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6inch F8 refractor fringing


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Hi all,

recently bought a F8 6 inch refractor helios ( ithink) second hand scope. NOw i realised would have some CA with it and bought a cromo corrector filter . Had first light on Monday. Very happy with its performance on stars and dso , in this case it was orions nebula which showed reasonable detail despite light pollution. HOwever , went to look at jupiter and had a nasty fringe on it with poor disc features , so stuck the croma corrector in the eyepiece and left with a nasty yellow fringe instead of purple . NOw i had heard adding a yellow filter could help (i only had a green one which i added and didnt improve the situation) . When defocused the pattern seemed symmetrical i.e. didnt seem to be a collimation problem with the scope. NOw i realise CA a problem on acromats , and i did used to have a 80mm apochromat for DSOs until i changed it for a 12 inch reflector (work in progress) as much of my dso stuff was done with camera lenses for photography, but had thought would be usable for visual for planets (i.e i thought the shorter F 5/6 6 inch ones would be unusable , but the f8 be ok)

So, will the yellow filter help? If not what F ratio is uable in a refractor for planetary work?

I have a celestron C8, 12 inch dob and now 4 inch and 6 inch refractors , it was bought because wanted big refractor so its not a big problem just wanted to see if situation could be improved

Steve

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I've used a few 6" F/8 achromats. In their native state they do produce some CA around brighter objects which can reduce the contrast and smear the detail a bit - I've still had enjoyable views of Jupiter, Saturn and the moon through them, despite this.

You say you have a chromo corrector filter - what sort is it ?. There are a number around which, from what I've read, reduce the CA by a noticable amount although they can introduce a tint of their own to the image. The Baader Fringe Killer seems to be one of the most effective.

I believe a yellow filter will do something similar but again you will get some tint.

The best solution for CA, but not an easy one to apply, is an Aries Chromacor which reduces CA by 80%-90%. I've owned a couple of these things and currently have a Meade 6" F/8 with a Chromacor in it waiting for first light (seems a long time coming !). The Chromacor adds no tint to the image and corrects under / over corrected optics as well. Not an easy solution to find to be honest though.

To get lower levels of CA in an acrhomat without filtering you need a slower scope - an F/12 or F/15 would show much less but i) they are hard to find and ii) they need equally impressive mounts.

Edited by John
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yes i have the baader finge killer, which to be fair took out the purple fringing , leaving a nice yellow in its place. jupiter was nice and bright put poor contrast and surface detail

yeah the larger F ratio scopes probably need a prohibitively expensive moung , at least for me, as the 6inch f8 is big enough!

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chromacor looks expensive, however i suppose if it improves the CA that much much less costly option than an apochromatic of that price !

I'd be interested in hearing how the first light goes !!

how do you test to decide which model you need !

They were expensive when they were available new - around £500. I picked the ones I've bought (used) for less than half that. Still more expensive than a filter but not so bad when you consider that they can get the performance of a humble chinese achro to the level of a decent 6" ED doublet refractor - one of those (a Vixen) recently sold for £1,800 used.

My cunning plan is to achieve ED-levels of performance in a 6" refractor for under £500 all-in :eek:

Here is some more info on the Chromacor:

http://www.astrosurf.com/luxorion/Documents/chromacor-doc.pdf

And my own thoughts pasted here a while ago:

http://stargazerslounge.com/attachments/equipment-reviews/39531d1279755811-chromacor-chromatic-abberation-corrector-ccreview.pdf

Edited by John
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You could increase the f ratio for planetary use by stopping it down to, say, 4" to make it f12.

But that would defeat the point of having a 6" refractor. I think an achromat should indeed be around F12 or slower to be really good on planets. Otherwise I would just use the C8 or any other big reflector you have on Jupiter. A 12" newt would surely beat a 6" achromat!

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You can still use it at 6" for dimmer objects. I don't think it defeats the object really. If you had a 4" f12 (if you can get hold of one at a reasonable price) you wouldn't be able to use it at 6". Also stopping down means you are using the centre of a lens and, from what I recall, it's the edges of an optic that usually tend to suffer the worst aberrations so you could actually be improving the quality of the view in other ways too.

If you haven't got the money or desire for 2 scopes a 6" F8 refractor can be almost 2 scopes in 1. Have to say though in Steve's case that doesn't seem to be an issue :eek:.

Steve how does it compare to the 120mm Evostar?

Personally I use a Baader Semi Apo filter on my AR-6 and don't find the colour fringeing a problem - it's a bit of personal thing I think.

My 14" dob (once cooled) definitely beats my refractor on Jupiter but my refractor is on a tracking mount and is a lot more convenient at high magnifications. I find there's just something nice about a refractor on solar system objects.

Edited by haitch
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I used to use a Semi APO filter with my ST80 clone, both for visual and imaging. It's not as good at cutting the CA out as the Fringe Killer, but leaves a more natural colour, personally, I was happy with a little CA and the right sort of colour...

this might help... http://stargazerslounge.com/equipment-reviews/67599-baader-ca-filter-comparison.html

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You can still use it at 6" for dimmer objects. I don't think it defeats the object really. If you had a 4" f12 (if you can get hold of one at a reasonable price) you wouldn't be able to use it at 6". Also stopping down means you are using the centre of a lens and, from what I recall, it's the edges of an optic that usually tend to suffer the worst aberrations so you could actually be improving the quality of the view in other ways too.

If you haven't got the money or desire for 2 scopes a 6" F8 refractor can be almost 2 scopes in 1.

Personally I use a Baader Semi Apo filter on my AR-6 and don't find the colour fringeing a problem - it's a bit of personal thing I think.

My 14" dob (once cooled) definitely beats my refractor on Jupiter but my refractor is on a tracking mount and is a lot more convenient at high magnifications. I find there's just something nice about a refractor on solar system objects.

Fair enough, but the C8 tracks, and in my experience beats an a fast 6" achromat any day even in contrast on solar system objects at high magnification (a 6" APO is another matter:rolleyes:)

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I have to be honest and say havent had 'first light' with the evostar 120 yet. HAve most of the bits and pieces now to get the 12 inch skywatcher newt on a dobsonian base so looking forward to that.

I put the 6 inch on the tracking mount and it fared ok keeping jupiter in the eyepiece pretty well. DOnt get me wrong i like the 6 inch acro, its a nice hunk of metal :-) and was very happy with the views of orion and stars , also havent had the chance to look at the moon yet so we will see.

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