Regarding the distance between mirrors of a RCT scope.
This web blog has the most accurate data on the distance between mirrors, which I have been able to find out from the Internet:
Teleskop Austria's blog tells this:
Like all Cassegrain Systems, in the Ritchey-Chrétien ist the spherical aberration is dependent on the distance primary to secondary, there is only one optimal distance. At the same time the backfocus (distance focal plane to tube) also changes with that distance. If the secondary is brought nearer to the primary, for each 1mm the backfocus increases 10mm. Sometimes more backfocus is needed, so how changes the spherical aberration respectively? To get an idea about that I measured some primary-secondary distances, and just read the terms for spherical aberration alone to avoid influence from test impurities. Here are the resulting Strehl values, distances given are from secondary holder to spider (means I changed the secondary only, inverse to mirror distances):
6mm - 99%
8mm - 99%
9,5mm - 98%
12mm - 93%
14mm - 79%
With decreasing distance secondary to primary, the undercorrection increases. Interesting, there ist no linear behaviour, first it goes slightly off, then rapid. If you ever need to adjust the distance, I advice to use a Ronchi grating, as zone errors can be deceiptive if you plan to use the star test. They are less careful in polishing under the secondary! Btw: At delivery the scope came with correct distance, just the collimation was fairly off, so it performed only around the diffraction limit. I also remember that I rotated the secondary to get rid af some astigmatism when collimation was otherwise perfect.
And I have confirmed that to be true with own previous 10" RCT scope too, but with a more experimental way (with the usage of CCD Inspector software).
Here's status for "somewhat correctly" collimated 10" RCT, when mirrors are too far apart from each other:
Star fields for a similar (but not the same) images (sorry):
Not that perfect star shapes there... eh, but over then (14-10-2015) my collimation accuracy was not that great either?
Then here's image change with appox. 1...1,5 mm (0,03937 to 0.0590551 inches) closer distance between those mirrors:
Field had "flattened" there...
NOTE: Here's CCDI:s definition for "curvature" in its' 3D Curvature Plot Viewer: (its' not actual field curvature we're talking here, but different aberration amounts)
CCDInspector extracts thousands of stars from each image and computes their FWHM.
Then, a polynomial function is fitted to the distribution of FWHM values. This is then plotted on a 3-D surface, with varying colors, as follows:
Black: lowest FWHM
Blue: slightly defocused
Green: more defocused
Red: highest defocus
And here's some warning for the above test too.
That curvature measurement in CCDI changes also with the collimation accuracy of those RCT mirrors!
I know that CCDI's 3-D curvature plot flattens out, when collimation accuracy for RCT mirrors gets better and better.
Here's just some comparison with that: (15-10-2015)
(star field is from the middle image there)
After some time (29-03-2016) I took once again the distance of those mirrors into a closer inspection with more "managed" results.
I wrote over then this story into our Finnish astronomy forum:
(You can click on the images to get full size images and watch out how star field changes a bit in the image corners.)
There I presented how the distance change between mirrors was affecting to the CCDI's 3-D curvature plot, when about one total turn for those secondary mirror collimation screws was done (and then secondary mirror was moved closer to the primary mirror by that one-screw-turn amount). The collimation accuracy was kept about the same, while doing that mirror distance shift. I loosened central locking screw a bit, then turned all three collimation screws 90 degrees each. Took collimation images and collimated the view. Then again loosened the central screw and turned collimation screws 90 degrees, collimated again. etc. etc. until full turn was done. If you scroll that message thread lower you can read about the 04-04-2016 update for further (minor) distance adjustment done for the mirror distance. You can watch there, how the star images got better and collimation accurary was getting nearly perfect. That's a long-time process there with my RCT collimation, but just like they say in that Teleskop Austria's blog there is only one optimal distance for this...