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so i got my "new" (secondhad) st80 yesterday, and spend a while yesterday evening looking at the moon. i was expecting CA, and there it was, though to be honest, it's not that much of a big deal really, is it?

using my BST explorers, I can get 22x, 33x, 50x, and 80x mags, which seem sensible for this scope, and the views so far are really nice. i need to try it out on some star clusters now, as i think this it where it'll really shine.

anyway, has anyone else noticed in this scope, or scopes like it, that the CA is worse at some mags than others? i.e. there's some at 22x, more at 33x, but less at 50x, then more again at 80x? any ideas why this is?

thanks

stephen

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It's not a big deal at 80mm but increases with aperture so the 100mm, 120mm and 150mm Startravel's progressively show more CA. It's not an issue for deep sky viewing but, in my view, does start to degrade planetary a lunar performance a bit at much over 100x although the views are still nice. Some people find CA more bothersome than others. I'm not keen on it personally.

Edited by John
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Hello Stephen.

I don't really know the answer to your CA observations but there are a couple of possibilities. Although you are using the same brand of eyepieces, quite often a couple in the range are not so compatible with the telescope in use and perform differently. As CA usually becomes more noticeable as the magnification increases it could be that the lowest power is too low to show CA, the slightly higher power stars to show it and then maybe, and this is the maybe, the exit pupil of the eyepiece compared to the exit pupil of the eye is such that it masks the CA slightly, magnification beyond that should follow the usual increase in CA. As you mention, the scope should shine on star clusters, I think you will find that the star clusters will shine on your scope. :smiley:. Good buy!

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It's not a big deal at 80mm but increases with aperture so the 100mm, 120mm and 150mm Startravel's progressively show more CA.

Isn't it also dependent on focal length? So, all other things being equal, a 120mm f/6, say, will show more CA than 120mm f/12?

Or have I got the wrong end of the stick?

James

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I have a Startravel 150. The CA was ok at first, but gradually ate away at me. I sorted most of it out with a Baader Semi Apo filter. The slightest amount remains when imaging, and sometimes a little when observing.

I bought a Celestron X-Cel ED EP not long ago, and I just don't see any now.

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Isn't it also dependent on focal length? So, all other things being equal, a 120mm f/6, say, will show more CA than 120mm f/12?

Or have I got the wrong end of the stick?

James

That's correct. I got told before by someone about a ratio that can be used to roughly work out how much CA will be present. I can't remember it, but I know that it involved aperture and focal ratio.

Clear Skies

Luke

Edited by lw24
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Isn't it also dependent on focal length? So, all other things being equal, a 120mm f/6, say, will show more CA than 120mm f/12?

Or have I got the wrong end of the stick?

James

Yes, CA is better controlled at slower focal ratios. The point I was making (probably badly :smiley:) was that increasing the aperture of an achromat at the same focal ratio also increases the amount of CA. So a 120mm F/5 will show more CA than an 80mm F/5.

Sorry if I confused things by mentioning the effect of larger aperture :)

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.... I got told before by someone about a ratio that can be used to roughly work out how much CA will be present. I can't remember it, but I know that it involved aperture and focal ratio....

This is quite an interesting table:

http://www.cityastronomy.com/CA-ratio-chart-achro.jpg

A 6" F/18 would be a bit of a handful !.

Edited by John
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I have used both the ST80 and ST120. The CA is worse in the 120, but I use this for deep sky viewing so it's not really an issue. I've never had much problem with the 80. I did use the Baader Fringe Killer at first with the 80, but I didn't like the greeny-yellow tint it gave to the overall image.

Ed

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Never used a ST scope but had a 120mm f8.3 refractor and the CA was not too bad on that. It bothered me less on the moon than on planets even though it was more obvious on the moon. It seemed to affect the sharpness more on planets.

I am now a fully committed newt man but have been considering a ST80 as a wide field/travel scope and instead of a pair of bins which I have never got on with for photography.

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Apparently my ST120 has "unacceptable" levels of CA. Hmm, I'd definately say it's filterable. :smiley:

The levels of CA seem to vary hugely from sample to sample. Basically with QC issues i suppose. The best ST i've owned was an older Helios ST120. The worst by far was a Skywatcher ST150. Had mixed results with the numerous Celestron/Orion/Skywatcher/Helios ST80 and ST102 i've owned down the years.

I found the Astro Engineering Achro filter worked quite well. It wasn't as aggressive as the Fringe Killer and didn't give such a noticeable colour shift.

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when i finally get myself out after midnight i'll see how jupiter looks with it. those astro engineering filters look nice - for £25 at scopes n skies i might get one the ca bothers me (anyone have a cheaper link?). thanks.

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Hi Stephen

glad you are enjoying and getting some use from the ST80. In use I found that cheap EP's increased the CA whilst quality EP's will not add to it but did not reduce it.

I also found 80x about the limit of the ST80's optical quality. This was achieved using a Vixen 10mm EP and a Tal 2x Barlow. Anything greater will reduce the exit pupil too much and hence a dimmer image that sort of defeats the ST80's intended use.

It is impossible to remove CA from short tube refractors (even F10 tubes) , their weight , portability and ease of use are the trade off for some CA being present. I have not used filters so cannot comment on them.

Paul

Edited by Polar Bear
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