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DPAC a Questar?

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#1 Chuck Hards

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Posted 31 October 2017 - 11:37 AM

I have a Questar 3.5" on-loan, and as of right now I am on the fence as to whether I should test it using DPAC.

 

I don't want my last Classic bubble burst.

 

I've tested just about all the classic Japanese refractors, classic American reflectors, SCTs, Chinese Maks, even a Zeiss.  A large majority of them have been at least a little disappointing, some shockingly disappointing.  So-far, Swift remains at the top of the heap, with the most consistent high-quality objectives.  I've tested 8 Swifts and there isn't a lemon in the bunch.  The only other brand with consistently good test results is, surprisingly, Jaegers.   Those are the only two brands that I would buy without testing the objective in advance, anymore.  

 

Royal Astro?  Unitron?  Asahi?  Nope.  None come close to Swift and Jaegers for consistently good optics, based on my tested samples.   The only two brands that I feel comfortable saying that you have a decent chance of getting a high-quality objective via the luck of the draw.

 

 

 

So, should I, or shouldn't I test the Questar 3.5"?


 

#2 Joe1950

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Posted 31 October 2017 - 11:48 AM

Yes! You've probably done a star test? Yes? I'd bet the DPAC is good if not very good. 


 

#3 starman876

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Posted 31 October 2017 - 11:50 AM

Cumberland optics does a DPAC in every set of optics they provide Questar.  I have watched them do it.   Some they provide are 1/10 wave.  


 

#4 Stew44

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Posted 31 October 2017 - 11:53 AM

What year is this Questar?


 

#5 deepwoods1

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Posted 31 October 2017 - 11:54 AM

Of course you should. You have the equipment and the inkling.


 

#6 starman876

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Posted 31 October 2017 - 11:58 AM

however, I think you would be surprised at the results.  The lines might not be a straight as you would expect from Questar.


 

#7 Chuck Hards

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Posted 31 October 2017 - 12:06 PM

1979 model, Cer-Vit Primary, broadband (enhanced silver on primary) coatings.  Powerguide II added in 1998. 


 

#8 Joe1950

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Posted 31 October 2017 - 12:11 PM

Yow!

 

Go for it.

 

We'll wait.


 

#9 Chuck Hards

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Posted 31 October 2017 - 12:23 PM

I'm at work right now, the test bench and the Questar is 12 miles and at least 7 hours away.


 

#10 Richard O'Neill

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Posted 31 October 2017 - 12:28 PM

When you eventually test it I think you'll find your sample decent enough but not perfect across the board. Still, charm counts for something and a Q is cute in lipstick. flowerred.gif


Edited by Richard O'Neill, 31 October 2017 - 12:29 PM.

 

#11 photiost

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Posted 31 October 2017 - 12:33 PM

If you do ... pls post some images.


 

#12 DAVIDG

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Posted 31 October 2017 - 12:35 PM

"I've tested just about all the classic Japanese refractors, classic American reflectors, SCTs, Chinese Maks, even a Zeiss.  A large majority of them have been at least a little disappointing, some shockingly disappointing."
Oh no, it has happened ! You've seen what I have. The Emperor really doesn't have new cloths,  Soylent Green is made from people and that book the Aliens gave us, it's a cook book !  I'm sorry that I've  taken your optical innocence away.  Quick, smash that optical flat and never talk about this again  and people will forget.  Just keep saying that every telescope has 1/20 wave optics because the manufactures say so and it will be alright. Never visit this forum again, just hang out in the refractor forum. 

 

                - Dave  


 

#13 Stew44

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Posted 31 October 2017 - 12:44 PM

Cumberland optics does a DPAC in every set of optics they provide Questar.  I have watched them do it.   Some they provide are 1/10 wave.  

Of the roughly 25 Questar 3.5s that I've owned or that have passed through my hands the worst I've seen is roughly 1/3 wave PV by star test to the best I've seen that is 1/26 wave PV by interferometer.  Again, those are PV, not RMS numbers.   And I am sure that each and every set of optics was bench tested by Cumberland, but not so sure that they were all DPAC'd across the 60 plus years that Cumberland was involved with the optics manufacture.

 

I like the odds that Chuck will be pleased with his DPAC results.  Braymer died in 1965.  The company continued to do well until the early to mid 90's when it entered bankruptcy, so no real reason to believe any cost cutting measures were required in optics or mechanical build.  And innovation continued with newer materials and mechanical and optical design.

 

I'd go ahead and test it.


 

#14 Stew44

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Posted 31 October 2017 - 12:47 PM

Oh, you will want to go through the camera port.  Please don't keep the diagonal in the path and certainly not the barlow.  And you will need some back focus so possibly will need an extension to the camera adapter.


 

#15 DAVIDG

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Posted 31 October 2017 - 01:01 PM

 Chuck,

    I'm sure you know this but test in green light since the Gregory Mak design does have spherochromatism like refractors.  Let it temperature stabilize, the thick BK-7 corrector needs time to settle in and have the mirror set to the infinity focus position since if not it will show spherical aberration.  

 

                             - Dave  


 

#16 davidc135

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Posted 31 October 2017 - 01:13 PM

You've got to test it now.

 

David


 

#17 Darren Drake

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Posted 31 October 2017 - 01:56 PM

Here's some Questar dpac results from a range of time periods...
https://translate.go...Y_5KB3gaISDYW4A

Edited by Darren Drake, 31 October 2017 - 01:59 PM.

