TMB 130 with CNC Tube

TMB130 Refractor with new "black trim" CNC tube

Specs

Aperture: 130 mm
Focal length: 780 mm
Focal ratio: F6
Tube weight: 20lb, 9kg
Transport length: (retracted dew cap) 620mm
Operating length: 740mm
Tube-ring set weight: 2.2 lbs, 1 kg
Focuser: 3.5" Starlight Feathertouch, travel 113mm, rotating, 50mm finder-scope bracket

After doing a side-by-side comparison with a friend's 6" TMB APO and my Intes Micro 7" F15, 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" F15 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 con's:

1) Astro Physics 130mm F6 - 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 F6 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 F7 - 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 F6 also made it quite attractive for imaging. Anyway, that money saved could be used for another project like finishing off the 14" F5 Dob (now for sale on this forum ) 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, it's 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!)

Here's are my impressions of observing with the TMB 130 F6. I'm not the type to keep notes, but certain memories stand out, and they're the ones to mention:

Bright stars: I don't look at stars much but of course I've looked at Vega, Spica and all the usuals to check out the optics. Racking the focus back and forth, there is no colour visible in the airy disk, which is perfect. See my notes about binoviewers later on though.

Deep sky sights:

Needle Galaxy - fantastic view at low powers, the central dust lane very obvious with direct vision. Really nice view putting the object in context with it's surroundings.

M13 - stars visible right to the core with perhaps a little averted vision needed, fine very sprinkling against a nice dark background, just so with M3.

Dumbbell - very well formed "keyhole" visible with mottling inside, overall nebula well formed with indirect vision.

Ring Nebula - very well formed and very punchy especially with the IDAS Narrowband Nebula photo visual filter.

Pleiades - from "Deepest Darkest Devon" (thanks to Graham for allowing us to enjoy your wonderful farm). Incredible wide view with the dusty nebulosity around the brighter stars immediately obvious. You could stare for ages at
this with the Pentax XL's.

Orion nebula - simply amazing for a 5" scope. I really couldn't believe this was a 5" I think that the contrast in a refractor really brings out deepsky detail. Even with the Denkmeier binos which dim the view a bit, I actually saw more detail, with an illusion of artificial depth to the "cusp" and "batwing" shape of the nebula.

Saturn: - Much better contrast than any Cassegrain I've ever looked through. There is almost no "haze" around the object, as the tube is well baffled all the way through, so one can pick up moons easily. Saturn easily shows off several bands of varying gradations and I think I've seen 2 layers around the polar cap. A lot of subtle colour visible for a 5" aperture.

Very crisp ring shadows and Cassini division. Crepe ring is visible. Saturn's globe really looks 3D, and hangs in space against a very dark background. I have used this scope alongside an 18" Obsession which clearly showed the Encke division that morning and although I suspected a darkening, I could not see it with the 5" aperture and suspect this isn't possible.

Jupiter: - Faint features are much more visible, and are a lot more contrasty than in my previous scopes. Zones and belts (and the GRS) are immediately visible and very separate at low powers, with ragged edges turning into swirls and so-on as I increase the power. Circulation "curl" patterns in the Great Red Spot are visible and the outer gap between it and the South Tropical Zone is easy at lower powers. Festoons, barges and swirls are all easy to see, along with regular sets of ovals after the GRS (up to 6 in a row) caused by turbulence trailing the GRS. I've suspected white ovals North of the NTZ and the STB southwards once or twice with peripheral vision. Jupiter's moons are easily visible as discs in decent seeing. I have seen 2 moons next to each other, and I recall their different size and colour were clearly visible and quite striking. Satellite shadows are jet black like holes through the planet. I can follow most satellites across the disc but I have lost them sometimes in the middle. This so called widefield scope
is brilliant on Jupiter and planets. Can't wait for Mars!

Power (and over use of it): The scope performs well at up to about 280x on bright objects like planets. You can push the power higher with little image breakdown, but as the image becomes bigger, it also becomes much dimmer, so there is no real point in exceeding 250x with this or any other 5" scope. I can't see any more detail simply because the image becomes dimmer the higher the power from here, and it's practically at the resolution limit for this aperture in any case. The old 50-60x per inch rule seems valid, and this has nothing to do with optical quality, but aperture alone.

