Atmospheric Dispersion Effect

[PeterAnderson]

This is something for the high resolution observers and imagers. Until relatively recently I just avoided low altitude objects unless I had to. the images were always bloated or bubbly and scintillating.

When I got my C14 it was a bit out of alignment and I played with adjusting the secondary. (I now have it about right - you know the stage where it is pretty good and you feel any more might just make it worse.) Anyway, whilst I was doing this I particularly noticed the colour fringes to the brighter stars - blue on top red below.

I immediately knew what this was - everyone knows about the 'green flash' and that bright stars flash different colours at low altitude. Our atmosphere smears these images to a decreasing degree right up to 60 degrees altitude and above.

A friend of mine using a large SCT thought there was something wrong with the optics when no amount of tweaking would remove this pesky colour!

I did some investigation and discovered:
1. The effect only becomes a real nuisance above 20cm aperture.
2. There are corrector devices available around $500 amateur and ten times the price professional models.
3. Adjusting the device is either visual or by test images, and is done (say) every half hour as the altitude of your target object varies.
4. Top planetary imagers - Damian Peach, Leo Aerts etc. use them.

I have included a few images. At the moment I am still tossing up whether it is worth $500 or so to me.....

[Maurice]

Hi Peter

Maybe this will be of interest at $128US?

http://astronomy-imaging-camera.com/products/accessories/zwo-adc/

I've used a homemade one for the past 10years or so, but I've just ordered the ADC in the link above because of AR coatings etc..

regards
Maurice

[PeterAnderson]

Thank you Maurice, that is great. The price is now at a reasonable level. (After all it is a fairly straightforward item of equipment, though the production numbers would likely be fairly low. That is a great find!

Please let us know how it performs after you have tried it out.

I am attaching a monochrome image of this effect with altitude.

[rustigsmed]

interesting stuff looking forward to hearing more, when I used to take more planetary images i found it happened a lot particularly after my DSLR was astro modified.

Jupiter was low in the sky back then though too...

example
https://www.flickr.com/photos/80336656@N07/8292567156/in/dateposted-public/lightbox/

photoshop "correction" by manually moving red and blue channels into alignment (Registax also has an auto function to do this as well)
https://www.flickr.com/photos/80336656@N07/9500145825/in/dateposted-public/lightbox/

Would these items work better than post processing?
EDIT i see the ZWO example shows what happens with rgb align vs the product but is it accurate or marketing?

[Maurice]

Hi Russell

'Shifting' layers in post processing does help, but the representation on the zwo website is accurate. The smear is far worse in the blue part of the spectrum than the red.
With Mars at 35 odd degrees up, I have a smear in red data of about 0.25", whereas in blue its about 0.9". This is significant when you are trying to get sub arc-sec detail.
As the dispersion is varying across the spectrum, an ADC does help more; you don't get the smear across each bandpass showing.

Cheers
Maurice

[Shiraz]

definitely helps, but only in the best seeing. In general, I found that seeing was mostly poor when at low altitude, so there was not very much benefit from having an ADC to correct for the dispersion - the images were slightly less crap with an ADC, but still not good.

Over a year of Saturn and Jupiter imaging, the ADC was valuable on 2 occasions. However, if they have come down that far in cost, why not?

[Merlin66]

We have similar problems is spectroscopy....
The entrance slit image has to be orientated along the Parallactic angle to ensure all the refracted star energy is collected

[Exfso]

I have had a pair of these "Risley Prisms" for over 10 years, originally purchased off Adirondak in the USA (now not a going concern I believe), they work a treat.:thumbsup: