- Milky Way, Credit: Ralph Clements
By Ralph O. Clements
When I was invited to write about this subject for Astronomysource.com, I must say I was flattered and a bit flabbergasted too, as I do not consider myself an expert on the subject, nor a writer by any means, but just a guy who likes to go out at night and take pictures of the sky. I stumbled into this hobby when my wife brought home an old 4” Meade reflector telescope with a manual equatorial mount from a yard sale that she paid $60 for.
I took that thing out in the country and set it up (completely wrong, I now understand) and as darkness approached, held my point-and-shoot camera up to the eyepiece and took a picture of Venus. Well, now that was very interesting…it was certainly not a very good photo and I have learned it is hard to get a good one of Venus, but I could tell it was not a star, it was not round but had a semi-circular shape. Wow! I took a picture of another planet! That got the gears turning in my head and I just had to do more….I mean who would think I could take a picture of another planet, with a point-n-shoot camera and an old yard sale telescope?
“Camera Only” Images
I do take images with newer telescopes and a decent equatorial mount which I have acquired since. Imaging galaxies and nebulae is an ongoing goal and interest, but I have learned that it is time consuming, tedious and has a fairly steep learning curve. My view of the sky at home is very limited. So for the time it takes to drive out to the country and get all that gear set up and working, I am limited to weekends and then only weekends when the sky is clear. Since clarity of skies does not always happen on Friday or Saturday night, I often image without the telescopes at all. All the tips and advice offered here is just what I have learned and I expect others may have better ways of doing things.
My research indicated that Canon cameras are preferred for astrophotography and the T1i is what I use for everything. I also have an older Nikon DLSR with two lenses, a 180mm fixed focal length and a 75-300mm zoom, for which I bought a Canon adaptor, but the 18-55mm “kit lens” that came with the Canon is what I use most often.
If you read up on astronomy and astrophotography equipment you will note it is often said that the mount is every bit as important as the telescope. My camera tripod is my mount and I fully agree that a sturdy tripod is a must. I am fortunate to use a tall Berlebach tripod with hardwood legs. The cheap aluminum department store tripods are not stable enough.
- Figure 1: Orion at Peaks of Otter, Credit: Ralph Clements
Widefield & Star Trails
Camera only astrophotos with a static tripod fall into these three general categories:
Widefield – Single Shot
These include what would be considered “scenic” or “landscapes” in daytime photography, that is, including some portion of the Earth, as well as constellations and shot of the Moon (See Figure 1). I try to shoot as long as possible without having oblong or streaked stars. A high ISO setting helps with this and I often use 3200 ISO unless it is twilight or too much man made light is around. Figure 1, Orion and the Peaks of Otter, is a 10 second exposure and the stars are a bit oblong but not too bad.
Widefield – Stacked Images
Images that are composed of multiple single exposures, stacked and aligned in the computer to reveal much more of the faint light features than what is visible to the naked eye. Sagittarius (Figure 2) was taken as series of short, 6 second shots and stacked in the computer using Deep Sky Stacker. The exposure time for shot like this can vary depending on the target, its location in the sky and ambient light conditions. I find that targets nearer the poles may allow a little longer exposure than those on or near the celestial equator, which appear to move more due to their location.
- Figure 2: Sagittarius, Credit: Ralph Clements
Star Trails Shots
Long Exposures or combined multiple exposures that show the apparent rotation of the stars above the Earth. Of course, the stars just appear to rotate because we are riding on the Earth which is really doing the rotating (Figure 3).
Taking star trails images is fun, easy and I like the look of them. Although a star trails image of say 40 minutes can be done on the “bulb” setting with a single exposure, this requires a remote timer and more importantly, a very, very dark site as the least ambient light will over expose the shot during that time. So I just take a series of 30 second shots and combine them using “Startrails” software, another useful and free program. This software is definitely easy to use and produces good results, although I do not notice much improvement when I use dark frames with it. For noise reduction I use “Noiseware Community Edition” in the final images instead. I recently became aware of another free software to do this, “Starstax”, and will be trying it soon as it offers more features.
On these star trails shots, sometimes it is good to have some moonlight on the subject and I will go out under a quarter to half moon and shoot them. I find a full moon makes it too much like daylight for my taste and if I lower the ISO under a full moon the stars don’t show up much. So depending on the amount of moonlight, artificial light and desire ground detail, I take these star trails shots at ISO setting of 800, 1600 or 3200. Generally, I try to get 40 to 60 minutes total exposure. Less than that and the trails are too short, more than that and chances are airplane will mess it up.
