Quote:
Originally Posted by toyos
Hi all, thanks for all the feedback.
The collimation of the 12" dob is fine. The big difference in image contrast between the two is what surprised me the most. Everything is brighter including the background through the 12" dob, whilst the 6" refractor shows a slightly dimmer but much clearer Omega Centauri with a much darker background allowing me to see details better. I used the same set of Plossl eyepieces 6, 9, 15, 18, 20, 25, 28 & 32 on both telescopes.
I should probably take the dob to some place darker since most of my observation time was more conveniently done in my backyard.
Mike, thanks heaps for the invitation. I'll be there if I can manage to first find a bigger car than my 2-seater.
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Hi,
The lack of contrast you describe predominantly stems from the fact that the GSO dobs are very poorly baffled when they come from the factory. While the flat black paint used on the inside of the tube in theory should be "non reflective", However, an enormous amount of stray light bounces around the inside of the tubes on these scopes. You can rectify this yourself pretty inexpensively. You can use "flocking paper" or any one of a dozen other methods to properly baffle the telescope tube itself. Flat Black epoxy paint with crushed walnet shells mixed into the paint is another option.
Some suggested mods which will improve the contrast of your dob by up to 40% :-
1) Baffle the tube. Flock the entire tube or as a minimum flock a large section opposite the focuser. In addition you can add a tube extension which helps a lot.
2) Blacken the edges of your secondary mirror with a black texta pen.
3) Baffle the inside of the focuser. You can do this by using some 1/4" self adhesive foam weather stripping.
4) You can go a step further and install annular baffles (ring knife edge baffles) into the tube at the top on either side of the focuser and at the bottom just above the primary mirror to prevent stray light entering the tube.
I have done all of the above mods to my 10" newtonian and several more; and it gives "refractor like" views in terms of contrast. It will beat the daylights out of any 6" refractor on the planet. There are plenty of IIS members who have looked through it to attest to this. Whilst a refactor gives its maximum performance straight out of the box, unfortunately these GSO dobs while having the makings of a very good scope, need quite a bit of tweaking to deliver of their best.
I have some photographs on my work computer which I will post tomorrow from work, showing exactly the modifications I have done. If you go to the Pony Club on Saturday night I will be happy to explain them all to you at the scope. I can't bring the scope because that would mean leaving the 18" behind.
Cheers,
John B