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capturing star test

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#1 rob1986

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Posted 27 October 2021 - 07:31 AM

which settings on a digital camera are recomended for capturing the results of a star test through the EP?

 

(using canon ixus185



#2 Steve Dodds

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Posted 27 October 2021 - 08:56 AM

Wide open aperture, depending on brightness of star, 30th or 60th sec.


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#3 dan chaffee

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Posted 27 October 2021 - 11:43 AM

I've had the best results with a cheap Canon by zooming 3x to 5x and

making short videos. Then go through the videos frame by frame and

capture the best ones.


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#4 rob1986

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Posted 27 October 2021 - 01:49 PM

my canon seems to be too smart. it wont stop trying to focus on the out of focus star.

 

but i'm pretty sure the results are pretty good. there seems to be a lot of seeing trouble, and my astigmatism seems to be impacting the view as well, small though it may be.



#5 darksky4us

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Posted 27 October 2021 - 01:50 PM

Here is out-in star test of my 16.5" f/2.9 with 7mm Nagler and 2.5x Powermate which I made using my iPhone on Sirius. I handheld it behind the eyepiece and made a few shots using its normal autoexposure, and choosing the best ones. Even these good lenses show a lot of chromatic aberration at this speed!

Attached Thumbnails

  • st-powermate (2).JPG


#6 MKV

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Posted 27 October 2021 - 02:14 PM

Here is out-in star test of my 16.5" f/2.9 with 7mm Nagler and 2.5x Powermate which I made using my iPhone on Sirius. I handheld it behind the eyepiece and made a few shots using its normal autoexposure, and choosing the best ones. Even these good lenses show a lot of chromatic aberration at this speed!

You should never use a Barlow to judge your optics. Barlow lenses add overcorrection.and other errors.


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#7 MKV

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Posted 27 October 2021 - 02:17 PM

I've had the best results with a cheap Canon by zooming 3x to 5x and

making short videos. Then go through the videos frame by frame and

capture the best ones.

How do you know that the best ones represent the best wavefront your optics are capable of? Let's say that your optics are 0.25 wave  undercorrected and the atmospheric turbulence at the "best image" is ~ 0.23 overcorrected the resulting optical path difference is 0.02 waves (1/50 RMS!) -- a perfect image -- for a fleeting moment at least, but you can't be sure if that's your optics or just a fluke coincidence.

 

The only way one can be certain it to take a knife-edge null foucogram of the wavefront using an extended exposure, so various atmospheric effects average out, as Texereau did when testing large observatory Cassegrains. This way he was able to demonstrate that his optics were capable of delivering superb performance averaging ~ 1/8 wave PV, and certainly better than 1/20 wave RMS).

 

He was also able to document the excellence of his optics as well as to debunk the notion that a fleeting "best image" is an unbiased or reliable measure of optical quality. The best part is that this test can be accomplished by any amateur with a knife-edge eyepiece and a camera.

 

texerau_ke_star_test.jpg

Source: Jean Texereau, How to Male a Telescope, Willmann-Bell, 1984, 2nd Edition, p. 313


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#8 darksky4us

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Posted 27 October 2021 - 02:25 PM

You should never use a Barlow to judge your optics. Barlow lenses add overcorrection.and other errors.

Yes! As you can see in my image the left side (outside focus) shows the more overcorrection than the right side (inside focus). So, the barlow adds the overcorrection which is not visible directly visually with say a 3.5mm Nagler. Plus it adds the awful chromatic aberration at fast speeds.



#9 ngc7319_20

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Posted 27 October 2021 - 03:23 PM

Here is out-in star test of my 16.5" f/2.9 with 7mm Nagler and 2.5x Powermate which I made using my iPhone on Sirius. I handheld it behind the eyepiece and made a few shots using its normal autoexposure, and choosing the best ones. Even these good lenses show a lot of chromatic aberration at this speed!

I would say F/2.9 is going to be difficult for most eyepieces or barlows or PowerMates

 

If you want to see spherical aberration... stick an F/1.4 camera lens in front of your favorite eyepiece....


Edited by ngc7319_20, 27 October 2021 - 03:41 PM.


