This is an animation. To view it please click on "original" below. The first image is the earlier M13 as shot by a Canon 1Ds using 1200mm focal length (full 1Ds frame); the second image is a stack of 29 ten-second exposures shot through a Canon 20D that has been modified by Hutech with clear glass over the sensor (full 20D frame shooting through a C14 at prime focus, 4125mm focal length). The sensor cover-glass has only anti-reflection coatings. This 20D consequently drinks all the photons available, from near Ultra Violet (350nm) to near Infra Red (950nm) and thereby has sensitivity that is up to 2-stops faster than an unmodded 20D. The frame size of the 1Ds is approximately 1.6x larger than that of the 20D. The images are shown at their relative frame sizes.
I wanted to check out three items:
1. Does the UV and IR lead to star bloat when one uses an SCT? Answer is NO. Certainly not within the constraints imposed by seeing and my less-than-precise tracking.
2. Does a C14 SCT have a flat-enough image field for a 20D, let alone a full 35mm-format 1Ds? The answer is again NO. Stars away from the center show mis-shaping quite quickly. Even on the 20D (smaller) frame edges they are already more distended than the Canon lens has on the edges of a 1Ds frame. Conclusion: a C14 is not the OTA to use for large prints, but should be satisfactory for web presentations.
3. Can the Celestron CGE mount be used unguided much beyond 1200mm focal length for DSO imaging? The answer is unfortunately again NO. I did try using a 0.63x reducer with the C14. The vignetting, with the optical train I was using, even on the 20D frame was awful. I'll have to try again one day, simplifying my optical train and shortening back focus. Not having the correct adapter handy, I switched to prime focus. There is very little vignetting on the 20D frames, easily flat-fielded out. I had to reduce exposures to 10 seconds each, frankly not much use for DSOs. M13 consists of stars, hence relatively "bright" compared to nebulae. Even 5-second exposures show some trailing, despite rejecting more than a third of them. Basically I suspect a C14 is simply too heavy an OTA for DSO imaging with the CGE mount, too much trembling in the slightest breeze. At 4125mm focal length autoguiding is of course essential, but even that will likely struggle in any breeze. It remains possible that with a lighter OTA the CGE may be well up to the job at, say, a focal length of 2000mm. But note that the set-up I used has an absurdly over-ambitious image scale, addressed in the next slides.