724 replies to this topic

[DAVIDG]

Yes, I had forgotten that important point.  Is your light box using a CFL?

 The light box is an UniLamp that uses a mercury bulb  and green plexiglass to only allow the green mercury line at 546nm through so the light is monochromatic. 

   You need monochromatic or semi-monochromatic light when testing surfaces by contact interference to make the fringes become visible.

  When doing double pass testing of refractors you mainly test in green light  but it doesn't have to be monochromatic just fairly narrow in the bandpass. A green LED works well.  The reason is that achromats and APO all have spherochromatism. Which is they show some level of spherical aberration at different wavelengths. Most lens are designed to have no spherical aberration in the green. So if they test well in the green then the other wavelengths will have the  amount of spherical aberration  that the lens was designed to have. If it tests with problems is the green then the other wavelengths will be worse and  both spherical aberration and  chromatic aberration will increase 

 

               - Dave 

[Scott in NC]

I'm not going to take any more DPAC pics with the iPhone.  The iPhone lens has so much distortion that it makes the lens look much worse than it is.  Here's one with my Canon DSLR with a 50mm f/1.8 lens, held as steady as I could by hand.  I'll get better at taking images with practice.

 

This one is outside focus with the SV80S.

 

[DAVIDG]

Here is something that's illustrated better then what we can do through an editor here on CN

 Those illustration are correct for both mirrors and lens tested in Double pass  for over all spherical aberration but the pattern you see from a Ronchi test of other errors  on mirror are interpreted the opposite in a lens. For example if the Ronchi pattern in DPAC shows a hill on a mirror , if you see the same pattern on a lens it is hole. A hill on a mirror  causes the light to diverge  and focus longer than the  rest. A hole on a lens, since light is passing through it also causes light to diverge and focus long.  Since one system is reflective and other refractive the pattern you see is the same for what the light is doing but the causes is the opposite error on the surface from a mirror to a lens. 

   The bottom lines is you want to see three dead straight Ronchi lines both inside and outside of focus. That means you have a spherical wave front and all the light is coming to a perfect focus at the wavelength your testing in.

 

                   - Dave 

[DAVIDG]

I'm not going to take any more DPAC pics with the iPhone.  The iPhone lens has so much distortion that it makes the lens look much worse than it is.  Here's one with my Canon DSLR with a 50mm f/1.8 lens, held as steady as I could by hand.  I'll get better at taking images with practice.

 

This one is outside focus with the SV80S.

 

IMG_7945.JPG

 This image shows a very good lens ! This isn't hard at all and there is nothing to measure. You can see if there are problems or not. 

  Just a word of caution. Your lens tested very well in the green but depending on the design and the F-ratio it can still have a 1/2 wave of chromatic aberration. Typical achromatic when they are faster then F/10 can have this amount of chromatic aberration even though in the green they show perfect spherical correction. A typical example is binocular objective that are usually in the f/3 to f/5 range. They still have a large amount chromatic aberration but since the magnification is fixed it is not objectionable. 

   So one needs to understand that both spherical correction and color correction go hand in hand. If spherical is off, color will be greater then the design calls for and there is only so much color correction you can get with two or three pieces of glass. So again you test a lens that shows excellent spherical correction but because of the design and the F-ratio still have a fair amount of  chromatic aberration. 

 

                  - Dave 

Edited by DAVIDG, 18 February 2023 - 11:08 AM.

[Scott in NC]

I found that the best camera settings to capture these images are 1/15 second exposure, f/1.8, manual focus (with my 50mm f/1.8 lens, at least).

 

SV80S f/6 LOMO triplet inside focus:

 

 

At focus:

 

 

Outside focus:

 

 

[Scott in NC]

Now for my AT80EDL f/7, which I had thought was a pretty nice scope for the money.  It gives pretty good visual images of the skies, limited by its small aperture of course.  The DPAC images clearly don’t look as nice as those of the SV80S though.  In fact, they’re downright scary.  You may want to shield your eyes!

 

[Scott in NC]

AT80EDL f/7 (FCD-100 / Lanthanum), guaranteed minimum Strehl 0.95 (hmm...)

 

Inside focus:

 

 

At focus:

 

 

Outside focus:

 

 

[Scott in NC]

So, turned edge and maybe a bit of under correction?

 

Interestingly, the DPAC images make the scope look worse than it really appears to be when viewing the skies through it.  

