While I've owned an Aries ADC for a few years now, I've recently had the chance to test out another model from Astro Systems Holland (ASH) for a friend who can't run it through it's paces until the snow disappears around here--- if ever. Fortunately I have my scope permanently mounted in a roll off roof observatory, and I'm only limited by all too few clear, relatively steady nights during this New England Winter.
Most of the currently available ADCs like this ASH unit and the Pierro Astro ADC use counter-rotating Risley prisms to offset the effects of atmospheric dispersion, as does my somewhat limited production Aries ADC. I now consider an ADC an essential tool for both visual and imaging use--- In visual use under good seeing, I can see an improvement with Jupiter even when it's at 60° altitude. Dispersion varies with altitude--- lower targets have the most dispersion, while the problem is reduced with a gain in altitude. During the recent Jupiter triple transit, tweaking my Aries ADC setting as the shadows of Io and Callisto merged allowed a noticeably sharper view of the short-lived partial eclipse by Callisto's shadow on the N hemisphere of Io. Jupiter was 63° in altitude at that time.
During my typical RGB imaging sequences an ADC certainly helps as there is still noticeable dispersion within the bandpass range of each filter, particularly the B filter as dispersion is strongest towards the shorter wavelengths. I feel that results from a one shot color camera benefit even more in part because it's much easier to focus on the planet itself once the ADC is adjusted correctly.
The attached images were part of a test of the latest version of the ASH ADC, a recently updated design that can now interface with common T-thread systems like the Baader T2 adapter series, as does the Pierro Astro ADC:
For these test images I used an ASI120MC color camera with a Baader UV/IR cut filter on my 14.5" D-K scope at it's native f18, about 6650mm FL. The planet was 44° in altitude during the video captures, each of 2 minutes duration. Seeing was not great, but probably about 5 of 10 on the Pickering Scale on average. Video capture settings and processing was identical for the left and middle images. There was no WinJupos derotation used as the videos were short enough to tame much of the rotational smearing. The images are presented at a scale of 75% of the original. Please click on the thumbnail below.
On the left is Jupiter with the ADC set to neutral--- no correction. It's actually mislabeled 'W/O ADC' as the ADC is indeed in place, but set to zero correction which essentially acts as if there was no ADC in line with the camera. You can easily see the atmospheric dispersion causing a blue fringe at the top of Jupiter; and a red fringe at the bottom. Features are also slightly smeared top to bottom throughout Jupiter's disk. In the middle image, The ADC was properly set to correct for dispersion, and you can see the improvement in fine-scale features along with the disappearance of the color fringing. The image to the right is a copy of the left image with one more processing step--- the RGB channels were realigned at the sub-pixel level in MaxIm DL 5 to remove the color fringing, but you can see that the fine scale features are still not as distinct as in the ADC corrected middle image.
Edited by John Boudreau, 10 February 2015 - 03:25 AM.