#10 BGRE

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Posted 27 October 2021 - 03:53 PM

Get rid of the eyepiece and the camera lens, they are not necessary when imaging the star test.
This eliminates any aberrations contributed by the eyepiece and/or camera lens.
You are essentially capturing the images used in the Roddier test so check the guidelines for image capture given for this test.
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#11 MKV

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Posted 27 October 2021 - 04:14 PM

 

Get rid of the eyepiece and the camera lens, they are not necessary when imaging the star test.
This eliminates any aberrations contributed by the eyepiece and/or camera lens.
You are essentially capturing the images used in the Roddier test so check the guidelines for image capture given for this test.

waytogo.gif

 

A knife-edge test at the prime focus is simpler and doesn't require software, but Roddier is quantitiave..


Edited by MKV, 27 October 2021 - 04:15 PM.


#12 ngc7319_20

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Posted 27 October 2021 - 06:27 PM

Get rid of the eyepiece and the camera lens, they are not necessary when imaging the star test.
This eliminates any aberrations contributed by the eyepiece and/or camera lens.
You are essentially capturing the images used in the Roddier test so check the guidelines for image capture given for this test.

Agreed (mostly).  My experience is that simple eyepieces (orthos, etc.) aren't too bad for performing star test.  But camera lenses can be dreadful.  I've seen patterns of rings, etc., that moved with the camera lens -- star test images depended on what part of camera lens was in the optical path, etc.  If you can image the star test from scope straight to the sensor -- thats the best situation.


Edited by ngc7319_20, 27 October 2021 - 06:28 PM.


#13 Stargazer

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Posted 27 October 2021 - 06:46 PM

my canon seems to be too smart. it wont stop trying to focus on the out of focus star.

My Cannon camera lenses have a button for auto or manual focus on the lens so you can switch them off the auto focus and avoid that hassle.



#14 dan chaffee

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Posted 27 October 2021 - 10:29 PM

How do you know that the best ones represent the best wavefront your optics are capable of? Let's say that your optics are 0.25 wave  undercorrected and the atmospheric turbulence at the "best image" is ~ 0.23 overcorrected the resulting optical path difference is 0.02 waves (1/50 RMS!) -- a perfect image -- for a fleeting moment at least, but you can't be sure if that's your optics or just a fluke coincidence.

 

The only way one can be certain it to take a knife-edge null foucogram of the wavefront using an extended exposure, so various atmospheric effects average out, as Texereau did when testing large observatory Cassegrains. This way he was able to demonstrate that his optics were capable of delivering superb performance averaging ~ 1/8 wave PV, and certainly better than 1/20 wave RMS).

Reasonable question; It's not a problem though. The advantage of the video is you will

have many images to compare and look for repeatability in the frames. Some have frame

stacking software that is particularly useful for this. Even with unsophisticated video processiong,

it's not hard at all to discern the turbulence induced aberrations for what's in the optics.  Evenso, I have

spoiled myself by setting up an artificial star at appropriate distance in any wavelength

I like to work with. That gets the lion's share of the turbulence out of the picture, although

sometimes not all of it.   For the  7inch instrument I'm presently working on, P-6/10 diffraction

pattern is the low end in such a setup if done at night or on a cloudy day. Even a P-6 

diffraction pattern will easily show 3rd order correction errors as bad as .25 wavefront

without any question; and considerably less.

 

Since larger apertures rarely, if ever get near perfect diffraction patterns, it does

necessitate numerous images to weed out the spurious stuff. Definitely better results

with an artificial star....if you can get the distance required. In fact I'd say it's pretty

necessary for consistently good star test image captures in apertures 12 inches and beyond.

 

The foucogram is certainly unambiguous done as Mladen described.  I've been working with

the wire test at focus using an artificial star on the f/18 7" achromat I am refiguring

and the results are astonishing. I'll go so far as to say it is giving the a fairly stable star test a run

for its money and I have the images to back that up. I'll be posting the progress of

this monstrosity in the near future, including vidcaps. Now, this is largely due to how slow

the focal ratio is with the more than generous tolerances, but it is encouraging. Imagine

doing a foucault knife edge or wire test at ROC on a 7 inch f/18 mirror; the sensitivity

is over the top. I expect a wire test null on a distant stable point source to be nearly

the equivalent for any type of scope of that aperture and focal ratio and again the

video will be better than taking individual shots. My camera is giving me admittedly much

less than great images, but they are good enough to do the job.
 


