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How DPAC your refractor

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#26 Bomber Bob

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Posted 22 September 2018 - 10:27 PM

This is an APM 150 ED scope.   Lines are clearly not straight, but how bad is the error on this scope? If I paid $3000 for it, should I be concerned?

 

It could one of the early models with the defective cell.  I ran my new APM 152 ED through DPAC this morning -->  https://www.cloudyni...19#entry8849902   and got straight bars that resemble my high-performing Royal 76mm F15.  One of my better refractors -- but not my very best.    Once again, DPAC correlated with my star & sky tests, as well as the detail in my planetary imaging.



#27 starman876

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Posted 23 September 2018 - 07:05 AM

This is an APM 150 ED scope.   Lines are clearly not straight, but how bad is the error on this scope? If I paid $3000 for it, should I be concerned?

 

It could one of the early models with the defective cell.  I ran my new APM 152 ED through DPAC this morning -->  https://www.cloudyni...19#entry8849902   and got straight bars that resemble my high-performing Royal 76mm F15.  One of my better refractors -- but not my very best.    Once again, DPAC correlated with my star & sky tests, as well as the detail in my planetary imaging.

Glad your 152ED tested OK.   DPAC is such a great tool.   



#28 Jimmy462

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Posted 23 September 2018 - 09:03 AM

How many here would like to know how to test your optics.  It is not really that hard and you could test your scope as soon as you get in the mail and know what to expect when you view with it.  Also, it would enable you to know if you got a dog or not  and ship it right back.   Please let me know and I will start posting instructions.  

Hi starman876,

 

Sure, add my name to the list of interested parties! I'm always curious to see how folks adapt and innovate methodologies to suit their needs, talents and budgets! ;)

 

:)

Jimmy G



#29 Daniel Mounsey

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Posted 23 September 2018 - 03:11 PM

Flats can found on ebay for about $100 if you keep an eye on what comes up for sale.  The flat is not required to be perfect.  Do not listen to what others have said requiring the flat to be perfect and certified.  You are not running a certified test lab. what a basic set up will tell you is all you need to know about how good a lens is. 

 

The flatness, not neccesarily so much, but quality of the flat does matter and should be tested for smoothness to make sure it is free of zonal errors. That's coming from Peter Ceravolo and it doesn't get any better than that. 



#30 Terra Nova

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Posted 23 September 2018 - 06:16 PM

You do say to yourself that the refractor certainly seems to have the edge in these tests. Of all the scopes I have tested using DPAC I must admit I have not found a compound scope that has impressed me.  I have C14's, C11, C8. C9.25 and Intes 6 and 7" Maks,  Meade Mak, skywathcer 6" and 8" Maks  and others and none of them have really impressed me.   The C9.25 seems to have the best DPAC results of compound scopes.

Surely you are not including your Questar in this list of unimpressive compound telescopes? 



#31 CHASLX200

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Posted 23 September 2018 - 06:26 PM

I best most C14's bomb out on any test.  I say forget the DPAC and let the eyes tell me the story.



#32 Bomber Bob

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Posted 23 September 2018 - 06:52 PM

On my DPAC rig, the flat is vertical & rests on 6 screws with plastic rollers.  My first few tests, I rotated the flat to see if the pattern changed significantly (the flat's coatings are unevenly worn).  It didn't.

 

My tests are qualitative, not quantitative, and I'm comparing the patterns for the lenses -- not extracting numbers for analysis software.  I'll leave that JPL-level work to the professionals.  This is a hobby.  

 

DPAC between my refractors is like Side-by-Side observing sessions -- except I can run the tests indoors, unaffected by continuous cloudy nights.


Edited by Bomber Bob, 23 September 2018 - 07:36 PM.

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#33 Daniel Mounsey

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Posted 23 September 2018 - 08:20 PM

It is not really that hard and you could test your scope as soon as you get in the mail and know what to expect when you view with it. 

And I'm not trying to take away from this thread as I think you should continue to encourage others, but this isn't going teach anyone what they should expect when they view with it if they're just learning this. It will only help them notice that one optical system may or may not be better than another while inside their room. Whether they see it themselves in the image during observation is entirely another story. This is another example of why you see so many others nitpicking over these silly Strehl reports. Those reviews take place indoors on paper and I realize others just may want some sense of knowledge that their optics are reasonable, but unless you learn these aberrations and how they actually may or may not manifest themselves, then there's really no practical way to equate them into visual reality outside under the sky. It's like handing someone the same three guitars. I'm willing to bet a very experienced guitarist will know the difference in how each one plays. You don't get any special privileges or grants to learn to play it over night and that's the problem with a lot of this current, get it quick, mentality.


