26 replies to this topic

[Scott in NC]

This weekend I got the opportunity to test a Takahashi TSA-102S. We've had some much needed rain in our area this week, so I haven't had the opportunity to observe with this fine scope yet. The TSA-102S is an Ohara FPL-53 based f/8 apochromatic triplet, and the "S" designation means that it has a sliding dew shield, whereas the TSA-102N has an "N" for "non-sliding," or fixed dew shield.  This particular TSA-102S was created by Takahashi in 2014.  I've never had the opportunity to observe with or test a TSA-102 before, so I've been really eager to see how this compares to my prized FS-102 from 2001 which I've owned for 10 years this month.  

 

Here's a picture of the scope set up on my usual optical bench prior to testing.

 

[Scott in NC]

Now I'll present the Ronchigram images:

 

Green inside/outside focus:

 

 

Red inside/outside focus:

 

 

Blue inside/outside focus:

 

 

White inside/at/outside focus:

 

[PatientObserver]

I was wondering when we would see a post about this scope. Looking forward to reading more after testing is completed and you have time to use it under the stars.

[Scott in NC]

Overall I think this looks pretty nice.  I see slight overcorrection in green, perhaps very slightly less overcorrection in red, and slightly more in blue. I'm not sure why the degree of spherical aberration is so much easier to see in the outside-focus images as compared to the inside-focus images, but suspect that it may have to do with slight differences in the positions of the draw tube when the intra-focal vs. extra-focal images were taken. The at-focus image shows two very shallow zones in the center of the lens and at about 40-50% of the radius. Chromatic aberration is astonishingly low, as one would expect from a high-end FPL-53 based apochromatic triplet.

 

The bottom line is that this scope tested very nicely, and I'm happy with it. Thanks for reading!

[Scott in NC]

I was wondering when we would see a post about this scope. Looking forward to reading more after testing is completed and you have time to use it under the stars.

Well, it just arrived 3 days ago! This thing called work got in the way.

[Scott in NC]

One astute member of the audience just texted me to ask why the outside focus images appeared more clear than the inside of focus photos.  
 

I think this is the answer: I manually focused the camera lens at the beginning of my image-taking and then didn't readjust when I took the other photos. Next time I'll try to refocus the camera whenever I adjust the scope's focuser.  

 

I'm still learning each time I do one of these tests, and everyone's questions and comments really help with the learning process, so thanks, and please keep them coming!

[Scott in NC]

 

I think this is the answer: I manually focused the camera lens at the beginning of my image-taking and then didn't readjust when I took the other photos. Next time I'll try to refocus the camera whenever I adjust the scope's focuser.  

 

Upon further reflection, I'm pretty sure that this is what happened.  

 

I'm not sure why the degree of spherical aberration is so much easier to see in the outside-focus images as compared to the inside-focus images, but suspect that it may have to do with slight differences in the positions of the draw tube when the intra-focal vs. extra-focal images were taken. 

After examining the images further, I think that this is indeed the explanation. Note how the Ronchigram images from the intra-focal position are slightly smaller than the images from the extra-focal position. This would indeed indicate that the intra-focal images were taken slightly farther inside the focal point than the extra-focal images were taken outside the focal point. And that will make the curvature of the bands look slightly more pronounced in the extra-focal images compared with the intra-focal images.

 

Please note that none of this reflects the accuracy of the test itself, but more so the accuracy of the images that were taken to record what was observed during the test. I hope that makes sense.

[PatientObserver]

Scott, do you know of any good links that explain DPAC and how to interpret the images? I am traveling so I only had time for a quick search before I got back on the road.

[davidc135]

Upon further reflection, I'm pretty sure that this is what happened.  

 

After examining the images further, I think that this is indeed the explanation. Note how the Ronchigram images from the intra-focal position are slightly smaller than the images from the extra-focal position. This would indeed indicate that the intra-focal images were taken slightly farther inside the focal point than the extra-focal images were taken outside the focal point. And that will make the curvature of the bands look slightly more pronounced in the extra-focal images compared with the intra-focal images.

