Manually Guided DSLR Astrophotography
I decided on the onset to stick to manual guiding, as I wanted to build my knowledge and skills on some of the basic principles of astrophotography before moving to auto guiding equipment. Furthermore, I currently do not have frequent access to dark skies and cannot justify the investment in auto guiding cameras as yet.
I started out with a Canon Digital Rebel (300D) and in the last two months have switched to the Canon 20D. Both of these are superb cameras with affordable price tags and decent low noise for exposures up to 4min. I’ve had excellent luck with either of these cameras. The Digital Rebel is very light weight and couples to the OTA without needing sensitive rebalancing. The 20D is heavier but offers a reduced electronic noise level and the ability of automatic dark frame subtraction (Custom Function=2). In addition, the 20D accepts Canon’s useful TC-80N3 remote with programmable exposure settings while you will have to get a modified version of this remote to use for the Digital Rebel. For long exposure astrophotography, mirror lock up is not an important requirement, but just for information’s sake, the Canon 20D has a mirror lock up feature useful for short exposure planetary/moon photography while the Digital Rebel does not. There is a “Russian hack” program for the Digital Rebel that enables mirror lock up that I’ve hesitated in employing.
I decided to use my Takahashi FS-102 4” Apochromatic telescope at f/6 for imaging. I’m sure the Takahashi build and optical quality is familiar to all, and my scope is no exception. I decided to image at f/6 because it is more forgiving to manual guide for a wider field and a faster photographic system. There were however several accessories that the scope had to be outfitted with for photography. The CA-35 adapter and EOS T-ring are necessary accessories to couple the EOS cameras to the scope, but the Takahashi Camera Angle Adapter is a particularly valuable option as it allows one to freely rotate and frame the shot without changing focus. In fact, I use the CAA at all times even for visual observation as it conveniently allows me to rotate the eyepiece to a comfortable viewing position without needing to undo the circular clamp. To reduce the optical system from f/8 to f/5.9, I use the focal reducer. It is rather expensive but it is a high quality item and will give you superb edge-to-edge pinpoint images. Finally, the Takahashi 10:1 Micro Edge Focuser is a useful addition for my FS-102 for fine focusing.
I decided on a lightweight field mount as I observe outdoors at different locations very frequently. In general, serious astrophotographers employ very stable mounts, like Takahashi’s EM200, NJP, AP’s 600, 900GTO’s and Losmandy’s G-11. I had a tight budget and as I am imaging a wide field f/6 system, I decided to compromise some by using Vixen’s Sphinx equatorial mount. Other similar mounts in this class are the Losmandy G-8, Vixen’s GP-DX and Takahashi’s EM-11. With the last firmware upgrade to the Sphinx, I ended up with a usable and responsive system for manual guiding. I have found that the manufacturer’s load rating of a mount is to be taken as a guideline. For astrophotography, the actual useful load is more conservative as compared to when using the setup for visual observation. If you already have a mount and scope you intend to use, set it up and put in an illuminated reticule eyepiece and center a star in the middle of the cross hairs. If you find yourself making extensive drive corrections despite a good polar alignment or if the star shudders at every breeze, your mount might not be suitable for imaging without further work.
I mounted the guide scope side by side with the Takahashi FS-102 using Vixen’s large accessory plate. There are other manufacturers who sell side-by-side mounting plates. I prefer this configuration compared to mounting the guidescope on top of my Tak as it greatly reduces the risk of me touching the DSLR/Tak during imaging. I use an Orion 80mm f/11 for guiding, and Vixen’s superb GA-3/4 illuminated reticule guider. This is very unique compared to your standard 4mm or 10mm illuminated reticle as the GA-3 allows you to use your own eyepiece for guiding. The device projects a clear and logical cross hair target and is fully adjustable: you can move the cross hair pattern over the entire view of the eyepiece and also rotate the cross hairs to orientate it to the axial movement of your mount for easy guiding. The general quality of the guidescope is not as important compared to its focal length and ease of positioning. I find it useful to keep a small 6x30mm finder on the guidescope as it helps greatly when I have to reposition the guide scope to a suitable star for guiding.
E. Other useful things
It is almost impossible to manual guide properly standing up. One needs an adjustable chair and the tripod height needs to be chosen to coincide with a comfortable sitting position when the scope is towards zenith, as imaging objects near the horizon often leads to dismal image quality due to atmospheric effects. A dew controller system like that by Kendrick ensures a dew-free night, and I drive both my mount and the Kendrick controller with 18AH Kendrick sealed batteries. A laptop is very useful for a number of reasons. There are focusing programs that one can use (Image Plus, DSLR Focus, both Astromart sponsors) with the DSLR’s. I have found these to be most useful in achieving good focus. There is nothing as frustrating as spending the whole freezing night imaging and going home to find that all the photos were just a little off focus. Been there and done that. I highly recommend these focusing aids as a sound investment. Canon also sells an angled magnifier for the viewfinder and some have found that useful. But for critical focus I still employ software driven image evaluation programs. At this point of writing (Dec 2004) DSLR Focus is still working towards focusing support for the 20D.
