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Chromatic Abberation filter - what size?

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#26 AxelB

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Posted 21 February 2019 - 06:14 PM

Key words: "Because I have another better scope for planetary observations, I rarely push my AR127 above 80x" A 6" F/8 Dob is a much better scope for planetary and costs $300. You already have a nice wide field (that you can also look at the Moon with) Buying things that make your scope live up to it's potential makes sense IMO. If you buy anything for your AR 102, maybe it should be the widest field best corrected 2" EP's that you can afford. (And not waste money changing purple to yellow) $300 6" Dob beats the planetary detail that can be seen in a $3000 4" APO. AR 102, best in 4" class widefield. 6" F/8 Dob best in ($300) class planetary. I might eventually switch to an ST 120 for wide fields, but the 6" Dob has to stay as the cheap planet killer on my to buy list. https://www.amazon.c...2/dp/B0018IWQC8 Halfway to the price of something that could actually see way more planetary detail. A filter or another scope that could see deeper DSO's as well. Edit: If the color is objectionable send it back for a refund (save up a little more $) and try the A.T. 102 ED or SW Pro 100 ED, or even the new F/11 ED from Altair astro. If you like refractors


I said that but I still experiment with expensive filters on the AR127 at high power nonetheless because I’m an equipment junky who derives as much fun looking at the sky as with testing different pieces of equipment lol.gif .


Edited by AxelB, 21 February 2019 - 06:15 PM.

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#27 russell23

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Posted 21 February 2019 - 10:30 PM

The 2" is more convenient if you have a 2" diagonal.  With a large fast achromat I just put the filter on the diagonal and leave it there for all types of observing - especially deep sky because the filters help so much to sharpen up the star images and allow for much sharper star images at higher magnifications.

 

A #8 light yellow filter will reduce the purple fringing about 50% and a Baader 495 Longpass will completely remove the purple fringing.  Both filters are yellow - the #8 gives a mild color shift to yellow and the 495 LP gives a stronger color shift but with a ridiculous improvement to the sharpness of the scope. 

 

You don't really realize how much CA is limiting the deep sky performance of an achromat at higher magnifications until you try a CA reducing filter. 


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#28 CeeKay

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Posted 23 February 2019 - 12:24 PM

Thanks to all for your input...

 

Chris



#29 mikeDnight

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Posted 24 February 2019 - 08:07 AM

Personally Chris, I'd accept it as it is. It's an achromat, so there will be some colour fringing, but I've found filters always have some subtle, but detrimental effect on the image, and that the pure view is always the best. I wouldn't bother with a CA filter of any kind!

Sometimes CA can be reduced by your choice of eyepiece though it will always be present to some degree. If you use Naglers for example, don't expect colour free views. Sometimes simpler is better and you might find old Celestron Ultima's give well corrected, very clean, high contrast, high definition views. 


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#30 russell23

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Posted 24 February 2019 - 08:36 AM

Personally Chris, I'd accept it as it is. It's an achromat, so there will be some colour fringing, but I've found filters always have some subtle, but detrimental effect on the image, and that the pure view is always the best. I wouldn't bother with a CA filter of any kind!

Sometimes CA can be reduced by your choice of eyepiece though it will always be present to some degree. If you use Naglers for example, don't expect colour free views. Sometimes simpler is better and you might find old Celestron Ultima's give well corrected, very clean, high contrast, high definition views. 

The problem with CA is really not about the purple fringing.  People focus on purple fringing and if you have a filter that reduces the purple fringing then the real benefits are actually the more subtle benefits that don't get talked about enough. 

 

An achromat struggles to bring stars to a pinpoint sharp focus.  This problem increases as magnification increases.  The defocus of light causes a blurring of stars and a general loss of sky background contrast as that light gets smeared across the field.   This becomes quite severe in large fast achromats.

 

When you use a CA reducing filter, you can get much sharper star images which is especially noticeable at high magnifications and the sky background becomes clearer/cleaner.   This improvement is not even close to subtle with a larger, fast achromat such as a ST120.  It is dramatic.  And yes, when you use a CA reducing filter the, the purple fringing is visibly reduced or eliminated, but it is the improvement in the image sharpness that really matters.

 

The only effect on the image that a CA reducing filter has that some find negative is the color shift.  But there are options.  If you want complete removal of the purple fringing then the Baader 495 LP filter accomplishes that and also provides the most improvement in sharpness of star images.  That filter does cause a strong yellow color shift.  If you want beneficial improvement without a strong color shift then the #8 Light yellow or Baader Fringe Killer will give you a roughly 50-60 reduction in visible fringing - both the brightness of the fringe and the linear extent away from the star are reduced by that amount.

 

If you don't like the color shift with the filters and you don't like the purple fringing of the achromat then you seriously need to consider upgrading to an APO or a mirror based telescope.


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#31 penguinx64

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Posted 26 February 2019 - 02:02 AM

I have a 1.25 inch diagonal, not a 2 inch diagonal.  I keep a Baader Contrast Booster filter attached to the diagonal all the time.  I tried the Baader Fringe Killer filter, but the Contrast Booster filter works better for me.  I have it on a TS 80mm f7.5 achromatic refractor.  



#32 Ernest_SPB

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Posted 26 February 2019 - 04:28 AM

The problem with CA is really not about the purple fringing.

And yes, when you use a CA reducing filter the, the purple fringing is visibly reduced or eliminated, but it is the improvement in the image sharpness that really matters.

 

It is a problem of "CA filters" that they mostly focused on purple fringing making wide bandwidth filtering. 

To improve resolution of fast achromats is required much more radical spectrum filtering - green color filter that seriously reduces brightness of image. 


Edited by Ernest_SPB, 26 February 2019 - 04:32 AM.



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