 

Please note that none of this reflects the accuracy of the test itself, but more so the accuracy of the images that were taken to record what was observed during the test. I hope that makes sense.

There may also be an optical reason for rgrams being better defined beyond compared to within focus. Or so it seemed to me, doing the test visually. But your explanation can be checked out too.

 

David

[Scott in NC]

Scott, do you know of any good links that explain DPAC and how to interpret the images? I am traveling so I only had time for a quick search before I got back on the road.

Here’s one to help with interpretation of Ronchigrams. This is for a mirror though, and so the interpretation needs to be reversed for refractor lenses:

 

https://nicholoptica...i-patterns.pdf 

 

Here’s a good thread started by peleuba, who participates in this forum quite regularly:

 

https://www.cloudyni...-bench-images/ 

[peleuba]

Here’s a good thread started by peleuba, who participates in this forum quite regularly:

 

https://www.cloudyni...-bench-images/ 

 

Yes, that's a good thread.  Lots of opinions and some factual information discussed.   The discussion gets into the deep end of the pool quickly pertaining to aberrations that can be detected in DPAC.  Curvature in the bands as seen in the static images most of the DPAC practitioners post to CN highlight the spherical correction, zones and the edge condition (and sometimes coma and alignment errors) of the lens being tested.  For me, astigmatism is easier to detect and quantify via the star test.  Yes it can be seen in DPAC, for sure.  But its difficult to photograph for evaluation later because to see it, you need to watch the bands "chock" and distort as you move through focus.  Its nearly impossible to capture this well in a photo.

 

I have tested a very nice TSA102 back in January of 2020.  Might have been a little better then this sample...  but I will look on my server in my DPAC image repository.    

Edited by peleuba, 30 June 2024 - 11:30 AM.

[peleuba]

I have tested a very nice TSA102 back in January of 2020.  Might have been a little better then this sample...  but I will look on my server in my DPAC image repository.    

 

Here are images of the sample I tested in early 2020 its a nice telescope and very close in appearance to the one Scott tested.  If I am picking nits -  and evaluating these tests is most definitely an exercise in subtly - the main diff between it and Scott's is the the zone near the edge is slightly more prominent on Scott's then mine, but the scopes are very close and neither is a ⅒ wave lens.   If interferometry was performed, these would likely test out with a Strehl in the mid .90's (.960?) and about ⅙ wave PtV.

Attached Thumbnails

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[Scott in NC]

Thanks, Paul. If you have more images from that TSA-102 please feel free to post them either here or in another thread (frankly, here would be just fine so that all the info is together on one place). I can then place a link to your test results to add to the CN DPAC repository here:

 

https://www.cloudyni...on-refractors/ 

 

For those who are unaware, we’ve compiled DPAC results on over 100 refractors here. Thanks to everyone who has participated in this project!

[peleuba]

Thanks, Paul. If you have more images from that TSA-102 please feel free to post them either here or in another thread (frankly, here would be just fine so that all the info is together on one place). I can then place a link to your test results to add to the CN DPAC repository here:

 

https://www.cloudyni...on-refractors/ 

 

For those who are unaware, we’ve compiled DPAC results on over 100 refractors here. Thanks to everyone who has participated in this project!

 

 

I do have more images, but I will not pollute your thread.  I don't like when that's done to me as it creates confusion as to what images are for what scope...  I only did it above and then only with two green images just to see how close two random TSA102's are.   And, they are VERY close.  I found it interesting that both scopes have an edge zone, so the polishing techniques are likely similar - which makes sense. 

 

I tested this scope 4½ years ago so will likely not create a new thread.   

 

Its interesting that the TSA102 design has some residual coma left in that the reducer/flattener corrects.  It would seem TAK designed the TSA102 for best correction when taking photos. 

 

I've seen a number of these TSA102's for sale recently.   I've always wanted one to pair with my TSA120.  

[Scott in NC]

I've seen a number of these TSA102's for sale recently.   I've always wanted one to pair with my TSA120.

There were three on AM recently. A friend of mine in NC and I both bought one, and as far as I know the third one might still be available but I haven’t bothered to go back and look. The one that I bought is 8 years newer than the one that’s left, not that that should really make a difference for scopes that have been well cared for.

