Quote:
Originally Posted by ngcles
Hi Bobby,
It isn't really about the increased focal length, but more about the smaller secondary mirror. As I said, once the size of the secondary is reduced to below the 20% mark (by diameter, not area) its effect on visual contrast is shrinkingly small. Higher than 20%, particularly once it passes 30% there will be a noticeable difference on nights of very good seeing. Probably 9/10 nights, it isn't really detectable but on that 10th night, you'll go "Oh"!
I know this is a bit off-topic but all commercial Schmidt-Cassegrainians have comparatively (to an Newtonian optimised for visual observing) quite large secondary mirrors -- in the order of 35% by diameter. Commercial Maksutov-Cassegrainians are similar-ish but come out at about 28-29%. Though there is a lot to like about both those designs, particularly when it comes to dimensions and portability, contrast on planetary images isn't their long suit.
Unfortunately (also), many of the less-expensive commercially-made Newtonians are sold with slightly over-sized secondary mirrors. I think it is because it renders the telescope a tad more forgiving of slight mis-collimation of the secondary mirror and also on the basis that a larger secondary is much more desirable for imaging and photography (even though 99.9% of those telescopes will never be used that way).
If you are prepared to put in a little effort, buy a smaller secondary and swap it in for the original it will make a slight improvement on nights with very good to excellent seeing. On deep-sky objects, it would be a virtually undetectable difference.
The trouble is, sourcing a size that is exactly what you want. Usually if you want "X" you have to choose between something like 0.9X and 1.2X.
Antares Optics (in the U.S) sell various sizes, and for the 10" f/5 GSO 'scope, the 54mm would be a good choice and provides a 21% central obstruction with a good-sized 100% illuminated field for about $100 USD. Unfortunately the next size down, -- the 46mm is probably a tad small and will result in a very tiny 100% illuminated field -- not optimal for observing deep sky stuff at low magnifications. You could overcome that to some extent with a lower profile focuser than the original equipment So you'd probably be better off with the 54mm. It would be a matter for the owner to weigh-up whether such an investment would be worthwhile.
In the alternative, BINTEL sell a GSO 50mm (in a perfect world I'd probably want 52mm) for just $49- AUD.
Best,
L.
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That was an excellent post !
Just a question regarding say a F/5 system verse a F/6 with both having the same aperture , the faster scope will need to be more accurately collimated ' may suffer coma & be harder on eyepieces plus with the larger central obstruction = softer contrast , would you agree
Seems the owner of the scope below must have liked looking at planets
Thanks for the info about Bintels 50mm secondary .
Cheers
Bobby