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How can I fix tilt with DSLR, field flattener, objective?

DSLR Software Imaging
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#1 Alsterling

Alsterling

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Posted 07 May 2021 - 05:00 PM

I can't seem to hit answers for search here on CN or wider on Google. Maybe I'm not asking the question properly..... Is there a tutorial/trouble-shooting manual on steps to take and what to look for?

 

I have tilt. Stars are better on one side and slowly get worse towards the other. 

I recently bought an Askar Triplet ED 40mm aperture 220mm f.l. that comes with a triplet field flattener/reducer to f/4.5. Good news is that it is better than my Canon 70-200mm f/4 set to f/6.3. Frustrating news: the aberrations are not symmetrical one side to the other, implying tilt. The Canon 5Diii seems to give me symmetrical aberrations with Canon lenses, so I'm leaning to the conclusion the sensor is reasonably perpendicular and my issue is the Askar lens system. A local astro machinist measured my T-ring to be quite flat. 

 

I put in a shim (first metal, then Astrodynium plastic) that improves the image! But, still not symmetrical. When the shim is between the T-ring and lens, it is best at one particular angle, but when the shim was between the FF and the objective, it was best 180 degrees the other side. Interestingly, the chromatic aberration is switched. This seems to imply in my inexperienced brain that if I shim both they might cancel out. [currently waiting 3 nights until next clearing to try].

 

But it's a lot of trial and error. I've looked but can't seem to figure out if CCD inspector will actually help me diagnose the source of the tilt, for a DSLR (Canon 5Diii). I'm not expecting perfection, but when one sees a result that is asymmetrical, it implies that something is out of line, and straightening is likely worthwhile.
Advice is appreciated!
Alister. 

 

 



#2 Alsterling

Alsterling

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Posted 13 May 2021 - 03:41 PM

I decided to try laser collimating in the basement using reflections. It improved the situation, but I felt there was too much chance.

Then I realized I could do the reflections on the field flattener and objective separately.

1. I spent time making sure the laser and the lens barrel were aligned by having only the autocollimator at the back of the barrel. After a bunch of iteratively smaller shifts, I had the reflection back on the laser and the laser through the autocollimator hole on the back wall centered in the shadow. So the barrel is now straight onto the collimator. Some Newton's rings visible.

2. Field flattener went on which adds reflections. Initially I left it loose so could wiggle it around to see what moved. In my case it looked like tight screwed on was indeed straight/collimated.

3. Put the objective on: more reflections. Again with it deliberately loose I moved it around to see how the reflections and shadows behaved. Noticed which angle was best. Tightened the screw-on barrel but inserted a shim to tilt one of the off-center reflections as reasonably close as I could. 




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