724 replies to this topic

[scooke]

Someone please correct me if you think otherwise, but I see a very small turned up edge and mild overcorrection.

 

No this isn't bad, but I'm frankly shocked given the beautiful views that this scope provides of the heavens.  I realize that DPAC is an especially rigorous test, but I can't fully reconcile these test results with the scope's actual star test. Now I'm almost afraid to test my other two prized Taks, the FS-128 and FS-152 (although the 152 may be a little too large for my 6" flat).

I am laughing so much.  I felt the same way when I started testing my scopes.  None of them look great in dpac except my questar.  Go figure.  You are seeing what they really are despite showing awesome images visually.

[AstroApe]

After watching all these fine scopes get tested, I've come to the conclusion that I'd rather be blissful in my ignorance of my own equipment, especially if I'm attached to a scope. As you said, it's definitely not for the faint hearted, and I'd hate to hurt my ego over a scope I thought was a 10/10 that turns out to 6-7/10. But as scooke said above, at least my Questar might show a decent result.

[Jeff B]

DPAC Nervosa and DPAC Depressive Syndrome as closely related and may be co-indicated.

[Scott in NC]

After watching all these fine scopes get tested, I've come to the conclusion that I'd rather be blissful in my ignorance of my own equipment, especially if I'm attached to a scope. As you said, it's definitely not for the faint hearted, and I'd hate to hurt my ego over a scope I thought was a 10/10 that turns out to 6-7/10. But as scooke said above, at least my Questar might show a decent result.

I’ll admit that when I first saw the Ronchigrams on my “perfect” FS-102, I was a little disheartened.  But the more I go back and look at those images, the more I realize that they still reflect a quite well-corrected scope. And when I correlate those findings with the views that I see when I actually observe through the scope, I realize that anything closer to perfection *might* make a difference when doing AP on a perfect night, but it’s doubtful whether I’d see a meaningful difference when visually observing in the typical skies in my typical observing location.  This is the same discussion that we get into when debating whether a 0.99 Strehl scope is going to show you much (or anything) more than a 0.95 Strehl scope.  The true answer is that they’re both fantastic. The FS-102 is still one of my favorite scopes, and is still not for sale at any price at this time.

[Scott in NC]

I have a second LZOS 115/805 lens set (long story about why I have it, that I’ll save for another day) that I’ve wanted to test out for a long time, but until now didn’t have a good way of doing so.  So I unthreaded the lens cell from my SV115 LZOS whose test results I presented above and replaced it with my second set of 115/805 optics.  I’ve never checked this optics set out under the stars, and to do so I’d have to add a thick layer of felt around the circumference of the lens cell in order to allow the scope’s dew shield to fit.  One day I’ll try that, but today I was able to subject it to DPAC testing.

 

Here’s my setup:

 

 

 

[Scott in NC]

LZOS 115/805

 

Inside focus:

 

 

 

At focus:

 

 

 

Outside focus:

 

 

 

[Scott in NC]

Pretty nice IMO.  Minimal spherical aberration (very minor overcorrection) and possibly a very small upturned edge). Nothing to complain about though.  Once again keep in mind that DPAC makes any deviations from perfection look twice as bad as they really are, and I doubt that this could be distinguished from that elusive "perfect optic" during visual observation.  I'd be happy to own such a lens (oh wait, I do!).

 

Edit:  Okay, let's try that one again.  At first I was amazed that the Ronchigrams looked virtually identical to those from my other LZOS lens set (the one that came with my SV115), but then I figured out that I had uploaded the wrong set of pictures in the post above.  I've now replaced them with the correct images.

 

In addition to what I stated above (which still appears to be true for this set of optics), I see some minor notching from what I believe is a spacer at the 2:00 position.  One day I'll test this scope out under the stars, but I suspect that that wouldn't be seen *at focus*, but could possibly be seen while star testing inside and outside of focus, and possibly during AP. Regardless, I still think this lens looks pretty good, but I'd love to hear comments from those who are more knowledgeable.

 

Edit #2: Of course, David is correct when he says in the post just below this one that the edge may be slightly turned down, not up as I said above.  Thanks for catching that!

Edited by Scott in NC, 14 May 2023 - 08:19 PM.

[davidc135]

Another beautiful lens with a minute departure from perfection. I'd say that the extreme edge was slightly turned down.

 

David

[Alan S]

Scott, nice work! I'm looking at this on my phone, so small screen...is there a center spot in these images? Assuming some artifact?

[Scott in NC]

Scott, nice work! I'm looking at this on my phone, so small screen...is there a center spot in these images? Assuming some artifact?

Alan, I think that's an artifact as I've seen it on other photos that I've taken with other lenses, and not posted here.  I believe it to be a reflection of the LED light.  But I'll have to do some more investigating to say for sure.

[Scott in NC]

Does it mean anything that the left-most Ronchi band appears to be slightly thicker than the right-most one? I'll have to go back and take a look at my test images from other scopes, but I suspect that this may just be related to camera tilt (as I've done all of my shots with the camera hand-held).

[m0bius]

I wanted to mention that this has really been interesting, you've made excellent progress too! Now I wonder if you'll be testing any optics which aren't pretty much perfect? How about a *meh* finder scope or a junky 'hobby killer'?  I think that might provide some perspective or a baseline in comparison, might be interesting.

