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Refractor color correction issue ?

Astrophotography Refractor
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#1 cedric_d

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Posted 07 April 2021 - 04:15 AM

Hi all,

 

New to astrophotography, i recently bought a Sharpstar 94 EDPH with the 0,8 reducer.

I use it with a ASI533MC camera + Astronomik UV-IR-Block L-2 2" + ASI AIR PRO and ASI120MM mini for guiding.

I process my images with Siril + photoshop.

 

Focusing and tracking should be ok now (autofocusing is done by ASI AIR Pro), but my images keep suffering from blue halos aroung the stars.

 

In my last image (https://astrob.in/lfps3f/B), i noticed big differences between the red, green and blue channels. Smallest stars have sort of an airy disc in the blue channel.

 

Can this difference come from my setup or processing ? Or could it be a scope quality issue ?

 

Thanks for your help !

 

Sharpstar94EDPH.jpg


Edited by cedric_d, 07 April 2021 - 05:21 AM.

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#2 cedric_d

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Posted 07 April 2021 - 07:13 AM

This scope is a triplet APO. It should have very low chromatic aberation.

http://www.sharpstar...n/edph/329.html

 

I use the dedicated 0.8 flatener/reducer that comes with the scope. It is supposed to be high quality too.

 

The scope + reducer is 414 mm / F4.4

 

It does not seem to be a field curvature issue, as the stars are round all over the image. Backfocus is 55.3 mm (+0.3 for the 1 mm UV/IR filter).

 

It looks like the colors don't focus in the same plane ? Which should not happen with an APO ? Or am I missing something ?


Edited by cedric_d, 07 April 2021 - 07:20 AM.

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#3 Simoes Pedro

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Posted 07 April 2021 - 07:21 AM

No expert, but focus is clearly towards the red. If you can, use a blue filter and look at stars images as you did.



#4 Supernova74

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Posted 07 April 2021 - 07:22 AM

This scope is a triplet APO. It should have very low chromatic aberation.

http://www.sharpstar...n/edph/329.html

 

I use the dedicated 0.8 flatener/reducer that comes with the scope. It is supposed to be high quality too.

 

The scope + corrector is 414 mm / F4.4

 

It does not seem to be a field curvature issue, as the stars are round all over the image. Backfocus is 55.3 mm (+0.3 for the 1 mm UV/IR filter).

 

It looks like the colors don't focus in the same plane ? Which should not happen with an APO ? Or I am missing something ?

Yes I definitely agree with you especially being a triplet is it possible you can try the scope visually this may rule out if the fault is with the optics!? If is all ok I’m thinking it’s something possible further down the imaging train.


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#5 MartinPond

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Posted 07 April 2021 - 07:28 AM

I hadn't really thought of this before, but it's true:

blue must (neccesarilly) show more diffraction fringes (in this case, 1)

 than longer wavelengths.  In a related phenomenon, the Airy Disk

 differs by wavelength.....of course (equation says so but we all miss it). 

 

Something obvious but we ignore it

 when concentrating on  lens chromatics alone. The fine diffraction is

  itself...chromatic.  Physically chromatic. Obvious...but such a surprise.

     The focus is likely balanced OK, it's just

  that blue+violet is a very long range.....much longer than red-green.

The Airy Disk must be larger...no choice.

Since the eye, with or without film, sees blue+violet almost together, 

   trimming off the violet would be a start....reducing the "blue tail".


Edited by MartinPond, 07 April 2021 - 07:33 AM.


#6 KBHornblower

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Posted 07 April 2021 - 08:35 AM

I hadn't really thought of this before, but it's true:

blue must (neccesarilly) show more diffraction fringes (in this case, 1)

 than longer wavelengths.  In a related phenomenon, the Airy Disk

 differs by wavelength.....of course (equation says so but we all miss it). 

 

Something obvious but we ignore it

 when concentrating on  lens chromatics alone. The fine diffraction is

  itself...chromatic.  Physically chromatic. Obvious...but such a surprise.

     The focus is likely balanced OK, it's just

  that blue+violet is a very long range.....much longer than red-green.

The Airy Disk must be larger...no choice.

Since the eye, with or without film, sees blue+violet almost together, 

   trimming off the violet would be a start....reducing the "blue tail".

