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(How much) Does mirror figuring influence pincushion/barrel distortion?

ATM equipment imaging mirror making optics
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#1 JoeVanGeaux

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Posted 13 September 2020 - 03:01 PM

Hi All,

I am on a time-sensitive search to determine how much the inherent figuring of a craftsman-style Newtonian mirror influences the occurrence of pincushion distortion.  I am considering buying an OTA that has a 10" mirror made by a craftsman that has no "certificate" or independent data to "certify" the mirror quality.  The seller provided a terrestrial (cellphone) image taken with through a 13mm fl - 82° FOV lens that showed a nearly 3% pincushion distortion (by my crude estimation) at the outer 20% zone (estimated from the approximated center). Forgive my lack of technical knowledge on the subject of mirror figuring and measurement Also, bear in mind that I estimated the percent distortion by reviewing a limited supply of web pages on the subject

I don't know enough to tell if this distortion is influenced by the mirror figuring or if its from the eyepiece.  My readings on the subject suggests the eyepiece is the dominant source but I cannot afford to buy a mirror that contributes to such effects that I can avoid by getting a mirror yanked out of a used "store-bought" OTA. 

My intent for the OTA is for use in spectroscopy and the quality of the mirror is my prime interest, but the scope will be used to a lesser degree in casual observation and, of course, photography will only be employed to document spectra, etc.  The scope OTA is currently unmounted so I think some additional deep sky images won't be available in the time I need to make a decision.  Also, if I commit to buying its a done deal and I cannot (and will not) back out as the seller is traveling a distance (a coincidental excursion) with his equipment mingled among his luggage.

Anyone have any insights as to how I may get some clarity (pun unintended) on the quality of the mirror from a terrestrial photo - taken with, maybe, a different eyepiece or of different subject?

All the best!

-Joe
 



#2 photomagica

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Posted 13 September 2020 - 03:21 PM

The mirror should have no influence on the distortion you are seeing. This will be all in the eyepiece and the camera - most likely the camera. Even if the figure on the mirror is not perfect, that should have negligible influence on the distortion over the usable field. I've done tests on eyepiece components on an optical bench and in some designs the distortion is highly influenced by the spacing of eyepiece elements. When the distortion is zero in the eyepiece - none shows in the telescope.

 

Only a focogram or a ronchigram will give you an idea of the actual quality of the mirror. A terrestrial image photographed through an eyepiece tells little about the mirror quality.

Bill


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#3 MitchAlsup

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Posted 13 September 2020 - 03:47 PM

A properly figured mirror has field curvature (Petzval), but displays almost no distortion;

Distortion is almost entirely from the eyepiece of camera lenses.


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#4 stargazer193857

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Posted 13 September 2020 - 04:03 PM

Pin cushion is an eyepiece aberration, not an objective aberration. It is from spherical aberration of the lenses in the eyepiece. With a negative lense element curved properly, you can cancel it out. Explore Scientific prioritises that. Televue priorities eliminating astigmatism, for the sharpest stars at the fastest f numbers. And they have pincushion I saw without warning.

#5 stargazer193857

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Posted 13 September 2020 - 04:07 PM

A properly figured mirror has field curvature (Petzval), but displays almost no distortion;
Distortion is almost entirely from the eyepiece of camera lenses.


True. And the petzal surface contributes to field curvature, which is a different phenomina, where the stars at the edge are not in focus when the stars in the center are.

With pincushion, the tubes of light coming out of the eyepiece don't intersect at the same exit pupil. This causes blackouts too during the daytime. It also makes the edge of field more more magnified than the center, and looks like you are viewing through a fish eye.
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#6 JoeVanGeaux

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Posted 13 September 2020 - 05:27 PM

Thanks, ALL.  This helps a lot.  Now, I have a little more of the proper vocabulary to aid my research.

 

-Joe



#7 ed_turco

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Posted 22 September 2020 - 12:15 PM

One must not be confused by applying a generalization, that testing distortion is the same as observational distortion.

 

The distortion observed during testing is not the same as distortion seen when observing starry nights, nor are they related in any way.



#8 BGRE

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Posted 22 September 2020 - 07:57 PM

As long as the aperture STOP is located at the primary mirror the primary makes no contribution to distortion at least in the Seidel approximation.




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