First off the bat, there are a number of things that have to be meticulously done to avoid a frustrating night of imaging. The entire equipment setup must be carefully checked for loose and unsecured parts (cables connections, guidescope rings, stability of ground, tripod clamps, dovetail interface clamp, camera bayonet mount etc.). Any loose part will easily mess up your system. Check and check again, as they say in the Army. You will be richly rewarded for your diligence here and mercilessly punished for your sloppiness. Next, polar alignment must be as accurate as possible. All good quality mounts have build in polar scopes. While drift alignment may not be necessary for 4 to 5 min exposures as field rotation effects are absolutely minimal, it certainly helps and does not hurt if you do it. I frequently check my polar alignment several times a night, and often find it has shifted slightly. Once again, constant diligence pays off. For mounts with PEC capability, it might be beneficial to invest 10 minutes of the night to train the drive. For manual guiding, you might have to adjust the backlash compensation of the drives to ensure smooth and responsive directional changes during guiding. The most effective manual guiding occurs when one only has to make occasional gentle changes.
After the setup, I typically slew to a bright star and center all the optical elements on it (finder scope on the Tak, camera view, guidescope illuminated cross hair, guidescope finderscope). Next, I align the cross hairs of the illuminated reticle so that they correspond precisely with R.A. and DEC movements. I test guide to make sure that the system is responds as it should be. I then slew to the object I want to photograph. I first frame it exactly on the camera viewfinder using fine R.A. and DEC adjustments and also the Camera Angle Adjuster. If it is a very dim object that I can’t see very well, I set my camera setting to small jpg and do an unguided 1 min exposure. That lets me see where it is exactly before committing to a real exposure.
Now for the actual photo taking! I typically shoot in raw format and process later in MaxIm DL. For the Canon Digital Rebel, I typically shoot for 4 min at either ISO 400 or ISO 800. If you are not after perfection, and if you intend to try your hand at some dark frame processing, you might be able to get away at ISO 800 for the Digital Rebel up to 4 or 5 minutes. I have found that with these non-temperature regulated imaging chips the dark noise can vary unpredictably even if a similarly timed dark exposure is taken immediately after the imaging exposure. For the Canon 20D, I typically shoot at either ISO 800 or ISO 1600 with the dark frame reduction feature turned on (CF=2). I found that the auto dark frame reduction is definitely beneficial but for long exposures at ISO1600 a hint of horizontally stripped artifacts can be seen across the entire image fame. Still, I use this feature as the reduction in noise is nothing short of dramatic. With the 20D, I typically take 4 or 5 images of about 4 min duration each and combine them in MaxIm DL. The disadvantage of using the auto dark frame reduction is that the batteries are used up very quickly. It is advisable that one keeps 3 or 4 of these. I purchased a 12V-110V car charger and charge my batteries as they run out, so I always have a fresh supply.
The actual manual guiding portion is pretty simple if great pains have been taken during preparation. I always find that I need to be very conscious where my body parts are. A little kick on the tripod or a nose tap on the guiding eyepiece can easily change alignment and give you barbell shaped stars instead of pinpoints. As the night goes on and you get more and more tired and punch drunk, this becomes a consideration. Sometimes I wish I had an SBIG STV!
Finally, a note on weather: steady windless nights are ideal for imaging. Windy nights such as those caused by transitional weather patterns are typically disappointing nights for imaging. I typically find that two or three nights after the passage of a front to be ideal. If you have a lighter duty mount like mine you will definitely need wind at a minimum. Granted that an f/6 imagining system is pretty forgiving compared to an f/11 SCT, one still gets the best quality images when the air is steady.
For those of you interested in wading into the exciting and addictive area of DSLR astrophotography, I hope that this short article has given you a taste of the equipment needs and approach. May you be successful in your imaging and I hope to see your superb images on Astromart!
Vixen Sphinx Equatorial Mount
Canon Digital Rebel
Cyanogen MaxIm DL V4
Kendrick 12V 18AH Sealed Lead Acid Battery
Takahashi Wide Mount T-Ring Canon EOS
Takahashi Camera Angle Adjuster for FS-102
Takahashi 2.7”/4” Reducer for TOA 130
Takahashi 10:1 Micro Edge Focuser for FS Series Refractors
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