21 replies to this topic

[peleuba]

A few weeks back I was asked to take a look at an FS102 that the owner believed was not performing as good as it could.  So, for the last week or so I have been tweaking/adjusting/testing.  Specifically,  I performed the following:

 

•  Corrected paint issues on OTA
•  Cleaned outer surfaces of front and rear lens
•  Adjusted focuser to remove as much focus shift as possible
•  Collimated focuser
•  Collimated lens cell
•  Performed suite of optical bench tests

 

The above work may seem like a lot, but in reality, the telescope was in very good "used" condition when I received it.  All that was really needed was some typical maintenance and TLC.  Afterall, it was a 2003 model and it had been used regularly the last 17 years.

 

Takahashi paint is soft, not blind criticism, rather, its reality.  Honda automobiles are the same way, too, as some of the other Japanese auto makers especially compared to German and to a lesser extent American automobiles.  I've previously wondered if paint formulas in Japan are purposely mixed this way.   Anyway, I was able to use a host of different compounds and dual action polishers to remove all defects except those that pierced the clear coat and underlying color going directly to metal - there was only one of this type.  The scope, in person, now looks show room new.  The owner will be very happy.

 

I have owned three TAK scopes - FS152, Sky90 and now an excellent TSA120.  The current TSA120 is my favorite.  But, even so, I am not a huge fan of the brand.  Along with the paint being soft, I don't like the focusers.  And, If wanting compression fittings, I find it overly complex to do simple things without the use of extension tubes and adapters.  The Baader Qick-Lock adpaters solve a lot of the headache here.

 

However, optically, every TAK I have owned has exceeded my expectations.  The FS152 showed me what 6 inches of unobstructed aperture could do; the Sky90 was better then almost all the reviews said it would be; and the TSA120 is best of breed in it's aperture class.  This was the first FS102 I had ever tested extensively over the course of several days on a bench in a controlled environment.  

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Edited by peleuba, 26 July 2020 - 03:21 PM.

[Nippon]

So? Is it a typical Takahashi optically or not?

[peleuba]

Here is a summary of the Bench tests showing overall correction of the lens in 3 wavelengths plus white.

 

==

•  White:  The telescope shows VERY slight spherical over-correction with a smooth lens surface.

 

• Green:  Telescope exhibits a smooth lens, with VERY slight spherical over-correction with a hint of a central circular zone at the 60% radius.

 

•  Blue:  Obvious central zone with over-correction.

 

•  Red:  Very smooth well corrected with only a slight under-correction balancing out the over correction seen in blue.

==

 

Overall its a very smooth lens slightly over corrected except in red.  I have an excellent Vixen Fluorite (.980 Strehl) and this TAK has slightly more color error and slightly more spherical over correction.  Nevertheless its a high performing objective with a Strehl > = .950.  It should be a fine planetary telescope only limited by its aperture.  The residual over correction noted in white and green is of little consequence.  Additionally, the doublet Fluorite design displays some sphero-chromatic aberration however its somewhat balanced between red and blue.

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Edited by peleuba, 26 July 2020 - 02:48 PM.

[peleuba]

I also extensively star test the lens in double pass configuration, indoors.  This is a critical test as it doubles the amount of error seen.  Even so, this lens held up well.  Its very smooth and its (now) well collimated.  The photos below are pictures of the telescope being star tested and the device I used to generate the artificial star.

 

I don't know if this is typical of the FS102.  I have only critically tested this one.  But I have seen another FS102 tested in double pass here:

 

http://skywatcher.sp.../ak_ronchi.html

 

The scope I tested is better corrected then this one the link.

 

===

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Edited by peleuba, 27 July 2020 - 08:20 AM.

[Etendue645]

Very excellent bench test and images. I just did a DPAC on my Vixen/Celestron 90mm and posted a review just a few minutes before your post!  My tester is just in blue LED but I will be trying it out with the white LED like yours and separate out the channels. 

