I've attended a couple of sessions within the last year led by renown planetary imagers (Damien Peach, Agapios Elia) in pursuit of what I can do to refine my setup. One thing that both mentioned rather high up on their list was using an Atmospheric Dispersion Corrector (ADC) in the imaging train. The premise is that the atmosphere bends blue light differently than red light, causing a slight smearing of the "white" light we get at the eyepiece. An ADC, which is a set of adjustable prisms, allows you to correct the dispersion, yielding a sharper image.
I had hesitated on acquiring one, in part because both individuals use large SCT units for their imaging - would a Newtonian benefit? Second, I had assumed (incorrectly) that the amount of smearing would be minimal once the planet got above 30° (see this). And finally, the Autostakkert software supposedly performs a correction for this when stacking.
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| Autostakkert "RGB Align" |
I ended up convinced that it might be helpful so I got the ZWO model which was a past Sky & Telescope "hot product" award winner. The unit arrived about a week after ordering it, and as forewarned by my research, there were no instructions on how to use it. Fortunately other brave souls have sorted it all out and been kind enough to post video tutorials.
The unit is placed between your Barlow and the imaging camera. There are two steps for having it correct the atmospheric distortion:
- Use the bubble level to ensure that the unit is parallel with the horizon. That will obviously fall out of order as the evening progresses and your scope tracks the planet across the sky, so re-doing this step every half hour or so may be needed.
- Adjust the two silver levers (on right in picture) in equal but opposite directions until you can no longer detect color fringing
It is that last step that can be tricky. One approach recommended by Martin Lewis is to overexpose the planet on your screen to make the fringing more obvious. You can then iteratively adjust the levers until the fringing is minimal (or at least you are unable to say which side is reddish one and the other bluish). Many of the imaging capture programs (e.g., FireCapture, ASI Capture) also have a utility to help assess your alignment. In general, when the circles overlap then you have achieved merging the spectrum back into white light. But given how wildly the two circles will bounce around on the screen it is still a bit of a swag.
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| Performing ADC Alignment with Jupiter |
This past week I decided to give it a go and placed the unit between Barlow and camera. Given the height of my set up, I had to use the step ladder to reach up to perform the leveling and then to gradually spread the levers, constantly referencing the laptop screen to see the impact. Of course another trick is to do it gently enough to not knock the planet out of the field of view. After about 5 minutes I felt the bouncing circles were are concentric as I might achieve. I did a focus and then started a couple of captures.
Sadly, what I noticed about 20 minutes in was that my focus was a bit soft. Looking at the focuser read out I noticed I was racked fully in due to the extra back space the ADC introduced. And sure enough, the actual focus point was just a tad further in. No amount of ADC will substitute for mushy focus, so I pulled it off and resumed imaging with a sharp focus. My task now will be to remove one of the spacers from the Moonlight focuser so that we can get to that focus point with the ADC in place.
Will it all be worth it? Hard to say, but given the feedback from well respected imagers I certainly am committed to finding out. And empirically it makes sense to try to correct this dispersion as the image is captured rather than to depend in post-processing algorithms to fully compensate for the effect. That's the game now - incremental improvement wherever I can achieve it.
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| Jupiter without ADC - can I tease out more with an ADC? |
