Reducer Spacing fine-tuning

[AngryDonkey]

Hello,

I was hoping someone might have a good way to fine-tune camera distance from a reducer. I have a Takahashi FSQ85 with reducer and am pretty sure I have correctly calculated the distance required from the reducer to the CCD (including correction for the filter). The results I get with my Moravian G2-8300 are pretty good but there are elongated stars in all four corners of the images pointing away from the image centre and I think a bit of fine tuning might be required. First of all I would like to ask, is what I am seeing likely down to incorrect spacing (or could there be other reasons)?

If it is down to spacing what would be a good way to go about finding the correct distance? At the moment I have included a 2mm spacer which brings me exactly to the theoretical distance. I have various other spacers down to 0.3mm so can take it step by step. I take it it is just a case of trying out different spacers e.g. initially in 0.5mm steps to see how things improve or get worse? I have PixInsight and am just running a trial of CCDInspector, what values should I look out for? Or should I just look at the images visually?

Thanks for your help!!!

[AKB]

I, too, am searching for an answer to this!  I know how to tell whether it's too close or too far.  I use CCD Inspector to tell me the measured field curvature.  But I'm still none the wiser as to how MUCH to move it!

[vlaiv]

Good old trial and error approach?

Although distance is real number with infinite possibilities for correct distance, in the end, there is finite combination of spacers and resulting distances - just go thru a set of spacer combinations, for each combination do really precise focusing of star in center of the field and examine star shapes in the corners (and calculate FWHM value). Choose distance that gives best looking stars / the least FWHM value for corner stars. You can go a step further and plot your FWHM values against the distance - interpolate samples and see what value of distance would give minimum for such graph?

[AngryDonkey]

Sorry forgot to reply to this. Thanks for taking the time to answer, yes I'm sure trial and error is the way forward. I just wanted to make sure I'm not missing something, my last attempt at this didn't go so well but this might have been down to poor seeing conditions. Additionally I just read in another tread that the reducer (for the Tak FSQ) I am using has a pretty poor mechanism with three grub screws which could lead to the field being off centre so that might have some input as well. My plan is to get better screws and try again once that's been resolved.

[Uplooker]

I bought one of these as it gives greater flexibility to adjust the distance without having break everything down.

https://www.altairastro.com/m48-17-29mm-variable-spacer-set-3-part.html

Thinking again, this may not be that helpful to you as Taks have some very funny thread sized ?

[AKB]

2 hours ago, Uplooker said:

bought one of these as it gives greater flexibility to adjust the distance

Ah.  In the end, I went for one of these (much the same concept) ...

https://www.firstlightoptics.com/adapters/baader-varilock-29-lockable-t-2-extension-tube.html