[PeterAnderson (Peter)]

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.