32 replies to this topic

[Scott in NC]

I just received a "new" TMB 100/800 a few days ago and finally had a chance to put it to the test last night.  A few years ago I posted a comparison of my prized Takahashi FS-102 vs. an Astro-Physics 105mm Traveler. As nice as the AP Traveler was, that scope is long gone, but the Tak remains in the fold.  And my decision to keep the Tak in lieu of the AP had nothing to do with any issues regarding the optical quality of the AP, but more that I found the Tak a better value proposition for my needs. I had such a good time doing that comparison that I decided that the Tak would be the perfect scope to put my new TMB up against.  Their optical prescriptions may be quite different, but given their similarities in aperture and focal length (and mainly due to the fact that they're my only 4" refractors), I really wanted to see how the new TMB stacked up against my tried-and-true Tak that I've now owned for nearly 9 years.

 

First a little background on the two scopes. The Takahashi FS-102 is a 102 mm, f/8 doublet apochromatic refractor with an outer calcium fluoride element, inner mating element of unknown glass, and 820 mm focal length.  My particular sample was manufactured in 2001, and having been produced prior to the last iteration of that scope (the -NSV version), has a fixed dew shield.  While I still have the original focuser, the scope currently is outfitted with a FeatherTouch FTF3035 3" R&P.  The TMB 100/800 is a 100mm f/8 triplet apochromatic refractor with an ED element of OK4 glass produced by LZOS in Russia, and unknown mating elements (probably known by some here, just not to me).  My TMB sample was created in 2006, and was reportedly one of the original ones that Thomas Back personally checked over before it was shipped to its first owner.  It’s outfitted with a very nice FeatherTouch FTF3545 3.5" R&P focuser, which although quite bulky, is probably my all-time favorite focuser.

 

For last night's "shootout" I set the TMB up on a DM6 mount and Berlebach Planet tripod, and the Tak up on a DM4 mount and Berlebach Uni-18 tripod.  Both scopes were outfitted with Astro-Physics 2" Maxbright dielectric diagonals.  I used Ethos 4.7-21mm eyepieces with the TMB and Pentax XW 5-30mm eyepieces with the Tak.  If this had been a really rigorous review, I would have made an attempt to use the same eyepieces with both of the scopes, but my intention was also to have fun observing, and I really didn't want to have to keep switching eyepieces back and forth between the two scopes.  

 

Here's a photo of the two scopes cooling down awaiting nightfall.  The sky was mostly clear, but more gray than black, as the full Moon washed the sky out enough that it wasn't worthwhile looking for any but the brightest DSOs.  Fortunately the Moon stayed below my eastern tree line for the entirety of my nearly 2-hour observing session last night.  The only two planets visible were Venus and Mars.  Seeing was so-so, not perfect, but not bad either, and about average for my area.  

 

 

 

[Scott in NC]

The business end of both scopes (OK4 in the TMB, and fluorite in the Tak):

 

 

[Scott in NC]

The first object that I viewed with both scopes was Venus, in approximately 60% gibbous phase.  At -4.2 magnitude, this is a very harsh test for chromatic aberration.  Despite that, in focus I saw very little CA with either scope, and I'm not entirely convinced that most of this wasn't simply atmospheric dispersion rather than true CA.  And to be fair to the TMB, this triplet with a heavy lens cell takes a bit more time to thermally equilibrate than the Tak doublet with its simpler, lighter lens cell.  I saw a fair amount of light scatter around Venus, and I'd judge the level of scatter to have been similar with both scopes.  Perhaps a prism diagonal would have cleaned this up some, but I didn't think of trying that last night. Despite this, I didn't see anything overly objectionable with either scope, and frankly Venus isn't one of my favorite objects to view anyway due to its brightness.  I did not get the opportunity to recheck Venus later on in the night after the scopes had both reached equilibrium, as by that time Venus had already set below my western tree line.

 

Next up was Mars, and at magnitude 1.3, CA was essentially nonexistent with either scope.  Maybe I could convince myself that I saw a tiny bit of it if I cranked up the magnification and really went out of my way looking for it, but who really does that unless they're rigorously testing a scope and trying to find faults that don't really exist?  I was surprised that I could make out a very small amount of surface marking on this diminutive planet at 5.3" angular diameter, and that little bit of albedo would tend to come and go with the seeing.

 

As tonight was clearly not a night for galaxies given the rising full Moon, I spent much of the next couple of hours looking at some of the brighter DSOs, double stars, and the carbon star X Cancri.  My favorite object was probably Porrima (Gamma Virginis), a bright double with relatively equal magnitudes, and 4.1” angular separation.  By this time the TMB had properly cooled, and I was treated to what looked like two small, perfectly round "headlights" with faint surrounding diffraction rings, and an inky black space between the two. This was the absolute best view that I've ever had of this pretty double.  Porrima looked very nice in the Tak as well, just not quite as sharp and well-defined.  In fact, this was the only object which I can clearly state looked more pleasing in the TMB than in the Tak.  Having said this, had I not had the two scopes side by side, and had just been relying upon memory from observations performed on different nights, it's possible that I might not have recalled a significant difference.

 

On star testing using a fairly bright star (Procyon, at magnitude 0.4), I could detect no significant CA with either scope while in focus.  Inside and outside of focus the Tak exhibited the expected level of CA for a well-controlled doublet, and the TMB exhibited none to my eyes.  I saw no evidence of astigmatism, zoning, or objectionable spherical aberration, but as I've said before, star testing isn't really my forte, and I've continued to experience difficulty noting any but the most glaring defects on the star test.  That's one reason that I've learned DPAC testing, but that's another story for another thread.  The inside/outside focus Fresnel ring pattern was much closer to identical on the TMB than the Tak, with the pattern on one side of focus with the Tak (I forgot which side was which) showing a little more "smearing" due to violet-colored CA than on the other side. Still, the star testing pattern on the Tak was quite pleasing, and was what I typically expect from a well-corrected doublet.

 

Overall this was a wonderful night despite the presence of the full Moon, and I really enjoyed getting the chance to test out two high quality apochromatic refractors.  I guess you could say the TMB "won" the shootout, but really there wasn't a huge amount to distinguish the two scopes optically, and frankly I'm amazed that the Tak came so close to the TMB despite being a mere doublet. And in case you're wondering whether the Tak might have come close to the TMB because I have an exceptional sample of the Tak and a suboptimal sample of the TMB, my DPAC test results suggest otherwise (I can share that later if anyone is interested). The Tak is probably a better value proposition, as many more of them were produced, and I believe the TMB (if you can actually find one nowadays) has risen quite a bit in value due to its rarity.  The Tak is also lighter, and I imagine that if it had been wintertime and I only had an hour or so to observe, I'd probably rather be out with the Tak due to its quicker thermal equlilbration time.  I'm really pleased to have had the opportunity to own both scopes, and although my long-term keeper Tak probably gives more "bang for the buck," the TMB was a wonderful find, and is not going anywhere anytime soon.  

 

That's all that I recall from last night's observation session right now.  I thank all of you who have read through this lengthy report, and if you have any questions, please just ask!

[CHASLX200]

The 100/800 i took a peek at the sun with had to be about the sharpest fract i ever used. This was at a star party back around 2000.  I always wanted to buy one.

[bobhen]

Nice write up. Very enjoyable read.

 

However, winning is winning. Even if close, it might mean the difference between seeing something and not seeing it. Also, I would expect the differences to be slight because these are both 4" scopes and both of high quality. I would not expect any differences to be all that great. But better is still better.

 

I think the Tak TSA 102 would be another story. It would also be interesting to see how the TMB, Tak TSA and new Tak FC 100dz would compare. 

 

Yes, I would hang-on to that TMB. LZOS optics are excellent. I owned a LZOS 105 F6.2 triplet and it also had a superb lens. 

 

Bob

[Scott in NC]

Thanks, Bob.  While the TSA-102 is also on my "wish list" to try out one day, I'm afraid it won't be anytime soon.  That nasty "money" thing always seems to get in the way, lol.  And I've been curious about the LZOS 105mm f/6.2 for a long time, but the TMB/LZOS 100mm f/8 really fits in better with my observing style (purely visual, no imaging).  I'd wanted to try out a TMB 100/800 for over a decade, and I feel very fortunate that one just happened to come my way.

[Kitfox]

Scott, great write-up. Tak fluorite and a LZOS lens Mr. Back touched himself is a great start to a 4” collection.  I get the sense these will stay in your hands for a long time!  

 

TBH, that TMB is one I wish I had beaten you to!    And yes, I think a few of us here would love to see the boring DPAC testing images to drool over, if you are willing to share. Maybe continue this comparison thread with a little time showing how they each do on the dining room table?

[Scott in NC]

I’ve previously posted the DPAC data from the FS-102, but I think I only had the capability to test in green light back then.  I’ll need to redo it in white, red, and blue one of these days.  Alas, my optical bench (a.k.a., dining room table) is currently decorated for my mother-in-law’s 84th birthday party tomorrow, and my wife has strictly forbidden its use!

[Scott in NC]

Fortunately, I’ve already DPAC tested the TMB 100/800.  I may not have had the chance to observe with it until a week after receiving it, but of course I DPAC tested it the day it arrived!  Let me go find those images.

[Scott in NC]

TMB 100/800, Green inside focus:

 

 

At focus:

 

 

Outside focus:

 

[Scott in NC]

Red inside focus:

 

Outside focus:

 

[Scott in NC]

Blue inside focus:

 

 

Outside focus:

 

[Scott in NC]

White inside focus:

 

 

At focus:

 

 

Outside focus:

 

 

[Kitfox]

Booooorrrrriiiiinnnnnggggggg…   My eyes can’t focus blue, but that scope sure can!
 

You have my number, let me know before you sell it 

Edited by Kitfox, 06 May 2023 - 11:32 AM.

[Scott in NC]

To sum it up, with the TMB 100/800 we see near-perfect spherical correction in green, almost the same in red, and slight fall-off in blue. Frankly though, I think that most of us would be happy to have a scope that tested as well in green as this one does in blue, and these results seem to correlate very well with the scope’s reported Strehl of 0.989.  The white light test results show very nice control of chromatic aberration.  

 

Even though I had to wait a few days to get to observe through this scope, I was thrilled to get these DPAC results the day that it arrived, so had no doubts that it would turn out to be an excellent performer under the stars.

[Scott in NC]

Booooorrrrriiiiinnnnnggggggg…   My eyes can’t focus blue, but that scope sure can!
 

You have my number, let me know before you sell it 

LOL.  I like boring DPAC results.

[Bill Barlow]

Nice comparison, Scott.   Maybe try another when the moon is not up in the sky for some deep sky objects for sharpness and contrast.  The TMB DPAC analysis shows it is an excellent lens for sure.

 

Bill

[Scott in NC]

Maybe try another when the moon is not up in the sky for some deep sky objects for sharpness and contrast.

Yes, that’s certainly my plan. But I’m pretty sure that the next new Moon, it will either be raining or I’ll be working or have other obligations. That’s typically how it works.

[Alan S]

Yes, that’s certainly my plan. But I’m pretty sure that the next new Moon, it will either be raining or I’ll be working or have other obligations. That’s typically how it works.

Ship it to me (I'll pay the shipping).  I'll be at a dark sky in 10 days and will evaluate it for you on a myriad of targerts.  And just to be safe, I'll hang onto it until you come pick it up in person so we can avoid shipping it twice

[Erik Bakker]

Great report Scott.

 

Many years ago, I compared my Vixen Fluorite FL 70 and TMB 80/600. While the TMB looked better corrected for CA, color saturation and contrast went to the fluorite doublet. The TMB triplet looked color free, but a bit milky/hazy in direct comparison. Not quite as saturated or contrasty. Spherical correction was about perfect on that TMB. Only my Tak FS-102 NSV paired the level of spherical correction of that TMB and the superb contrast and color saturation the 70mm fluorite doublet to a somewhat higher level of overall detail-richness.

 

Would be interesting to see how your Tak FS and TMB 4" compare in contrast/color saturation.

[Deadlake]

The f6.2 does fold up quite small, however I suspect the f8 is better balanced and slightly better Strehl, not that I can source one to compare. 
Edit. Well I can, see https://astrograph.n...s/AG0800LZAPOHS however hard to justify.
 

This one has custom printed tubes from TS in Germany, brings the weight down as the ring weigh around 120 gram each. 

The Strehl is around .98, but since LZOS tested in the green line I do not feel the need to DPAC test it. 

Amazing views for wide field if the cloud would give up...

One question I have for the OP, would you swap the LZOS for the AP 105 mm Traveller?

Edited by Deadlake, 06 May 2023 - 01:14 PM.

[dawnpatrol]

Very nice writeup, Scott. I'm looking forward more as you get more time at the eyepiece of the TMB and under moon-less skies. We should all be fortunate enough to have scopes with such boring DPAC results.

[CHASLX200]

I think the one i took a peek thru was like my CNC 105/650 Version with the gray paint if i remember right.

[Scott in NC]

 

Would be interesting to see how your Tak FS and TMB 4" compare in contrast/color saturation.

I didn't notice any significant color differences between the TMB and the Tak when looking at the carbon star X Cancri, or when looking at Iota Cancri, the "spring Albireo." Of course, your eyes might see things differently from mine.

[Scott in NC]

 

One question I have for the OP, would you swap the LZOS for the AP 105 mm Traveller?

Great question, and one to which I'd say, no, not at this time.  I've owned an AP Traveler before and compared it against the Tak FS-102 (report from 2019 here), and I couldn't tell much difference optically.  The main reason I'd look for another Traveler would be if I ever needed something much more portable than the FS-102. Frankly though, the FS-102 is light and portable enough that I don't wish to pay the price that Travelers are currently going for just to gain that extra bit of portability.  And the current market rate for a Traveler now seems to be about $1000-1500 more than I sold mine for just a little over 3 years ago. I'm not saying that they're not worth it, just that I personally don't value the increased portability factor enough to want to pay the going rate for one.  My recently purchased TMB did cost quite a bit more than I paid for my Tak 9 years ago, but still cost quite a bit less than the Traveler did 4 years ago. Having said that, if someone offered to trade me a mint Traveler straight up (without any extra cash involved), I'd have to think carefully about that.  Sure I could flip the Traveler and make some money on the deal, but then the likelihood of my ever finding an original TMB 100/800 in this nice condition ever again would probably be slim to none.  So I don't think I'd do that.