 

Tak quality is amazing! Love your bench test setup!

 

Best Regards,

 

Tony

[Etendue645]

I also extensively star test the lens in double pass indoors.  This is a critical test as it doubles the amount of error seen.  Even so, this lens held up well.  Its very smooth and its now well collimated.  The photos below are pictures of the telescope being star tested and then as well as the device I used to generate the artificial star.

 

I don't know if this is typical of the FS102.  I have only tested critically tested this one.  I have seen double pass bench images here:  

 

http://skywatcher.sp.../ak_ronchi.html

 

But the scope I tested is better corrected then this.

 

===

Good website, wow that CFF refractor on his site is terrible...

[peleuba]

Good website, wow that CFF refractor on his site is terrible...

 

Thanks.  Glad you like the tests...   The CFFis not that bad.  Its a very fast, large, oil spaced  APO.  When you have these variables, you need to aspherize the lens to control spherical and color.  The manner in which you do this is to put your thumb on the lens as its spins on the spindle.  Your thumb, with cerium or some other grit puts those rings into it.  Yes, its rougher then I like to see but its due to aspherization and not carelessness.

 

Regarding your setup your best bet is to get a green LED.  

Edited by peleuba, 27 July 2020 - 08:21 AM.

[donadani]

seems to be a very good sample! 

[peleuba]

seems to be a very good sample! 

 

Yes - I think so.

 

One thing I was surprised at - the lens was slightly over-corrected.  I've heard for awhile now, from experienced FS102 users, that these telescopes are typically very slightly undercorrected.  And, indeed, when star testing without filters, the lens certainly appears slightly under corrected.  But this is primarily due to spherochomatism smearing the diffraction pattern with some unfocused color.  It presents itself with a sharper Fresnal pattern inside of focus and a somewhat less distinct pattern outside of focus.  If you look at Suiter's book this indicates undercorrection.  But, for this lens, per my testing, the reality is slight over correction.  This is why Roland Christen laments that for inexperienced star testers Suiter's book can be deceptive - because it glosses over (basically ignores) the effects of spherochromatism in Apochromatic refractors.  Roland is correct.

Edited by peleuba, 28 July 2020 - 02:52 PM.

[Etendue645]

Thanks.  Glad you like the tests...   The CFFis not that bad.  Its a very fast, large, oil spaced  APO.  When you have these variables, you need to aspherize the lens to control spherical and color.  The manner in which you do this is to put your thumb on the lens as its spins on the spindle.  Your thumb, with cerium or some other grit puts those rings into it.  Yes, its rougher then I like to see but its due to aspherization and not carelessness.

 

Regarding your setup your best bet is to get a green LED.  

Hello,

 

I disagree with the use of thumb to apply correction on a lens/mirror. That kind of roughness seen is unacceptable. I have made mirrors and would never accept that rough of a surface on Ronchi screen. Zambuto would agree;

 

https://zambutomirro...oopticalcd.html

 

You will also never see a Takahashi scope with that kind of surface roughness. Smoothness is an important factor in image quality. CFF scopes I would not trust based on the following:  

 

http://astro-foren.c...heit#comment-96

 

and

 

http://astro-foren.c...d-optisch-na-ja

 

If an optician wanted to aspherize a surface then a modified stroke or star lap would be best. A smooth surface is always desirable. 

[Suavi]

Thank you Paul for sharing results of your test. A very neat set-up  , one day I would not mind dipping my toes into optical testing.

[peleuba]

Hello,

 

I disagree with the use of thumb to apply correction on a lens/mirror. That kind of roughness seen is unacceptable. I have made mirrors and would never accept that rough of a surface on Ronchi screen. Zambuto would agree;

 

How long have you been researching aspheric optics?   And, have you ever figured any aspheric lenses?  Just because you disagree, does not make your assertions correct.  I would encourage you to do more research before expressing your opinions as fact.  You are sort of stating the obvious when saying smoother lenses are preferred. 

