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GSO Reducer on AT8RC: How critical is backspacing?

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#1 fewayne

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Posted 17 April 2024 - 05:33 PM

I tried out my new-to-me 0.75X GSO reducer with an APS-C sensor and am seeing some weirdness off-axis: Stars look like little tadpoles (will post example later). I have an Askar OAG in front of the T-ring adapter plus a few shims to make the guide camera wind up in the right orientation, so it's ~3 + 55 + 20. Back spacing is given as 80mm by the manufacturer IIRC.

Not a flattened, so I shouldn't be getting radial or circumferential distortion that you get when a flattened is at the wrong spacing.

Should ~78mm be close enough?

#2 bobzeq25

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Posted 17 April 2024 - 06:13 PM

Backspacing is not that critical.  But, it's also unlikely to be optimal at the manufacturers specified spacing.  Most imagers doing this have found that experimentation is necessary to achieve the best results.

 

And, if that reducer was not _specifically_ designed for an 8RC, ALL bets are off. 

 

Reducers are not generic.  Almost all reducers will also curve the field.  Is yours supposed to not do that?  Those are rare.  I know, I tried to find one.

 

To flatten a field, one has to consider how it is now curved.  That varies between types of scopes and, to a lesser degree, among specific examples of scopes.


Edited by bobzeq25, 17 April 2024 - 06:15 PM.


#3 fewayne

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Posted 17 April 2024 - 07:19 PM

Thanks Bob. Pretty sure this is the model that GSO intended to reduce (though not further flatten) their RCs. I've never been wrong before, as you well know, but there's a first time for everything. Maybe I just have to content myself with a smaller usable FOV with the thing, but I can muck about with spacing.

My SV-70's matched FF/R spoiled me, I guess. Screw on a T-ring, done.

Edited by fewayne, 17 April 2024 - 07:20 PM.


#4 arbit

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Posted 17 April 2024 - 09:03 PM

The backspacing by itself isn't critical in this case. Changing it will only change the reduction factor.

The bigger issue is that the GSO 0.75x is specced for sensors 15mm diagonal or less. An APS-C is way beyond spec. Probably the reason most prefer the AP CCDT67 for the RCs.

Sent from my SM-S908E using Tapatalk

Edited by arbit, 17 April 2024 - 09:12 PM.


#5 fewayne

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Posted 17 April 2024 - 09:49 PM

D'OH! Welp, I did not know that at the time. Thank you. It'll still be useful with my 183, then. Still beyond spec but looking at the results I'd say the 1"-diagonal zone will be acceptable.

 

And taking a second look, the stars do look very much like the "too close to sensor" FF/R examples one might see. Have to see if I can stuff in a tad more spacing too.




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