Hi Peter
I would have to support Greg on all three counts, but happy to stand corrected with convincing facts.
1. He didn't specify particularly limiting magnitude of stellar objects so we can assume that he had speed of imaging extended objects like nebulae and galaxies in mind . An F5 optics has an image intensity 2.5X greater than an F8 for the same aperture, so recording speed of extended objects will have to be similarly quicker. This applies to nebulous extended objects only.
2. Collimation of the GSO RC's depends on the optical center of the primary coinciding with the optical center of the squared on focusser. If there is any decenter error there from the factory , no amount of secondary angle tweaking will help. Are there any user adjustments for either of these mechanical adjustments ( SCT primaries are spherical so not as sensitive to decentering). There is nothing to stop manuafcturers making fixed primary mirrors in Newtonians so this potential 'feature' is not a privelage of RC scopes.
3. The short tube will help but an F8 scope is going to be more sensitive to vibration han an F5 whichever way you look at it as it is operating at high prime focus magnification.
Quote:
Originally Posted by Peter Ward
Greg, your thoughts are no doubt appreciated by many and while it may seem I'm on your case, mate, I'm not, this was simply not right.
This old chestnut is false. With CCD imaging the limiting stellar magnitude and extended object S/N ratio is determined by aperture alone. I think it was Russ Croman... who did a pretty comprehensive analysis on SBIG users as to the how and why.
Again false. The GSO RC's have adjustable secondaries only. (the rest it locked in at the factory) . Newtonians have both primary and secondary adjustments.
Again false. In all Cassegrains (ie RC's), the light path is folded back through the primary..OTA length = FL divided by 3 or about 21 inches. Newtonians don't, so the OTA's are similar in length to the FL , 8" F5 so about 40 inches.
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