74 replies to this topic

[Jeff B]

My first ever Tak scope, loaned to me by Larry (lwbehney here on CNs) when he was here with his Agema SD 150 (for testing and a video shoot of it).

 

Administrator Scott has DPAC tested one previously:

 

https://www.cloudyni...e-plunge/page-8

 

Larry's sample seems to be a very early sample, serial number 00006.  Does anyone know the time frame associate with this serial number?

 

It has the often maligned single speed 2.7" focuser, but this sample is quite smooth and tight with excellent alignment to the objective's center as shown in the laser test and the cheshire shows just one reflection dot indicating very good alignment of the objective's tilt with the center of the focuser.   I love the deep, rich, coating presentation.  

 

The OTA is very light weight and well balanced.  But I personally dislike straight through finders on smaller apertures, finding them, quite literally, a pain in the neck.

 

A nice, straight forward package.  I find a certain elegance in its simplicity.

 

Jeff

 

 

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[lwbehney]

Hi Jeff. The serial numbers are decoded in this manner:

 

The first two numbers are the last two digits of the year, and the remaining three digits signify the number of telescopes of this line made in that year. So, this particular telescope was the sixth FS-128 made in the year 2000.  

I have always enjoyed the cobalt-blue coating reflections of fluorite doublet refractors. IIRC correctly, the new TEC fluorite doublets have a similar reflection tone. This makes me wonder if fluorite lens elements all use the same coating material compositions. 

[Jeff B]

Another view in double pass and the white light inside/outside and green light at focus images.

 

The objective is over corrected overall.  There seems to be little secondary spectrum judging by the minimal fringing around the shadow bands on each side of the center shadow but there is some blue spherical error as seen by the blue fringing at the tips of those bands.  There is also a very narrow, mild edge roll that you see as "wings" formed by the shadows creeping in from the R/L edges and the very tips of the full R/L shadows, best seen with the inside of focus image.

 

The green at focus image shows a smooth, figure of revolution with a center zone with smooth transitions spanning about the center 30% of the diameter.  I believe this zone accounts for the over correction.

 

Jeff

 

Edited for my comments

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Edited by Jeff B, 06 March 2025 - 02:13 PM.

[davidc135]

The over-correction is noticeable and I suppose shifts the best correction to the red, but does it affect the views?

 

Increasing the spacing a little could make it near perfect.

 

David

Edited by davidc135, 06 March 2025 - 03:35 PM.

[Dean J.]

Hi Jeff. The serial numbers are decoded in this manner:

 

The first two numbers are the last two digits of the year, and the remaining three digits signify the number of telescopes of this line made in that year. So, this particular telescope was the sixth FS-128 made in the year 2000.  

I have always enjoyed the cobalt-blue coating reflections of fluorite doublet refractors. IIRC correctly, the new TEC fluorite doublets have a similar reflection tone. This makes me wonder if fluorite lens elements all use the same coating material compositions. 

Yes, correct.  THis was the sixth one manufactured in 2000.

[Jeff B]

Here are my usual inside/outside of focus color montages.

 

Best correction appears to be towards the orange/red with increasing amounts of over correction moving towards the blue.  I have found this characteristic in other APOs as well and it seems to be preferred by some visual observers when ambient temperatures drop during the viewing session.  As ambient temperatures drop, the cooling of the objective from the edge in, can apply some under correction to the figure, perhaps giving a more neutral figure in green/yellow.  Some folks, me included, can like this for some cooler weather observing.

 

There does seem to be a small amount of secondary spectrum in blue relative to green through red (blue focus is slightly different than that of the red and green).

 

I also include an interesting color montage of at focus images.  I used my white LED and with a deep green filter over the return beam on the Ronchi screen, then, I focused in green light, removed the filter and took the picture.  Using software, I separated the red, green and blue channels for display, just like I do with the inside/outside of focus images.

 

As you can see, the red image is zone free which corelates with the excellent red correction seen in the red inside/outside of focus images.  But I also see a Ronchi screen shadow intruding from the right in the blue image.  This is due to the spherical error in blue but some of the line intrusion may be due to secondary spectrum as well, something I will try to check with the AOS program and post later.

 

Jeff

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[Jeff B]

As the figure is smooth, a uniform figure of revolution with errors that are mostly spherical and the edge roll is tiny, I used my earlier version of Maciek's nice AOS program to estimate the spherical Strehl in green.  Very nice really coming in at ~.96 - .97 ish.