Optics: The optics on this scope are perfect. Star images are the same inside and outside focus as far as I can see with concentric diffraction rings and very little or no colour in the out of focus airy disc. The diffraction rings collapse perfectly into a star and remain concentric all the way until the star gets to it's smallest point. Stars are tack-sharp.

Use with Binoviewers: Add binoviewers without the corrector, and they will come to focus, as this scope has plenty of in travel. For low/medium power use, don't bother with the binoviewer "correctors". For those who are obsessed with colour or absence of it, there is some red or blue colour if out of focus without the corrector lenses at high powers. This is perfectly normal at this fast F6 focal ratio with prisms, however as soon as you're in focus the colour disappears. It really is a testament to how will this scope is corrected that there is no in-focus colour with uncorrected "raw" use of a binoviewer. I use Denkmeier 2's with an Astro Physics "BARCON" Barlow screwed into the front to increase the F ratio, and bring me up to planetary powers, and one can see that this removes just about all colour inside or outside focus. In the Denk II's, Jupiter and Saturn have to be seen to be believed, and are textbook, with high contrast details in Jupiter and Saturn very visible at about 250X.

How it handles poor Seeing: According to many aficionados, small refractors are not as affected by bad seeing compared to centrally obstructed or large aperture scopes. I live in London, and while the seeing can be surprisingly good on occasion generally it's 6 out of 10. My best views have been in Devon (thanks Graham G) and Kelling Heath in Norfolk. I think this scope handles average seeing much better than my 7" Mak did. I'm reasonably sure, though I admit it's not a objective thing. The real benefit is there are no visible tube currents, or currents formed by boundary layers on the optics. I suspect the oversize tube with walls further from the light path, plenty of baffles, plus the fast focal ratio, and lack of a light path all help to reduce thermal issues.

Focuser: The Starlight Feathertouch focuser is great, plenty travel (about 5 inches) very stable, very precise and I especially like the fact this it uses a self centering clamp for the 2" visual accessories. This has a knurled outer collar which is big and very easy to grip with gloves. No screws to come off in your hand or swinging diagonal problems you get with brass inserts and no de-centering of the optical train. By the way, I was interested to see the exact same focusers on the Royal Greenwich Observatory 28" Refractor.

Dew control: The scope dewshield is black flocked (a nice touch to improve contrast). It doesn't really stop dew if the humidity is high, so if you don't have a dew heater the next best thing is a decent flocked dew shield on top of the telescope one. No problems using the Kendrick 5" heater strip wrapped around the base of the telescope dew shield. The lens cell and optics absorb the gentle heat without the need to turn the duty cycle on the Standard Controller over 50% on most nights. I've had the whole scope dripping wet but the optics completely dry in Devon when dewing was very high.

Mounting: I've had a Losmandy GM8 which was perfect for this scope, recently upgraded to a G11 Gemini as I want to add another OTA, which unsurprisingly also works great. I considered a Vixen Sphinx but it won't take the weight of the scope at around 25 pounds with binoviewers, eyepieces, finder, tube rings etc.

Criticisms:

- Weight, it's a bit on the heavy side compared to the AP, OK they're hard to compare as the design philosophy is different. To be fair, overall the TMB just has a beefier aesthetic. To be unfair, it would be nice to see these optics in a light tube which also looks nice.

- Baffling - there is plenty of baffling internally and it's all black edged, expect for one ring which has a shiny metallic inner edge. I'm not sure if this is an issue and the scope's performance certainly isn't affected, but why leave a shiny edge on it when the rest is perfect?

- Objective cap - the black paint is low quality, and comes off very easily, it would have been much better to anodise this or use paint which sticks better.

No serious issues, I really had to think about these and I'm nitpicking here!

Well that's about it. It's a great telescope, definitely the best quality instrument I've ever owned. It simply gives better images than a lot of scopes which are considerably larger and that's the quality you pay for.

Please APM, scale this design up and do us a 160mm F6 lightweight, they will outsell anything else out there!

Nick Witte-Vermeulen