive wooden TS Stativ Deluxe tripod, mine is designed to handle Newtonian reflectors and can be loaded (in theory) up to 120 kg. The GP-DX is mounted on a pointed Baader hard-wood tripod which I usually ram deep into the ground (if soft) to quickly damp any vibrations.

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For photography, both mounts can be controlled by a PowerFlex MTS-3SDI, which features everything an astrophotographer really needs: Fully programmable drive speeds, microstep mode, periodic error control, backlash compensation in declination, a (non-standard) autoguider port, PC interface and (at least in theory) GOTO capability, i.e. the ability to point automatically on objects, when connected to a computer. It is produced by Boxdörfer Elektronik in Germany. For visual observations an older version of the PowerFlex MTS-3SLP, which I also own, is sufficient.

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Currently I use two special CCDs for astrophotography: A monochrome and a color camera.

The monochrome CCD is an ATIK 383L+ which features a Kodak KAF-8300 CCD sensor with 3362 × 2504 effective pixels (8.4 megapixels). The pixel size is 5.4μm × 5.4μm, the bit depth 16 bit. This is a good sensor which worked right from the start and never gave me any kind of troubles. It has a mechanical shutter and cools down to a maximum of -40 degrees below ambient temperature, the chip temperature is regulated. The ATIK is used for taking deep luminance frames in the field, and in combination with narrow-band filters in my backyard (see below).

My color CCD is a ZWO ASI294MC Pro. It is equipped with the Sony IMX294CJK CMOS sensor which has 4188 × 2822 pixels (11.8 Megapixels). The pixel size is 4.63μm × 4.63μm and the bit depth 14 bit. The ZWO ASI is a one-shot color camera with an RGGB bayer matrix on the sensor. It can be cooled down to -35 degrees below ambient temperature, the chip temperature is regulated. I like to work with a gain setting of "0" to make use of the camera's maximum 63700 e- full well capacity, and take longer exposures up to 10 minutes when I have dark skies. The ASI is mainly used for taking color images in the field.

Older color images were taken with a QHY8 Pro. While basically a good camera, and cheap as CCDs go, it had a few drawbacks, most annoying was its tendency to collect dew (and occasionally ice) on the CCD window and - additionally - on the sensor itself once its temperature drops below 0°C.

To minimize thermal noise, I set the chip temperature to the minimum temperature possible in 5° steps. In summer, this goes up to -15°C; in the winter, down to -30°C. The 5° steps ensure that I have calibration frames for each chip temperature.

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Often I am astro-imaging in my backyard at home in Vienna, where the sky is heavily light-polluted. Nonetheless, I image emission nebulae there, and do so using a set of 2" Baader CCD Narrowband Filters, which consists of a Hα, an [OIII] and a [SII] filter with band widths between 7 and 8.5 nanometers, in combination with the ATIK monochrome camera. Exposures of emission nebulae through all three filters can be combined to "Near Natural Color" or "Hubble Palette" color composits.

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DSLR Cameras

The Canon EOS 350D (=Digital Rebel XT) was my main astro camera before I bought my first CCD and is still in use as backup and as secondary camera for piggy-back photography of larger areas of the night sky using my assortment of Nikon lenses, many of them older fixed-focal-length lenses out of chemical film days. I employ a Nikon Lens to Canon EOS Body Adapter to couple the Nikon lenses to the Canon 350D. The Canon 350D has a CMOS sensor with 3456 × 2304 effective pixels (8 megapixels) and a pixel size of 6.4μm × 6.4μm. To overcome the camera's weak Hα sensitivity, I have myself exchanged the original IR-cut filter by a Baader UV/IR cutoff filter, which was not an easy task, since the process voids the manufacturer's warranty and the risks of damaging the delicate parts in the camera's interior are high.

When astro-imaging with the Canon EOS 350D, I use a sensitivity of ISO 800. 10-minute exposures will yield a nice Signal-to-Noise (S/N) ratio and a sky background level somewhere between 10% and 30% of saturation level, and this ISO setting turned out to yield the best S/N for faint object detail if the conditions are good.

For startrail photos on a fixed tripod, I use a Nikon D7000, which I also employ for daytime imaging. It has not been modified. On the tripod, it is used with Nikon lenses, most often the Nikon 18-200mm VR lens. This is also the setup I use for time-lapse videos of the night sky, with the focal length locked down to 18mm. For those, I use very high ISO settings of 3200 and above.

The bit depth of both DSLRs is 12 bits per color channel in RAW mode. Both cameras can work in several color spaces, originally I prefered Adobe RGB, more recently I switched to sRGB because of its wide compatibility.

