- My primary imaging scope is an achromat with a fast focal ratio (f/5), which means there is significant chromatic aberration (CA) in images of bright objects and long exposures of stars. One form of CA that is common in long exposures is purple fringing, which creates blue and purple halos around bright stars.
- The mount is unguided and was probably never intended for making exposures that last longer than a couple of minutes.
- The DSLR that I use for imaging does not have a mechanism for cooling the imaging chip, so it generates a lot of heat-related noise.
- The IR-cut filter on my DSLR filters out the Hydrogen Alpha (H-alpha) wavelength of the spectrum, which is where a lot of the "action" is happening in the sky, color-wise.
All of these issues could be solved if I had more money in the astronomy budget. But, since that won't be improving in the foreseeable future, I try to make the best of what I have. (Which I've very thankful to have, by-the-way!)
The images below mostly represent failed attempts. One or two may look pretty, but I assure you that something in the process did not work as I had intended. Regardless, I learned something from each, as described below.
I avoid using my ShortTube 80 for lunar and planetary imaging--the chromatic aberration is just too severe to get a sharp image. It comes with a special lens cap that permits the scope to be stopped down to about f/9, but even at that focal ratio the scope still produces a blurry image on bright objects. The Orion SkyGlow filter seems to mitigate the purple fringing around stars a little, so I thought I'd see how well it performed with an image of the moon. The results were far from impressive, as the following image illustrates:
|Effects of chromatic aberration (colors have been enhanced). CA blurs images because the individual colors do not come to focus at the same point. The inset images highlight the blurring effect on the lunar limb and Copernicus crater.|
I separated the image into its red, green and blue components. It was out of focus in red and blue, but pretty close to focus in green. Here is the green channel in grayscale:
|Moon in Green, April 1, 2012.|
|Here are the same areas from the insets, above, except only in the green channel. Note that the focus is sharper.|
One night back in January 2011 I imaged NGC 2264, the Christmas Tree Cluster and Cone Nebula, hoping to improve on an earlier attempt that I made with the Epsilon-200. I took a few test shots and wasn't impressed with the results, so decided not to continue. Recently, I was going through my imaging archives and came across the test images and decided to see if I could salvage something out of them. Here is the result:
|NGC 2264, the Christmas Tree Cluster and Cone Nebula; ST80 on Vixen SP; 3x180|
Below is the earlier image that was taken with the Epsilon-200:
|NGC 2264; Epsilon-200 on NJP|
Finally, the image below was taken while experimenting with visual solar imaging using a full aperture solar filter on my ST80:
Focusing on the Sun is tough! The image constantly shimmers due to the atmosphere being heated by solar radiation, and the image in my DSLR is rather small. Add in the effects of CA, and getting a sharp image is nearly impossible. This image was heavily processed to bring out the surface granularity and the details around the sunspots.