Quote:
Originally Posted by Somnium
Hi John
Just one thing you mentioned that was new to me was around the focusing of cassegrains. you say that all cassegrains achieve focus by manipulating the distance between the primary and the secondary, i understand that is the case for SCTs but with a CDK the primary and secondary are fixed. if extra travel is needed there is no manipulation of the primary to secondary distance, you just move the eyepiece further back. so how would this impact the optical performance or am i fundamentally missing something here?
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Hi Aidan,
You didn't miss anything. I didn't word my post very well at all. I should have said "a lot" as opposed to "all"; cassegrains achieve focus by changing the distance between the mirrors". I don't even know why I said that because I have used several cassegrains that had a moving focuser train as opposed to moving the mirrors.
On the basis that the intercept distance between the mirrors remains fixed there will not be any introduced spherical aberration. The only issue which will affect the high power images is the large secondary obstruction which will affect the MTF curves and cause diffraction.
I have prepared some simulated images of Jupiter using Aberrator software, which is a free download from this linked website, to attempt to show the likely impact on the high power images of the large central obstruction.
The left side simulated image below shows Jupiter as you would expect it to appear in good seeing using an unobstructed 368mm (14.5") telescope, assuming perfect optics.
The right side simulated image below shows Jupiter as you would expect it to appear in good seeing using a 368mm (14.5") telescope, assuming perfect optics, but in this case introducing a 51.7% Central Obstruction.
If you're not a discriminating visual observer you may well be very happy with the images you get, notwithstanding they won't equal what you would get from a high quality 10" to 14" newtonian. The only way to find out is to throw an eyepiece in it and see what happens. As I mentioned previously low to medium power should be fine, you just lose some light gathering from the large Central Obstruction, high power you just have to suck it and see if you're happy with it.
I have used other Corrected Dall Kirkhams (CDK's) for visual astronomy including a Takahashi 250 Mewlon and a 14" Planewave. The Tak Mewlon has a relatively small central obstruction (~28%) compared to other CDK's and performs very well as a visual instrument and IMO is just a little behind a high quality equal aperture Newtonian. The Mewlon also has a slower F Ratio (F10) compared to the specialist imaging CDK's. The 14" Planewave, which has a ~49% Central Obstruction, came up well short of the best 14" scopes I have used. That doesn't mean however that you wouldn't have been happy with it. Over a long period of time I have used some of the best visual telescopes on the planet, so I can be a bit more discriminating at times than some observers.
Cheers,
John B