 

#18 davidc135

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Posted 31 October 2017 - 02:16 PM

Here's some Questar dpac results from a range of time periods...
https://translate.go...Y_5KB3gaISDYW4A

Very interesting set of photos. David


 

#19 starman876

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Posted 31 October 2017 - 02:35 PM

 Chuck,

    I'm sure you know this but test in green light since the Gregory Mak design does have spherochromatism like refractors.  Let it temperature stabilize, the thick BK-7 corrector needs time to settle in and have the mirror set to the infinity focus position since if not it will show spherical aberration.  

 

                             - Dave  

Dave

would the focus setting needing to be at infinity be true for all sct's and maks?


 

#20 DAVIDG

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Posted 31 October 2017 - 03:01 PM

 

 Chuck,

    I'm sure you know this but test in green light since the Gregory Mak design does have spherochromatism like refractors.  Let it temperature stabilize, the thick BK-7 corrector needs time to settle in and have the mirror set to the infinity focus position since if not it will show spherical aberration.  

 

                             - Dave  

Dave

would the focus setting needing to be at infinity be true for all sct's and maks?

 

 

 In theory yes because there is only one spacing of the mirror to the secondary that results in the best correction. If the spacing is not optimal you should see either smooth over or undercorrection but no zones. So the pictures of the DPAC testing on the website mostly show excellent results with  only a few with a small amount of spherical aberration  That could be real or from the focus not being  set for infinity. 

 

                        - Dave 


Edited by DAVIDG, 31 October 2017 - 04:10 PM.

 

#21 starman876

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Posted 31 October 2017 - 03:44 PM

 

 

 Chuck,

    I'm sure you know this but test in green light since the Gregory Mak design does have spherochromatism like refractors.  Let it temperature stabilize, the thick BK-7 corrector needs time to settle in and have the mirror set to the infinity focus position since if not it will show spherical aberration.  

 

                             - Dave  

Dave

would the focus setting needing to be at infinity be true for all sct's and maks?

 

 

 In theory yes because there is only one spacing of the mirror to the secondary that results in the best correction. If the spacing is not optimal you should see either smooth over or undercorrection but no zones. So the pictures of the DPAC testing on the website mostly show excellent results with a only a few with a small amount of spherical aberration  That could be real or from the focus not being  set for infinity. 

 

                        - Dave 

 

So are all SCT's and Maks designed to that that optimal mirror spacing is obtained and the infinity focus of where we would have it when viewing the stars?


 

#22 Richard O'Neill

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Posted 31 October 2017 - 04:06 PM

Just exactly where is the infinity focus designed to be on a 3.5" Q in relation to the rear port? Are all optics made sufficiently close to the same spec to reliably define a reasonably exact position and what would the max variation be between samples? I assume they're close but I wonder how close?


 

#23 rolo

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Posted 31 October 2017 - 04:08 PM

I've tested two and both were very good. I tested them through the rear port but wasn't able to photograph the results. With my set up only part of the image was visible. Visually I could see the whole thing. Pretty straight lines (not perfect) butt with a very smooth finish that I haven't seen in the ETX's or SCT's. The smoothness and spherical correction is what stood out the most and in the star test as well. In the two ETX 90's the spherical correction was definitely not as good and almost identical in both. While not as smooth as the Q they were still pretty good.


 

#24 DAVIDG

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Posted 31 October 2017 - 04:08 PM

 If an optical system is designed for astronomical view it  should be design to have highest optical correction when focused at infinity. If not it  would show at least spherical aberration if not other aberrations as well. 

 Questar makes a Long Distance Microscope like the QM-1 that focuses from around 2ft to 5ft. If you look at the tube length it is longer then the  standard Questar. Part of that is that the corrector is spaced farther from the primary mirror to add under corrected spherical aberration that you would see when using it on an object that is at infinity.  Since the optics are undercorrected for objects that  are at infinity they have much less or no spherical aberration for an  object that  is close by.

   You use a parabolic mirror to focus light that is at infinity and it when perfectly figures shows a null then tested with parallel light. Test that same parabolic mirror at it's radius of curvature ie the object is 2x the focal length away and it shows overcorrection. Take a spherical mirror and test it with parallel light from object that is at infinity and it shows undercorrection. Take that same spherical mirror and test with a object at it's radius of curvature and it tests perfect with no spherical aberration.

 So when you move the mirror in a Mak, SCT or Cass your changing the focus of were the object is and that is changing the correction of the system. Like I said there is only one spacing that results in the best correction and that depends what the system was designed to view, be it something close or something far away. 

 

                     - Dave 


 

#25 DAVIDG

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Posted 31 October 2017 - 04:17 PM

I've tested two and both were very good. I tested them through the rear port but wasn't able to photograph the results. With my set up only part of the image was visible. Visually I could see the whole thing. Pretty straight lines (not perfect) butt with a very smooth finish that I haven't seen in the ETX's or SCT's. The smoothness and spherical correction is what stood out the most and in the star test as well. In the two ETX 90's the spherical correction was definitely not as good and almost identical in both. While not as smooth as the Q they were still pretty good.

 Questars have aspherized primary that removes the third order spherical aberration that would be present if the optics were all spherical surfaces. I believe the ETX's 90 use the all spherical design so they would have these aberrations. When Meade made the 7" Mak they stated the primary in those was aspheric. If they tried to use an all spherical system at F/12 it would show a fair amount of spherical aberration. 

 

                 - Dave 


 


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