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FS-128 vs TEC 140 vs Achro w/chromacorr

Started by adamsdp, 03/25/2004 04:43PM
Posted 03/25/2004 04:43PM 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

TEC 140

Advantages: triplet - slightly? better color correction,
10 mm greater apeture, better focuser - smoother, fine focus knob and rotatable.

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 12:30PM #1
The chromacorr is a doublet lens designed to correct the chromatic aberation (essentially turning an achromat into a petzval 4-lens system). It should do considerably better than a minus-violet filter (it is supposed to actually refocus the chromatic abberation back into the image, not just block it).

From the reports I have read, the chromacorr does a pretty good job, but not quite as good a true APO, since in a true APO the lenses are designed and tested together as a single system, which generally means tighter tolerances. There are some reports on Cloudy Nights you can read. If you go this route, I would certainly order a matched scope and chromacorr from Markus, rather than try to estimate the spherical abberation yourself.


Posted 03/27/2004 01:54AM #2
Dave, I've had this baby since 1991. No spurious color that I can detect at 50+ power per inch and no oil leak issues. Dom Q.

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