Quote:
Originally Posted by ausastronomer
I've looked through dozens of different SCT's over several decades (yes I can even go back to the crapola produced in a rush for Haleys Comet in late 1975 and 1976) and I am yet to get a lunar / planetary view in a SCT that comes remotely close to what you get from a high quality equal aperture Newtonian. The laws of physics preclude a SCT from being the optical equal of a Newtonian and that is consistent with my under the stars experience. Planetary imaging is a whole different ball game where the size of the secondary and some of the other factors that affect SCT's visually don't come into play. You might be more easily pleased than me, in which case the size of the CO wouldn't matter. If you're trying to eek out the last bit of performance for a given aperture then it does and ideally it should be under 20% and not under 25%. Under 20% is where the MTF curves of an obstructed scope approach those of an unobstructed scope. At 25% CO they aren't close. My 10"/F5.3 Suchting mirrored SDM has a 1.83" Protostar Quartz secondary and I can guarantee you there isn't a SCT on the planet that gets remotely close to it as a lunar / planetary scope. I don't even let them on the same field 
It will all be academic if Stuart sticks with the original Coulter primary as it's quality is unlikely to be high enough for the secondary quality or size to make much of a difference.
Cheers
John B |
I wasn't suggesting an SCT will rival a Newtonian, merely a Newtonian with 28% obstruction is much less than an SCT will offer. I have a C9.25 as well as a 10.1" f6.4 Newtonian (Suchting), the latter with both 2.1" secondary and 1.83" secondary. I prefer the 2.1" for better illumination for my wider field stop eyepieces. The Newtonian with the larger secondary still gives exquisite views of planets with either Tele Vue offerings or my personal favourite for planets being Clave Plossl. I have teased out very faint detail on Jupiter with my 6" f5.5 Newtonian using a Vixen 5mm Ortho (0.965") before I had any Clave or other eyepieces and this with a 1.83" secondary.
I use Mel Bartels secondary sizing routine. Mel used to talk off axis illumination in percentage often referring to aim for 70% illumination at edge of field of widest field stop eyepiece you intend on using. Now he refers to magnitude drop (about 0.4-0.5 mag. drop).
Of course 20% is better than 25% and 15% is better than 20%, (f ratio dependent to avoid too much illumination drop). A smaller obstruction is much more easily achieved with a larger telescope than a smaller one. 25% obstruction has been talked about on other forums and is an acceptable target to aim for but of course less is better but whether you even notice it on most nights or at all is up to a number of factors.
I have looked through a 16" f5 (Suchting) with a 2.6" secondary and low profile Moonlite focuser and the image noticeably dimmed at edge of field. Obstruction was 16% and yes it gave a superb view of Saturn that night but as a general purpose telescope it could have done with one size bigger secondary at least. (100% illumination was very small).