 

I know Carl.  I have owned 8 of his mirrors and currently own two - in a Portaball and a StarMaster.  You are mixing apples and oranges when speaking of parabolic mirrors and aspheric, oiled  apochromtic  lenses.   While I understand your concern about surface roughness, you are failing to realize that in double pass it makes the errors appear twice as bad.  So, the roughness is on order of about 1/10 wave.  Really - no kidding.  The images you see in single pass on Carl's website in Foucalt are much larger then what is apparent on this lens tested in DPAC.

 

You mention Takahashi...  And I am glad that you did (So is Edif300)...    Takahashi does not apsherize ANY lens they make - there is no need to as ALL TAK lenses are very accurately machine polished, air-spaced and of moderate focal ratios.   You would only need to aspherize a shorter FL oiled triplets.  Again, comparing these to TAK is another apples to oranges scenario.   You simply just cannot compare a sphercial air spaced lens to an oiled aspherized lens.

 

 

As of today there are only two manufacturers who aspherize their lenses - Astro-Physics and CFF.  I would encourage you to communicate with Roland Christen and Pal Gyulai to understand the nuances of aspherization and the methods employed to figure an aspheric.  PRO TIP:  Roland uses his THUMB!

 

 

 

 

If an optician wanted to aspherize a surface then a modified stroke or star lap would be best. A smooth surface is always desirable. 

 

I don't think you understand how a lens is made when you say a "modified stroke".   This is not a mirror.  The only way to efficiently aspherize a lens in a small production shop is to do it by hand.

 

A word to the wise:  I would take any test results you see on the internet, including mine with a grain of salt.  I can show you several test on Rohr's website that are inaccurate.   Ask Yuri Petrunin about the TEC160ED that was stored horizontally and tested out very poor by Rohr.  Then, miraculously the scope, after a few weeks of resting horizontally, tested in the highs .90's   Another Pro Tip:  orientation has no bearing on test results no matter what Rohr says.  It had to do with thermal equalibration...

 

 

Best Regards.

Edited by peleuba, 03 August 2020 - 08:07 AM.

[Jon Isaacs]

Paul:

 

Interesting, nice work.

 

It would have been interesting to see how the lens performed before your tune up. 

 

Jon

[peleuba]

Paul:

 

Interesting, nice work.

 

It would have been interesting to see how the lens performed before your tune up. 

 

Jon

 

Hi Jon - thanks!   

 

The lens was out of collimation so the DPAC test showed the bands to be "keystoned"  They started out OK, then fanned out in a diagonal pattern.  Also, lens was dirty and the dust in double pass made getting a nice photo very hard to do.  The focuser noticably had some image shift, thankfully this was easy to adjust out.  

 

Its a nice telescope and its 17 years old - clearly it had been used, and I think thats terrific!

 

Best Regards.

[Jon Isaacs]

Hi Jon - thanks!   

 

The lens was out of collimation so the DPAC test showed the bands to be "keystoned"  They started out OK, then fanned out in a diagonal pattern.  Also, lens was dirty and the dust in double pass made getting a nice photo very hard to do.  The focuser noticably had some image shift, thankfully this was easy to adjust out.  

 

Its a nice telescope and its 17 years old - clearly it had been used, and I think thats terrific!

 

Best Regards.

Paul:

 

So how much do you charge for your services? 

 

Jon

[Jeff B]

Sensi Paul is indeed correct concerning refractive optic "roughness" .  Here is a DPAC of my CFF 160 F6.5 at focus and in B, G, R inside focus.  It's a beautifully corrected, well balanced lens that gives truly excellent views even on Jupiter's and Saturn's low contrast cloud structures.  

