i was very careful to tighten/loosen in pairs, but I did tighten it all down to begin, so i'll have to take this next step
just to make sure I understand, What I should now be looking at is the amount of curvature (in addition to proper collimation) as an indicator of correct (or at least better) SM-PM distance, correct?
Hi Brian, pleased to hear that I could help you a bit.
To your questioning, yeah... you need to get good results for every aspect of that CCDI measurement to get a decent starfield. CCDI's "curvature" measurement flattens out when you get nearer to the accurate collimation point (without mirror spacing adjustments), but the accurate spacing between RCT mirrors will be the limiting factor there too. You can read more info of that from this blog writing:
Scroll down until you can read 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."
After that you can find a practical table of mirror spacing:
"...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."
As you can see, there's also a reference for the usage of Ronchi grating while collimating a RCT scope, so I can verify that here.
But I personally have used only CCDI for those measurements (with averaged test image measurements) and the excellent DSI RCT collimation procedure described in this document:
Adjust the mirror spacing according that blog quide and then finish the final RCT collimation without changing the mirror spacing too much (possibly with that DSI guide).
Then you should be just fine with your 10" RCT scope's star field ie. no disturbing optical abarrations can seen in your seeing limited test images. Good luck there as that needs finally only very minimal collimation adjustments with those screws to get significant changes into your star images