There was a thread several months ago about these units. The cheapest (I think), is available for US$128 from ZWO in China, but, without all this hassle I bought one last month from Bintel for $179 plus $12 postage. - A good price. (European ones are around AUD$500 plus and professional ones run into the thousands.)
Basically they comprise of two prisms that are adjusted to counteract the refraction effects of the atmosphere on the different wavelengths of light. This effect results in the upper edge of high resolution images having a blue fringe and the lower edge a red fringe. (A spectacular example of this is the 'green flash' phenomena on sunset.) Telescopes over 8 inches in aperture are particularly prone to showing these colourations and the effect does not become insignificant until an object reaches over 60 degrees in altitude.
To use the unit, and longer focal ratios are preferred (say F20), the corrector is inserted after the barlow, but before the eyepiece or camera. The unit is aligned parallel to the horizon, and when in use, a further adjustment about every 30 minutes is called for as the object changes in altitude.
All the top lunar/planetary imagers appear to use them. I wont rabbit on further, but show a few images why the unit is necessary. (C14 images.) These are NOT good images, being merely chosen to show the pernicious effect of this distortion.
Hi Peter, I also have a Zwo adc and it is extremely effective on my big dob. However, on an equatorially mounted scope I have yet to figure out an easy way of keeping it aligned with the horizon. I'd be interested in hearing how others tackle this!
Cheers
Andrew.
I had that trouble aligning it with the horizon. First remove your eyepiece so you see the image of the mirror against the sky, then pass your hand, a sheet of paper, a young child, or something in front of the lower part of the lightpath. When you look through the drawtube, you will see the partially obstructed lightpath. This will tell you where the bottom (hence horizon) is. Then align the ADC so it is parallel to the horizon. Replace eyepiece and away you go. If I was using a newtonian, or small SCT it would be dead easy because I could reach. For the C14 I made the horrible contraption in the attached image...
Hmmm...
Wouldn't the simple software tool, like PT lens do the same job, possibly cheaper?
It works very well for lateral CA, and it has controls to correct atmospheric dispersion as well.
IRIS is another way of dealing with this.. Firstly, chromatic channels should be separated, then re-aligned and re-combined. IRIS is free BTW.
Or Gimp..
I don't know. I will leave answers to the experts in the field. What I do know is that I have seen images where the colour channels have been separated and then correctly recombined, and these still fall far short of the correction by the ADC BEFORE its gets to your camera, or eyepiece for that matter, rather than trying to correct the image later. (In fact the ADC people use such 'recombined' images in their advertising to show their product is superior.)