 

As red seems just about perfect, I didn't bother with using AOS as I know it will be ~.99 ish.

 

Jeff

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[Jeff B]

Blue is more interesting.  There does seem to be a bit of blue secondary spectrum relative to green as the only way I could get a good simulation was to also vary the "Dist from ROC/Foc" parameter from that of green.  The out of focus blue, relative to green, will lower the blue Strehl relative to green.

 

Jeff

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[Sacred Heart]

Last year I was lucky enough to look through that scope, Takahashi 128, I found it to be a wonderful scope, excellent views.  I am not as schooled as some of you,  know very little of these DPAC tests,  but like my SD130 on that same night the differences very very subtle.   My opinion, for what it is worth, both scopes are keepers.

 

Joe

[The Ardent]

Here is what I did with the FS128 five years ago . (I can’t believe it’s been that long)

https://www.cloudyni...8-march-part-1/


There is some blue/purple fringing with the brighter stars exaggerated by the camera. This comports with many reviews I’ve read of the 128 and my own experience.

I traded the color-free TSA 120 for the FS128. I did not regret that at all. They are different scopes but both were superb for viewing. Based on this and other comparisons I prefer a doublet over a triplet refractor even if the color correction isn’t perfect.

I also completed about half of the AL Multiple Star program with the 128.

[CHASLX200]

I liked my 128 the least out of all my Taks.

[Lagrange]

I believe the peak (design) Strehl should be around 600nm since that's what it is with the FS-102. That correspond to yellow/orange light which agrees with these DPAC results showing best correction somewhere between yellow and green.

[peleuba]

Hi Jeff - You've had an embarrassment of riches, lately, in scopes to test.     And, I think your correct in the overarching analysis which indicates this lens is is a nice all around performer.

 

My sense, though, in monochromatic green, this is not ⅒ wave lens as AOS would indicate.   See the photos below that I cropped from your AOS output.  Rather, I think this is closer to ⅙ wave and .950 Strehl.

 

I think pure spherical correction could be approaching this value, but the mild zone does have some impact on the overall PtV and Strehl.

 

In either case, its a nice, classic, scope.

 

 

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Edited by peleuba, 07 March 2025 - 09:54 AM.

[davidc135]

The mild central 'zone' is a function of the shorter wavelengths SA, which is why it disappears in red light.  David

Edited by davidc135, 07 March 2025 - 10:07 AM.

[Jeff B]

The mild central 'zone' is a function of the shorter wavelengths SA, which is why it disappears in red light.  David

Agreed.  I find it interesting to have a couple of visual perspectives to show that.  Just one more use for double pass. 

 

Jeff

[Jeff B]

Hi Jeff - You've had an embarrassment of riches, lately, in scopes to test.     And, I think your correct in the overarching analysis which indicates this lens is is a nice all around performer.

 

My sense, though, in monochromatic green, this is not ⅒ wave lens as AOS would indicate.   See the photos below that I cropped from your AOS output.  Rather, I think this is closer to ⅙ wave and .950 Strehl.

 

I think pure spherical correction could be approaching this value, but the mild zone does have some impact on the overall PtV and Strehl.

 

In either case, its a nice, classic, scope.

 

 

Thanks sensei Paul.   There are more in the pipeline too.  

 

Yes, what AOS generates in this case would be the very best strehl with no other warts in the figure, but I was expecting it to be about .95 - ish too.  As there are manual manipulations and iterations of parameters when using AOS, I personally put a tolerance of sorts of +/- .01 on my AOS generated strehls, not bad really.  A very useful tool.

 

That .95 strehl expectation was from the remarkable measured similarity of this FS-128 to my other friend Paul's similar vintage AP 102 pre-ED StarFire, F8, which is shown below.  Its green spherical only strehl comes in close to .95, its blue is ~.7 -ish.  The AP does seem to have a bit more blue spherical but with similar blue secondary spectrum as those of the Tak.  Paul finds the scope quite sharp and color free.

 

Historically, AP's literature at that time did make comparisons of their Starfires to fluorite doublets.

 

I was able to get in a couple hours of observing with the Tak FS-128 last night.  Seeing was pretty good but I was limited to ~150x with my CZAS/Denk OCS/Denk power switch/UO 18MM orthos bino-viewer set up. 