 

Also, speaking of Roland and the rule of thumb, here is the at focus green DPAC image of a 5" F8 pre-ED Starfire objective Roland recently redid for me.  No ED glass in sight but Roland hand, or maybe thumb, retouched it to bring best correction towards the green (from yellow) and rebalance the red and blue.  Does the image look familiar?  A strehl of .985 no less, and it shows at the eyepiece.

 

But this is a diversion.

 

Excellent work there Paul and a great rescue!

 

Jeff

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Edited by Jeff B, 02 August 2020 - 11:25 PM.

[edif300]

Smoothness is an important factor in image quality.

So true. BTW nowadays the main diff factor from the in “China” sourced optics.

[Etendue645]

I apologize to get off topic with my comments on surface roughness. I quite often look at Rohr's website on optical testing in DPAC and when I see surface roughness like that it makes me cringe.

 

I get that an aspherized surface will not be "as smooth" as spherical optics on a machine that can naturally generate smooth spherical optics; if set up properly. I do however believe aspherized optics (mirrors or lenses) can be produced smoothly using star laps, petal laps, and/or with modified strokes.

 

You make up your own decision's on the matter. I personally tend to side with Rohr and his testing experience. I am sure you have plenty of testing experience too based on your posts, and you have clearly shown your opinion on the matter. I will post this final link (use google translate) and image link from Rohr's website and bow down of the rest of this discussion.

 

[Jeff B]

Paul, I copied you top white light montage of the 102 and used my software to split out the R,G, B, channels separately.   This gives me some information as to how close the three colors focus relative to each other.  For this sample, they are very close to each other.  I bet this sample is completely color free visually.  

 

Jeff

[peleuba]

Paul:

 

So how much do you charge for your services? 

 

Jon

 

This was for a friend - no charge.   He uses the telescope - a lot.  A while back ago he was complained that "something was not right".  I was able to have the scope for a period of time to check it out.  Collimation, focus shift and a dirty lens was the culprits.   In fact the rear element, specifically the R4 surface, had a light coating of grease in spots. that gave star images a dim halo.  After cleaning, collimating and adjusting the focuser I decided to test it.  I have never spent much time with an FS102.  These scopes have a legendary reputation but I have a similarly designed Vixen Fluorite that always brings its "A" game so never felt the need for another 102 Fluorite telescope.  

 

The most fun of the whole experience was bringing the finish back.  Another one of my hobbies is paint/gel coat  correction and enhancement.  The paint really pops now.  Too bad I can't replicate the "new car" smell.

 

Your point about charging for the service is a good one...   If someone is just interested in learning about telescope performance and how it all "fits together" I will spend all the time needed as others have done it for me in the past.  And, it exposes me to to telescopes that I would otherwise never get to see - like this FS102.  But, if someone wants their telescope evaluated/tested/cleaned up for the sole purpose of selling it, I will charge for my time.  

[peleuba]

You make up your own decision's on the matter. I personally tend to side with Rohr and his testing experience.

 

Do so at your own peril...     Take online tests with a grain of salt - including my own.  And, tou need to do some additional reading and speaking with folks who actually make aspherized lenses.  I gave you the names to ping in an earlier post.

 

FWIW, in your research - if you choose to do it -  you will undoubtedly come across some Zeiss lenses are not terribly smooth either, but no one is saying anything about their performance or how their rough surface contributes to scatter.  

Edited by peleuba, 03 August 2020 - 09:13 AM.

[peleuba]

Paul, I copied you top white light montage of the 102 and used my software to split out the R,G, B, channels separately.   This gives me some information as to how close the three colors focus relative to each other.  For this sample, they are very close to each other.  I bet this sample is completely color free visually.  

 

Jeff

 

Thanks.  And, thanks for chiming in vis-a-vis CFF.  I thought about PM'ing you, but I saw that you have your plate full with the 11" Zambuto/Parallax.

 

I can send you the RAW images if you'd like, they are quite large, however.

 

Yes, the FS102 is color free visually, but just like my old FS152 when you get even very slightly out of focus you get a strong yellow/blue pattern.