223 replies to this topic

[starman876]

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.  

[Element79]

Buy the book.

 

http://www.willbell.com/tm/tm5.htm

[starman876]

Buy the book.

 

http://www.willbell.com/tm/tm5.htm

Hard to do on rainy days.  DPAC can be done even on your dining room table.   It allows you to test your scope anytime night or day.  wonderful tool.

[Element79]

Then buy these too:

 

http://www.willbell....icalTesting.htm

 

http://www.willbell....erferometry.htm

 

 

My library has all of these and many more.  I haven't done any DPAC yet because I don't have the flat mirror...

[Kent10]

I do.  I do. 

[PirateMike]

I would like to know how DPAC (what's that?) is done. Please post away!

 

I need to be able to test my scopes as I feel that one is not performing up to standard.

 

Thanks,

Miguel

Edited by PirateMike, 21 September 2018 - 09:00 PM.

[Bomber Bob]

Here's my DPAC rig, testing my vintage Swift (Takahashi) 50mm F14 objective:

 

 

From left to right:  The Flat Holder, the Lens Holder, and the Ronchi & LED holder.

[starman876]

Here's my DPAC rig, testing my vintage Swift (Takahashi) 50mm F14 objective:

 

BBs DPAC Gear S31 (Swift 838 Testing).jpg

 

From left to right:  The Flat Holder, the Lens Holder, and the Ronchi & LED holder.

and that is a nice setup.

 

The basic set up just requires the flat to be sitting on something to hold it steady.   The scope lined up with the flat and a light source (green LED) shinning through a ronchi screen lined up at the focuser end of the scope.   This is oversimplified, but it is just about how it is done.  

[starman876]

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. 

[deepwoods1]

I would.....

[PirateMike]

please continue. 

[Axunator]

Definitely interested. Never understood people who ”don’t need no stinkin’ tests, ’cause night under the stars tells everything” they need to know.

Well, I find optical testing & theory 1) interesting and fascinating on its own right (this is a hobby, remember. If it’s fun and harmless it should be OK - if you’re not interested, fine, but perhaps you don’t have to be so vocal about not finding other people’s interests worthwhile), 2) useful for getting the best out of the equipment I use to enjoy the universe.

While point #1 is a purely personal one and a matter of taste, point #2 makes me wonder why anybody interested in a good view of the heavens above would NOT be interested in some testing and theory of their scopes.

Edited by Axunator, 22 September 2018 - 01:03 AM.

[saptharishi]

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.  

I request you to post it as a document so that we can keep a copy for offline reference too.  Thank you 

[bob midiri]

Me too Johann , please elaborate . What size flat would be needed

Edited by bob midiri, 22 September 2018 - 06:27 AM.

[starman876]

The size of the flat is dependent on the size of the lens you want to test.   A 5" flat will test 4" scopes and smaller just fine.   The bottom line is the size of the flat needs to be a bit larger than the optics you will be testing.   I have flats from 6" to 16" .  I have even found that flats used for laser projects will work just fine.  Those can be found rather cheaply.   I will post  a picture of the simplest set up I have used.   It consists of the flat just sitting on my granite counter.  The scope sitting on the granite counter and using a 35mm film canister in the focuser of the scope.   The 35mm film canister has the Ronchi screen and the green LED in it.   It is all rather simple and it will tell you in a heartbeat if your optics are good or not.   I have tested many lenses this way and the test has never lied.  There are plenty of other people I know that has been testing lenses this way.   We can thank DaveG for getting a bunch of us interested in doing DPAC testing.  His knowledge of optics is awesome and he is willing to step in and help anyone all you got to do is ask.    The reason why we use a green LED is because most lenses are figured to be best in the green wavelength of light because that is where our eyes are the most sensitive.   I have done DPAC in blue, red and even in white light.    It is interesting when you do DPAC in these frequencies of light because only the very best lens will test well in these other wavelengths.   I have not checked the Chinese doublet ED lenses in the other  colors, but I have a feeling they would not so as well as an AP or a Tak.    AP scopes are really made for imagers and CCD cameras are more sensitive in the blue wavelength of light so the lens of a scope used for imaging would have to be figured better than a lens for a visual scope.  In other words the different wavelengths of light in an imaging scope would need to come to focus at the same point. If they did not the CCD imager would show blue halos around the image.  