The Airy disk is smaller at the shorter wavelength of blue or violet light.  If we are getting larger spots in that range at a particular setting of the optics, it is defocus, not diffraction.



#7 MartinPond

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Posted 07 April 2021 - 10:45 AM

The Airy disk is smaller at the shorter wavelength of blue or violet light.  If we are getting larger spots in that range at a particular setting of the optics, it is defocus, not diffraction.

That's way off-target....

 

It looks in-focus, then in-focus, thenn not a fatter or smaller disk, but a -->multifringe pattern<----

 

If you focus for an Airy disk, which is a single visible fringe pattern, at red and green,

    there will usually be, in a short barrel,  no such thing as a "disk" at violet.  It splits into a multi-fringe pattern,

    exactly as you see above. 

Take a look at the center of the violet pattern in the first post:

     it is a smaller disk, sure, but your troubles have only begun....you now have multiple nodes.

     "Disk" ceases to describe what is happening....it is 'rings'.

 

  A disk at lamba is

     becoming multiple rings at lamba/2 .  That's the Physics of it.

    That's the truth of diffraction.   The "Airy Disk" is just a subset. 

 

A little diffraction science:

 

https://quantummecha...tes/node63.html

 

 

The concept of an "Airy Disk" is a monchromatic concept.

It is also a long-barrel approximation.

It is a diffractive concept that does not care what glass you have.

Doing a star test can help show what is going on.

 

BTW:    For the same aperture, the shorter the barrel, the more fringes it can generate

             across the spectrum.   Look at the entry above and note how the difference

             from side to side changes more quickly with a shorter triangle. 

..I think I just figured out why this happens...the short barrels.


Edited by MartinPond, 07 April 2021 - 11:44 AM.

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#8 cedric_d

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Posted 07 April 2021 - 12:15 PM

Thanks a lot for your replies !

 

Considering what was said, il will proceed with some more testing.

 

- i will take a few shots with slightly different focus, to see what happens in each channel. (I have only a color camera, so no blue filter to force autofocus to blue).

 

- i ordered the L-3 version of Astronomik UV-IR-Block filter. It has a higher blue cut (420µm instead of 400µm) and might help filtering bad correction in far blue.

 

I'll come back with more data... as soon as the weather is better.



#9 MartinPond

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Posted 07 April 2021 - 03:03 PM

Aha...looks like the Airy Disk is still called that when it has

    turned to many light/dark bands.  It can be called the Airy Pattern as well.

Anyway, I think that's what is happening to violet in the OP.



#10 cedric_d

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Posted 08 April 2021 - 02:02 AM

Waiting for clear sky... a interesting read :

http://www.csun.edu/...land/color.html



#11 Petripher

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Posted 08 April 2021 - 02:57 AM

May I suggest a (boring & time consuming) procedure to help pinpoint the problem? You need to select a blue star (spectral class O) high in the sky, then:

  1. Take a picture of the star at the center of the sensor frame directly from the objective (no flattener/compressor, filter...)
  2. Take a new picture with the flattener/compressor in place, but no filter...
  3. Add the filter you will use (UV-IR cut, contrast enhancer...)
  4. Add any other "gizmo" you will use...

By comparing the pictures taken, you should be able to see when the problem arise/worsen and maybe find a way to troubleshoot the annoying effect in your picture.



#12 cedric_d

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Posted 08 April 2021 - 03:23 AM

May I suggest a (boring & time consuming) procedure to help pinpoint the problem? You need to select a blue star (spectral class O) high in the sky, then:

  1. Take a picture of the star at the center of the sensor frame directly from the objective (no flattener/compressor, filter...)
  2. Take a new picture with the flattener/compressor in place, but no filter...
  3. Add the filter you will use (UV-IR cut, contrast enhancer...)
  4. Add any other "gizmo" you will use...

By comparing the pictures taken, you should be able to see when the problem arise/worsen and maybe find a way to troubleshoot the annoying effect in your picture.

I will do that, thanks for advice.



#13 F.Meiresonne

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Posted 08 April 2021 - 05:54 AM

Check it out visualy...a bright moon would be a good try..

 

This really shouldn't happen with a scope in that price range...


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