Moon colour

[jaygpoo]

I have a skywatcher travel star 120 with a cannon 500D camera . I have been taking photos of the moon using my cassegrain 127 skywatcher eariler and they are very good however the moon shots taken on the travel scope show a purple ring around the moon edge. Is this because its the cheaper version of the evostar ed epo and the lenses are not that good or is it something i am doing with my equipment. I did have a Tal 100mm and the photos from this were also great but I found it to big to handle and decided to go for the better F5 600mm length of the travel star. The Tal also had the same type lenses but the photos were good. Do I have to have a Achromate lens to banish this colour ring?

[jimmyjamjoejoe]

The star travels are achromats, and ad such will suffer from chromatic aberration. In general, the faster the scope, the worse the CA will be, which is why these fast refractors are generally recommended for widefield work rather than planetary. At around f10 CA is usually pretty well controlled, but at f5 it's to be expected. You can try filters, but i dont think they're too great. Maybe try editing it out? I think i recall hearing about a photoshop plugin that did something similar... Other than that, it's either a slower achromat or an expensive apochromat i'm afraid.

[jaygpoo]

Thanks for the knowledge . On that info I should have kept the Tal instead of giving it away on ebay. Will have to keep an eye out for a super scope.

[laser_jock99]

Unless you pay mega money for a well corrected triplet - all refractors suffer from some degree of chromatic abberation (even some doublets advertised 'apochromatic' are not immune). A point which is often overlooked is that mirror based scopes are inherently apochromatic and so are often favoured by planetary imagers.

[jaygpoo]

I take it sub dwarf that you are refering to relectors and dobsonian scopes ( mirror based) Interesting in that I have spent since Feb researching on the net and on this site and the conclusion that I came to was that Reflector and cassegrain scopes were best for looking at planets /clusters where as much light as possible gave a good image but for imaging a refractor was the scope most imagers went for as optic size was not as important as the image area need by camera chips was so much smaller than the human eye. Of course the quality of that optic remains the issuse as my results have shown . One other problem I have come across using the travel scope is the focus. On the tal the focal adjustment was massive and my cannon camera was almost always able to focus on an object. With the travel scope if I connect the camera in the diagional I need to wind the focuser in and it stops before I am in focus. Connecting the camera direct to the tube again does not always result in good focus as the camera appears to be too close and needs an extension of some type. Any ideas ?

[nptncodge]

I take it sub dwarf that you are refering to relectors and dobsonian scopes ( mirror based) Interesting in that I have spent since Feb researching on the net and on this site and the conclusion that I came to was that Reflector and cassegrain scopes were best for looking at planets /clusters where as much light as possible gave a good image but for imaging a refractor was the scope most imagers went for as optic size was not as important as the image area need by camera chips was so much smaller than the human eye. Of course the quality of that optic remains the issuse as my results have shown . One other problem I have come across using the travel scope is the focus. On the tal the focal adjustment was massive and my cannon camera was almost always able to focus on an object. With the travel scope if I connect the camera in the diagional I need to wind the focuser in and it stops before I am in focus. Connecting the camera direct to the tube again does not always result in good focus as the camera appears to be too close and needs an extension of some type. Any ideas ?

hi i have the same problem with my 120mm refractor, hope you get a solution, N/C