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Star Test--How Important Are the Inner Rings?

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#26 Eddgie

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Posted 28 June 2016 - 10:17 AM

Yes, exact quantificaition can be difficult, but obvious problems are easily detected.

 

The problem I had was that without the obstruction, color aberration in a refractor makes the ring brightness hard to judge.  I have tried filters, but still because the peak intensities can shift due to spherochromatism, it becomes difficult to make inside and outside look very close, and the eye is a horrible discriminator of brightness differences and what one observer sees as a larger difference, the other may say is a subtle difference, to there is no easily applied uniform standard.

 

The obstruction allows enough sensitivity reduction to counter some of the other issues but higher order sperical aberration that has no impact on the in focus image can still make the balance between the ring brigtness appear different.

 

This is why I suggest trying to ignore the appearance of the rings.  The SA test really only depends on the secondary shadow size ratio between in and out of focus, and will show a larger error quite easily (1/6th wave), but when you get into tiny differences, it can be more difficult to estimate small errors.

 

And it bears repeanting. I do not know if this particular scope has been aspherized to reduce spherochromastism, but at this aperture and this speed, there is going to be some present, and you cannot dismiss the possibility that this is being presented in the star tests..

A question.. Do you know the design polychromatic Strehl?  Not talking about the finished instruments fabrication quality but rather the absolute polychromatic Strehl for the glasses, aperture, and focal ratio used.   I would be curious to know exactly how much light would not go into the Airy Disk even for a perfect instrument.    



#27 Kent10

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Posted 28 June 2016 - 10:57 AM

Kent:

 

Does your star test on Vega look like this?  Both my TEC 140 and 200ED look a lot like this.

 

Jeff

Sort of, yes.  On Vega, the outer ring is brighter like the image on the right in your picture when outside focus.  I am not sure it is that bright though.  Then when inside focus the outer ring is not as bright as the image on the left in your picture.  The inner rings are more distinct when outside focus.  The color in your picture, at least on my monitor which is calibrated, looks green but in my star test, outside focus the outer ring is white and the inner rings "yellowish" and inside focus the outer ring is "yellowish" and the inner rings white.  So the colors are reversed.  These are subtle colors though not bright like in an achromat.  This is using a 3.5mm Pentax XW which would be about 360X or 50X per inch.  Also, the outer rings on mine don't appear to be different widths.  In your pic on the left the outer ring looks a little wider compared to the right picture than what I think I am seeing in mine.  Very difficult to describe and analyze though as I have found out.

 

Thanks for posting this, Jeff.  Where is this star test from?  Is it actually from one of your Tecs or another scope?



#28 BillP

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Posted 28 June 2016 - 01:30 PM

I have only a few thousand hours of star testing/ bench testing experience and I'm still learning (sometimes surprising) things here and in the field. Sometimes I do focus the scopes and actually observe and have found that sharp images aren't always equated with only perfect star tests:)

 

 

Amen to that!  Nothing like a field test as no bench can account for all the variables happening in the field.  I tend to get more excited over a sharp image of a real and familiar target as opposed to what diffraction rings may or may not be doing around a point source in the presence of a multitude of un-accounted for field variables...but that is just me :lol:


Edited by BillP, 28 June 2016 - 01:31 PM.


#29 Uwe Pilz

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Posted 28 June 2016 - 11:50 PM

In general, the star test allows to give the Strehl if there is only one error present. The combination of different deviations is hard to estimate.

 

If false color plays a role, you may use color filters for the non colored errors and focus shift for estimation of the secondary color. Spherochromatism, however, cannot covered well with the star test. As far as memory servers, Suiter doesn't describe this error in his book.



#30 daquad

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Posted 29 June 2016 - 08:40 AM

In general, the star test allows to give the Strehl if there is only one error present. The combination of different deviations is hard to estimate.

 

If false color plays a role, you may use color filters for the non colored errors and focus shift for estimation of the secondary color. Spherochromatism, however, cannot covered well with the star test. As far as memory servers, Suiter doesn't describe this error in his book.

True.  And his treatment of star testing an APO is scant, at best, probably because of the many reasons stated here.



#31 Asbytec

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Posted 01 July 2016 - 01:57 AM

 

I have only a few thousand hours of star testing/ bench testing experience and I'm still learning (sometimes surprising) things here and in the field. Sometimes I do focus the scopes and actually observe and have found that sharp images aren't always equated with only perfect star tests:)

 

 

Amen to that!  Nothing like a field test as no bench can account for all the variables happening in the field.  I tend to get more excited over a sharp image of a real and familiar target as opposed to what diffraction rings may or may not be doing around a point source in the presence of a multitude of un-accounted for field variables...but that is just me :lol:

 

 

I'm in that camp, as well, but do like to star test my scopes for, if for no other reason than, the halibut. My scope's tests are startling, but the images sharp. With relatively favorable field variables, I've seen albedo differences on Ganymede's surface. No one's complaining here...

 

I personally think a smooth optic accounts for a lot. As long as it is reasonably well corrected as evidenced either by the shadow break out or reasonably high contrast and circular in and out of focus patterns. So long as you avoid that textbook 1/4 wave "cave" appearance, I think you're okay. 


Edited by Asbytec, 01 July 2016 - 02:01 AM.


#32 Kent10

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Posted 01 July 2016 - 10:39 PM


A question.. Do you know the design polychromatic Strehl?  Not talking about the finished instruments fabrication quality but rather the absolute polychromatic Strehl for the glasses, aperture, and focal ratio used.   I would be curious to know exactly how much light would not go into the Airy Disk even for a perfect instrument.    

Hi Ed,

 

Here is a test that Rohr did of one Tec 180FL, not mine of course.  It lists the Strehl in some colors.  My Ronchi tests don't look like that.  The lines for mine are smooth but on one side of focus the lines bow out slightly.  I would say it looks close to the picture in blue but I am not using any filters.

 

You can also see in the star test that on one side of focus the outer ring is brighter than on the other side of focus.  Mine is similar to that.

 

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