Celestron gurus, C 9.25 question

[The_bluester]

After a bit of possibly obscure knowledge!

Does anyone know what the maximum practicable field stop diameter is for an eyepiece before I will see vignetting turning up from the baffle tube or any other part of the light path?

I would be using a 2" visual back as the threads on the back of the scope support this, I have actually just got in the mail today my nice new SCT format 2" mirror diagonal.

The scope is a run of the mill CPC925 GPS with XLT coatings.

[casstony]

I think they have the same rear opening as the C8 Paul, which is 38mm - you could measure that yourself. If that's correct eyepieces with 38mm field stops will give the widest low power field (eg. 35mm Panoptic). You can go about 2mm or 3mm bigger on the eyepiece field stop without seeing noticable vignetting.

[The_bluester]

Actually I just measured it and the opening in the end of the baffle tube is around 45mm, so if I can go around the diameter of the baffle tube without significant vignetting then that should allow for some real wide field possibilities.

[casstony]

In that case you could use 40mm 70 degree afov eyepieces without vignetting.

[casstony]

One other thing Paul, the nose of the SCT screw-on type diagonal will vignette eyepieces with larger field stops. Refractor style diagonals are preferred if you want to use a 41mm Panoptic for example.

Is your scope a new one? I'm wondering if older 9.25's had smaller baffle tubes?

[The_bluester]

Mine is only about 9 months old.

Not sure why the nose of the diagonal would cause vignetting on really large field stops. To my eye it is the same diameter as the baffle tube. But then I will not get to measure it now until Christmas day! It arrived today and I was allowed to check we had the right part and that is the last I will see of it til next weekend.

Edit, looked at the 41 pan specs and the field stop is bigger than the baffle tube so I suppose that would be a problem in any case. I doubt my budget is Going to go there, but if it did I would be tempted to look at a 31mm Nagler.