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Old 28-06-2021, 06:30 PM
kosborn (Kevin)
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Back focus adjustment

Hi all,

I am having difficulty adjusting the back focus on my Esprit 100 with a Starizona Apex 0.65 reducer and ASI 2600mc. The documentation for the reducer says I need 58mm of back focus (which I have exactly using 1mm shim discs) but the stars in the corners are still distorted. I remember once seeing a web site that described whether back focus was too long or too short based on the shape of the distorted stars but I can no longer find that.

Attached is a zoomed image of the top left corner. All corners show similar appearances with elongated stars pointing towards the center of the field. Can anybody suggest whether I should lengthen or shorten back focus?

Thanks, Kevin
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  #2  
Old 28-06-2021, 08:04 PM
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billdan (Bill)
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Hi Kevin,

From what I have attached it looks like your reducer is too close to the camera.

Cheers
Bill

EDIT: My charts are for a field flattener, I think its the same for a reducer.
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Old 28-06-2021, 11:01 PM
kosborn (Kevin)
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Quote:
Originally Posted by billdan View Post
Hi Kevin,

From what I have attached it looks like your reducer is too close to the camera.

Cheers
Bill

EDIT: My charts are for a field flattener, I think its the same for a reducer.



Hi Bill,


Thanks for that. Now I just need to find a couple more 1mm shims...
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Old 25-07-2021, 09:48 AM
stephen2615 (Stephen)
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Does every corner have that exact same pattern where the stars are oblong away from the centre? If not, then you might have a problem with tilt. Problems with tilt mean the stars might look good on one side but have oblong stars on the other side. It's painful to stop.

Last edited by stephen2615; 25-07-2021 at 04:28 PM.
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Old 25-07-2021, 12:30 PM
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Stonius (Markus)
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Also, I wonder if the backfocus is exact? When my Esprit 120 is set up, the focal length as solved from actual images works out at ~860mm, not the spec'd 840mm. This is for the total scope/flattener setup, of course, but I've never had issues to bother with figuring out which was to blame.



It's a bit out of the box, but maybe you want to dial it in by spending an evening dialing in a v-curve by shooting 1mm increments in back focus and seeing which one gives the best results?


Markus
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