- Figure 3: Startrails, Credit: Ralph Clements
Foreground and Framing
I try to pick a good site with some interesting foreground , although “fore-ground” in this case doesn’t mean close to the camera, rather, it means the part of the Earth that is shown. I try to frame the shots so that the sky covers roughly ¾ of the frame, since the sky is the real subject and the foreground is really just a reference or point of interest.
To get crisp focus on the stars and the ground, anything in the image needs to be as far away as your camera’s “infinity” focus distance, which varies with the lens. So I try to take scenes that I would focus to infinity on if I were shooting them in daylight, such as the farm you see in Figure 3. For all my images I use the camera’s “live view “. This feature lets me zoom in on a bright star, or the moon and focus. If your target is too dim, aim at a brighter one or an artificial light a long way off and focus on it and re-aim at your target. Make sure your camera is not set to “auto-focus”, use “manual”. The Nikon I was using did not have “live view” but I used the same method, only I looked through the view finder at a bright star or light. Sometimes a few test shots were needed to get it right.
As for general advice for other beginners, I offer the following
- Read your camera’s instructions, particularly the section on manual control.
- Learn to work your camera’s controls in the dark, the corollary of which is….
- Don’t be afraid to experiment. I use the trial and error method, with lots of trial and plenty of errors. That’s okay though as I am having fun and try to learn from my mistakes, and I don’t have to buy film for a digital camera, so I don’t mind deleting the ones that didn’t come out.
….just do it! Have fun with it.
Eyepiece projection is a great way to take detailed shoots of moon and planets. Photographed objects in these images are considerably larger and show more detail than such taken with prime focus shots. Prime focus techniques replace the camera lens with a telescope OTA (no diagonal, no eyepiece), but eyepiece projection adds an eyepiece into the optical path, increasing focal length and magnification considerably. The image below shows the typical eyepiece projection setup.
Greater magnification and increased focal length come however at a price. Higher focal length (at the same aperture) results in a higher focal ratio number (1/f). The higher the focal ratio number the fainter the image becomes. This demands longer exposure times or higher ISO speeds to achieve a decent image brightness. Furthermore, constantly moving air layers diffract incoming light. That means, with stronger magnification distortion is magnified as well. The same is true for any mount and telescope shake or vibration.
Typical eyepiece projection setup with refractor telescope an DSLR camera.
How to do it?
The following paragraphs describe equipment that is needed and such which is additionally recommended to make photographer’s life easier. I will share some experiences that I had to learn the hard way; it will help you getting good results sooner.
- The mount needs to be strong and sturdy. It has to carry all the weight of telescope, camera and all accessories, furthermore it has to stand steady, even with light breezes.
- Many manufacturers are quite “generous” when listing weight capabilities of mounts and tripods in their data sheets. Unfortunately, this leads often to unsatisfactory imaging experiences.
- Never max out a mount load. The old astrophotographers’ rule still applies: actual equipment weight should not exceed half of the mounts specified load capability.
- Many astrophotographers do not extend the tripod legs for better stability and minimal vibration.
- Balance the mount very carefully with camera and all accessories attached.
- Polar align German Equatorial Mounts (GEM) with great care. It helps “keeping the object in the field of view”, even with highest magnification.
Telescope & Accessories
- Finder scope and main scope axis need to be perfectly aligned. This helps to “find” the object and framing it in the very narrow field of view (FOV).
- Screwed accessory connections,like tube extensions, are preferred over slide-in joints. Screwed connections offer better stability, less flex and are less receptive to shake and vibrations.
- Eyepiece projection requires usually significant focuser back travel, particularly with refractors. The required length can exceed the telescope’s focuser travel, which will render the projection out of focus. One or two 2” extension tubes provide the required additional focusing way. My telescope has sufficient travel way but I still use extension tubes because it keeps the, relatively heavy, focuser tube more inserted. This has the advantage that the telescope’s weight distribution is somewhat closer to the center of the mount (less vibrations).
Astrophotography: Typical Eyepiece Projection Assembly with DSLR T-Adaptor
Note: M42 and T-thread accessories have different threads. While the diameter is the same their thread pitches are different (M42: M42x1mm and T2: M42x0.75mm). Accessories with M42 and T-threads should never be mated.
- Remote control for the camera is strongly suggested. Pressing the shutter release manually will cause shake and vibrations. If your camera does not have remote capability use your longest shutter release delay, minimum is 10 seconds. Some cameras offer only 2 seconds shutter delay. This time is usually too short because many mounts are still shaking 2 seconds after the shutter button has been pressed.