Edited by Daniel Mounsey, 23 September 2018 - 08:21 PM.

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#34 Auburn80

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Posted 23 September 2018 - 09:33 PM

There's always the danger that information can be misinterpreted by the uninformed or inexperienced. But, having information (assuming it is accurate) is usually better than not having it. A laudable goal would be to help those gain the experience to make an educated judgement; whether it's telescopes or guitars.
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#35 starman876

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Posted 23 September 2018 - 09:38 PM

And I'm not trying to take away from this thread as I think you should continue to encourage others, but this isn't going teach anyone what they should expect when they view with it if they're just learning this. It will only help them notice that one optical system may or may not be better than another while inside their room. Whether they see it themselves in the image during observation is entirely another story. This is another example of why you see so many others nitpicking over these silly Strehl reports. Those reviews take place indoors on paper and I realize others just may want some sense of knowledge that their optics are reasonable, but unless you learn these aberrations and how they actually may or may not manifest themselves, then there's really no practical way to equate them into visual reality outside under the sky. It's like handing someone the same three guitars. I'm willing to bet a very experienced guitarist will know the difference in how each one plays. You don't get any special privileges or grants to learn to play it over night and that's the problem with a lot of this current, get it quick, mentality.

You can play the guitar indoors on cloudy nights and still enjoy the sound. You can not test your telescope when the clouds wont let you see the stars.  That is what is so great about DPAC.  All you need is a table and you can test a scope to see how good the optics are. You can even make a portable rig so you can test a scope you are about to buy.   Daniel, you never did say how you test a scope before you ship it to make sure the optics are OK?


Edited by starman876, 23 September 2018 - 09:39 PM.

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#36 Daniel Mounsey

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Posted 23 September 2018 - 10:06 PM

I compare telescopes against others. Would be nice if you guys were able to do the same but I understand it’s hard when the weather doesn’t allow. Regardless, I like what you are doing here and I think it’s very nice that you are trying to help others learn to test their telescopes. It’s better than not doing anything at all with bad weather. waytogo.gif


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#37 Mike Spooner

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Posted 24 September 2018 - 07:24 AM

I've used DPAC for 35+ years. Its sensitivity is f/ratio dependent. Line frequency matters. For null optics it can be quite powerful. But even with a scope that seems nearly flawless on the bench, there can be apparent differences in a field star test that have no effect on the infocus images. Have on scope that has higher order errors on the bench and star test. An optician friend of mine called it one of the best telescopes he had ever looked through but I'm cursed with "knowing" its limitations - perceived more than real.

Mike Spooner
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#38 starman876

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Posted 24 September 2018 - 07:44 AM

I've used DPAC for 35+ years. Its sensitivity is f/ratio dependent. Line frequency matters. For null optics it can be quite powerful. But even with a scope that seems nearly flawless on the bench, there can be apparent differences in a field star test that have no effect on the infocus images. Have on scope that has higher order errors on the bench and star test. An optician friend of mine called it one of the best telescopes he had ever looked through but I'm cursed with "knowing" its limitations - perceived more than real.

Mike Spooner

The curse of knowing the truth about your optics.  I think Chuck Hards said it best. DPAC has ruined  more optics for people lol.gif


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#39 punk35

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Posted 24 September 2018 - 08:29 AM

Is it possible for one of you folks that DPAC your scopes to make a video of the process and post it on YouTube? 

 

How do you see and record your results? 

 

Are there times when the DPAC results don’t correlate to what you see visually when using your scope outside? 

 

Thanks folks.

 

Wade.


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#40 starman876

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Posted 24 September 2018 - 08:45 AM

Is it possible for one of you folks that DPAC your scopes to make a video of the process and post it on YouTube? 

 

How do you see and record your results? 

 

Are there times when the DPAC results don’t correlate to what you see visually when using your scope outside? 

 

Thanks folks.

 

Wade.

about the only lens I have tested that appears to be much better than tests show is the Meade 178ED I have. Optics look to be around 1/4 wave or a bit better the best I can tell.  However, it puts up stunning images.   every other lens I have tested if it is a lemon DPAC shows it in a heartbeat.  I have tested lenses in green, blue and red light.  Blue light is a little more stringent because most lenses are figured to be best in the green wavelength.   To me the blue wavelength is where I would want a lens to be figured the best if you do a lot of imaging because that is where CCD cameras are the most sensitive. 


