Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Strehl colour correction

  • Please log in to reply
8 replies to this topic

#1 Jo85

Jo85

    Lift Off

  • -----
  • topic starter
  • Posts: 3
  • Joined: 11 Jul 2020

Posted 14 August 2020 - 01:01 AM

What roll does the strehl ratio play in color correction of a scope ?

If a scope was 98% strehl for example would that be better colour corrected than a scope at 95% ?

Also comparing two scopes -

98% green 94%blue 88%green
93% green 91%blue 92%red

I’m assuming the scope with the higher strehl in green is best for visual but the other scope would be between for photography as the ratios are closer ?

#2 TOMDEY

TOMDEY

    Cosmos

  • *****
  • Posts: 8,765
  • Joined: 10 Feb 2014
  • Loc: Springwater, NY

Posted 14 August 2020 - 01:44 AM

You would have to ask the person doing the testing whether they refocused for each color in testing or analyzing a design. The true polychromatic Strehl should be based on one single selected best focus and an appropriately-weighted sensor or visual spectrum. It also depends on how you will use the scope. For example, if you use your green data for luminance, then the other two only colorize. It's pretty easy to overthink this stuff, and usually with the vendor and customer making different tacit assumptions. Most often the vendor's quoted Strehls properly apply design spherochromatisms, but not the chromatic aberration itself (i.e. they are assuming that you are refocusing for each RGB color). The interferometry itself is usually measured at only one wavelength, and then differential design spherochromatisms applied for the three presumed RGB that will be used. Gets messy. Often, the guy doing the testing doesn't even know how the backouts and analysis are being applied! I did interferometry at work and half of the production metrology clowns had very little idea what they were doing... not my clowns, just other departments' clowns.       Tom


  • Aleksandr Naumov likes this

#3 tonyt

tonyt

    Vanguard

  • -----
  • Posts: 2,216
  • Joined: 01 Sep 2009
  • Loc: Australia

Posted 14 August 2020 - 03:32 AM

What roll does the strehl ratio play in color correction of a scope ?

If a scope was 98% strehl for example would that be better colour corrected than a scope at 95% ?

Also comparing two scopes -

98% green 94%blue 88%green
93% green 91%blue 92%red

I’m assuming the scope with the higher strehl in green is best for visual but the other scope would be between for photography as the ratios are closer ?

Those figures might be derived after refocusing for each colour. The polystrehl would not be as good, though some premium scopes have a flat polystrehl curve that doesn't roll down towards red and blue.

 

For visual a well figured ED doublet is fine and a triplet is better for imaging, generally speaking.  



#4 MalVeauX

MalVeauX

    Cosmos

  • *****
  • Posts: 8,523
  • Joined: 25 Feb 2016
  • Loc: Florida

Posted 14 August 2020 - 08:09 AM

What roll does the strehl ratio play in color correction of a scope ?

If a scope was 98% strehl for example would that be better colour corrected than a scope at 95% ?

Also comparing two scopes -

98% green 94%blue 88%green
93% green 91%blue 92%red

I’m assuming the scope with the higher strehl in green is best for visual but the other scope would be between for photography as the ratios are closer ?

Unfortunately there are too many things at play. You can have a scope that is very well corrected at specific wavelength, but poor in others. A good example that is common for this is many scopes with good 0.95 strehl in the green domain, but if you were to see the performance of the figure, correction, spherical aberration, longitudinal focus, etc, in near UV wavelengths or longer IR wavelengths, you wouldn't like what you see. This doesn't matter obviously for a visual observer, but it matters a lot for an imager.

 

Very best,


Edited by MalVeauX, 14 August 2020 - 08:11 AM.


#5 peleuba

peleuba

    Mercury-Atlas

  • *****
  • Posts: 2,738
  • Joined: 01 Dec 2004
  • Loc: North of Baltimore, MD

Posted 14 August 2020 - 08:21 AM

Often, the guy doing the testing doesn't even know how the backouts and analysis are being applied! I did interferometry at work and half of the production metrology clowns had very little idea what they were doing... not my clowns, just other departments' clowns.       Tom

 

This has been my experience, too, and its not just limited to optical metrology.   Many times, the techs simply don't understand the big picture.  Though, in fairness, its not always necessary that they do.    