- Most cameras allow shooting movie clips (avi). Even if the movie mode may provide less pixel resolution, shoot movie clips, particularly for planetary imaging. Movie clips consist of many single frames and software like RegiStax convert the movie clip into a string of single images, which can be stacked. With a frame per second rate (fps) of typically 10 fps to 30fps, a 10 second clip results in a large number of single frames. This is important because air movement and other distortions will blur many images. The probability of getting a few good ones increases with the number of available images.
- Stacking good images helps to pronounce object features and texture.
- If your camera has no movie (avi) feature take at least 30, better 50 (or even more) images to increase the probability hitting some really good ones with little of no air movement.
- DSLR cameras use mirrors that flip up during the exposure. If shooting images (not movie clips) use mirror lock if available. Even if the mirror is very light, the fast movement can create enough momentum to cause shake, which again blurs the image.
- Jupiter – Image taken with eyepiece projection technique (telescope: 900/120mm, eyepiece: 20mm)
- Take shots at planets when they are high in the sky rather than low at the horizon. Positions high in the sky minimize air refraction distortion. Light that travels through the atmosphere is scattered by aerosol droplets and absorbed by dust. These effects cause diffraction rings and reduce the image brightness. High in the sky, light’s atmospheric path is much shorter, reducing distortion effects significantly.
- There are also disadvantages of high object positions. Particularly when shooting with a large refractor, the camera position is very low. Also, a large refractor with extension tubes and camera mounted may hit the tripod legs in this position. Make sure enough space is left when moving the telescope to the desired object.
- Remote controlling the camera with a computer is strongly suggested, particularly with a large refractor. Looking in upright position at the computer screen is simply much (!) more convenient than crawling on the ground trying to peek in the – very low hanging – camera screen or finder.
- The image on a much larger computer screen allows more precise focusing.
- Take your time when focusing. High magnifications combined with moving air layers can make this quite a challenge.
- Re-check with some test shots that the focus is still optimal.
- Check the histogram and ensure that neither end (black or white) is clipped. If data is lost (clipped) it is lost for good, and can no longer be used to build the image. Even the best post processing effort can not bring lost information back.
- Shoot several movie clips. My recommendation is 10 by 10. Ten clips each ten seconds long. Depending on the fps rate this will provide you 1000 to 3000 single frames, a good base to work with.
- Some photographers prefer much longer clips to increase the probability of catching better results. With very long clips it is more likely that shake, vibrations and drift errors are introduced as well. CCD chips get hotter and start to introduce additional noise and hot pixels. Besides, long movie clips result in very large files, making processing somewhat cumbersome.
- Powerful software like RegiStax (freeware) converts the movie clip (avi) into single images. Furthermore, it aligns the images, selects the best ones and stacks them for best detail. It allows improving the resulting image even more with a great set of post processing features.
Question for Power
It is possible to calculate how much more magnification we get with eyepiece projection over a simple prime focus setup. To determine this, we need to know some dimensions: focal length of telescope and eyepiece, and the telescope aperture. Furthermore we have to measure the distance from the eyepiece lens to the camera’s CCD chip.
The dimensions used in the following example are from an actual eyepiece projection setup that was used when I shot the Jupiter image: Orion EON 120ED refractor with 20mm Eyepiece, 2 extension tubes each 2 inch ( about 50mm) and a Canon EOS T1i DSLR camera.
Focal length of telescope (FLtele): 900mm
Focal length of eyepiece (FLep): 20mm
Distance eyepiece to CCD (Depccd): 100mm
Telescope aperture (TA): 120mm
Magnification over prime focus set up (Mopf)
Mopf= (100mm-20mm)/20mm = 4
The image is 4 times larger than that of a prime focus setup.
Focal Length overall EP setup (FLoEPs)
FLoEPs = Mopf * FLtele
FLoEPs = 4 x 900mm = 3600mm
This setup has a focal length of whopping 3.6 meters (141 inches)! The number shows that eyepiece projection focusing can really be a challenge and has to be done carefully in minute steps.
Focal ratio overall EP setup (1/f oEPs)
1/f oEPs = FLoEPs / TA = 3600 / 120 = 30
The original telescope focal ratio of 7.5 has now become 30. The image will be much darker than that of a prime focus setup. Higher ISO speeds particularly for planetary images may be necessary.
Is it worth the challenge?
Most definitely: YES. Eyepiece projection astrophotography is for more advanced star shooters. It is easily among the most challenging processes in amateur astrophotography, not because of the setup but because of the effects that have to be considered and factored in. But with the right equipment and some practice it can be mastered – and the results speak for themselves: clearly visible features of the moon landscape, surface coloration and visible ice caps of Mars or detailed cloud bands of Jupiter make eyepiece projection imaging indeed quite rewarding.