Edited by starman876, 24 September 2018 - 08:46 AM.


#41 Bomber Bob

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Posted 24 September 2018 - 08:45 AM

I'm cursed with "knowing" its limitations - perceived more than real.

 

I try not to read too much into the results.  Recent case in point:  1950s Unitron 75mm F15 versus 1964 Royal 76mm F15.  The Royal had a cleaner DPAC pattern; and, visually, it also resolves finer details at the same magnification.  HOWEVER, if I'd never owned the Royal, I would've been happy with both the Unitron's DPAC pattern and views (& digital images!).



#42 starman876

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Posted 24 September 2018 - 08:48 AM

I'm cursed with "knowing" its limitations - perceived more than real.

 

I try not to read too much into the results.  Recent case in point:  1950s Unitron 75mm F15 versus 1964 Royal 76mm F15.  The Royal had a cleaner DPAC pattern; and, visually, it also resolves finer details at the same magnification.  HOWEVER, if I'd never owned the Royal, I would've been happy with both the Unitron's DPAC pattern and views (& digital images!).

However, the test results of DPAC agreed with the field tests. The better lens as shown in DPAC put up the better images. 



#43 Bomber Bob

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Posted 24 September 2018 - 09:01 AM

Yeah, I don't think I've yet had a lens with a great DPAC and bad views.  In fact, I like to DPAC a "new" scope BEFORE its first star / sky test -- in case the elements aren't aligned correctly / flipped.  Very easy to spot with DPAC.  And much easier to fix indoors!


Edited by Bomber Bob, 24 September 2018 - 09:02 AM.


#44 starman876

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Posted 24 September 2018 - 09:11 AM

Yeah, I don't think I've yet had a lens with a great DPAC and bad views.  In fact, I like to DPAC a "new" scope BEFORE its first star / sky test -- in case the elements aren't aligned correctly / flipped.  Very easy to spot with DPAC.  And much easier to fix indoors!

That is a good point.  Also, if the new scope does not do well in DPAC than I would say it would be time to send it right back to the vendor.   Many of us spend considerable money on our scopes and it is not like buying a car.   The car I drive over to the dealer the scope I have to pack up and ship back.  We all know what happens when you have to ship a scope several times.  with each trip your luck starts to run out.  



#45 Bomber Bob

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Posted 24 September 2018 - 09:16 AM

DPAC also helps when you've owned the scope for a while.  Refractors typically hold alignment well for years.  But the views in my excellent Royal 76x910 (Lafayette Galactic) fell below par.  I put it on the DPAC, and... yep!  I had the retaining ring too loose, and the elements rotated out of line.  Got them straightened-out (literally), then taped them together.  No more slips!



#46 SandyHouTex

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Posted 24 September 2018 - 09:28 AM

And I'm not trying to take away from this thread as I think you should continue to encourage others, but this isn't going teach anyone what they should expect when they view with it if they're just learning this. It will only help them notice that one optical system may or may not be better than another while inside their room. Whether they see it themselves in the image during observation is entirely another story. This is another example of why you see so many others nitpicking over these silly Strehl reports. Those reviews take place indoors on paper and I realize others just may want some sense of knowledge that their optics are reasonable, but unless you learn these aberrations and how they actually may or may not manifest themselves, then there's really no practical way to equate them into visual reality outside under the sky. It's like handing someone the same three guitars. I'm willing to bet a very experienced guitarist will know the difference in how each one plays. You don't get any special privileges or grants to learn to play it over night and that's the problem with a lot of this current, get it quick, mentality.

I don't see anything wrong with testing your optics.  It will let you know what you have.  I star test all of my scopes right after I get them.  If they're not diffraction limited (1/4 wave of better), back they go for a new one.

 

If everyone did this, manufacturers would probably tighten their testing, and do a better job making good optics.  If you accept a lemon, then the manufacturer never knows that they produced a bad one, and they don't get hit in the pocket with return fees and end up with a bad scope that they don't know what to do with.


Edited by SandyHouTex, 24 September 2018 - 09:31 AM.


#47 Jeff B

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Posted 24 September 2018 - 12:35 PM

Well, with the gracious help from Paul (Peleuba), I assembled my first 2" DPAC Eyepiece.  It has a slot for my Ronchi Screens .Com gratings and removable LEDs.  