Edited by starman876, 22 September 2018 - 08:19 AM.

[Eddgie]

I would like to know how DPAC (what's that?) is done. Please post away!

 

I need to be able to test my scopes as I feel that one is not performing up to standard.

 

Thanks,

Miguel

DPAC is "Double Pass Autocollimation".

 

People test with a Ronchi Tester, but in single pass, it really is not sensitive enough to show anything but the most severe errors.  Even the inventor of the test, Vasco Ronchi, came to conclude that the sensitivity of the test was to low to be acceptable for precision evaluation.

 

The Double Pass tests does what the name implies, which is to fold the light back so that it passes the objective twice, and this greatly improves the sensitivity of the test.

 

Here though is the limit of the test and it is (to me anyway) a very serious one.   It makes the errors far easier to see, but it does not tell you much about the severity of the error.  In other words, if you see the lines curve, you can't easily say how much error that represents without using a software program to help you quantify the error.

 

Likewise, zonal errors or surface roughness are both difficult to quantify (I don't know any program that will quantify surface roughness or zones discovered in a Ronchi Double Pass.  You can see them, but you simply have to use a total guess to say how much the error is, so seeing the error but not knowing how bad it is kind of makes the test somewhat frustrating.    ( A program to do this may exist, but I simply don't know about it).

 

The good news though is that if the lines are perfectly straight and seem pretty sharp, you probably have a very good optic.  But what happens if you see an irregularity?  How bad is it?  Should you be concerned if you see the indications of a zone?  Is that curve in the line bad enough to worry about?

 

Lacking a big enough mirror, you can also do a double pass by pointing the telescope straight down over a pan filled with water and a few drops of oil to make an oil film.  This produces a flat of remarkably high quality.

 

For the quantification reason, I have to say that it is still good to know how to do both Star Test and/or Roddier test.  These allow you to quantify the errors (especially the Roddier that produces a full wavefront and MTF value, and this again is the major shortcoming of double pass. You see the errors easily, but there is no easy reference that you can use to estimate the severity. 

 

And while it was implied in my first paragraph, let me be explicit.  The problem with a single pass Ronchi test is that it is not sensitive enough to reveal anything but pretty serious errors.  A optic can be pretty bad and still show lines that look straight and that is because by the time you get to the three or four lines needed to get the sensitivity required, the edges of the lines are so muddy and indistinct that it makes it hard to see if they are curved or not.

 

Suiter addresses this in his book on star testing, and regardless of how one wants to test, it is my own opinion that if you care about testing telescopes, his book is an essential reference.  See, knowing that there is an error does not tell you how that error affects the view.  Suiter explains every error and how it impacts the wavefront.   This is how I know why a turned edge is such a serious error.   I read the book.

Edited by Eddgie, 22 September 2018 - 10:54 AM.

[Kent10]

Does using a Barlow with the Ronchi eyepiece also act like a double pass test?

[punk35]

This is an interesting topic. Following. I think being able to test your own equipment is a definite plus.

[starman876]

Does using a Barlow with the Ronchi eyepiece also act like a double pass test?

Sorry no it will not.

[DAVIDG]

"Here though is the limit of the test and it is (to me anyway) a very serious one.   It makes the errors far easier to see, but it does not tell you much about the severity of the error.  In other words, if you see the lines curve, you can't easily say how much error that represents without using a software program to help you quantify the error."

 

Sounds like you never used this test because if you have you would know that if to have 1/10 wave optics you would have to have very straight Ronchi bands. So if you easily see bowing then your looking at 1/4 wave or worse.  So while the test doesn't give you an exact wavefront it  will tell you in seconds if your anywhere close to  1/10 or 1/4 wave Unfortunately most of the time it turns out to be 1/4 wave.  Double pass has a  huge advantage that there are few sources of errors. So it is very difficult to get a false positive  ie testing bad  optics and the test shows they are good. With many other tests this very common because of  lack of understanding  how to do the test and not knowing the type and magnitude of errors in the result. 

 

   "Lacking a big enough mirror, you can also do a double pass by pointing the telescope straight down over a pan filled with water and a few drops of oil to make an oil film.  This produces a flat of remarkably high quality."