 

I saw no astigmatism or coma associated with Castor or Pollux, with round, white-ish stellar cores with no blue/purple fringing, just the usual tint differences between the inside/outside of focus rings.  The glides into and out of focus were round and even.  Even without a green filter, spherical seemed fine.  I did find I had to be rather precise with focusing, which was a little difficult as the single speed focuser's action was stiff with the cold weather.  I can see why some owners go for the 3.0" FT conversion.

 

The lunar "X" was easy with the no fringing seen at all on brightly lit crater rims or mountain peaks just getting sunrise, detail was sharp.   Jupiter was superb (!) with "rich" colors and quite sharp too.  I neglected poor little Mars.

 

Now I have to also say bino-viewers typically disturb and filter the blue to a degree, something I've documented here on CNs.  However, the CZAS viewers have, by far, the least such disturbances, especially visually.

 

I'm now going to attempt an indoor star test of this scope.

 

Jeff 

 

Edit:  changed "not" to "now" in the last sentence...a big difference 

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Edited by Jeff B, 07 March 2025 - 11:42 AM.

[Jeff B]

Here are the green AOS images for both.

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[RichA]

Another view in double pass and the white light inside/outside and green light at focus images.

 

The objective is over corrected overall.  There seems to be little secondary spectrum judging by the minimal fringing around the shadow bands on each side of the center shadow but there is some blue spherical error as seen by the blue fringing at the tips of those bands.  There is also a very narrow, mild edge roll that you see as "wings" formed by the shadows creeping in from the R/L edges and the very tips of the full R/L shadows, best seen with the inside of focus image.

 

The green at focus image shows a smooth, figure of revolution with a center zone with smooth transitions spanning about the center 30% of the diameter.  I believe this zone accounts for the over correction.

 

Jeff

 

Edited for my comments

Beautiful.

[Scott in NC]

Very nice, Jeff! And thanks for linking back to the test that I did of mine a little over 2 years ago. When I did that test I only had the capability of testing in green, and my technique has improved quite a bit since the early days.  I really need to DPAC test my FS-128 again in R, G, B, and white one of these days.

[Scott in NC]

I was surprised to see the amount of CA that your DPAC test shows. In actual use I typically see very little, even on challenging bright targets like Sirius and Venus.

[peleuba]

I was surprised to see the amount of CA that your DPAC test shows. In actual use I typically see very little, even on challenging bright targets like Sirius and Venus.

 

These classic TAK doublets don't size up gracefully when considering color correction.   In all fairness, the camera is way more sensitive to color then the eye.

 

Here is a star test of an FS152 I did some years back.  The scope had to be perfectly focused for me not to see objectionable color.  Even so, the infocus image was very pleasantly warm in tone and the spherical  correction was dead nutz on.

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Edited by peleuba, 08 March 2025 - 11:46 AM.

[Joe G]

What did you use to take those pics?

[lwbehney]

The mild central 'zone' is a function of the shorter wavelengths SA, which is why it disappears in red light.  David

I would agree; I can see the red tint of Castor C and Barnard's Star with this FS 128 and the carbon star V Hydrae has a fantastic deep red color. 

[PKDfan]

These classic TAK doublets don't size up gracefully when looking at color correction. In all fairness, the camera is way more sensitive to color then the eye.

Here is a star test of an FS152 I did some years back. The scope had to be perfectly focused for me not to see objectionable color. Even so, the infocus image was very pleasantly warm in tone and the spherical correction was dead nutz on.

Aperture is merciless on chromatics.

That figure looks ideal Paul so its performance must have been literally out of this world.

Spherical is the 2nd most important aberration to solve and chromatics a very distant third.

My 62mm Evo has none out-of and my 100ED the very slightest of violet and yellow on either side.

That FS-152 looks exactly like my F/6.8 70mm semi-ED Pronto whose SA was also perfect as is both my present scopes with the 100ED level just astonishing.


And Great work Jeff !

I could also use your services one day- hopefully.


CSS
Lance

[peleuba]

Aperture is merciless on chromatics.

That figure looks ideal Paul so its performance must have been literally out of this world.

 

 

Hi Lance,

 

It was the scope that showed me what 6" of unobstructed, well corrected, aperture could really do.   I could, somewhat routinely, watch Galilean satellite transits.  I am not speaking of the shadow transit, rather the actual Galilean moon go across the face of Jupiter.   Without good seeing and a high contrast objective, this is impossible.

 

 

Jeff, Scott and I could test your scope.  Choose whoever is closest.