Flamsteed Astronomy Society

TMB130 refractor — A user review

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TMB 130 F6 Triplet Apochromat Refractor with new black trim — by Nick Witte-Vermuelen

Specifications:

Aperture: 130 mm

Focal length: 780 mm

Focal ratio: F/6

Tube weight: 20lb, 9kg

Transport length: (retracted dew cap) 620mm

Operating length: 740mm

Focuser: 3.5" Starlight Feathertouch, travel 113mm, rotating, 50mm finder-scope bracket   Tube-ring set weight: 2.2 lbs, 1 kg

After doing a side-by-side comparison with a friend's 6" TMB APO and my Intes Micro  7" F/15, I caught the refractor bug. I decided to get myself the largest aperture APO refractor I could practically use for planetary observing from my balcony in London, yet be portable enough to take away to deep sky sites too - inside a car boot with other luggage.  Why a refractor? The Intes Micro 7" F/15 was very good when collimated and cooled.   It performed just as a 6th wave 7" with 25% obstruction should, and definitely out-performed several big name Cassegrain telescopes of larger aperture on Saturn.   Thing is, it just didn't have that wide field too, although the planetary contrast was the highest   I'd ever seen on a Cassegrain telescope. I did  consider a quartz optics 7" Maksutov  Newtonian (which on a night of good seeing IMHO outperformed a 6" APO on Jupiter), however the size, awkward observing position, high weight, cool-down issues, IM focuser  and general ergonomics just put me off - as I'd never get to use a scope this big regularly.  In any case, I wanted an un-obstructed telescope which looked like a telescope should.

I know there's a lot of debate about the whole issue of central obstruction vs. clear aperture but in my experience, refractors of similar size always gave better visual impact because of that extra contrast and wide field - even when the resolution was less. The 2 Maksutov’s mentioned above are probably better value in most scenarios, but I had the “refractor bug" by then, and that was that.   So after agonising for ages, here were my criteria for the new scope:  - must have a clear aperture for contrast and a wide field  - must be able to cool down in a reasonable time, within 45mins  (the M715 cools down in this time with the fan running)  - must be more or less the same size and weight as my previous scope  - must have a wide field or view, yet also be a top notch planetary performer  - Price limit of £4.5K  - 5" or more aperture, which I consider the minimum for my planetary observing needs  - Not too long, must fit into a small observing space, easy to transport  - Little or no colour whatsoever, and proven high optical quality

 

I considered a couple of telescopes in the 5" APO triplet range, and ranked them with a few pros and cons:

1) Astro Physics 130mm F/6 - fulfilled most criteria except for the micro-focuser, extremely light (how do they do it?), nice portable size. 5 year wait, or pay a couple of grand over the retail price to someone who's (astro-lottery) number had come up with AP. I have one on order anyway, as well enough AP kit to bankrupt me if it all comes at once.

2) TMB 130mm F/6 in new CNC tube - smallest of the lot (620mm), but quite heavy, at over 23lbs with rings. Starlight Feathertouch focuser and an interferometer test report showing the exact structure & performance of the lens for that little bit of extra reassurance! Beautiful tube design - yes I know cosmetics have nothing to do with performance but they are simply the best looking scopes around other than Taks.

3) TEC 140mm F/7 - a more reasonable weight-for-aperture ratio than the TMB. Starlight Feathertouch dual-speed focuser. A bit large and reaching my "limits" of portability.  1-2 year wait payment in advance if TEC and the UK prices were a grand too high.  No interferometer test report, but a calculated Strehl of 99%.

4) TMB "lightweight" version of the 130mm F6 - I decided I just didn't want the TMB  lightweight tube assembly as it was longer, and to be honest, it didn't look half as  nice as the CNC tube (OK, call me a snob!)

 

I really had my sights set on the TEC 140 at first, but then I found out it was a  barebones deal without all the accessories which are normally part of the scope  package to get started (£4,600 without), I couldn't justify paying £1500 more than  the £3,500 TMB130. Especially for 10mm of extra aperture. Seeing as the TMB was smaller, and came with an extension tube, CNC rings and finder mounting, my mind was 75% made up. The fact that the TMB is an F/6 also made it quite attractive for imaging. Anyway, that money saved could be used for another project like finishing off the 14" F/5 Dob or high end eyepieces.

 Before placing the order, I made some enquiries with other amateurs who have had experience with these two scopes. Well it's hard to find folks who have used both but after some trawling on groups and chats with folks at the Kelling Heath Starparty I found two people. They went through the same process as I, and thought that the difference in planetary performance was pretty insignificant, and not really worth it.  They both preferred the colour correction of the TMB130 at high powers, its portability, and most significantly, they both actually ended up with the TMB130 CNC tube and a “lightweight" tube version respectively after having gone through virtually the same decision process. My mind was now 100% made up.   So I called APM and ordered it on the poor old credit card (ouch!)

 

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