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FS-128 vs TEC140 vs synta w/chromacorr

Started by adamsdp, 03/25/2004 04:46PM
Posted 03/25/2004 04:46PM Opening Post
On my quest for the "scope of a lifetime", I think I have narrowed the choice to a Tak FS-128 or TEC 140. I have compiled the following information and am hoping for comments.

Tak FS-128

Advantages: lower weight 16.5 vs 18.8 lbs for TEC140, flourite doublet- faster cooling?, air-spaced- don't have to worry about oil leaking or oil degrading over time, less expensive on used market, dew shield easier to move,
not sure if available in shorter binoviewer version.

TEC 140

Advantages: triplet - slightly? better color correction,
10 mm greater apeture, better focuser - smoother, fine focus knob and rotatable, can specify tube length 80mm shorter for use with binoviewer w/o compensators for no extra cost.

I haven't heard many people buying synta's matched with a chromacorr so am thinking there performance must be a notch below a true apo like a TAK or TEC but would welcome any comments on this. I saw that APM used to sell 120 mm synta's matched with chromacorrs for ~1,500 or so.

I thought a 2 element air spaced lense would cool faster due to less mass up front but someone mentioned to me that the air space acts as a thermal insulator so that the inside lense takes a long time to cool. Has anyone compared the cooldown times of an oil spaced triplet to an air spaced doublet?

Also, on another list, someone mentioned that the oil in a triplet can degrade over time affecting the performance of the scope. The degredation mentioned was not great but enough that owners of some oil spaced triplets payed to have them refurbished with new oil. Has anyone heard of this issue?

Dave Adams

Posted 03/26/2004 03:22PM #1
Hello Dave,

What do you mean by "dew shield easier to move" on the FS128? It threads on and off, but otherwise cannot slide or adjust. It cannot reverse for small storage, like an AP, or slide to a small overall profile like the TMB or TEC140's.

The FS128 is pretty long, certainly longer than the 140 f/7, especially in an 80mm shortened version (which is what I would get). I have heard Texas Nautical can adapt FS128 into a shorter binoviewing configuration, probably for bucks of course.

Color correction comparison between the FS128 and TEC140 is hard to know just from online reading. Yuri has said he left a little bit of color in far blue/violet in exchange for better overall correction. Tak doublets also have some far blue/violet. Maybe they are close in color correction? I dont know.

I've owned oiled triplets, air spaced triplets and air spaced doublets here in Alaska for 4 years and I have seen no practical differences in cool down time. However, I always let me scopes acclimatize for about an hour before I uncap them and begin observing. A nice kind thing to do to your precious optics anyway, right? I dont think cooldown comparisons are worth much.

I have owned a Synta 6" f/8 with a Chromacor, and indeed, photography with it showed incredible color correction! Just amazing considering the cheap Chinese objective. See comparison here:
But visual use was another matter, the true APO scopes are a pleasing experience the Chromacor would never match for me. As your head or eyes move around you can see color effects in the Chromacor. With the APO, it's all corrected, all the time, gorgeous. YMMV, IMHO, etc. I sold the 6" f/8 and Chromacor once I used a true APO, the TMB 100/8, which was absolutely color free to my eyes.

Posted 03/26/2004 04:25PM #2

Hi Dave!

TEC-140: I've used one of these for about a half
hour. I was just incredibly impressed with the
star test and mechanics.

I've also done the 6" Synta + Chromacor exercise.
Took a while to get everything matched up properly,
but I've been very happy with the result. I've
put it up against an AP155 side by side on planets,
and details visible were virtually the same.
Not bad considering the modest $1K investment
(we won't count the hours star testing and
adjusting the Synta...). Of course detailed star
test is better in the AP and theres no comparison
for build quality. But even so, the price difference
is hard to ignore.

I have seen examples where triplet APOs were
challenged by thermal issues. I once had a
4" F15 Unitron achro side-by-side with an
AP Traveler 4" oiled triplet. A front had come
through and the temperature was falling all night.
The Unitron gave superb images on Mars, but the
Traveler just never reached equilibrium and was
far inferior -- not even close. Thin lens in
light tube vs. thick triplet in short heavy
tube. But I'm sure this doesn't happen often,
and is nothing to worry about.

I expect the oil in current scopes is extremely
stable. I think the ones you hear about are
older Zeiss scopes.

- John B.
Posted 03/26/2004 08:29PM #3
While on the topic:

could the new Vixen Sphinx support the TEC 140 for visual?

any thought

thanks howard