 

  You don't use  water because the viscosity is too low and you don't add a few drops of oil to water.  Since oil is non polar and water is very polar they don't mix anyway and would just separate . What you do is use a  pan with  about 1/4"  of pure oil in it.  Vegetable oil works well and it is non toxic.  You are correct that the oil flat is remarkably flat something like 1/300 of wave. Here is  a link to  a thread here on Cloudy Nights on how it  is done https://www.cloudyni...llimation-test/

 

                     - Dave 

[Eddgie]

Does using a Barlow with the Ronchi eyepiece also act like a double pass test?

No. 

[Eddgie]

Here are some Ronchi double pass tests; over 100 of them.

 

http://teleskop-spez...-teleskope.html

 

I would invite the readers to look at these tests and ask themselves what happens when the lines are not straight.    Is the telescope only so-so?  Is it still a decent scope?  Is it a total lemon?

 

As will be seen, in a great number of these tests, the lines are not straight.  The question then becomes "How bad is it?" 

 

I think Ronchi double pass is outstanding for showing errors.  I just don't think it is that easy to get a good handle on the total wavefront without using other tools. 

 

Scope 54 in the above tests is interesting.  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?

 

To see the number of the scope, scroll about half way through the page and then click on one of the pictures.  This should open a numbered index so that people can make sure they are looking at the same scope.  So many of the refractors seem to have straighter lines than this one.  Is this one a lemon?  Should the owner be concerned that they got a scope that was not as good as their friend's scope? The lines here are not almost straight. They are clearly curved.  How bad is it though?  Don't ask me.  I am not good at judging SA from a Ronchi double pass.   I would have to use a 33% obstruction test to feel like I could tell much about the severity.   

Edited by Eddgie, 22 September 2018 - 05:01 PM.

[Steve D.]

I would have to use a 33% obstruction test to feel like I could tell much about the severity.   

 

Here’s an example of the “33% obstruction test”.

 

https://www.cloudyni...tv85/?p=7500206

 

PS:   The best part of this test is that you don’t need special equipment other than maybe a spark plug gapping tool to get the defocus the same on either side of focus.

Edited by Steve D., 22 September 2018 - 05:34 PM.

[Eddgie]

Scope 134 in the link above is also interesting (a 120mm f/8.3 achro.  Clearly this scope has a pronounced zonal error but how bad is the error in terms of wavefront?  Of course this is not the kind of scope most people would use for super high resolution observing, but I only pick it as an example of how seeing the error and judging the severity of the error are two different things.   Probably an OK scope for general use, but people test not to see if they have an OK scope but to see if it has errors and if they are bad enough to be concerned about.   Again, in this scope, not going to matter because the CA is the overpowering limit to contrast transfer on this instrument, but that does not mean you could not make it worse by further damaging the quality of the view with a severe zonal error. 

 

Some appalling MCTs and SCTs in these tests.  Even someone that does not know the first thing about testing telescopes should be able to ID the biggest dog of pack of barkers in this test page.  There are some miserable scopes shown in these tests.   Most though are well, Ok-ish to quite excellent.  Some miserable scopes tested here though.  Double Pass can make some of them look worse than they are though because the lines are curved enough to kind of make one wonder where the limit should be. 

[starman876]

Scope 134 in the link above is also interesting (a 120mm f/8.3 achro.  Clearly this scope has a pronounced zonal error but how bad is the error in terms of wavefront?  Of course this is not the kind of scope most people would use for super high resolution observing, but I only pick it as an example of how seeing the error and judging the severity of the error are two different things.   Probably an OK scope for general use, but people test not to see if they have an OK scope but to see if it has errors and if they are bad enough to be concerned about.   Again, in this scope, not going to matter because the CA is the overpowering limit to contrast transfer on this instrument, but that does not mean you could not make it worse by further damaging the quality of the view with a severe zonal error. 

 

Some appalling MCTs and SCTs in these tests.  Even someone that does not know the first thing about testing telescopes should be able to ID the biggest dog of pack of barkers in this test page.  There are some miserable scopes shown in these tests.   Most though are well, Ok-ish to quite excellent.  Some miserable scopes tested here though.  Double Pass can make some of them look worse than they are though because the lines are curved enough to kind of make one wonder where the limit should be. 

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.