[CHASLX200]

There were three on AM recently. A friend of mine in NC and I both bought one, and as far as I know the third one might still be available but I haven’t bothered to go back and look. The one that I bought is 8 years newer than the one that’s left, not that that should really make a difference for scopes that have been well cared for.

Much better than a FS102?

[peleuba]

There were three on AM recently. A friend of mine in NC and I both bought one, and as far as I know the third one might still be available but I haven’t bothered to go back and look. The one that I bought is 8 years newer than the one that’s left, not that that should really make a difference for scopes that have been well cared for.

 

Yep, I saw them...   I passed on both.  One had been "autographed" by some folks at Takahashi, and the other had cosmetic issues.

[Scott in NC]

Much better than a FS102?

That remains to be seen, as I haven’t yet had the opportunity to observe with this scope that I’ve only owned for 4 days. Certainly the chromatic control is better on the triplet TSA than the doublet FS, as they have the same aperture and same focal length, but the FS is already exceptionally well corrected for a doublet.

[Scott in NC]

Yep, I saw them...   I passed on both.  One had been "autographed" by some folks at Takahashi, and the other had cosmetic issues.

My friend bought the autographed one prior to my purchase of one of the other two. And after careful consideration, I decided that I wanted the one with minor cosmetic issues on the tube rather than the one that appeared to have better cosmetics. The minor cosmetic blems were the result of an improperly fitting set of non-Tak rings. Fortunately the three small spots are mostly covered by an original Tak clamshell ring and the position of the dovetail plate when the scope is mounted. Plus I also bought some touch up paint from TNR for an even better solution. I’m pretty picky about the condition of my scopes, and despite that, this didn’t really bother me.

[peleuba]

My friend bought the autographed one prior to my purchase of one of the other two. 

 

 

I seriously considered purchasing the scope with the signatures as soon as it was posted.  Both had been on the market for awhile maybe a month, and the only thing keeping me from pulling the trigger was the signatures.  I've previously used Isopropyl alcohol to remove decals and black ink from an OTA's.  But if the ink has been there for a long time, the pigment has likely been absorbed by the paint and faint outlines of the signature would remain.   Unfortunately, the TAK paint is on the fragile side so nothing like acetone could be used as this would strip the clear coat.

 

When testing and collimating an FS102 for a friend I decided to do a full restore on the cosmetics...   I ended up ceramic coating it.  The tube had some love marks and ring rash that I buffed out with some polishing compound then touch up paint.  It turned out excellent. 

 

Anyway, its interesting as the TSA series will only be signed if the scope goes back to Japan (for repair or otherwise) or if the owner requested it to be signed at NEAF while the TAK Japan team was there.  I think the early TOA series were all signed...  Wonder why this one was signed?

 

When you touch up the paint on your TSA102, clean it with Isopropyl first.  Then touch it up.  A few days later, hit with a polishing compound to level it, then use some type of polymer sealer on it.  Should look really good.

Edited by peleuba, 01 July 2024 - 07:46 AM.

[Scott in NC]

Thanks for the advice on the restoration, Paul. I’ll PM you.

[gjanke]

Here's the link.

 

 

https://www.cloudyni...102-bench-test/

 

 

I regrettably sold that one but luckily I picked up another one and one day hope to have Paul put it through its DPACing paces.

[SandyHouTex]

I have a TSA-102S and it is optically excellent.

[Jeff B]

I have a TSA-102S and it is optically excellent.

Prove it!

 

Sorry, couldn't resist.  

[peleuba]

I regrettably sold that one but luckily I picked up another one and one day hope to have Paul put it through its DPACing paces.

 

 

I, of course, will test it for you.  I am starting another optical project this week that may take about 6 weeks to complete.  Then I've got another large aperture APO to test and will be attending both the Black Forest Star Party in early September for a week and then a week at the Staunton River SP in early October.  We can do it after that if you still own the scope .  

 

I am wondering if any of these scopes that were recently purchased were previously owned by you or if the one for sale now was yours at some point?

Edited by peleuba, 01 July 2024 - 12:41 PM.