THE CCD REVOLUTION INFECTS DEEP-SKY IMAGING
- by Luis Eguren
- continued -
A Since my "baptism by fire", the images from my backyard have surprised me. OK, they are not Hubble, but it is interesting to see M13 or M57 on the laptop with more contrast and detail than I would at the eyepiece in real-time. This is especially true of the globular clusters and some planetary nebulae which are quite evident and show detail not observable at the eyepiece in Montreal skies. Galaxies don't come through as well, although they too are more visible than through the eyepiece once the images are processed. These two images of the relatively bright M31 - Andromeda Galaxy clearly show the benefits of a dark sky vs a bright, city one. But, even in the city, it does come through on the image.
The software that comes with the SAC7b, ASTROVIDEO, is actually quite powerful and as I become more familiar with some of its functions, my results have improved. In fact, I can image in alt/az mode (with no equatorial mount or tracking, just putting the camera with lens on a tripod) and the software will automatically align and stack several frames together to get a decent combination image.
I tested this feature and found that placing the object to be imaged ² from the edge of the field, I can get 10 to 15 frames of 5 sec (longer exposures result in star trailing) each with a 50mm camera lenses. The software automatically stacks the frames.I then reset the object when it approaches the periphery of the field and take another set of frames. I do this about 20 times and the final image is actually pretty good (10 frames X 10 sec X 10 reps = about 17 min exposure).
If you have an equatorial mount, this feature will let you work around poor polar alignment and the period error inherent in your mount. I usually do this when in EQ mode; I polar align as best I can and start imaging. The software then automatically stacks the images. My backyard exposures for something like M13 or M57 run 10 to 15 seconds (anything more and the sky background and light pollution totally saturate the CCD - white out the image). I usually get about 100 images for a total time of 1,500 sec (about 25 minutes). If I want I can then initiate another run of any amount of images I desire. See M13 and M57 images.
While imaging, I donāt have to watch the laptop the full time Iām imaging. I usually "multi task", doing other things inside the house but checking the screen every few minutes to ensure that the object I'm imaging is still in the frame (using a polar alignment scope usually keeps the object within the central area of my frame for more than 10 minutes). If it gets too far toward any of the frame edges, I guide the object back toward the center without stopping the exposure. Once my total exposure is complete (or at my convenience) I can delete those frames that show too much star trailing or aren't sharp as a result of periodic error or bad seeing.
Result, I usually end up with 30 to 40 OK frames from my 100 frame run. I do this many times, say 3 or 4 runs and stack the best frames from my runs either in Astrovideo, FitsX (another SAC image processing S/W bundled with the camera) or Registax. The beauty of this is that I can use images using the same setup or focal length from several nights to stack together. The results are as if I shot everything the same night. Stack permits you to build up the image, whether in one sitting or over many nights.
All this sounds easy. It's not! But, it's not difficult either. Itās a lot easier than imaging with film; and, I can see my results almost in real time. The best cost/benefit analysis is one where I can measure tangible results and estimate the effort required to get those results. This is a no brainer; my film based camera and telephoto lens cost me more than the entry price of the CCD equipment. The results are far superior, and I can attest to that by looking at my astro images over the years. If you're like me, who can't always get to a dark site to image or observe, the CCD actually lets you see some objects on the laptop after only a 10 to 15 second image.