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12" f/3 DPAC photo trouble

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#1 ccaissie

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Posted 08 July 2020 - 05:29 PM

I can't get my lens up close enough to the Ronchi grating, so can't get a full disk view outside of focus to photograph. 

Images inside show reasonable figure slight undercorrection in center, slight long outer zone.

 

Coating is junk on both the conic blank primary and 4" m.a. secondary.  Secondary is supposed to be 1/10 wave, but I judge it more like 1/8.

 

I'm using a laser pointer aimed at a ballbearing in the perforation of the 12.5" flat to make a pinpoint glint.   Laser light shows every speck of stuff doesn't it?  Maybe I'll try a half dead battery to get it below the lasing level and just have a red diode.

 

133lpi Ronchi, inside focus.

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#2 Steve Dodds

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Posted 08 July 2020 - 06:15 PM

Don't use a laser even non lasing you get that  awful speckeling, use a diffuse led through one half of the grating and return on the other half.

I use a DSLR with a 50mm f/1.8 film lens I found on ebay for $30 and I can get right up to it.  Have to run it in manual mode though.


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#3 BGRE

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Posted 08 July 2020 - 06:49 PM

Actually a laser diode that's operating below threshold in LED mode doesn't produce speckle.
Achieving below threshold operation can sometimes be problematic depending on the Laser and/or its driver electronics.

Its often possible to reduce the speckle contrast so that its not detectable even with a laser.
However the usual method results in a finite size spatially incoherent source.

In practice, unless a source with high temporal coherence and brightness is required, a LED source is both simpler and cheaper

#4 ccaissie

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Posted 09 July 2020 - 09:10 PM

I used this setup because the usual led/grating setup on my Foucault rig doesn't fit.  Using it on a 10f/4 setup now.  I might rebuild a led setup for this one and get a little better photo....

 

I find the Ronchi lines to be very interesting...the diffraction within the bands.

 

The mirror star tested well but the tube and setup was never rigid enough to hold collimation for astrophotography, so the mirror and all came back to me for retest, documentation and goes up for sale.  

 

It's a stunning visual scope at f/3..like you read about.




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