Edited by m0bius, 25 February 2023 - 08:21 PM.

[Scott in NC]

After I get done with all my apos, it'll be time to move on to my achromats.  I could also test an Orion Starblast 4.5" dob, but would need a 2" to 1.25" step-down adapter for that, as that scope only has a 1.25" focuser and my Ronchi eyepiece is 2" in diameter.  I just told my son this, and he says he'll try to 3-D print an adapter just for this project.  Isn't he great?!?

[Scott in NC]

For fun I tested my FS-102 again, this time with a TOA-1.6x Extender added.  Here's a link to the images and my interpretation, but the bottom line is that it just magnified the errors seen in the original DPAC testing without the Extender in place.  It didn't add any additional new insights.  Hey, I'm still learning!

 

https://www.cloudyni...152/?p=12524043

[Villa_il_Gioiello]

For fun I tested my FS-102 again, this time with a TOA-1.6x Extender added.  Here's a link to the images and my interpretation, but the bottom line is that it just magnified the errors seen in the original DPAC testing without the Extender in place.  It didn't add any additional new insights.  Hey, I'm still learning!

 

https://www.cloudyni...152/?p=12524043

Interesting… I wonder if one would get similar results with the FOA60 and the FOA60Q. Or the FS60 vs the FS60Q. 

Edited by StarAlert, 25 February 2023 - 10:33 PM.

[Scott in NC]

Interesting… I wonder if one would get similar results with the FOA60 and the FOA60Q. Or the FS60 vs the FS60Q. 

I’m thinking not, as my FS-60Q looked nearly perfect on DPAC, and I can’t imagine that my base FS-60 would have been as well corrected, given its fast f/5.9 doublet optics.  I think that the 1.7x Q module for that scope also serves as a field flattener, and I’m not sure that the TOA-1.6x Q Extender serves the same function.  But I haven’t tested my base FS-60 on DPAC, and what I’ve guessed about the function of the Extender may not be correct, so I don’t really know for sure.

 

Here’s how the FS-60Q tested: https://www.cloudyni...nge/?p=12506195

[scooke]

You all will quickly learn that NOTHING is perfect on DPAC.  It is just too sensitive.  The nice thing is nothing has to be perfect on DPAC to give great images.

[Maciek_Cz]

Scott, I think a DPAC of FS-60 with and without an extender would be interesting.
I'm curious what it would look like for red, green and blue.

[Scott in NC]

I agree, Maciek.  I’ve never removed the 1.7x Q module from my FS-60Q in the 7 years that I’ve owned it, but think I’ll do so just for DPAC testing purposes.

 

Right now I only have the capability of testing in green light, but am working on the parts to upgrade my testing rig for white, red, and blue light.

[Maciek_Cz]

Scott, it will be a great experiment with the FS-60 with and without the extender.
I bet dollars to nuts that the extender improves the quality of the FS.

[CharlieB]

Thanks to DavidG, I started testing my 40mm-76mm scopes a few years back because a surprising number of my classic Japanese scopes didn't perform as well as I had anticipated.  The test quickly shows the problems with a given lens - in my case, it was often simply a flipped lens element. 

 

Two of my favorite examples are from an early 50's Soligor 60/1000 scope and a Unitron 105 (50/700) from the 80's.

 

The Soligor just would not give a decent view and achieving a good focus was impossible.  After testing it with my DPAC setup, I could immediately see why.  No amount of playing around with lens elements ever improved it.

 

 

On the other end of the spectrum was the Unitron lens.  It's about as good as any doublet I've seen tested and the spectacular views confirm this.

 

Inside, outside and null in green.

 

 

   

 

It's also respectably well corrected in blue

 

  

 

Charlie

 

 

 

 

[CHASLX200]

That one lens is a twisty mess jess.

[Scott in NC]

That's a nice Unitron lens, Charlie--thanks for sharing that!  I appreciate the pics of the Soligor lens too.  It's good to see something like that to put everything into proper perspective.  While we're fretting about minor deviations from perfection on our high end scopes (deviations too small for the eye to even detect under normal use under the skies), it's good to see what something to fret about really looks like!

[sunrag]

Two of my favorite examples are from an early 50's Soligor 60/1000 scope and a Unitron 105 (50/700) from the 80's.

 

The Soligor just would not give a decent view and achieving a good focus was impossible.  After testing it with my DPAC setup, I could immediately see why.  No amount of playing around with lens elements ever improved it.

 

soligor dpac.jpg

 

On my first and only attempt at DPAC, I got an image like this.

I was using a Gerd Neumann 10 L/mm Ronchi Eyepiece on which I taped a green LED over the lower half. I have a 6" round coated flat (unknown flatness). I placed it on table and put the 4" refractor on it and took a quick look. The DPAC image looked very bad, but the refractor (which is homemade using a 25x100 bino lens) star tests ok at 95x. I thought that the reason for the twisted lines was because of improper alignment of the flat with the objective.

[Scott in NC]

I tested the Takahashi FS-60 with and without the 1.7x Q module.  For those who are unfamiliar with the specs of this scope, the FS-60 is an f/5.9 fluorite doublet, and adding the 1.7x Q module turns it into an FS-60Q, an f/10 fluorite quadruplet.

 

FS-60 inside focus:

 

 

 

At focus:

 

 

 

Outside focus: