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

First DPAC Test AT92

  • Please log in to reply
42 replies to this topic

#26 moshen

moshen

    Apollo

  • *****
  • Posts: 1,298
  • Joined: 17 May 2006
  • Loc: San Francisco, CA

Posted 07 February 2020 - 02:18 PM

It's amazing how sensitive the test is.  Star testing matches the Zygo on the SV125.  And the Zygo does show a very mild turned up edge.  It sure seems a bit more obvious with my eyes on the test than the picture shows.  However, during observing it behaves as well as any 5" aperture scope.  It supports 487.5x (3-6 zoom Nagler at the 4 setting and 2x barlow) on the moon without image breakdown.  It makes me appreciate the fact that seeing is far more important than the last few digits on the strehl ratio. 

 

It would be interesting to study how much error in this test it takes to really see a difference in viewing objects.

Nice testing, see if you can attach the Zygo anyway and check the outer diameter tested (OD %) stated in the test - in the past SV's Zygo tested 95% OD leaving out the last 5% diameter of edge.


  • peleuba likes this

#27 scooke

scooke

    Mariner 2

  • *****
  • topic starter
  • Posts: 216
  • Joined: 14 Mar 2009

Posted 07 February 2020 - 11:06 PM

Nice testing, see if you can attach the Zygo anyway and check the outer diameter tested (OD %) stated in the test - in the past SV's Zygo tested 95% OD leaving out the last 5% diameter of edge.

Thanks.  Yes, that was the first thing I checked when I got the report.  It is 100% of aperture.  I'll try cropping the Zygo into a couple of parts.  That should get the size down enough.



#28 starman876

starman876

    Nihon Seiko

  • *****
  • Vendors
  • Posts: 20,545
  • Joined: 28 Apr 2008
  • Loc: VA

Posted 08 February 2020 - 04:55 PM

Very nice DPAC  test of that lens  waytogo.gif



#29 scooke

scooke

    Mariner 2

  • *****
  • topic starter
  • Posts: 216
  • Joined: 14 Mar 2009

Posted 08 February 2020 - 08:06 PM

Here is the Zygo for the SV125

IMG_0130.jpg



#30 peleuba

peleuba

    Vanguard

  • *****
  • Posts: 2,038
  • Joined: 01 Dec 2004
  • Loc: North of Baltimore, MD

Posted 10 February 2020 - 04:17 PM

Here is the Zygo for the SV125

attachicon.gifIMG_0130.jpg

 

Virtually all of the error in this lens is at the very edge.  Too bad as that is the worst place to have an issue because the surface area of the edge is larger then if it was in the center.  According to the surface plot the error is around the entire circumference.



#31 YAOG

YAOG

    Surveyor 1

  • *****
  • Posts: 1,686
  • Joined: 12 Oct 2015
  • Loc: Southern California

Posted 10 February 2020 - 07:00 PM

That much error completely around the lens can't be right, could this be a testing error? It seems to be shown in the DPAC so maybe it is real and not a Zygo anomaly?   



#32 peleuba

peleuba

    Vanguard

  • *****
  • Posts: 2,038
  • Joined: 01 Dec 2004
  • Loc: North of Baltimore, MD

Posted 10 February 2020 - 08:00 PM

That much error completely around the lens can't be right, could this be a testing error? It seems to be shown in the DPAC so maybe it is real and not a Zygo anomaly?   

 

Ha.  No - the Zygo interferogram is probably correct.  When making a lens to a price point, one of the ways you can cut corners is to not take the steps to insure the edge is fully corrected in hopes the beauty ring (retaining ring) will cover most of the issue.

 

In the APM 152ED this is pretty common - all the error on the lens is basically at the edge.  


Edited by peleuba, 10 February 2020 - 08:23 PM.


#33 scooke

scooke

    Mariner 2

  • *****
  • topic starter
  • Posts: 216
  • Joined: 14 Mar 2009

Posted 10 February 2020 - 09:02 PM

What I find interesting is although testing, both DPAC and Zygo, reveals the issues with the SV125 lens, it is at a degree low enough that I find it difficult to tell much if any difference in the views between it and what I consider to be as good as 5" gets, the AP130GTX. 

 

While edge issues are the worst, how bad does it have to be before it matters in what you see of the most minute details at high power?

 

This DPAC testing is very revealing and I would love to look at a test and be able to predict what to expect in the eyepiece.



#34 peleuba

peleuba

    Vanguard

  • *****
  • Posts: 2,038
  • Joined: 01 Dec 2004
  • Loc: North of Baltimore, MD

Posted 11 February 2020 - 09:17 AM

While edge issues are the worst, how bad does it have to be before it matters in what you see of the most minute details at high power?

 

 

Hey Scott,

 

You should be able to see this error in the star test - unless the retaining ring masks (most) it from detection.  

 

Below is an exaggerated, computer generated image of a star test through a telescope with a turned edge.  The outer ring will appear drastically different as you run the telescope through focus.

 

Typically an edge issue will be noticeable and can be detrimental to the high power performance of a telescope because the edge of a lens (or mirror) has a lot of surface area.

 

At what point it will affect the view is a subjective question - I am a pretty critical type of observer so the little things are noticeable (and drive me nuts).  From what I can gather, for most folks, ignorance is bliss.  I would have to test the telescope myself to give a more concise opinion.  

Attached Thumbnails

  • edge1.jpg

Edited by peleuba, 11 February 2020 - 09:21 AM.

  • YAOG likes this

#35 scooke

scooke

    Mariner 2

  • *****
  • topic starter
  • Posts: 216
  • Joined: 14 Mar 2009

Posted 11 February 2020 - 12:48 PM

Hi Paul,

I should have been more clear on my question.  I was really wondering about looking at objects in focus.  I can see the edge issue in the star test but it is very subtle.  At first the inside and outside look nearly identical but after studying the image long enough you can see a very slight version of your attachments.  Nowhere near as pronounced.  I read Suiters book years ago and it has been invaluable to evaluate optics when the atmosphere cooperates, however, even with visible issues in the star test some scopes still show amazing detail at high power.  In the last few years, I've leaned more on seeing fine low contrast detail and crisp edges when observing Jupiter, Saturn, and the Moon at high power (.5 or so exit pupil) as the ultimate test of what will give satisfying views.  I'm sure with more experience I could predict what to expect after evaluating the optics using the star test or DPAC but I'm still learning and having fun. 

What really got me thinking about this more is when I posted the DPAC of the SV125, the reaction it received might make someone reading it think it was a bad lens.  Using it in focus on objects it performs beautifully and I have owned and used some of the best for comparison.  I think it's an interesting field of study.  Thanks for again contributing your expertise.



#36 YAOG

YAOG

    Surveyor 1

  • *****
  • Posts: 1,686
  • Joined: 12 Oct 2015
  • Loc: Southern California

Posted 11 February 2020 - 01:14 PM

Ha.  No - the Zygo interferogram is probably correct.  When making a lens to a price point, one of the ways you can cut corners is to not take the steps to insure the edge is fully corrected in hopes the beauty ring (retaining ring) will cover most of the issue.

 

In the APM 152ED this is pretty common - all the error on the lens is basically at the edge.  

 

 

Hey Scott,

 

You should be able to see this error in the star test - unless the retaining ring masks (most) it from detection.  

 

Below is an exaggerated, computer generated image of a star test through a telescope with a turned edge.  The outer ring will appear drastically different as you run the telescope through focus.

 

Typically an edge issue will be noticeable and can be detrimental to the high power performance of a telescope because the edge of a lens (or mirror) has a lot of surface area.

 

At what point it will affect the view is a subjective question - I am a pretty critical type of observer so the little things are noticeable (and drive me nuts).  From what I can gather, for most folks, ignorance is bliss.  I would have to test the telescope myself to give a more concise opinion.  

Scott,

 

Can you measure the ID of the beauty ring on this Access 125? Maybe like Paul says you are not seeing the effects due to the lens being masked just enough to hide the Access 125's obvious lens problems. If the smallest opening in front of the lens is at least 125mm then maybe making a 115mm cardboard mask and painting it black to center over the objective to see if you can see a difference will improve the view at the eyepiece. If you see any difference in the masked observations I would send it back. I know they were a good deal and all but when I buy something I want the best copy of it I can get. I'm picky and cheap, what can I say? 



