Oh Say, Can You CCD?
Author's note: This was originally written in June, 1993. Clearly, I have done a lot of work since writing this article. More information may be found elsewhere in this web site.
First, some background. I had never before attempted "through the telescope" astrophotography. All of my experience had been to piggyback my camera to the frame of my 8" Kramer Newtonian and then create hand-guided exposures of several minutes. Although successful for wide area pictures, this was clearly inadequate for high magnification or long exposures of dim objects.
When I read about the SBIG ST-6 camera in Sky & Telescope in late 1992 I decided it was time to return to active observing and leap into computer-based astrophotography. Even then, I realized that my skills were nowhere near the level of professionals such as Dennis di Cicco or Jack Newton. How far off was I? Well...
Although I had completed my Messier list with it, my Kramerscope was not designed to be clock-driven. The solution was to buy a used Meade 2120 (10") f/6.3 scope. I picked the lower focal ratio on the recommendation of SBIG. The idea is to increase the field of view so that small (faint) star images are about the same size as the CCD pixels.
I ordered the camera in October 1992, before the club trip to New Mexico, expecting to have it by Christmas.
Mar 5, 1993 (Day Zero)
It arrived (they had a large backlog). I spent the day reading the manual and trying to find a serial cable long enough to run the camera from inside my basement. No luck.
Mar 6, 1993 (Night One)
Since it was too cold to move my desktop PC outside, I borrowed an old 8086 portable from a friend. Its temperature stayed reasonably warm as I kept it in its case. Naturally, the 8086 made everything quite slow, in particular the serial port connection to the ST-6 control unit. One thing I couldn't fix was the narrow view of sky from my driveway. Imagine,if you will, an observing dome that can't rotate its opening. That is the view from my driveway.
Most of my time the first night was spent learning to focus the CCD camera. ST-6 focusing was designed to be easy, but I was so far out of focus that it appeared that the procedure was not working. I finally realized that what I was seeing was the fuzzy doughnut image of the secondary mirror itself. Oops. Once I had it reasonably in focus I attempted a picture of Orion. The Trapezium was obvious, but my polar alignment was bad and caused the image to drift and smear. As bad as it was, I used this picture as an example during the April 1993 meeting.
Definitely an evening to chalk up to experience.
April 3, 1993 (Night Two)
Still too cold to put the PC outside, so I moved it into the laundry room and ran a serial extension cable outside. Jack Kramer came over and we experimented focusing the camera. Not perfect, but much better than the first night. Since my polar alignment skills were suspect, I realized that I needed to utilize the Track And Accumulate feature of the ST-6. This works by taking an image, selecting a bright guide star, and then letting the ST-6 take a series of images and adding them together while adjusting them for any movement by the guide star. The images of the Beehive cluster and M65 that I showed at the April meeting were taken this way. This was an improvement, but I still had a lot to learn.
May 8, 1993 (Night Three)
(Notice my one-day-a-month observing schedule). It was now warm enough to move the PC into the garage, making movement between the PC and the scope a lot easier. One problem with the April pictures was a gray fuzzy area in the center of the images. Realizing that this was due to vignetting by the f/6.3 scope, I learned how to take a Flat Field image. A Flat Field image is literally a "blank" picture. You make one by taking an image of a gray card, or the inside of your dome (hah!), or the twilight sky. This image records any imperfections in the telescope optics, and can be used later to subtract these defects from your other images and give them a true "flat field". This would eliminate the gray fuzz in my pictures.
The ST-6 instructions indicated that creating a Flat Field image was necessary each time the CCD camera was installed or the optical configuration changed. This meant that once I took the twilight image, I could not remove the camera. Therefore, I had to align the scope by first getting Polaris in the CCD frame, focusing, aligning the finderscope and Telrad on Polaris, and then polar aligning the scope itself using the polar alignment circles in the finderscope. This resulted in the best polar alignment I had ever achieved. This also greatly helped the accuracy of the NGC-Max digital setting circles.
The pictures from that evening were wonderful, several of which were printed in the June 1993 issue of our Night Times newsletter. All the deep sky images were 2.5 minute exposures using the Track And Accumulate feature. The Moon was 0.01 seconds. These objects (globular clusters) are fairly easy to find, even in light pollution and a near-full Moon. However, I was unsuccessful at finding galaxies M51 or M101. All in all, my first truly successful CCD pictures.
June 5, 1993 (Night Four)
Even though it was only one night past Full Moon, I decided to try to image the galaxies and nebulae near the "Big Dipper". I focused and aligned as in May. Since I was looking for objects that were invisible in the finder (or even through an eyepiece had I removed the camera), I had to set the ST-6 to an automatic focus mode using full images. This meant that it was continuously taking 1-3 second images and downloading them to the computer screen. I would move the telescope a bit, check the image, and move it again. This allowed me to "scan" the approximate area of the intended object. Using this method I was able to find M108, M97 (Owl Nebula), M101, and M51 (Whirlpool Galaxy). I took 5 minutes images of all these as well as a 14 minute image of M51. Once again a very successful evening! These images accompany this article.In a future article I will discuss CCD sensitivity and temperature, removal of light pollution from images, processing techniques to improve images, and how I convert ST-6 pictures into Windows bitmaps.
June 11, 1993