 

So the first scope I tried it out on was one I've known for some time was of high quality based upon its star test and FREAKING GOOD images, my TEC 140 ED from Yuri's third run.  

 

It nailed the test. 

 

I'll use the "diamonds on black velvet" equivalent for DPAC which is "jail bar straight" lines, even with the 200 LPI grating and even coasting down to just two bars.  At focus, there was just a subtle hint of a small, maybe 1/2" zone right in the middle.  And that's it.

 

Excellent and backs up my visual observations.

 

So what camera settings do you all use as I found I need to put my eye right up to the screen?

 

Jeff


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#48 peleuba

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Posted 24 September 2018 - 12:45 PM

The flatness, not neccesarily so much, but quality of the flat does matter and should be tested for smoothness to make sure it is free of zonal errors. That's coming from Peter Ceravolo and it doesn't get any better than that. 

 

Actuall, Daniel, this reference you mentioned above came from me.   I pointed you to the Peter Ceravolo documentation in another thread as its what I use it as a reference to construct my interferometer, and its mentions optical flats and the need for regularity rather then absolute flatness. 

 

 

 

I compare telescopes against others. Would be nice if you guys were able to do the same but I understand it’s hard when the weather doesn’t allow. Regardless, I like what you are doing here and I think it’s very nice that you are trying to help others learn to test their telescopes. It’s better than not doing anything at all with bad weather. waytogo.gif

 

I would venture to say that most of the telescopes you test are imported from China.  The sample to sample variability has to make testing telescopes against each other a somewhat futile process.  With DPAC, that goes away.  You test the telescope against itself.  Its the perfect Null test.

 

Whats your reference telescope and has that been tested?  Is it with you on every test?  In DPAC you don't need a reference telescope.

 

In every example, I have been able to have my star test results in with what I see on the DPAC bench.  Its a beautiful thing.


Edited by peleuba, 24 September 2018 - 12:48 PM.

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#49 starman876

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Posted 24 September 2018 - 12:45 PM

Well, with the gracious help from Paul (Peleuba), I assembled my first 2" DPAC Eyepiece.  It has a slot for my Ronchi Screens .Com gratings and removable LEDs.  

 

So the first scope I tried it out on was one I've known for some time was of high quality based upon its star test and FREAKING GOOD images, my TEC 140 ED from Yuri's third run.  

 

It nailed the test. 

 

I'll use the "diamonds on black velvet" equivalent for DPAC which is "jail bar straight" lines, even with the 200 LPI grating and even coasting down to just two bars.  At focus, there was just a subtle hint of a small, maybe 1/2" zone right in the middle.  And that's it.

 

Excellent and backs up my visual observations.

 

So what camera settings do you all use as I found I need to put my eye right up to the screen?

 

Jeff

I assembled one using a microsoft webcam that when taken apart will fit into a 35mm canister which fits into a 1.25" focuser. 

 

can you post pictures of the assembly you made with a 2" eyepiece.

 

It is really not that hard.  Micro center has a good selection of LED's in their hobby section.  RONCHI film can be bought at a couple of places.    

 

My webcam setup allows my results to be displayed on my laptop.  Very handy for aligning optics.  



#50 starman876

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Posted 24 September 2018 - 12:52 PM

Actuall, Daniel, this reference you mentioned above came from me.   I pointed you to the Peter Ceravolo documentation in another thread as its what I use it as a reference to construct my interferometer, and its mentions optical flats and the need for regularity rather then absolute flatness. 

 

 

 

 

I would venture to say that most of the telescopes you test are imported from China.  The sample to sample variability has to make testing telescopes against each other a somewhat futile process.  With DPAC, that goes away.  You test the telescope against itself.  Its the perfect Null test.

 

Whats your reference telescope and has that been tested?  Is it with you on every test?  In DPAC you don't need a reference telescope.

 

In every example, I have been able to have my star test results in with what I see on the DPAC bench.  Its a beautiful thing.

I agree, DPAC is a beautiful thing. Well, as long as you get beautiful results which I have found to be very hard with Celestron and Meade SCT''s.  DPAC has really made me a refractor lover.   One thing I have gathered through all this is that optic masters can figure large optics really well.   Therefore, do not buy large  optics from an optician that is not a master of their art.  Royce, Zambuto, AP, Tak, TEC, and others are true masters in their art.   They have shown constant consistency in the optics they produce.   I doubt this is true of Chinese imports.  I would bet that out of 10 scopes tested each one would be different tested with DPAC.   


Edited by starman876, 24 September 2018 - 12:56 PM.



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