Spacing: does this look flat?

[Filroden]

I think I've just got my spacing right for the ASI1600MM-C which was 66mm for the Skywatcher field flattener plus an additional 1mm for the filter (1/3rd of the 3mm filter thickness on the Astrodon 3nm HA). I achieved this spacing with:

• 11.0mm Skywatcher spacer ring (included with FF)
• 9.0mm FLO M48 to M42 adaptor (https://www.firstlightoptics.com/adapters/flo-m48-to-t2-adapter.html)
• 1.0mm delrin spacer
• 7.5mm Baader T2 extension tube (https://www.firstlightoptics.com/adapters/baader-t2-extension-tube.html)
• 10mm spacer supplied with ZWO camera
• 2mm male to male connector supplied with ZWO camera
• 20.0mm ZWO EFW
• 6.5mm ZWO ASI1600MM sensor distance inside body

So with the addition of the 6.5mm inside the camera body, this is spaced at 67.8mm (probably a little less as it tightened a little when attaching it to the scope).

Here's a single 120s image (Ha) and the analysis showing the corners. Does this look flat?

 

[alexbb]

Hi Ken,

Looks flat to my eye. I wonder though, why you also put the additional 1mm spacer? Without it you would have 66.8 which is closer to the 67 than 67.8. I don't think it would make a difference, but just asking.

Clear skies,

Alex

[Filroden]

37 minutes ago, moise212 said:

Hi Ken,

Looks flat to my eye. I wonder though, why you also put the additional 1mm spacer? Without it you would have 66.8 which is closer to the 67 than 67.8. I don't think it would make a difference, but just asking.

Clear skies,

Alex

Thanks Alex. I did it mainly to avoid locking adaptors together. You will not believe how many I've locked so far! I could swap it out for a thinner one. 

[r3i]

Ken,

I ran your image through CCD Inspector and results in the image.  Looks pretty flat.

[Filroden]

Thank you Mike. I wasn't expecting it to be that flat! Such a difference since I got the right spacers. 

[Filroden]

And in reading up about CCD Inspector I've discovered that there is a PixInsight script (FWHMEccenticity) that does part of this, even generating its own FWHM and eccentricity map (choose the support button to generate the images):

Although the absolute measures are different to CCD Inspector, I think I can use this to see if I can improve the flatness by removing/reducing the delrin spacer.