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Esprit 0.77 focal reducer edge performance...

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#1 tocster

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Posted 28 May 2024 - 06:11 AM

So I recently picked up a second hand Esprit 120, and it came with the 0.77x focal reducer.  While the specs say it should have a backfocus of 55mm (with the included spacer) Im finding that even at about 61mm it is giving me stretched stars in the corner of an APSC sensor.  Im still experimenting with it, but wanted to check in if others have noticed this?

 

 

I have attached an example from NINA - its actually not terrible I guess, but I suspect this would be poor on a full frame sensor.

 

 

 

Attached Thumbnails

  • Screenshot 2024-05-28 211225.jpg

Edited by tocster, 28 May 2024 - 06:17 AM.


#2 tocster

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Posted 28 May 2024 - 07:09 AM

So I did some experimenting tonight - I added additional 10mm of backfocus, and was right at the limit of the focuser - I'm using the guide here:

 

https://optcorp.com/...rect-back-focus

 

I was not able to get the star images to resemble the 'sensor too far way' pattern.  They always seem to appear pointing towards the centre of the image, no matter what I do shrug.gif

 

btw this is the spacing set to 55mm - Im wondering if my issues are just tilt?

Attached Thumbnails

  • Screenshot 2024-05-28 221004.jpg

Edited by tocster, 28 May 2024 - 07:13 AM.


#3 gpom

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Posted 30 May 2024 - 08:56 AM

I bought my Esprit 120 new and bought the 0.77x reducer used from the classifieds. I never was able to get good stars from it and did the exact same thing as you, in fact my stars looked almost exactly the same as yours. I then bought a new reducer from Skywatcher and was able to get almost perfect stars with 56mm of back focus (using 1mm thick filters so added an additional 1mm). So based on my experience, I'd say you have a bad reducer and your options are either buy a new one or use the scope at native focal length. And as an aside, using a reducer will always impact the quality of the stars. How much depends on your tolerance, I'm not looking for 100% perfect, just good enough.



#4 Kevin_A

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Posted 31 May 2024 - 11:48 AM

Add more until they stop being elongated. Since they look even there is no tilt. Nothing to lose. 
I have the 100ED and it is perfect. My backfocus is 63mm stock but the scope comes with an 8mm spacer to give me 55mm to fill. 




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