Bahtinov advice please

[lux eterna]

Hi all,

Yesterday I took a few test images to see if my focal plane is reasonably perpendicular to the optical axis. I looks ok to me, but I want to ask you if anyone can explain why one of the diffraction beams (clock position "8 to 2" if we call the middle one "9 to 3") is so weak in the middle right and lower right positions, while another (10 to 4) is weak in the middle left and lower left positions ?

I slewed Albireo to these positions and snapped 9 pics, then merged them in PS with lighten blending mode. Scope was 8" Meade LX200-ACF with Astro Physics CCDT67 reducer (thinking about it now, I should have tried it without reducer). Camera was Nikon D7000.

I also did a collimation test, slightly off focus (iso 100 0.25 sec). Resized 8X in PS :

Ragnar

[MarsG76]

I imagine this would be due to slight distortion introduced by the FR... most mirrors would start to distort toward the edges too...

[lux eterna]

Maybe you are right. I will try it out next time, thanks.

Am I the only one having noticed this behaviour ?

Ragnar

[Steve Ward]

Albireo's a pretty bright double which might account for the irregularity , might be worth repeating on Vega or some similar single point source.

[lux eterna]

My bad, I made a typo... The star was Arcturus, not Albireo, sorry. Thanks Steve for your input anyway.

Ragnar

 

[Stub Mandrel]

I imagine it reflects the vignetting in the system.  In the corners/edges different areas of the mask will be partially vignetted, weakening the image according to which of the three sets of slits are affected.

[lux eterna]

I was thinking vignetting too, but could not figure out why the corners are so differently affected. Now when you mention it, I can see that the worst ones are closer to the edges than the best ones. So I think you are right - Thanks Neil.

Ragnar

[newbie alert]

I'd make sure the collimation is pretty spot on ..did you  aclimatise the tube ..try at least an hour beforehand..i use my planetary camera and software for this.. you're looking for concentric circles