#6 peleuba

peleuba

    Mercury-Atlas

  • *****
  • Posts: 2,738
  • Joined: 01 Dec 2004
  • Loc: North of Baltimore, MD

Posted 14 August 2020 - 09:06 AM

Unfortunately there are too many things at play. You can have a scope that is very well corrected at specific wavelength, but poor in others. A good example that is common for this is many scopes with good 0.95 strehl in the green domain,

 

On the surface, you are describing an achromat.  And, it would matter visually as well as photographically.  The hairs get split once ED doublets are added to the discussion and/or the focal ratio get longer as the focus points of RGB are closer together.

 

At the end of the day for the OP, when you see a strehl ratio listed on an interferogram its only a single wavelength, unfortunately, its usually in red, though some are in green.  Its less expensive to equip an interferometer with a red laser then one in green.

 

The vast majority of polychromatic strehls that are listed for specific refractors are not actually measured, they are based on the design and generated from software that takes into account, glass type, number of elements, prescription, spacing etc. and the results are generally best case scenario assuming perfect figuring.


  • TOMDEY likes this

#7 TOMDEY

TOMDEY

    Cosmos

  • *****
  • Posts: 8,765
  • Joined: 10 Feb 2014
  • Loc: Springwater, NY

Posted 14 August 2020 - 09:10 AM

This has been my experience, too, and its not just limited to optical metrology.   Many times, the techs simply don't understand the big picture.  Though, in fairness, its not always necessary that they do.    

Hi, Paul! Yeah, it's actually an efficient system for manufacturing good stuff efficiently. My first job at B&L ~1964 was as a coater operator apprentice kid. I quickly found out that the seasoned machine operators understood almost nothing about how coherent coatings worked... but they were exquisitely skilled at producing great coatings in huge quantities! The techs understood more, the engineers more, and the scientists a lot more. Only the highest-ranking scientists approached ~world-class~ ... and (thankfully) very few of those are needed!    Tom



#8 peleuba

peleuba

    Mercury-Atlas

  • *****
  • Posts: 2,738
  • Joined: 01 Dec 2004
  • Loc: North of Baltimore, MD

Posted 14 August 2020 - 09:39 AM

Only the highest-ranking scientists approached ~world-class~ ... and (thankfully) very few of those are needed!    Tom

 

I have always admired how you understand as much as possible about processes ...  bow.gif

 

This is such a lost art in today's world where premiums are placed on specialization.


Edited by peleuba, 14 August 2020 - 09:41 AM.


#9 TOMDEY

TOMDEY

    Cosmos

  • *****
  • Posts: 8,765
  • Joined: 10 Feb 2014
  • Loc: Springwater, NY

Posted 14 August 2020 - 10:10 AM

I have always admired how you understand as much as possible about processes ...  bow.gif

This is such a lost art in today's world where premiums are placed on specialization.

Thanks, Paul!

 

I drove my parents and the nuns nuts with the perpetually-repeated question, "But why?!" My memory for rote recipe tasks was terrible. I felt that I could only perform if I understood, could derive what was percolating under the surface. It slowly dawned on me that most people are bluffing their way through life... and that that's OK! That conceit ultimately worked out well, because I wound up doing research, where one actually seeks the nooks and crannies of our ignorance. Even among ~professionals~ the formula leans heavily toward rote. Next time you are listening to an expert talking head... ask yourself, "Does he really know what he's talking about, or is he just blowing hot air?" Politicians, moguls, lawyers, M.D.s, scientists ... are driven by biases and agendas, just as much as anyone else... even more so!  Tom


Edited by TOMDEY, 14 August 2020 - 10:12 AM.

  • peleuba and eros312 like this


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics