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RGB and chromatic aberration


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This is probably going to turn out to be one of those really stupid thoughts I have, but ...

If one has an achromatic refractor (as I do), with its inherrant CA problems, and one uses a mono camera with RGB filters to get an image, does the fact that you are only capturing one bit of the spectrum at a time mean that the scope now works like a fully corrected scope?

Or am I still going to have to plan on an ED/APO purchase at some point?

Thanks

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Yes is the answer:hello2:

if you ensure best focus with each filter.

That's why Lunt get away with a single "magnifying glass" objective on the Ha scopes ( and why I recommend the Vixen A80mf for the pst mod) - in narrowband imagiing there is no CA!!!

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You would have to refocus each channel - that would limit the aberration, but still more processing would be needed as each chanell would have slightly different focal length (due to the focus change) - so stars could have bit different sizes. On the other hand LRGB images would fix even that (Ha-RGB for example or other very narrow band as L would totally kill CA).

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Demon

The simple answer is yes it does. But you will need to re-focus after each filter change. Although you might be lucky and get away with not having to re-focus if your scope is on the slow side

If you stick with RGB then CA becomes a non-issue when imaging with an achro. However, if you start to image in LRGB, there is a tendency for some minor star bloating but nothing too serious

Steve

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TBH, even if your filters are parfocal and your scope is perfectly color corrected I think it's good practise to re-focus between each filter anyway :rolleyes:..

If you stick with RGB then CA becomes a non-issue when imaging with an achro.

I'm curious though, if there's no real issue with using achros then why are using scopes with better colour correction? My understanding was that because achros are only corrected at 2 points of the colour spectrum (as opposed to an APO's 3 points) then you'd have issues with the colour that isn't corrected. Is that right or am I missing something?

Tony..

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I'm curious though, if there's no real issue with using achros then why are using scopes with better colour correction? My understanding was that because achros are only corrected at 2 points of the colour spectrum (as opposed to an APO's 3 points) then you'd have issues with the colour that isn't corrected. Is that right or am I missing something?

Tony..

In simple terms, the design of achros prevent them focusing all wavelengths at the same point. However, when taking separate R, G & B images, the spread of each filter's wavelength is much less than for 'white' light and thus avoids this problem. In other words, that part of the colour spectrum which hasn't been corrected is effectively ignored. The downside is that re-focusing is a must-do. However once you want to start imaging LRGB and the problem part of the spectrum isn't being filtered out then the problem returns to some extent.

The design of APO's is such that they dont have these compromises ... or rather, they are much much less obvious

Steve

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When you use narrow band filters these "uncorrected" wavelengths are filtered out so the problem dissappears with them!!

But the RGB filters are pretty broad pass band, and in the blue especially the dispersion in a fast achromatic is likely to lead to some blurring.

Another point is that in a fast achro the image scales in R, G & B will be slightly different.

Not so much of an issue for DSO imaging but, when imaging planets with RGB filters, the time spent refocusing is time wasted - achieveing critical focus can take several minutes so you simply don't have time to fiddle around refocusing between colours when you've only got 2 or 3 minutes to complete the set before rotational blurring ruins the result. Parfocalization & good colour correction is the key here.

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So I guess the obvious question to ask is why are we using apo's for imaging when something like the Skywatcher startravel series would apparetntly do the job for RGB?

Tony..

Edited by Whippy
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So I guess the obvious question to ask is why are we using apo's for imaging when something like the Skywatcher startravel series would apparetntly do the job for RGB?

Tony..

It's a fair question ... but I suspect there are criteria other than just the design of the optics.

As rik says, there's the little issue of the field being flat especially with the trend towards larger sensors. I'd also add the quality of the optics rather than their underlying design; the quality of manufacturing of the OTA; the imaging circle; the focuser design and quality and probably a few others

But of course all this costs extra. So you pays your money ...

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It's a fair question ... but I suspect there are criteria other than just the design of the optics.

As rik says, there's the little issue of the field being flat especially with the trend towards larger sensors. I'd also add the quality of the optics rather than their underlying design; the quality of manufacturing of the OTA; the imaging circle; the focuser design and quality and probably a few others

But of course all this costs extra. So you pays your money ...

That was my thoughts, there must be more to it than just color correction otherwise we'd all be using ST120's or Vixen Super Polaris' with a matched flattener/reducer.

Tony..

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I think the key thng is that once you have the three images (and as Brian says, the blue is likely to be a bit soft) what do you do with them? No point in admiring them individually! So then you combine them and unless you do some clever stuff you are going to be close to where you started. Combining in Registar, which can resize and do other clever geometric things, might mitigate some of the effects.

Losing the L channel would be a killer for me though. Apart from on clusters I find RGB only very vague. It is not like OSC data for some reason. Anyone know why?

Olly

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I have attempted RGB using an achromat and, sadly, refocusing is not the sum total of your problems. You get bloated stars in the blue channel which manifest themselves as halos in the reassembled colour image. You can attempt to remove the halos in post-processing, Noel's tools do a good job or you can run a star reduction action on the blue channel only, but if any of your haloed stars overlap nebulosity or other stars you are right royally snookered.

Narrow band works just as well with an achromat as an apo, except you have to re-focus for each channel. Ha and SII are usually close together, but OIII is way out.

