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Field Flattener adjustment


69boss302
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Ive spent the last 2 nights trying to adjust m field flattener. I have a William Optics Zenithstar 73 and the 73a adjustable flattener (not reducer). It is asking for a 55mm back focus, so I add the two extensions it comes with, add it up, and Im 19mm short. So I screw it in to the 73a at the 19mm mark and it should be OK. Well its not.  I tried a half turn one way, then a full turn the other way and was making no difference.  So I went to the extremes and went all the way out and took a picture (at the 20mm or so mark) and then screwed it all the way in to the 5mm mark... and there was absolutely ZERO difference. Apparently thats not where you adjust a field flattener?  The two shots below are at the extremes, but show no difference. What am I doing wrong?

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So you have the two ZWO spacers which are 16.5mm + 21mm 

+ 17.5mm from the top face of the ASI2600 to the sensor giving you the 55mm back focus.

I have the same flattener and I set it to 11.4mm + 55mm to my ASI294MC Pro. Are you sure the elongation is not a guiding issue?

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update: It seems I was missing one of the extensions on the camera. I remeasured everything, got some help from the ZWO forum and found I forgot to add one of the extensions on. Ill give it a try tomorrow night if the weather cooperates!

 

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5 hours ago, 69boss302 said:

update: It seems I was missing one of the extensions on the camera. I remeasured everything, got some help from the ZWO forum and found I forgot to add one of the extensions on. Ill give it a try tomorrow night if the weather cooperates!

 

I was just about to comment and say it looks to me like your sensor is still a little too close to your final lens element. All the stars are pointing in towards the middle which is a very good indicator of this.

This is a useful image to show what I mean...

Phil

Back_Focus_Spacing_Guide_1024x1024.jpg

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Just to follow up with you, I added that extra extension and last night I did some shots. Although the skies werent great, it was enough to see that it fixed the issue!  A VERY quick and dirty NGC7000 shows the corners as being much better. Thank you all for your amazing advice.

Bob

 

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