I attached the Nikon D50 to the C8 SCT last night for the first time. Of course I was excited to see what kind of pictures I could take with it, but I also had concerns about how I was going to focus the camera. The view port on a SLR is pretty small, which is challenging on its own. Add the fact that the camera will be attached to the bottom of a telescope pointing nearly straight up, and seemingly simple procedure like focusing becomes a significant challenge. So I did a little research and I found an interesting page on Arcturus Observatory / Comet Man web site that discussed using a Hartman mask to focus. Then I wondered: could I make a Hartman mask? Would it work?
Following that idea, I found a sheet of heavy weight art paper, set the C8’s lens cover on it, and traced a circle of the inner diameter of the OTA. Using a standard Swiss Army pocket knife I made three cutouts: two circles and a triangle. It was quick, cheap, and amazingly useful. Here’s the mask and the pad or art paper from which it was made:
Hartman Mask
My wife said it looks scary. Maybe that’s because I was holding the mask over my face saying, “ooooo,” but I don’t see how that could have any effect. Besides, the circles are much farther apart than my eyes, so I couldn’t see a thing.
Anyway, the real fun began when I set up the C8 and used the mask to focus on a star. I got distracted for a while just watching the pattern coalesce and split as I turned the knob. Of course the real test was whether it would work with the camera attached. I wanted to try getting a picture of Saturn and its moons. First I aligned the scope and slewed to Regulus, which was the nearest bright star to Saturn. Then I removed the eyepiece and the star diagonal and attached the camera to the visual back using the T-ring and T-adapter. Then I started capturing images.
The following focus trial images were taken at ISO 400 with a 2 second exposure time. It took several trials for me to establish a procedure and I won’t include the first six or seven photos for that process. Let’s just say there was not a lot of instant gratification here. In any case, I was still exciting to see if I could make the mask work.
Like many digital cameras, the Nikon D50 allows you to “zoom in” on a portion of an image during playback. After each image I magnified the diffraction pattern as much as possible to gauge the focus.
Hartman Focus - 1
As you can see, the pattern is pretty distinct. This means the image is not in focus. I turned the knob about 20°-30° and took another picture.
Hartman Focus - 2
The pattern is smaller and brighter. That’s good because it means I’m turning the knob in the right direction. After a little more knob action I took this one:
Hartman Focus - 3
Now the diffraction pattern has mostly disappeared (no rings or dark centers), but the spots are still distinctly separate. Notice the triangle shape of the bottom left spot.:
Hartman Focus - 4
In this image the star appears pretty well focused. Keep im mind I was looking at this outside, bent at the waist, squinting at a little LCD screen smaller than the image you see here. I gave the knob a little more twist and took another picture to see if the image could get any sharper.
Hartman Focus - 5
Oops! Too far! Notice how the triangle has inverted from the earlier images. I turned the knob back 3/4 of the way to its previous position.
Hartman Focus - 6
Now that looked like pretty good focus. Also, it was getting late and the mosquitoes were starting to gather in larger numbers. I was anxious to move on to imaging Saturn.
After slewing to the ringed planet I captured 17 images at different ISO settings and shutter speeds. The best picture of Saturn was taken at ISO 400 with the shutter speed 1/4 second.
Saturn Solo
The best picture of Saturn’s moons was taken at ISO 400 with the shutter speed 5 seconds.
Saturn's Moons
By this time it was getting late and the mosquitoes were driving me crazy. I disassembled everything and finished for the night.
The human eye seems to have a better dynamic range than the D50. I combined the two previous images as a composite that represents how Saturn appeared when observing visually. The picture is rotated 90° and made extra-wide to show the faint moon Iapetus on the left. The image size is reduced on this page, but you can click on it to see the full-sized composite image.
Saturn (composite image)
