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Hotech SCA field flattener back focus issue

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#1 blemasle

blemasle

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Posted 18 February 2020 - 06:19 AM

Hi,

I'm using an Altair 72EDF Deluxe (which is rougly the same as the AT72II) with a Hotech SCA field flattener for a few months now.
This is my first "real" astro equipment, so I'm prone to errors and mistakes smile.gif

Since I have both items, I cannot manage to get pin point stars across the FOV. Worse, they're really tend to quickly form comet like shape.
This is both confirmed by PixInsight FWHMEccentricity script :
fwhmeccentricity.jpg

And CCDInformation
ccdinformation.jpg

As a supposed reference for that flattener, see this review
ccdinformation-reference.jpg

This is so bad that you can even see it when looking at the picture as a whole (luminance from the stacked picture)
m45.jpg

m45-1.jpg m45-2.jpg

If you want to see more zoomed examples, you can use the picture on astrobin.
These were taken with a 58mm backfocus (versus the recommended 55mm, because it's even worse). The other night, I tried all the spacer combinations I could (I bought many trying to mitigate this issue) from 58 to something like 75mm without significant improvements.

Am I doing something wrong ? Is my flattener defective somehow ? Or could it be the scope itself ?
I'm desperate at the moment... Got a wide field scope on purpose, and I cannot get pinpoint stars outside a ridiculous small centered zone frown.gif

Regards

#2 Deesk06

Deesk06

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Posted 08 September 2020 - 07:24 AM

Sorry everyone to dig up an older thread. Did you ever get this issue figured out? I own this FF and it seems like I am dealing with some of the same issues. I contacted Hotech and got no response back. 



#3 blemasle

blemasle

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Posted 15 September 2020 - 06:39 AM

No, I never found a solution. Hotech went silent and did not get back to me in the end.
Right now, I'm using the TSFlat72 which is specifically made for the TSAPO72 (essentially the same scope as mine). It's working beautifully with the scope and after some trials and errors, I get almost perfect stars in every corner. I'm still looking for the perfect back focus distance, but it's getting pretty good !

 

Lower right corner :

lower-right-corner.png

 

Despite what is stated, I found many reports online of this flattener not being suited for small focal lengths scope. It seems to work very well for longer scopes though.

 




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