Quote:
Originally Posted by toyos
Coming back again to amateur Astronomy after 15 years on & off, and just purchased a GSO 12" Dobsonian and an imported 152mmx1200 refractor.
I owned several reflectors before but never had a large refractor. I always knew that refractors would provide clearer images with much better contrast, but I didn't expect that the 6" refractor would significantly outperform the 12" Dob (including many deep-sky objects) after comparing them side by side under the moderately light-polluted sky. Omega Centauri stands out crisply against a much darker sky background showing better detail.
I'm starting to wonder if there's probably something wrong with my 12" Dob, but everything seems fine and properly set up.
Any inputs?
Thanks
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Well toyos, there is no doubt that theoretically the 12" dob should easily outperform the 6" achro, though it is interesting to read asimov's review of his 6" job wherein he makes comparisons to his 12" dob in the IIS reviews section: I doubt whether asimov would take too kindly to certain individuals questioning his expertise and judgement, from my observations of him.....
What I can tell you is this: I own a 10" GSO dob and a 6" achromat, and first of all there are 2 things I can tell you about my dob that are unequivocal.
(1) it appears, from the small sampling I have been able to do with other GSO dobs, that its optics are par for this particular instrument (measured against 3 other newish GSO 10 inchers.)
(2) it is kept collimated to a very high degree: my own capabilities here are more than competent - I alllways perform star-tests to ensure the optics are kept properly aligned, and achieve excellent diffraction rings, uniformly concentric, before considering the scope fit for obbing.
However, on a wide range of objects, there is very little to separate these 2 scopes: I do use one of the Baader "semi-apo" filters and a quartz dielectric diagonal in the achro train; but with 13mm Televue T6 Nagler or 17mm Vixen LVW the above appraisal is unequivocally true; also when employing a Televue 2x barlow in each scope's optical train! These 2 scopes are often set up side-by-side on my ob deck, and although I don't have the luxury of 2 sets of these types of eps, I have frequently swapped the same ep between the 2 to compare views.
With focal lengths of 1200mm and 1250mm respectively for the achro and dob, the difference in magnifications are so close as to be insignificant. (the aforesaid eps give
approximate magnifications of 70x, 90x, 140x and 180x)
What is also unequivocal is the superior planetary detail revealed with the achro; specifically on Saturn and Jupiter. Other than the fact that my vision is excellent and I have been obbing on and off for over 50 years and don't need any johnny-come-latelies to lecture me on collimation, observing techniques et al, I have nothing more to add except that the clarity, contrast and definition of this (relatively) small achro refractor constantly amazes me with its ability to stand up against a (my own) significantly larger apertured, well maintained dob newt!
Regard, Darryl.