#37 Jeff B

Jeff B

    Fly Me to the Moon

  • *****
  • Posts: 6,397
  • Joined: 30 Dec 2006

Posted 11 February 2020 - 02:14 PM

Scott, maybe I can help with an example.

 

Here are quick and dirty green light DPAC images (you can tell by the keystoning of the lines and over exposures) taken from an old AP 127 F8 pre-ED Starfire.  This lens had been modified decades ago by the, now deceased, original owner, who, for some reason, replaced the spacing oil with a cement.  He did a pretty good job but the cement did not age well, putting stress on the glass, leading to a very easy to see turned edge, and some over-correction to the entire figure.  

 

I first noticed it as a rather soft image of Saturn (but color was fine).  The star test looked almost identical to the example Paul has shown.  At focus, the airy disk looked ok.  However, the intra and extra focal images were waaaaay different from each other, very much more so than other similar vintage APs I have.  I quickly suspected a turned edge and I went right inside and made a quick 115mm aperture mask out of paper and stuck it over the objective.  BINGO!  The image of Saturn was much sharper with a much more normal star test (that showed over correction though).  Back inside, I removed the lens block and stuck in DPAC which confirmed what I had seen visually.  The images in green are attached. 

 

So, this is an example of a lens with a very uniform, easy to detect turned edge. This one was obvious but I hope this example gives a perspective on edge issues and their subjective effects.  .  Like Paul says, the edge does matter.   

 

FYI, the lens went back to AP where Roland measured the as received correction in green at a .82 Strehl.  Once he separated the elements, let them "relax", re-oiled them and had another look the Strehl has improved to .93 in green, better in yellow.  Roland said he could do a bit better and refigured the lens, which then came in at a .98 Strehl (though with some very narrow, circular machine polishing marks/zones that I can see in DPAC).  It is now an excellent visual lens.

 

Jeff 

Attached Thumbnails

  • 127 F8, Before, Green, Inside Focus.jpg
  • 127 F8, Before, Green, Outside Focus.jpg
  • 127 F8, 115mm Stop, Green, Inside Focus.jpg
  • 127 F8, 115mm Stop, Green, Outside Focus.jpg

Edited by Jeff B, 11 February 2020 - 02:16 PM.

  • peleuba, eros312, Tyson M and 1 other like this

#38 scooke

scooke

    Mariner 2

  • *****
  • topic starter
  • Posts: 216
  • Joined: 14 Mar 2009

Posted 11 February 2020 - 05:52 PM

Jeff,

Thanks for sharing the DPAC images for comparison.

Chip,

I may just try the ring just for the heck of it and see how much better it is.

 

So going back to the DPAC images, .82 strehl level of error on the edge produced noticeably soft images on Saturn and .979 strehl level of error on the edge of the SV125 seemed very sharp.  In side by side tests with my AP130GTX, the AP was slightly sharper but only after a long time at the eyepiece and I'm sure it also due to a tiny bit more aperture.  Really good to get these examples to help quantify the level of error that we are seeing in the DPAC images.



#39 Jeff B

Jeff B

    Fly Me to the Moon

  • *****
  • Posts: 6,397
  • Joined: 30 Dec 2006

Posted 11 February 2020 - 10:14 PM

Scott, just to be clear, the edge issue was part of that low .82 Strehl number as the lens also showed moderate overcorrection.  However stopping it down did considerably improve the mage.

 

Jeff



#40 scooke

scooke

    Mariner 2

  • *****
  • topic starter
  • Posts: 216
  • Joined: 14 Mar 2009

Posted 12 February 2020 - 10:15 AM

Jeff,

Yes I saw the overcorrection as well but the edge issue seemed to be more pronounced so I assumed it contributed the most to the lower strehl, but I know I might be going into dangerous territory, trying to quantify errors in a qualitative test.

A question that has been rattling around in the back of my mind is:

From a peak to valley perspective, it's obvious that PV error on the edge is much more detrimental to the image a PV error in the center.

Since strehl is a measure of how much light intensity goes into the airy disk, does it matter where the error is? 

