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Refractor mods?

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#1 Timaoes



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Posted 25 February 2013 - 01:35 PM

A couple of weeks ago I bought a used Celestron Omni XLT 120 mm (achromat) on a home built and very sturdy altaz mount. I am really a dobsonian kind of guy and I have been observing under mag 6 skies for quite some time with a SW 10" and a GSO 16". Recently I have become curious about the refractor way of star gazing, so when this scope came my way with a nice price tag on it, I bought it.

My previous experience with refractors are very limited, and I must admit that during first light I was totally struck by the chromatic abberation introduced by the optics, especially observing Jupiter (using Hyperions and Baader Orthos).

So my question to the forum is quite simply what you can do about it (and an apo is not an option :-). I know there are fringe killer filters, and I guess that would be a start. But how about flocking the tube, or buying a better diagonal than the one that came with the scope. Or are there others mods worth doing (besides buying a better scope)? I haven't yet made a proper star test, but the collimation seems to be in order.


#2 dan_h


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Posted 25 February 2013 - 01:49 PM

I have a number of achromat refractors and to some extent, they all show CA. There are a number of cures if you find it bothers you. All CA fixes are filters. You cannot change the image by flocking, baffling , changing diagonals, etc. The amount of visible color is affected by the f ratio of the scope so an aperature stop can help to reduce CA but it also reduces resolution.

There are CA filters, VR-1, etc. These work by restricting the blue and red ends of the spectrum with minimal visual impact to the overall color of the image. They all work to some extent and it seems everyone has their favorite.

I have tried a Baader Contrast Booster and it defnitely reduces secondary spectrum but imparts a yellow-green cast to the image that I don't care for.

My favorite filter for Jupiter is a simple polarizer. No color fringing remains but the filter also reduces brightness so it works best with large aperatures.


#3 SeptemberEquinox


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Posted 25 February 2013 - 02:13 PM

Few filters that works for me while observing bright object is, green filter #56, a single polarizing filter, or one of those moon filter. It kills the CA when I'm observing with my 150mm f5 which suppose to get more CA than yours. But I don't use any filters now, I learned not to pay attention to CA. But if I have to, I use a polarizing filter for sure, it doesn't change color or it doesn't lose contrast much at all.

#4 BryanS


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Posted 25 February 2013 - 03:19 PM

Have you compared image quality with a good quality barlow in the optical path?


#5 Sol Robbins

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Posted 25 February 2013 - 03:39 PM

For Moon and planets, this:


I have been testing it out in my scope evenwhen Chromacor is not installed. Its dielectric.

#6 Timaoes



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Posted 26 February 2013 - 03:33 AM

Many thanks for your advice. I will try the apertur stop and the barlow fix on the next clear night. If that doesn't do the trick I will go for a filter of some kind. Thanks again.


#7 Jon Isaacs

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Posted 26 February 2013 - 08:04 AM

Many thanks for your advice. I will try the apertur stop and the barlow fix on the next clear night. If that doesn't do the trick I will go for a filter of some kind. Thanks again.


The Barlow does not affect the level of the chromatic aberration, in this situation, it's best thought of as part of the eyepiece, a 2x Barlow cuts the focal length of the eyepiece by two.

Chromatic aberration is the result of the inability of the objective to focus the different colors to the same point, when the yellow-green are in focus, the red-blue are out of focus, that is why you see purple. To actually correct this, the different colors of light have to be individually refocused. This can be done but it is very difficult and expensive to do. The Aries Chromacorr did this.

If a Barlow were able to refocus the out of focus colors of a achromatic refractor, it would defocus the colors of a color free scope so they would exhibit false color. This doesn't happen..


#8 GlennLeDrew


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Posted 26 February 2013 - 02:08 PM

The polarizing filter, moon filter and Barlow only reduce chromatic aberration by dimming the image overall. Make the CA fainter, and it's harder to see. But this is not a cure, really.

Regarding flocking. Perhaps the focuser drawtube should be the first to get this treatment. After that, perhaps the portion of the main tube ahead of the first baffle (behind that baffle--if there be one--the tube is very well shadowed already.)

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