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CA treatment

zhgutas

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zhgutas

zhgutas

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Posted

As should be expected, my f/5 acromatic refractor shows significant amount of CA. It is not a big problem with the moon, however, and i don't know why. But other objects, like venus, jupiter and bright stars, are badly affected. It does not annoy me too much, but when viewing stars at close (especially doubles, which, as I found recently, are lovely) it takes time and effort to get a decent view.

Question: which way is the best, to treat this "illness". (Though I know it is impossible to cure it perfectly without buying a 1200 Takahashi or sth). Is it a fringe killer, APO barlow or something else, that would make my life easyer :D ?

Thanks

Marius

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GazOC

GazOC

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There are quite a few fringe killers on the market, Baader and WO both do good ones. But be warned that they arn't a miracle cure, they block out light (the violet) so the image can take on a yellowish colour which looks unnatural.

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GazOC

GazOC

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Blast :? what about an APO barlow?, since I use barlow for doubles anyway..

Thats just a Barlow that won't introduce any more CA to the image. It doesn't correct whats already there.

"No such thing as a free lunch" as they say.... :D

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Whippy

Whippy

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I could be wrong here (it wouldn't be the first time), but an all an APO barlow would do is keep the light coming from the scope itself colour corrected, it doesn't correct any CA itself. The problem you have is the scope itself. My understanding of achromat refractors is that generally short tubes like yours, suffer more from CA than long tubes. Hence many older design 'fracs are f10-f14 to try and keep CA down to a minimum.

I know Antares do a long focal length (f14 I think) achromat, looks like one of those old designs you see from a while back, but it's £700 for the 100mm version :shock:. I would imagine that has very little CA.

Tony..

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Naz

Naz

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My 120 ST when viewed in the middle of the field shows very little CA. Move to the side it increases, but with the cap on and the stop removed, the CA virtually disappears. I still do use it often, even in preference to the 200 dob for the ease of use and mainly for wide field views. Each scope type has it's followers and each scope has it's place and use.

naz

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MartinB

MartinB

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Marius, the cheapest advice is to learn to live with it. Even a Takahashi triplet will show colour in some circumstances. If you look at a very bright object like venus in mediocre seeing colour is inevitable - so don't do that. If focus isn't perfect colour will be exagerated. If you don't look precisely through the centre of your EP you will see more colour.

When you look at the moon why would you want to look at the edge rather than the terminator - but that's just what people with refractors do, they obsess about whether there's a fringe when it really matters very little. Poor colour correction will give more light disperal and this reduces contrast esp when looking at the moon so that can be a real minus. If you have nice contrasty views of sharp craters then a bit of violet fringing isn't a big deal is it?

Colour isn't a great issue with DSOs.

All scopes have strengths and weakness, its a matter of working around them and if that's not possible training your brain to ignore them. Apos are a recent thing. Generations have revered achromatic refractors, they haven't suddenly become no good.

If you get a newt you've got coma, get an SCT and you've got loss of contrast, get a decent aperture apo and you've got a financial crisis and a purchasing decision to defend to the hilt.

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GazOC

GazOC

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When you look at the moon why would you want to look at the edge rather than the terminator - but that's just what people with refractors do, they obsess about whether there's a fringe when it really matters very little. Poor colour correction will give more light disperal and this reduces contrast esp when looking at the moon so that can be a real minus. If you have nice contrasty views of sharp craters then a bit of violet fringing isn't a big deal is it?

Thats a good point Martin, my 150mm f5 gives decent lunar views as long as I look at the terminator rather than the edge. I can live with the CA for DSOs but the two main problems for me are the areas Maruis mentioned - planets and double stars. Planets because you are losing detail to the smearing and double stars because a lot of their charm is purely aesthetic and the extra colour can mar that.

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zhgutas

zhgutas

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Thats very generous of you , Gaz. :shock: I knew you guys are helpful, but helpful like that?! It would be awesome. I just don't feel right "borrowing" your precious stuff :insects1:

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Cloudwatcher

Cloudwatcher

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I think Martin 'hits the nail on the head' with what he says. Until the 'perfect' scope for all seasons comes along we will just have to live with what we've got.

Anyway, if there was such a thing everybody woud have one.Think how boring that would make the members equipment gallery and we wouldn't have interesting discussions like this one! :D

CW

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GazOC

GazOC

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I think Martin 'hits the nail on the head' with what he says. Until the 'perfect' scope for all seasons comes along we will just have to live with what we've got.

Thats easy to say when you've got an equipment list like that!! :D

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barkis

barkis

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Marius, are there any diaphragms in the scope? tube, or are they in the focuser. Try putting a home made diaphragm over the lens of the scope.

It will obviously cut down the image brightness, but if you are observing a bright object like the moon, you could use a mask with a 2" aperture.

Make another with a 3" hole in it and try it on bright stars and see if the CA has reduced. It is not an Ideal remedy, if any at all, but worth a try

If only to reduce your headache. :(

Ron. :)

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