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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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#14 leemajors

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Posted 20 April 2021 - 04:59 PM

Waiting for clear sky... a interesting read :

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

Hows it going? Have you had a chance to try the L3 yet?



#15 daquad

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Posted 20 April 2021 - 06:44 PM

You said you recently bought the scope.  Is it still under warranty?  If so send it back.  You can tinker with it, but I doubt you will be able to correct the problem.  Triplets require very precise positioning/spacing with respect to each element.  Advice has been given as to how to diagnose the problem, but no one has told you how to fix it, or even what the potential fixes might be.



#16 barbie

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Posted 20 April 2021 - 08:24 PM

About 12 years ago, I had a triplet apo that showed a little color visually but performed excellently in all other respects but I ended up sending it back for a full refund from the vendor. A properly executed triplet shouldn't show any false color visually so I never tried it for photography. It could very well be a manufacturing flaw. I would think seriously about sending it back for an exchange or refund. As was suggested earlier, you could try it visually on the moon to see if the false color is there but I wouldn't try to tinker with it, especially if it's under warranty and because triplets are very tricky to adjust as far as lens element spacing, etc.


Edited by barbie, 20 April 2021 - 08:30 PM.


#17 cedric_d

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Posted 21 April 2021 - 01:58 AM

I tried the L3 but it's not better, maybe even worse. So I'am waiting for a clear night to do more tests without filter.

 

My last images with L3 : https://astrob.in/lm30ls/0 (this image is zommed x2).

 

-> daquad, barbie : i already sent 2 emails to Sharpstar but they did not answer. I will message them again after i have more results without filter.

 

For the moment i just had time to do a 1 sub comparison with and without filter. The only treatment is a sinh, hence the greenish color.

 

1 sub without filter + sinh

50-sansfiltre.png

 

1 sub with L2 + sinh

50.3-L2.png

 

1 sub with L3 + sinh

50.3-L3.png


Edited by cedric_d, 21 April 2021 - 06:42 AM.


#18 Tommohawk

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Posted 21 April 2021 - 03:45 AM

I bought the Mk 1 (doublet) version of this scope and the 0.8 reducer flattener. It was stated that this reducer was designed for the scope. If this were so, it's hard to believe the same reducer would work properly on a triplet. Maybe they redesigned it?

 

Also FWIW, the Astronomik Deep sky filters and L3 seems to mostly fix the CA with my set-up. 

 

Not sure that helps the OP much though! 



#19 leemajors

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Posted 21 April 2021 - 03:47 AM

I tried the L3 but it's not better, maybe even worse. So I'am waiting for a clear night to do more tests without filter.

 

My last images with L3 : https://astrob.in/lm30ls/0 (this image is zommed x2).

 

-> Surveyor 1 : i already sent 2 emails to Sharpstar the they did not answer. I will message them again after i have more results without filter.

 

For the moment i just had time to do a 1 sub comparison with and without filter. The only treatment is a sinh, hence the greenish color.

 

1 sub without filter + sinh

attachicon.gif50-sansfiltre.png

 

1 sub with L2 + sinh

attachicon.gif50.3-L2.png

 

1 sub with L3 + sinh

attachicon.gif50.3-L3.png

interesting, I dont think the way the 3 examples are presented really shows the issue well, but the link to the full size image you posted clearly shows the blue fringing is still there.

Would you mind either sending me or posting a dropbox link or similar to a single raw fits file from each filter? I was just about to order an L3 so looking at the raw subs would really help me decide.

im using the 61EDPHii not the 94, but I think they both have a similar amount of CA introduced by the reducer, so far ive found my Astronomik CLS ccd filter removes the fringing completely as it cuts everything below 450nm, though I dont always want such a strong LP filter, I also found my Baader fringe Killer almost corrects it but not quite as perfect as the CLS. 

Thanks

Lee


Edited by leemajors, 21 April 2021 - 03:51 AM.


#20 cedric_d

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Posted 21 April 2021 - 04:47 AM

The 3 pictures i posted are crops of the raw files. I changed nothing except i applied sinh for brighter stars. I will share the full raws when i am home.

 

Looking at these pictures i wonder if the fringing comes from blue or green (and is then "pushed" to blue by color balance in the final image). It would explain why some users have better results with green-cut filters like CLS or Moon and Skyglow than blue-cut filters.

 

That's why i want to test the scope with no filter at all. I will also reduce the exposure time : 180s might be to much for F4.4.

 

-> Tommohawk : my reducer is the 94EDPH dedicated model :

http://www.sharpstar...erface/346.html



#21 leemajors

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Posted 21 April 2021 - 05:00 AM

The 3 pictures i posted are crops of the raw files. I changed nothing except i applied sinh for brighter stars. I will share the full raws when i am home.

 

Looking at these pictures i wonder if the fringing comes from blue or green (and is then "pushed" to blue by color balance in the final image). It would explain why some users have better results with green-cut filters like CLS or Moon and Skyglow than blue-cut filters.

 

That's why i want to test the scope with no filter at all. I will also reduce the exposure time : 180s might be to much for F4.4.

 

-> Tommohawk : my reducer is the 94EDPH dedicated model :

http://www.sharpstar...erface/346.html

Im not familiar with what sinh is, as im not a pix insight user, looking forward to seeing the full raws.

 

Im pretty sure its blue, as my CLS filter only blocks blue, it doesn't block green at all. 

 

If I use my 61EDPH ii without reducer, the colour correction is much better, so I think the reducer either signifies it or is introducing it.

 

Lee



#22 Tommohawk

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Posted 21 April 2021 - 05:03 AM

 

 

-> Tommohawk : my reducer is the 94EDPH dedicated model :

http://www.sharpstar...erface/346.html

Doh! Got my threads muddled!



#23 cedric_d

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Posted 21 April 2021 - 05:15 AM

I have not tested the 94EDPH without reducer because i have no adaptor for the camera, but CA seems to come from the reducer too. It it is visible in the spot diagrams published by Sharpstar :
 
Without reducer
94edph-spot-diagram.jpg
 
With reducer
94edphreducer-spot-diagram.jpg

 


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#24 cedric_d

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Posted 21 April 2021 - 05:50 AM

Im pretty sure its blue, as my CLS filter only blocks blue, it doesn't block green at all.

 

CLS also cuts some of the green. Here i superimposed CLS filter and 533MC transmission curves.

AstronomikCLSCCD+533.JPG


Edited by cedric_d, 21 April 2021 - 05:50 AM.


#25 Ben the Ignorant

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Posted 21 April 2021 - 08:26 AM

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 ?

These are separate questions so they have to be tested separately. An ideal apo makes defocus figures like this on both sides of focus, identical rings and totally white.

 

tak.png

 

Take photos of the defocus disks and post them, any problem with the objective will blossom into view, but no filters.




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