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DPAC Test - AP 178 F9 Starfire.

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33 replies to this topic

#26 Jeff B

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Posted 12 February 2020 - 11:43 PM

What a great posting in #24 Paul.  Thanks!

 

I love your clear perspectives.  And man, is it ever hard to tell higher order SA from a turned edge in the star test (in fact, I gave up), which, I found, like you is ideal for testing for coma and astigmatism.  

 

Now how about this for a quick and dirty DPAC set up.  grin.gif

 

Jeff

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#27 peleuba

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Posted 13 February 2020 - 07:50 AM

Thanks Jeff.   Also, you’ve just solved a problem for me.   On my C9.25” I have been having alignment issues, not collimation, but rather, aligning the telescope with the flat.  
 

You’ve solved that for me.  
 

I never thought of doing the test vertical.

 

Thank you.   
 

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#28 t.r.

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Posted 13 February 2020 - 10:56 AM

Paul, did you get a Tak 120 to test yet? Looking forward to it...

#29 peleuba

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Posted 13 February 2020 - 12:01 PM

Paul, did you get a Tak 120 to test yet? Looking forward to it...

 

Yes.  I posted it a few weeks back...   See here:

 

https://www.cloudyni...f-a-tak-tsa120/



#30 precaud

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Posted 15 February 2020 - 12:42 PM

I thought it would be interesting to compare these RGB lines, extracted from the outside-focus white LED pic, to the lines derived from your separate RGB LED's.

 

It shows that the blue focus point is not the same as the red and green, which are only slightly different from each other.

 

BTW, what camera are you using? It appears to be attenuating the red quite a bit. I had to jack up the gamma of the red channel by a factor of 4X to get the brightness commensurate with the other two. The blue channel is by far the strongest. You might check to see that it's color correction is set to "Neutral" or "Cloudy", those will mess with the color balance the least.

 

EDITed to add the inside-focus extraction as well.

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Edited by precaud, 15 February 2020 - 12:58 PM.


#31 Jeff B

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Posted 16 February 2020 - 10:41 AM

John:

 

Thanks and pleased explain how you did that!  Only with my achromats have I been able to reliably measure focus differences between the colors, simply because they are large.  I knew the blue on this lens hangs out a bit and green through red are close, but it's nice to see that confirmed by what you did.  And I want to know how you do that.

 

Yes, my cell phone camera ( LG V30+) is a bit hot in blue as mentioned above, plus I adjust the white for each color for the individual color shots to better isolate that color.  Also, my red LED is very bright and I've been too lazy to solder in a resistor, so I go into the camera's menu to knock it down considerably.  I also now adjust the WB for the white LED shot to better match what I see visually through the screen.  So it does not surprise me you had to reverse manipulate the gammas around a bit.

 

Good stuff!

 

Jeff



#32 precaud

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Posted 16 February 2020 - 12:35 PM

Jeff, it's really simple, actually. You use the color balance sliders in a photo editor to eliminate the colors you don't want.

 

As you know, most camera sensors use RGB filters to isolate the color spectrum at adjacent pixel locations (most use a "Bayer pattern", see Bayer Filter), which are then combined by the software to determine the color at each pixel in the final image.

 

If one were a total purist, you would capture in RAW and only use the pixels from a given color for each image. In practice, that's not necessary. The info in the camera's .JPG file is quite sufficient.

 

I use XNVIEW (see Xnview homepage), it is freeware and has several features which are very handy for this sort of work. And the author has had, from day one, an obsession with accuracy in his routines. Here's a screen shot of the green being isolated by killing the blue and red channels.

 

Referring to the images in post #30, you can tell that the blue focus is noncoincident with the red and green, because its Ronchi lines are spaced differently. That's what creates the colors in the original "white" image. In the out-focus images, the blue is closer to the focus point (line spacing is further apart), while in the in-focus images the blue is further from the focus point (the lines are closer, and more of them).

 

I doubt if we can use this technique to accurately "measure" the focus point difference. But it can certainly detect the differences.

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Edited by precaud, 16 February 2020 - 12:47 PM.

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#33 Jeff B

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Posted 16 February 2020 - 02:44 PM

John:

 

Hah!

 

I have an ancient version of Paint Shop Pro and after rooting around a bit in the adjustable stuff, I found that exact adjustment feature.  Super easy and thanks!!

 

I imagine it might be possible to simulate the focus differences of the colors relative to each other with software somehow but that sounds like...work.

 

But this may very well be a good tool to see how, at least semi-quantitatively, introducing other refractive optics into the optical food chain affects performance.  For example, a prism diagonal, my Bino-viewers, and my Chromacors.  Using the white LED, I can see obvious differences when these things are introduced singly or in combination but I'm unable to go beyond that right now. 

 

I do want to make a 1.25" format holder so I can use it directly in my Bino-viewers.  I will also eventually make a more accurate 2" format holder with smaller LEDs placed as close as I can to the center axis.  This way I can get the source and return beams even closer together than I do now.

 

Thanks again!

 

Jeff



#34 precaud

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Posted 16 February 2020 - 04:58 PM

John:

 

Hah!

 

I have an ancient version of Paint Shop Pro and after rooting around a bit in the adjustable stuff, I found that exact adjustment feature.  Super easy and thanks!!

 

I imagine it might be possible to simulate the focus differences of the colors relative to each other with software somehow but that sounds like...work.

 

Oh, good. PSP should work just fine for that.

 

I agree, the math probably exists to decode the line spacing differences into physical dimensions, but that's beyond my pay grade... and I'm not sure how useful it would be, anyway. It's a qualitative measure, just like the Ronchi DPAC is.

 

I look forward to your further experiments using it, especially on the Chromacorrs!

 

EDIT: I should add, to do the work you're describing, it would be highly desirable to use a camera that does not mess with the color balance by applying corrections for various lighting conditions, or which does "auto white balance". If you look in the images in post #30, you can see that the shape of the red lines were altered by the boost in gamma I had to apply to get the contrast to a useful level. Subtractive filtering is fine; but additive filtering can have undesirable consequences...

 

Cheers.


Edited by precaud, 16 February 2020 - 06:08 PM.



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