Is this a backspacing issue?

[osbourne one-nil]

Last night, despite being at death's door following my Covid booster the day before, I set up my 8" newt and imaged the Crescent Nebula. It was my first time using an OAG so I was primarily interested in getting to grips with that (it's not difficult is it?!). I've never been very clear on the backspacing requirements of my Vixen scope and coma corrector. 

I've emailed Vixen to ask, and they say my Coma Corrector 3 requires 63.5mm of backspacing. Now, I assume this is 63.5mm measured from the base of the thread which I attach my camera to, and as such I have a 9mm spacer ring, OAG (16.5mm), ZWO filter holder (21mm) and then the 17.5mm of spacing offered by my ASI533 camera. This equals 64mm...close enough, especially with the relatively small sensor of my camera. However, when I inspected it using Pixinsight aberration thingy, I wonder if I've interpreted Vixen's answer correctly?

 

Perhaps it's just a collimation issue; it's f4 after all, but I'm normally ok with that. I wonder if the 63.5mm Vixen quote is from the face of the corrector. If so, once everything is set up the distance from the bottom of the thread actually measures 55mm, which seems to be standard. 

Any ideas?

Here's the actual image btw...it'll do for 15 x 3min exposures. 

 

 

 

 

[ian_bird]

These may help.

If the stars look like this you are too close:

If they look like this you are too far:

Good luck.

Ian

Edit. Sorry - just noticed you said Coma Corrector. The examples above are for a Flattener.

So basically - just ignore me! 😉

 

Edited November 4, 2022 by ian_bird
Correction

[RolandKol]

If you use autofocuser and NINA, it has nice aberration inspector,

It will run autofocus and will calculate the aprox distance to move sensor to/from Coma Corrector.

[osbourne one-nil]

Thanks both - even Ian (I do have a flattener on my refractor so the knowledge will still come in useful!)

12 hours ago, RolandKol said:

If you use autofocuser and NINA, it has nice aberration inspector,

It will run autofocus and will calculate the aprox distance to move sensor to/from Coma Corrector.

That sounds very clever. I've never used nina but there are plenty of awful nights when I can get to grips with it and perhaps there might even be a clear night when I could try it out. 

This certainly discourages me from rushing into a larger sensor camera....and emphasises the benefits of those lovely Petzvals!

[pete_gamby]

The optical path diagram for the R200SS (assuming that's what you have) shows 55mm back focus with Coma Corrector 3 in place (see below) so not sure where the 63.5mm comes from 🤔.

HTH

Cheers, Pete

[osbourne one-nil]

Hi Pete - sorry, missed this. I've been looking for a diagram like that but have never found it; where did you find that?!

As for the 63.5mm…that came directly from Vixen!

Edited November 24, 2022 by osbourne one-nil