Some older DSLR images which can still be found in my webpages were taken with a Nikon D70 camera, where I also had replaced the IR-cut filter, and which was my first DSLR, bought in 2004. Before that, I was astro-imaging with a Nikon F3 camera on chemical film.

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Polar Alignment

For rough polar alignment I use the polar scope of my mount. Fine polar alignment is done using the declination drift (Scheiner) method with CCD and notebook. Any camera control software which provides the ability to repeatedly download subframes around a star and offers a zoom tool will do. I quicky spot the star's drift when I put the mouse pointer over it and wait a few seconds, and I determine where north and south directions are in the CCD image by pressing the appropriate buttons of the hand controller and watching the star move.

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For rough focusing, I have marked the telescope focusers' drawtubes with the correct focal position for my standard CCD camera with Hα filter. Fine focusing with CCD is done manually using a Bahtinov Mask on a bright star within the frame. Most often I use a notebook for evaluating the star's diffraction pattern. Again, any camera control software which provides the ability to repeatedly download subframes around a star and offers a zoom tool will do. An alternative method I use at times for focusing the DSLR cameras employs the camera's LCD screens, with Live View and zooming in on a bright star as much as possible.

For the Nikon lenses, I have predetermined their exact focus spot by taking a test series of startrail images. For the zoom lenses, I have done that at several focal lengths. A microscale fixed to the focus ring of each lens provides reference.

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Exposure Times

When I have dark skies, the deep-sky exposure times are usually standardized to 10 minutes for individual exposures, with both CCDs and DSLRs. When the autoguiding runs exceptionally smooth, and the sky is exceptionally dark or when I image through narrow-band filters, I take 20-minute exposures.

To minimize noise in the final image I take as many individual exposures of each object as I can. A series should at least include 12 good exposures. Narrow-band filtering requires even more exposures.

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Since 2012 I am guiding my astrophotos with a Lacerta MGEN stand-alone autoguider. This is another one of these good devices which worked perfectly right from the start and never gave me any troubles. The MGEN is placed in the primary focus of a TS-Optics 8x50 Finder for all my telescopes. My prefered guider settings are: Mode 2, 0.3 pixel tolerance for both axis, and an aggressiveness of 30%-120% depending on the mount, local seeing and wind speed.

Previously, I guided my astrophotos with a Meade Pictor 216XT, which had just a 2-digit numerical display at its back, but could also guide stand-alone.

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Image Calibration

For both CCDs and the Canon camera, I take calibration frames: Dark frames and bias frames. Additionally, I take flatfields to correct for uneven illumination caused by the optics. To capture flatfields I use an Aurora Flatfield Panel with a diameter of 220 mm set directly in front of the optics. Flatfields should be taken every night, ideally immediately following the astro images, to be able to correct for uneven field illumination and dust spots on the optics. Dark and bias frames need only be taken once or twice a year, no telescope required.

For the startrail and still photographs taken with the Nikon camera, I take the simpler approach of using the automatic dark frame subtraction to subtract the so-called "amp glow" (actually it is not caused by heat, but by electroluminosity) in the upper left corner of the frame, and the dark current. Edge vignetting is corrected with the "Lens Correction" filter of Abobe Photoshop.

Currently, I do not use a special color calibration. Both DSLR cameras are set to the "full sun" white balance, that yields realistic color results.

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Image Processing

Basic data reduction (bias and dark frame subtraction, hot pixel filtering and flatfielding) for digital deep-sky images is done with IRIS, an excellent freeware image processing software written by Christian Buil. For registration and combination of many individual exposures, I use the DeepSkyStacker. For separating the stars from the background to process stars and background independantly, I use Starnet++. Further image processing is done in Adobe Photoshop. Getting the most out of astrophotos is a sophisticated process which takes a long time to learn. Jerry Lodriguss' Articles on Digital Techniques are an excellent starting point. Maybe I will publish some articles on my own digital processing techniques in the future (if I get enough requests from you ☺). The usual processing steps are: Basic data reduction, registration and combination, background flattening, noise reduction, deconvolution, non-linearization, local contrast enhancement, star size reduction, color correction, and final framing.

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Planetary Images

Images of planets, the sun and the moon were taken using the 9.5" Newtonian. Since the spring of 2010 I use an Imaging Source DBK 31AU03.AS single-shot color camera for planetary imaging, which features 1024 × 768 pixels and can be run at 30 frames per second (fps). A Televue 5x Powermate boosts the effective focal length of the telescope to about 6 meters for small planets. Older planetary images were taken with a Philips PCVC 740K webcam.

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You want to start in astrophotography yourself? Or you want to improve your astrophotography techniques? Read the well-written Digital Astrophotography Techniques by Jerry Lodriguss. Many thanks to him for his comprehensive descriptions, which where a lot of help to me, too!

© 2019 Walter Koprolin