I have often wondered if narrower-pass RGB colour filters would allow you to circumvent the blue halo problem.

As an imager of great ambition and not so much money, I am all in favour of cost saving solutions!

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Could we get some agreement then please?

That when using narrowband, an Achro is no different from an APO? (within reason) It might be worth just having one as a dedicated NB scope for when the moon is about, once its gone... switch back to the APO to add the colour.

An example would be using an ED80 and an ST120, both are the same focal length except the 120 runs at F5.

When you say that flatness would be an issue on larger sensors, then what would be the optimum size of chip? (ie: no bigger than a 285?). It would be nice to see any results from anybody who has tried it before I entertain the thought of having a go.

Edited by Uranium235
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This is interesting...

I use a achromatic doublet 40mm f5 from Edmund Optics as a collimator in a spectroscope. When the spectrum is formed it's basically a series of "narrowband" - like REALLY narrowband, images.

I can focus on the blue part of the spectrum and get a very tight focus on a blue emission line similarly I can focus in the red and NIR and get very tighted focused lines....what's the difference when imaging through a "similar" achromat (ie f5 doublet) telescope - if the narrowband filters are ONLY passing say 6nm around the OIII etc then there's not enough residual aberration to cause an issue??

Am I really missing something here??

I can supply spectral images if need to show the excellent focus.

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  • 5 years later...
2 hours ago, Wayne_Ford said:

Would adding a yellow long pass filter not tighten up the spread in the blue channel?

Wouldn't it just block the blue channel?

Olly

Edited by ollypenrice
Typo
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On 27.05.2016 at 07:43, Wayne_Ford said:

Would adding a yellow long pass filter not tighten up the spread in the blue channel?

I use such filter for planetary luminance channels, but with big apertures dispersion will still likely affect it.

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  • 7 months later...

What about parfocal RGB filters? They should be factory brought into the right focus without fiddling with the focuser knob at every change..., but I don't understand how the manufacturer can know the chromatic aberration of my objective. Are they parfocal to a simplee lens? Or to a flint/crown doublet? Or to an apochromatic objective? How do they take into account ED glasses? To me parfocal filters should come out of the factory as a precise match to the objective, something like obj/filter-set pairs, otherwise they would need focusing just like non parfocal ones.  

Or may I be wrong....?

cesco

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My understanding would be that for a given object, your scope (whatever its configuration) will have a position of the focusser knob that will always bring that object into focus [to all intents and purposes, this will be the same for all astronomical objects]. When you add a filter into the optical path, this alters the focus by a precise amount. Parfocal filters are made so that this 'precise amount' will be the same for each of them. Yes, if you use a different scope, this 'precise amount' might differ, but within one system, it will be constant across the filters.

At least, that is my understanding ;)

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On 19/01/2017 at 22:00, cesco said:

What about parfocal RGB filters? They should be factory brought into the right focus without fiddling with the focuser knob at every change..., but I don't understand how the manufacturer can know the chromatic aberration of my objective. Are they parfocal to a simplee lens? Or to a flint/crown doublet? Or to an apochromatic objective? How do they take into account ED glasses? To me parfocal filters should come out of the factory as a precise match to the objective, something like obj/filter-set pairs, otherwise they would need focusing just like non parfocal ones.  

Or may I be wrong....?

cesco

They are parfocal only with each other. They cannot correct non-parfocal (non-apochromatic) optics. They make no attempt to correct for optical defects which means they depend on the optics being parfocal.

7 hours ago, Demonperformer said:

My understanding would be that for a given object, your scope (whatever its configuration) will have a position of the focusser knob that will always bring that object into focus [to all intents and purposes, this will be the same for all astronomical objects]. When you add a filter into the optical path, this alters the focus by a precise amount. Parfocal filters are made so that this 'precise amount' will be the same for each of them. Yes, if you use a different scope, this 'precise amount' might differ, but within one system, it will be constant across the filters.

At least, that is my understanding ;)

This is only true of perfectly corrected optics (as in an all reflecting system). Any refracting element will introduce some variance between the focal points of different wavelengths. This may mean your parfocal filters operate parfocally within measurable limits or that they don't. Sara now uses a Baby Q which was once mine. I found it parfocal with Baader filters but Sara doesn't. She may be fussier than I am, of course, but I think the real difference is that she is using smaller pixels so what was effectively perfect for me is not effectively perfect for her.

Olly

Edited by ollypenrice
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8 hours ago, Demonperformer said:

My understanding would be that for a given object, your scope (whatever its configuration) will have a position of the focusser knob that will always bring that object into focus [to all intents and purposes, this will be the same for all astronomical objects]. When you add a filter into the optical path, this alters the focus by a precise amount. Parfocal filters are made so that this 'precise amount' will be the same for each of them. Yes, if you use a different scope, this 'precise amount' might differ, but within one system, it will be constant across the filters.

At least, that is my understanding ;)

Thus, the focus of each colour will be shifted, but not made to coincide, by the filter presence of the same amount for every channel. It sounds obvious: parfocal RGB filters do not remove the objective chromatica aberration, they just shift it. I will have to refocus after each filter change do the objective, not to the filters. Thank you for your answer

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