Hi Bobby & All.
Quote:
Originally Posted by Bobbyoutback
I think the OP would like answers to the following question 
I'll refer to the objective size of the OP scope ' 10" F/5
Question ' If that mirror was of the same quality but figured to F/7
would it show better contrast on planets ?
Cheers
Bobby |
The short answer is: Only if the secondary mirror was also smaller and therefore the central obstruction became smaller -- though there probably wouldn't be much in it and the difference would only be detectable on nights with good seeing. Because of the longer f.l, you can get away with a smaller secondary.We don't know the dimensions of the secondary mirror in this 'scope, but GSO 10" 'scopes are typically about 60-65mm for the f/5 and they are therefore about 25% obstructed.
Conventional wisdom says if the obstruction is kept below 20% its effect on contrast is negligible. A 10" f/7 could happily use a 40mm minor axis diagonal and the central obstruction would drop to just under 16%. Such a telescope, assuming a good figure on the glass and good collimation would be a superb planetary telescope -- almost refractor-like images. The extra focal length would also mean moderately high magnification (say x200) could be obtained with relatively simple eyepieces (like a 9mm abbe Orthoscopic for example) that would also offer reasonably comfortable eye relief. The f/5 to achieve the same magnification would need an eyepiece 5/7ths that of the f/7 -- about 6mm.
The downside to f/7 is that truly expansive fields (at very low magnifications) aren't going to be nearly as wide as the f/5. The other downside is the long and heavy tube about 1800mm would need a more substantial mount and the whole package would be less transportable.
Hope that helps.
Best,
L.