What's the canonical focuser upgrade for a C8 + Starizona SCTCORRV? [Deep Sky] Acquisition techniques · John Stone · ... · 8 · 204 · 0

Anyone?

Thanks.  Can you point to a couple links here in the US?

also, what’s the real improvement if the C8 doesn’t have mirror locks?

I though that the mirror was loose/floppy as a result of the design of how it focuses by moving the big mirror up and down a baffle tube.  I'm thinking it would still shift/flop around as my telescope pointed at different places in the sky (especially after a meridian flip), right?

If this is the case why do so many people put on draw-tube focusers?  What does that improve unless you somehow tap/drill your own set of mirror locks and set them after moving the primary mirror the optimal distance from the secondary that minimizes spherical aberrations?

Speaking of which, has anyone ever made their own mirror lock system for an SCT?  That would really make a lot of sense to me.   I wonder if there's a how-to guide to doing that somewhere.

Thanks for you help and replies.

Ahmed,

Let me see if I understand correctly; 

1.)  You can mitigate the wobbly SCT mirror by finishing focus with an anti-clockwise turn which takes up the backlash in the focusing mechanism against gravity and (mostly) holds the mirror from wobbling around during slews/meridian flips.

2.)  Moving the SCT focus knob during an EAF (or manual) focus run can cause the SCT mirror to shift/wobble a little bit, but by following #1 you can "pseudo-lock" the mirrors new wobbled position at the end of focus, then by using a draw-tube focuser you can avoid changing this now "pseudo-locked" mirror position for fine focus adjustments (like filter offsets).

3.)  During autofocus the EAF will make a large clockwise turn of the focuser followed by a series of small steps anti-clockwise to create a focus curve, then after finding the bottom of the focus curve will make another large clockwise turn, followed by an anti-clockwise "black-lash compensation" turn which can "pseduo-lock" the mirror in a different orientation which actually isn't the new focus point because of the wobble.

In NINA the focus routine does exactly as described in #3 and could miss the actual focus point because it changed because of the wobble during the back-lash compensation step;

But what what about a focus routine like the one in the ASIAIR that does two focus runs:  a larger first one to find the approximate focus point, followed by another smaller one to discover the actual focus point.   In this case do you still believe an external focuser would be required?

The critical focus zone on a F10 (or F7 reduced) SCT is pretty large.  Due you think the shifting mirror during the NINA-like final focus move be large enough to shift it out of the critical focus zone?   Is there a way to actually tell if this is happening?

This also begs the question:   How do the RASA scopes overcome this limitation?  There is no place to attach an external focuser there so they only rely on moving the main mirror.

So my take away is if you're a mono-imager then you'll have to do a complete ASIAIR style focus run after changing each filter, you can't do filter offsets unless you buy a drawtube focuser.

If you're a OSC with an ASI then perhaps you can avoid the draw-tube focuser?

Is there a particular test you can do to see if your EAF is missing the "real" focus point because of the mirror shift/wobble?

Thanks for all your help.