[Scott in NC]

Next the TV85 f/7 doublet.  Now I know why this scope cost 3x as much as the AT80EDL, which is a fine scope in its own right (at least for visual purposes).

 

 

 

[Scott in NC]

TV85 f/7 doublet

 

Inside focus:

 

 

At focus:

 

 

Outside focus:

 

 

[Scott in NC]

The lines look pretty straight to me, but do the shadows in between the dark lines mean anything?  I did use a slightly slower shutter speed, 1/10 second this time (compared with 1/15 second with the other two scopes) with ISO 800, f/1.8.

[Scott in NC]

SV80L f/7.5 LOMO triplet.  I think this is about as good as it gets for an 80mm refractor (at least one that I can afford).

 

 

 

 

 

[peleuba]

Some general thoughts...

 

The AT80 appears to have very good spherical correction, but there is a narrow turned edge.  The IF may not have measured to the absolute edge of the optic.  Also - this photo illustrates the edge issue well, but in Autocolliomatuin its twice as bad as in "real life" I would be interested in seeing or hearing what the star test looks like.   

 

The TeleVue85 is nice, with some slight overcorrection and a very slight edge anomaly.

 

One item of note, under the real sky using a Ronchi eyepiece on a real star in single pass mode, you would have never seen the overcorrection in the TV85.  This is the power of testing in Autocollimation.  

 

I made your power supply with a potentiometer so you can turn down the brightness to better match the camera or eye.   

Edited by peleuba, 18 February 2023 - 09:32 PM.

[Scott in NC]

SV80L f/7.5 LOMO triplet

 

Inside focus:

 

 

At focus:

 

 

Outside focus:

 

[Scott in NC]

Okay, I think that's all for today.  This has been great fun, and I thank you all for bearing with me.

 

Paul, you've unleashed a monster!

[Scott in NC]

 

The AT80 appears to have very good spherical correction, but there is a narrow turned edge.  The IF may not have measured to the absolute edge of the optic.  Also - this photo illustrates the edge issue well, but in Autocolliomatuin its twice as bad as in "real life" I would be interested in seeing or hearing what the star test looks like.   

 

So if it were masked down by a couple of mm, the lens would be much better.  Not that I’m going to do that though, as it gives a nice view visually, and odds are very high that I would damage it, and make it worse.

 

And to answer your question about the star test, I really didn’t detect anything unusual, just the typical star test for an ED doublet.

[Scott in NC]

Can anything be said about the in-focus images of any of the scopes?  Or is it not even worth trying to capture those images, as they don’t add any useful information that the intra- and extra-focal images don’t already provide?  

 

I’m wondering whether the bright ring around the circumference of the in-focus image of the AT80EDL provides more confirmation of the slight turned edge, or whether that’s just an artifact from the camera.

[Scott in NC]

Okay, I just had to do one more, and now I’m done for the day.  

 

Takahashi FS-60Q f/10 quad.

 

 

 

 

 

[Scott in NC]

Takahashi FS-60Q f/10 quad

 

Inside focus:

 

 

At focus:

 

 

Outside focus:

 

[peleuba]

So, turned edge and maybe a bit of under correction?

 

Interestingly, the DPAC images make the scope look worse than it really appears to be when viewing the skies through it.  

 

Not not always.  Turned up/down edge can be independent of the spherical correction of the rest of the lens.  

 

Yes, DPAC makes the lens look twice as bad.  The error you see are multiplied by a factor of 2.

[peleuba]

Can anything be said about the in-focus images of any of the scopes?  Or is it not even worth trying to capture those images, as they don’t add any useful information that the intra- and extra-focal images don’t already provide?  

I like the best-focus images.  Or try to show only 1 band.  This highlights the peaks and valleys of the lens.

[Scott in NC]

I noticed that the FS-60Q has thinner bands.  Is that due to lesser aperture, greater focal ratio, or a little of both?  Or none of the above?

[Alan S]

Scott-

 

Have you eaten today?

 

Seriously interesting stuff and fun to watch you start out on this path...I think that, like astrophotography, optical testing is it's own facet of our hobby and I enjoy following your efforts and learning from them.

 

Alan

[peleuba]

Paul, you've unleashed a monster!

 

That was my goal.  

[Jeff B]

That was my goal.  

I think you exceeded your goal.