Would two objectives provide the same view if they were of the same strehl ratio if one had all the error on the edge and the other had all the error in the center? 

Does strehl already take into account the degree of degradation regardless of where the error exists?

If true, I would think that it would require a larger error in the center to equal a smaller error on the edge.

 

Sorry for all the questions but I'm trying to reconcile the fact that the AT92 DPAC is almost perfect while the SV125 DPAC shows obvious edge error while showing only .004 difference in published strehl ratio.  



#41 YAOG

YAOG

    Surveyor 1

  • *****
  • Posts: 1,686
  • Joined: 12 Oct 2015
  • Loc: Southern California

Posted 12 February 2020 - 10:34 AM

Jeff,

Yes I saw the overcorrection as well but the edge issue seemed to be more pronounced so I assumed it contributed the most to the lower strehl, but I know I might be going into dangerous territory, trying to quantify errors in a qualitative test.

A question that has been rattling around in the back of my mind is:

From a peak to valley perspective, it's obvious that PV error on the edge is much more detrimental to the image a PV error in the center.

Since strehl is a measure of how much light intensity goes into the airy disk, does it matter where the error is? 

Would two objectives provide the same view if they were of the same strehl ratio if one had all the error on the edge and the other had all the error in the center? 

Does strehl already take into account the degree of degradation regardless of where the error exists?

If true, I would think that it would require a larger error in the center to equal a smaller error on the edge.

 

Sorry for all the questions but I'm trying to reconcile the fact that the AT92 DPAC is almost perfect while the SV125 DPAC shows obvious edge error while showing only .004 difference in published strehl ratio.  

It is funny how that works. We had a name brand U.S. ground lens that had a so-so lens but a magically good Strelh number value on the Zygo test sheet. The zones visible in the test report were clearly visible at the eyepiece. The owner was so proud of the nearly perfect number on the sheet he ignored what was obvious at the eyepiece. After that initial excitement and a critical evaluation the scope was sent back for a refund and another scope was purchased. That scope had no Zygo test sheet supplied but it was a textbook perfect lens. 



#42 scooke

scooke

    Mariner 2

  • *****
  • topic starter
  • Posts: 216
  • Joined: 14 Mar 2009

Posted 17 February 2020 - 12:41 AM

Just for fun, I made a 115mm ring and a 120mm ring.  I see no difference in resolution, just the brightness of the images.  I will try it with DPAC when I get home from the Winter Star Party.



#43 Jeff B

Jeff B

    Fly Me to the Moon

  • *****
  • Posts: 6,397
  • Joined: 30 Dec 2006

Posted 17 February 2020 - 09:58 AM

Hey Scott,

 

You should be able to see this error in the star test - unless the retaining ring masks (most) it from detection.  

 

Below is an exaggerated, computer generated image of a star test through a telescope with a turned edge.  The outer ring will appear drastically different as you run the telescope through focus.

 

Typically an edge issue will be noticeable and can be detrimental to the high power performance of a telescope because the edge of a lens (or mirror) has a lot of surface area.

 

At what point it will affect the view is a subjective question - I am a pretty critical type of observer so the little things are noticeable (and drive me nuts).  From what I can gather, for most folks, ignorance is bliss.  I would have to test the telescope myself to give a more concise opinion.  

And those images also look a lot like balanced higher order spherical aberration too.   

 

I first ran into outright difficulties distinguishing between them with my CFF F6.5 triplet.  In fact, I, at first, mistook what I saw in the star test for a turned edge.  However, lunar/planetary images were really very sharp and DPAC showed no turned edge.  What's going on here?    Quizzing Pal (CFF's optician) he mentioned that he balances the spherical content across the spectrum (C to F lines) with hand figuring.  I then dove into Vlad's most excellent site and found this:

 

https://www.telescop...t/refractor.htm

 

Figure 145.

 

Bingo.   Notice how similar even 1/10 wave of balance higher order SA looks like a turned edge.  

 

For me. this drives home Paul's point that the combination of star and DPAC testing is very powerful and tells me just about everything I need to know.  They even helped me to identify my complete ignorance concerning higher order SA.   

 

Jeff




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