Edited by dan chaffee, 27 October 2021 - 10:32 PM.


#15 dan chaffee

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Posted 28 October 2021 - 03:08 AM

Agreed (mostly).  My experience is that simple eyepieces (orthos, etc.) aren't too bad for performing star test.  But camera lenses can be dreadful.  I've seen patterns of rings, etc., that moved with the camera lens -- star test images depended on what part of camera lens was in the optical path, etc.  If you can image the star test from scope straight to the sensor -- thats the best situation.

On the other hand, I want to know how the system star tests with the workhorse

eyepieces I intend to use.  If I were making optics to sell them, or selling what I had,

the images w/o eyepiece would be the way to go.


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#16 darksky4us

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Posted 28 October 2021 - 03:49 AM

On the other hand, I want to know how the system star tests with the workhorse

eyepieces I intend to use.  If I were making optics to sell them, or selling what I had,

the images w/o eyepiece would be the way to go.

Eyepiece design is very important for accurate star testing, especially for the faster f/ratios. I have found the Naglers and Ethos are superbly corrected for any f/ratio down to at least f/2.8 and the Lunt HDC series are almost as good for half the cost.

 

The old Plossls and Orthos simply fail miserably once you drop below f/4-ish, apparently showing extreme spherical aberration, when in fact none exists!



#17 ccaissie

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Posted 30 October 2021 - 04:19 AM

Here is out-in star test of my 16.5" f/2.9 with 7mm Nagler and 2.5x Powermate which I made using my iPhone on Sirius. I handheld it behind the eyepiece and made a few shots using its normal autoexposure, and choosing the best ones. Even these good lenses show a lot of chromatic aberration at this speed!

Again, do not use a Barlow or any Amplified eyepiece.  I know you paid good coin for that super eyepiece, but when working at f/4 and f/3, I found that the Morpheus eyepieces I was using in the star test created extreme false results.  A Televue Barlow also induced strong aberrations.  (Strange that an Orion Barlow did not).  Only with a plain 5mm Orthoscopic eyepiece was I able to get star test results that corresponded with DPAC and with independent ZYGO tests.


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#18 rob1986

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Posted 30 October 2021 - 11:58 AM

Again, do not use a Barlow or any Amplified eyepiece.  I know you paid good coin for that super eyepiece, but when working at f/4 and f/3, I found that the Morpheus eyepieces I was using in the star test created extreme false results.  A Televue Barlow also induced strong aberrations.  (Strange that an Orion Barlow did not).  Only with a plain 5mm Orthoscopic eyepiece was I able to get star test results that corresponded with DPAC and with independent ZYGO tests.

I'm assuming because of the difference between capturing the focal plane (as a focal plane) and focusing it onto our curved retina (job of EP) and the tast of evaluating the light cone leading to and from the focal plane (star testing).

 

in changes needed to adapt the focal plane probably greatly twist the light cone outside of it.



#19 dan chaffee

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Posted 03 November 2021 - 12:22 PM

https://www.cloudyni..._15247_6585.jpg

 

Here are three video captures for a 7" /177mm f/18 achromat using an artificial star at a

distance of 76 yards/ 70m,  using a dominant wavelength of 560 nm. @335x.  The infra and

extrafocal images are at approx. six wavelengths defocus with the differences somewhat

exaggerated by the camera. The test was repeated at 589nm with identical results and

considering how sharp I want Mars to look, that's a relief.  The design is an air spaced

BK7-F2 aplanat with C-F correction

 

The shadowgram is the wire test at under one mm outside of focus.  Both tests

show overcorrection in the 90% and 15% zones in the presence of very slight

 undercorrection. For a lens, this means the outer zone is

turned up and there is a central depression, opposite from a mirror.  The central slope error is the

least of the problems and the outer zone needs to be polished down. 

 

Even with these errors, the doublet produces an ideal in focus diffraction pattern, a

testimony to how much tolerance there is for very slow systems.


Edited by dan chaffee, 04 November 2021 - 01:18 AM.



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