Perfect Afocal Digital Astrophotography, Part 1

Monday, June 25 2007 @ 10:29 pm EDT

Contributed by: nspace01

By Leonard Lynch

Figure 2
Top to Bottom - Coolpix 950, Coolpix 990, Coolpix 995, Coolpix 4500
A few months ago, I developed a renewed interest in astronomy and joined this fine club. I can''t express how fortunate I feel to have such a fine group of people to share an interest with. Every event that I have attended, from the Astronomy In The Park events last year, to the club meetings and star parties (that weren't rained out) this year, were like “Family Events.”

Figure 1
Example of Vignetting
One of my interests is astrophotography. I have an excellent Nikon 35mm camera and one day, I will buy the necessary attachments and take the time to figure out polar alignment. For now, Perfect Afocal Digital Astrophotography is my goal in life.

For the “newbies” that don't know the term, afocal astrophotography is simply the act of focusing an image in your eyepiece on the telescope, and then positioning a digital camera at the eyepiece and snapping away. The eyepiece focus, camera alignment, camera steadiness and camera/eyepiece proximity are all critical for good pictures, but I have seen some really good pictures taken by holding the camera to the eyepiece by hand.

I have a Sony CD500 digital camera, that takes wonderful pictures of everything terrestrial. When it comes to the afocal use of this camera, however, there is a problem. The lens moves in and out during focusing and manual zooming, and this, combined with the large diameter lens, causes a phenomenon called “vignetting” (pronounced vin-YET-ing.)

Vignetting means that the camera image looks similar to tunnel vision, that is, a small round image surrounded by blackness — like looking into a toilet paper roll. See Figure 1. I will not go into what causes this, as it is a little confusing to me, but I will try to work around it. Zooming the camera IN will eliminate most, if not all, of this in most digital cameras — but NOT with mine. Perfect Afocal Digital Astrophotography is not possible using this camera.

So, I searched the Internet to see how other like minded Amateur Astronomers were doing it. Most agree that the “only way to go” is with a Digital SLR camera. “Forget about afocal astrophotography, take your shots at prime focus, with a DSLR camera.”

Basically, like many of you, I cannot afford a thousand-dollar Digital SLR camera. I have too may other telescope accessories to buy, and recurring bouts of “aperture fever” to think about. Some of the most popular cameras for afocal astrophotography are the Nikon Coolpix 950, 990, 995, and 4500 model digital cameras. The 950 has a 2.1 mega-pixel chip and it is the oldest camera model. The 990 and 995 are 3.2 mega-pixel. The 4500 model is the newest model, with a 4 mega-pixel chip. The more mega-pixels, the more resolution the final picture will have. These cameras have small lenses that do not move externally when focusing and zooming, and have another feature that I like, a swivel in the middle of the camera so the LCD Screen can be adjusted to view the image when held at odd angles. Vignetting can be completely eliminated on these camera by zooming in slightly. Unfortunately, none of these cameras have been manufactured for several years. They are only available on the used market. See Figure #2.

Figure 3
Top to Bottom - Coolpix 4500/Vixen Adapter, ScopeTronix Digi-T System, WO eyepiece that threads directly to camera.
Figure 4
Universal Camera Adapter
Buyer Beware: The first Coolpix 4500 camera I bought a from an eBay auction was thoroughly worn out and I had to send back for a refund. Most eBay sellers will not give a refund on an used item, so be aware of this possible pitfall. The second one I bought is in pristine/like new condition. It came with rechargeable batteries, a charger, all hook-up cables for the computer and TV, and original users manual. I haven't been able to try it out yet at night, because of cloudiness, but I have set it up and taken close-up pictures of hubcaps from 300 yards by hand… real cool, but a little blurry from camera shake. I could almost read “Dodge” in the center and see little craters on the chrome. However, even at 84x, which is the lowest power my 10" SCT will support with my current collection of eyepieces, it doesn't take much movement to show very obvious blurring. The next step was to eliminate the need to have to hold the camera by hand and to stop the “shaky-shake” when I take a picture. Both problems are easily solved for me, with a little more money.

There several methods available to attach a camera to a telescope eyepiece, from several different manufacturers. These range from camera/eyepiece adapters (Figure 3,) to “Universal Camera Adapters” (Figure 4.)

I chose the Universal Camera Adapter from Scopetronix. These have a plate with a tripod-type mount that holds the camera in place and a clamp that connects around either 1.25” or 2” eyepieces. The adapter is adjustable to center the camera with the eyepiece and to position the camera lens as close to the eyepiece as possible. This type of mount will add more weight to the set-up and could be an issue on smaller telescopes. You may need to add some counterweights (if your scope and mount are large enough to accommodate them.)

Finally, the last problem that keeps me from achieving Perfect Afocal Digital Astrophotography. The camera still shakes when I press the shutter button. My SCT CG-5 mount is fairly sturdy, but as just about everybody that uses a telescope knows, it still takes a few seconds for the mount to stabilize after touching it.

One way to avoid this is to use the self-timer feature that is available on most digital cameras. Just turn on the feature, press the shutter button, and the camera will snap the picture after a short interval, say 10 seconds or so. In my case, it gives the scope and mount time to stabilize before the picture is taken.

Figure 5
Remote Control
It just happens that Nikon manufactured a wired “Remote Control” for the Nikon Coolpix 995/4500 cameras (Figure 5.) This serves the same function as the camera self timer, but gives the user more functions such as programed interval shooting and a “bulb” shutter feature for taking short time exposures. I am in the process of buying one of these. I think I now may have all my afocal bases covered.

There is a wealth of information available on user groups and other websites. Great pictures of the moon, bright planets and even a few deep sky objects are definitely possible using afocal astrophotography

Now… if I can just find a clear sky this time of year! GOOD LUCK!!

Part 2 of this article will cover the actual afocal astrophotography.

Comments (2)