What is an Atmospheric Dispersion Corrector and do I need one?

[Stub Mandrel]

Atmospheric dispersion is the 'smearing' of light as it enters the atmosphere at a low angle - directly comparable to how a prism spreads light into  a 'spectrum'.

 

This effect blurs details on images taken low down, especially when using extreme magnification, as for planetary images.

 

It is possible to use RGB align routines to reduce the effect, but even so each of the three colour bands, red, green and blue, will still be smeared to an extent - theoretically greatest in blue, but sometimes most noticeable in green when imaging Mars ( which doesn't have much of a blue signal).

 

The ideal solution is prism used to counteract the effect of the atmosphere. You can get filters that are fixed prisms to cancel the effect at various alt angles.

 

More sophisticated is the 'atmospheric dispersion corrector' which uses two moveable prisms to create an 'adjustable prism' you can tweak to maximise the effect.

 

Here are two examples, these are as-stacked (unsharpened) images of Jupiter taken through a blue filter using a mono camera, about five minutes apart. The first has poor detail and the moon Europa is clearly smeared into a vertical smudge. This image is with an ADC set to its 'neutral' position where it has no effect. The second image is with the ADC set to more or less eliminate atmospheric dispersion (it doesn't need to be spot on to make a big improvement). Note how Europa is more or less a dot, and there is much more detail and contrast - if you look closely Europa is still slightly oval, so an even better result could probably have been obtained.

 

 

 

The best way to adjust the ADC is by eye - remove your camera and use an eyepiece to observe how the planet 'sharpens up' and loses any coloured fringes. Alternatively it can be done on screen, setting it for the greatest contrast/sharpness (just like focusing).

 

Note that adjusting the ADC moves the image up and down, you may need to retarget. Also, the 'neutral axis'  should be parallel with the horizon.

 

An ADC should improve all high magnification imaging below about 30-40 degrees alt, except for narrowband which is not affected by dispersion.

 

ADCs can be costly, however the ZWO ADC is about two-thirds the price of older designs (comparable to a specialist 2" filter) and in my experience gives excellent results. It certainly gave me a step-change in the quality of my planetary images.

 

 

[geoflewis]

That's an excellent item Neil. I often use an ADC, even when imaging with a mono camara at low elevations, e.g. for Jupiter and Saturn currently, as there is still some dispersion in the fairly broadband Red, Green and Blue filters. However, I did find that they had a net detrimental impact on Mars last year when that was up at ~40 deg elevation, so it's definitely worth trying with and without at higher elevations.

[Stub Mandrel]

Agreed Geoff. Atmospheric dispersion is closely linked to the alt-angle. By 40 degrees the effect is minimal, and you are more likely to add it than take it away with an ADC.

 

In my second example above,  the ADC is actually set to slightly over-correct. I should have backed it off a little as I set it on Saturn which was about five degrees lower.

 

But that's the fun of planetary imaging - each improvement you make shows up a weakness somewhere else, and gradually your images improve.

Edited October 4, 2021 by Stub Mandrel