Atmospheric Dispersion Corrector

[Radders]

Just wondering if anybody had used one for planetary imaging? Only had a brief read up on them but isn't it the same as doing a RGB Align in Registax?

Paul

[sharkmelley]

I know someone using one with excellent effect - I've seen it being demonstrated.  It is much better than doing an RGB align.  The reason RGB align cannot do a full dispersion correction is that each filter has a wide bandpass so within each colour channel the data is already smeared and cannot be recovered.  The dispersion corrector allows unsmeared data to be recorded.

It will be very useful for Saturn in the UK this year - which is fairly low down in the sky.  Low in the sky is where most dispersion takes place.

Mark

[Radders]

Can they be used with colour cameras or are they better with mono? I'm always looking at ways to improve my images.

[Steve Ward]

Good article here ... http://www.skyinspector.co.uk/Atm-Dispersion-Corrector-ADC%282587060%29.htm

Whether they are 400 Euros better than RGB-Align is debatable ... http://www.teleskop-express.de/shop/product_info.php/language/en/info/p5992_Pierro-Astro-ADC-MKII-Atmospheric-Dispersion-Corrector-with-T2-connection.html

[riklaunim]

I'm using one. RGB align in Registax just aligns color channels so that you don't get color edges, but it doesn't refocus your image. That's the difference. Atmospheric dispersion moves the focus point of shorter wavelengths against longer.

With ADC I'm able to use L or yellow longpass filters for LRGB planetary images (thing you don't see usually). Color cameras would also benefit as they are imaging all the wavelengths at once.

[jambouk]

I have one and I don't like it at all. I have never seen any improvements when using it (with a OSC camera), and it is a pain in the bum to use. I don't think my other optics are of sufficient quality or precision [calibration] to get the marginal benefit I think these add. But I am lazy and if anything is a faff to set up I get bored anyway.

Damian Peach uses one.

James

[Tommohawk]

I'm struggling to understand the concept of the ADC.

Certainly thee will be wavelength dependent differential refractive effects caused by the atmosphere, and these will create varying degrees of image shift as described in the link given above. The effect is like a unidirectional chromatic aberration ie fringes top and bottom.

But surely applying a prism to this will shift all the images equally? Unless we are saying the prism itself suffers chromatic aberration, and therefore the prismatic effect is colour dependent? But then surely the whole image would suffer chromatic aberration?

Also, in the case of mono imaging, if the images are shifted, surely the stacking routine re-aligns them?

That link shows a supposed improvement in clarity of the blue image of Jupiter... but, assuming focus is corrected for each colour,  we arent talking about clarity, we're talking about lateral image shift. 

Slightly puzzled.

[jambouk]

An article by Peachy himself:

http://www.damianpeach.com/images/articles/JBAA%20dispersion%20Peach.pdf

James

[Tommohawk]

OK I think I can understand this a bit better now. 

The atmosphere itself acts like a prism, so that the RGB images are spread.. but of course there are multliple wavelengths involved, not just RGB. The prism acts in an opposite fashion, effectively re-uniting the images in a single position. I guess we could say the prism suffers chromatic aberration in a single plane - unilateral aberration - which counteracts the unilateral aberration created by the atmosphere. 

Using RGB or other filters reduced the problem, by correcting 3 specific bandwidths, but there is still smearing within each band.

So this is another bit of kit I need????? 

[jambouk]

I suspect the ADC makes minute improvements in the final image, they are not cheap, they add yet more glass for the light to pass through, and they are a pain to set up.

James

[Tommohawk]

For non-mono/colour imagers, one solution could be to split the final image into multiple channels rather than just RGB,  and then re-align. Is this feasible... maybe with layers?

Seems to me that the effect of the atmosphere varies from one AVI frame to the next, so it would probably be better to do per frame multichannel align, rather than RGB only on the stacked image.

Would this be possible within PIPP maybe??

[riklaunim]

they are not cheap, they add yet more glass for the light to pass through, and they are a pain to set up.

I find it very quick and easy to set up. I just put an eyepiece, center the planet, focus and then shift the levers until I can't see any color edges. Then the mono camera can go in. With color cameras it can be easily done with the camera.

For non-mono/colour imagers, one solution could be to split the final image into multiple channels rather than just RGB,  and then re-align. Is this feasible... maybe with layers?

If you focus on red then blue will be a blurred mess and vice versa. With a color camera you can't focus every channel at once. With a mono camera you could refocus between RGB filters.

