I have an old 1963 A.Jaegers 5" f:5 crown/flint RFT. They were stunning on the Sagittarius area when I was a kid and I finally found one to call my own. These old lenses were about the best in that era but suffered from comatic aberration. They weren't designed for lunar or planetary viewing but low powered views of nebulae and open clusters. However the filter can be applied to any focal length refractor and even those with longer focal lengths will suffer some color fringing. Last night I took my scope out and turned it on the first quarter moon, a good test for the violet, green and red halos around bright objects. With the filter out the edge of the moon showed plenty of fringing. However with the filter in the star diagonal so all eyepieces were protected, fringing all but disappeared. I had to look to see it...that's GOOD! BTW - I read about this on at thread on Cloudy Nights so if you have a non-ED/APO and want to get the maximum use out of it by all means this IS the recommended filter to use!! Highly recommended on CN as well as by me!!! And...FLO is a super place to purchase wherever you live in the world!!!

I bought this to use with my Skywatcher Startravel 120 and I have been very impressed. I had the fringe killer, but in my opinion this is better. It really is such an easy, simple solution, wish I'd got one sooner.

Used on Skywatcher Star Travel Achromatic Refractor 120 (f/5) Telescope.
Since adding William Optics 90-degree Erecting Prism and improved eyepieces I only noticed chromatic aberration (CA) on the brightest of objects like the big planets and on higher power around the limb of the moon where it is more apparent, but of lesser impact for observational astronomy for DSOs. I found that Baader Semi Apo filter when added practically removes (CA) on all objects, while It improves contrast on deep-sky objects and splitting double star observations. It also delivers a very neutral view, whilst retaining the object brightness. I now leave it on permanently. These addons helped turn an already good out of the box visual telescope into an excellent one.
FLO service was excellent. Will use again.

I bought a pair of these to fit my binocular telescope's eyepieces. The improvement is immediate and dramatic, tidying up the fringes on the image and improving the contrast. Makes moonwalking a breeze.

Bought this with my telescope reading the review and its absolute gem. I have tried few images without and with the filter and boy does it make a difference. Took a few images of a Moon without filter and there was a really distinctive blue line around the edges of the Moon clearly visible. I have placed a filter and took images again and as if by magic it was clear and crisp. Worth a every penny Thanks FLO for help and support. Dee
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I very recently bought a barely used Sky-Watcher StarTravel 120 achromatic refractor, which has a focal ratio of f5, and is naturally prone to chromatic aberration, giving some objects a purple outline, and even horribly discolouring stars at times, especially if your stock star diagonal is pretty cheap.

I upgraded to a wonderful William Optics Amici prism diagonal which helped a lot, but this little filter is an absolute gem.

Chromatic aberration is very noticeably reduced, and it certainly makes a difference in filtering out light pollution, and reducing the deleterious effect of a bright moon in the sky.

Colours look very natural, and you get none of the overall yellowy colour that various other light pollution filters can produce. The change in colours is so subtle that you just don't notice it, with a dark looking sky looking black as it should.

It is wonderful to get a filter that meets a dual purpose with distinction. If you have an achromatic refractor, do yourself a favour and get one of these, and upgrade your star diagonal if you haven't already if you got a relatively budget refractor. Your views will be markedly better, especially if you have a shorter focal length telescope.

Can't recommend this filter enough. It won't turn your refractor into performing like a high-end and very expensive apochromatic refractor at very high magnifications, but it will help you get the best out of your optics and help reduce the effects of light pollution into the bargain. Fantastic product.

Absolutely super filter, really does away with ca and sharpens up fine contrast in planets and lunar views. So very pleased with results, makes double star splits a pleasure. I can really recommend this for achromatic refractors.