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Ford52

Anyone own a skywatcher 4 inch achromatic and have purple fringing

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Hi all

A few years ago,I had a 4 inch achromatic from skywatcher. Didn’t use it that much as it had annoying purple fringes around planets and especially the Moon,was set up in the living room as a semi grab and go. Did anyone else have issues with this scope? Had also at the time an Orion optics UK 8 inch f6 Europa on a GEM mount,the optics were nice and I had some great views of the Moon,Jupiter and Saturn. Around about 2002 at one of Mars closest approaches got my first good views of Mars and saw Syrtis Major clearly not just a ‘hint’. The Europa seemed better than the achromat and I still have it now though gave up the GEM to go with a OMC 140 Mak Cass that I sold.

Clear skies

 

Ford52

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Depends how sensitive to chromatic abberation you are, but all achromats exhibit the fringing. Some worse than others, the lower the F ratio then the worse it is. My little 102 at F5 is quite objectionable on the moon....but at low power on star fields it’s very pleasing.

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I always feel that someone needs to develop a filter range ( maybe 2 different reds, a Green and several shades of Blue, sort of LRRGBBBB) )for these achros as some of them could be exceptionally good .

My 6" F8 achro grabs an amazing amount of light from DSO's its just needs a mono sensor with the above filters to be almost as good as an APO costing 10 times more.

The F5 version may need to be LRRRRGBBBBBBB to get the same effect.

 

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A 4 inch achromat has to have a focal ratio of F/15 or so to show no false colour visually. At around F/10 the amount of false colour is really quite small so that is a popular focal ratio because the scope tube is manageable (ie: not too long). The achromats that operate at F/5 or F/6 do show quite a bit of false colour but their main intended use is for low to medium power viewing of deep sky objects and in this they do a good job.

There are filters that reduce the amount of false colour that is visible but they do tend to add a noticable tint to the image. Some people find them useful and some don't like them.

A 8 inch F/6 newtonian will show no false colour at all and is likely to show excellent planetary views.

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I do.

I owned ST102 - that is 102mm F/5 scope and chromatic aberration was horrible on bright targets, but that is not what the scope was intended for - it is wide field scope. I sold that one and got Evostar 102 F/10. Yes, it does have blue fringing, but I found that Baader Contrast Booster helps a lot with that.

Moon is almost clear of CA with it and still has rather neutral tone. I briefly observed Jupiter with it on one occasion and again there was no bright purple halo with this filter.

You can use wratten #8 yellow as cheaper alternative but it will give you slight yellow cast on the image.

What you can further do to reduce chromatic aberration is to use aperture mask. It reduces aperture and increases F/ratio at the same time. It will limit max usable magnification, but will provide much clearer image. For example using 80mm aperture mask, you will get F/12.5 scope. If you look at above posted table - you are in the green zone and still have a scope capable of delivering around x160 magnification.

37 minutes ago, Kev M said:

I always feel that someone needs to develop a filter range ( maybe 2 different reds, a Green and several shades of Blue, sort of LRRGBBBB) )for these achros as some of them could be exceptionally good .

My 6" F8 achro grabs an amazing amount of light from DSO's its just needs a mono sensor with the above filters to be almost as good as an APO costing 10 times more.

The F5 version may need to be LRRRRGBBBBBBB to get the same effect.

 

I'm sorry but that simply won't work. You can image with such scope though and get some decent results. You don't even need mono camera for that.

There are couple of tricks that you need to employ in order to minimize chromatic aberration issues (one of problems is that you will no longer have F/5 scope, but that does not matter - it is aperture at resolution that determines the speed of the system).

Here is an example:

First ST102 F/5 scope without any "tweaking":

Crescent.png

Image is rather poor as it was one of my first attempts with OSC camera (small sensor) and this scope. Camera had no cooling and it was QHY5IILc I believe - so not a very good camera. But that is not the point - resolution is terrible because of CA issues and there is large blue bloat around the stars (and red around smaller stars).

Now here is the image done with the same scope and different OSC camera - but not too different. Still 3.75um pixel size, still uncooled - it is ASI185.

m42_wide_v2.png

And later same target, a bit smaller field but better color correction:

m42.png

As you see - no fringing and images are quite "decent".

Trick is to use aperture mask (66mm) and wratten #8 filter - or that is what I used. No special processing was done to remove bloat from the stars.

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Yes, I see it with my ST102 as well.  For some reason it seemed even worse on the ST120.

James

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I wouldn't class the '102 as a telescope for imaging though.....however for widefield visual it is a great little scope with the power level at around x20.

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7 minutes ago, Alkaid said:

I wouldn't class the '102 as a telescope for imaging though.....however for widefield visual it is a great little scope with the power level at around x20.

No, it's probably not ideal for imaging (narrowband, perhaps?).  Mine works very nicely as a guidescope though :)

James

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14 minutes ago, Alkaid said:

I wouldn't class the '102 as a telescope for imaging though.....however for widefield visual it is a great little scope with the power level at around x20.

I would not do that either, but if you are starting AP on a limited budget - you can in fact use it to get decent results. It is also good to know that it can be done - and more importantly why it can be done and under what circumstances.

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15 hours ago, JamesF said:

Yes, I see it with my ST102 as well.  For some reason it seemed even worse on the ST120.James

It would be. False colour increases with aperture if the focal ratio stays the same.

 

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I have a 120mm fl1000mm achro. It's pretty bad on planets but more than adequate at low powers. Unless you go for an apo, CA is always going to be a problem.

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