[jambouk]

I thought you were meant to adjust the levers whilst looking at a star not the planet.

I have tried with an eye piece, and with a OSC camera, but never found the device altered any dispersion whatever I did with the levers; the talk of the fringing reducing just didn't happen, largely as there was minimal fringing to start with and it didn't get better or worse whatever I did.

I might have another go with it, but I was TOTALLY unimpressed, sorry.

James

[riklaunim]

I thought you were meant to adjust the levers whilst looking at a star not the planet.

Nope, no point in doing that, plus it's altitude will be different and seeing dispersion effect also harder. Stars are often given as example drawings to illustrate dispersion.

This is Saturn taken with MS Lifecam without corrector:

The dispersion is clearly visible and can be also easily corrected.

But the best effect would be to use a mono camera and do LRGB images.

RGB vs LRGB (with ADC):

[jambouk]

I was told by http://astrograph.net/ to use a star:

"So use your [bahtinov] mask to get focus as accurate as possible on Spica. Turn the gain up to make the spikes as clear as possible.

Now look at Spica. If you can see the Red / Blue fringe (obvious in your images) then move the levers away from each other. If you move the levers and see no change, then put them together again and turn the ADC through 180 degrees and try again.

BTW, I am assuming that your scope is properly collimated. [i'm not sure it was/is]

If you get the fringing to disappear, then refocus and do it again. After that you should be able to just target the planet."

I will try again in a few weeks when I am off work and have a go with Saturn if the sky is clear.

If the differences between the images above is JUST due to the ADC, then I take my comments back about it making minimal difference.

James

[riklaunim]

That is pretty much pointless unless you want to take pictures of Spica. You just put it - horizontally with the lock screw pointing to the western side of the telescope, point at the actual object and set the levers.

[jambouk]

I can only report what I was told. This website also suggests tuning the ADC on a bright star at a similar elevation to the target:

http://www.cloudynights.com/page/articles/cat/user-reviews/the-atmospheric-dispersion-corrector-r2934

James

[riklaunim]

It's hard to see those colors on a star, plus you need a very very similar or equal height which is not easy. I always set it on the object I'm imaging.

[Tommohawk]

If you focus on red then blue will be a blurred mess and vice versa. With a color camera you can't focus every channel at once.

The idea of lateral shift I understand, and I can see the effect in my images - but surely if youre using a decent apo or a reflector, RGB should all be in focus, no? There shouldnt be longitudinal aberration? Or are we saying that atmospheric effects introduce longitudinal aberration?

[riklaunim]

Atmospheric dispersion is a function of object altitude above horizon. It's not related to a telescope.

[Tommohawk]

Atmospheric dispersion is a function of object altitude above horizon. It's not related to a telescope.

Yes I gather that!! The point I'm making is that you said:

"If you focus on red then blue will be a blurred mess and vice versa. With a color camera you can't focus every channel at once."

... and I'm not sure what you mean by that. Why can't you focus every channel at once? I'm not saying youre wrong, I'm just trying to undstand your point.

So far as I can see, atmospheric dispersion causes a lateral - or transverse if you prefer - shifting of the varying coloured images. The object  therefore "smeared" vertically. Are you saying that in addition to this there is longitudinal aberration ?

[riklaunim]

Dispersion moves focus points of wavelengths so there isn't a point where every wavelength focus at given point. Aside of prism-like rainbow effect the focus is also shifted due to that.

[sharkmelley]

Dispersion moves focus points of wavelengths so there isn't a point where every wavelength focus at given point. Aside of prism-like rainbow effect the focus is also shifted due to that.

I don't understand the point you are trying to make.  Dispersed blue rays still arrive at the optics parallel and dispersed red rays also arrive at the optics parallel.  Parallel rays come to focus on the focal plane and therefore both dispersed red rays and blue rays will come to focus on the focal plane.  They are displaced spatially but should still be in focus.

Mark

[riklaunim]

That could be my mistake, but there is some effect, maybe not connected to dispersion itself. When doing RGB on Saturn with a mono camera and focusing on R then the B will be noticeably not in perfect focus (although blue on Saturn is hard in general).

[si@nite]

I can only report what I was told. This website also suggests tuning the ADC on a bright star at a similar elevation to the target:

 

http://www.cloudynights.com/page/articles/cat/user-reviews/the-atmospheric-dispersion-corrector-r2934

 

James

Rik gives some good advice, I use one when imaging but prefer the colour cam route to set the adc, & would not be without mine!