Monday, 11 August 2014

Review: TwinStar 90 mm / 1,200 mm (f/13.3) Maksutov Telescope

I thought I’d post a quick review of a cheap 90mm Mak that I just picked up on eBay for US$149.99 (plus US$80.58 USPS priority international shipping to Australia).

I already had a 90mm short-tube refractor, which I bought as a basic “grab and go” travel telescope, but I found the limited focal length a restriction for lunar / planetary viewing, and the CA on bright targets is a bit of an issue (tolerable, but very noticeable). I recently developed  a hankering for an even more compact Mak to replace it, and started browsing eBay etc to see what I could find.

What I came up with is a "TwinStar" 90mm Maksutov, with a 1200mm focal length (f/13.3). It came with a very basic Alt-Az tripod, an 8x21 finder scope, a 45° “erect image” prism (intended mainly for terrestrial viewing), and two 1.25” eyepieces (Kellner 20mm and Kellner 9mm, both with storage containers).

The telescope is made in China, supposedly “manufactured in the very same factory that makes optical products for Bushnell, Tasco, Konus and many other name brands”  - whatever that means!

One thing that appealed to me is that the telescope back has two optical ports - a straight-through threaded port (which the supplied 45° prism screws onto - or you can screw on an extension tube adapter for photographic use, or just leave it blocked off with the supplied screw-on dust cap), and a 90° port which is intended to be the primary astronomical visual eyepiece port. The two ports are selected by using a built-in flip-mirror, with a lever on the back of the telescope.

Un-boxing and First Light:

The telescope arrived in a sturdy cardboard double-box about 1200mm long x 300mm x 300mm. It was well packed, and there were no marks or dings on the inner box. Inside the box were the OTA (in a styrofoam box), and two smaller cardboard boxes containing the tripod and other accessories.

Unpacking and assembling took just a few minutes. The OTA mounts onto the tripod using a standard camera screw mount - you could mount a dovetail bar (not supplied) onto the OTA if you want, but the camera mount works for my needs.

My first impression was that the supplied tripod and Alt-Az mount are rather flimsy for the focal length. In addition to the general problem of vibrations etc, the OTA is supported from the back end, and is quite “nose-heavy” (due to the length of the tube and the heavy front glass corrector), and the tilt-head with twist-handle struggled to stop the OTA from “drooping” whenever you try to point it at a target. However, when I tightened up all the threads on all the bolts and knobs, I found it can hold the telescope reasonably steady, especially if you hang a bit of weight on the hook under the centre-post of the tripod. 

The supplied mount is "OK" as a lightweight travel mount, but I put the OTA onto an EQ2 mount, and it was much better. (Having an even shorter tube than my 90mm short-tube refractor, it is even more stable than the refractor on this mount.) I now use the scope mainly on my EQ2 mount (or a Celestron SLT Go-To mount), but I'll keep the supplied Alt-Az as a Grab-and-Go mount, or for terrestrial use of stationary targets (with the supplied 45° erect-image prism on the rear port).

The supplied 8x21 finderscope is - well, lot's not mince words, it's pretty useless! It has low light-gathering power, it's very awkwardly positioned, and pretty well impossible to use for anything other than terrestrial viewing (with the OTA close to horizontal, and using the 45° erect-image prism on the back port). I replaced it with a generic Red Dot Finder, and this is a huge improvement - it sits at a much more comfortable position, and is much easier to use for targeting night sky objects. (The OTA back has a dovetail mount built into it, so the RDF fitted OK, although the dovetail mount seems to be slightly tighter than the one that came with the RDF - I solved this by filing a bit of plastic off the foot of the RDF bracket, and it fits fine.)

Focusing is by a decent-sized knob on the back of the telescope, which has a LOT of turns from one focusing extreme to the other - maybe 50 or 60 turns in total? It took me quite a while to even work out where “focus at infinity” was to be found (Note to self: First-time set-up and use of any new telescope is MUCH easier in daylight!), but once I had managed to get some visible light smudges which I presumed to be bright stars, I was able to get a beautifully sharp image. (The focuser knob is semi-recessed in the telescope back, so it might be a bit awkward if you are wearing heavy gloves on a cold night - being a Queenslander, I don’t even own a pair of gloves, so it’s not an issue for me!)

First visual impressions were very positive - especially when I switched to some other Plossl eyepieces, rather than the supplied Kellners. Unfortunately, the Moon had already set when I got outside (which would have been a good first test target), but Mars and Saturn were beautifully defined, as well as the Acrux double, Jewel Box Cluster, etc. 

After a Couple of Weeks:

I have been using the scope for a few weeks now, and I continue to be impressed with the optics. Experimenting with the supplied Kellner eyepieces a bit, I have found that the 9mm is perfectly adequate optically; pretty much indistinguishable from my Plossl 10mm in terms of image quality and ease / comfort of use. The 25mm also gives a good image as long as you are looking absolutely down the eyepiece axis, but “blacks out” as soon as you move just a bit “off-axis”. My 25mm Plossl has very similar image quality, but is much more forgiving of eye position, and since neither of the supplied Kellners has a rubber eyecup, I much prefer to use the Plossls, or my 7.5 - 22.5 mm zoom lens for general flexibility. I also use a 40mm Plossl to get a wider view (although with 1200mm FL, this is NOT a wide-angle telescope!)

Using two eyepieces (one in each port, the straight-through port having the 45° prism) means that I can put a relatively wide-angle eyepiece in one, and a higher magnification in the other, and swap between them easily. However, the two optical paths (one with a prism) have very different lengths, so it takes about 3-4  full turns of the focuser knob whenever you swap ports. It’s probably more convenient to just use one port and swap eyepieces in the conventional manner (requiring only fine-tuning of the focus between eyepiece changes), and reserve the other port for a camera or similar.

I have found that there is a slight image-shift when you use the built-in flip-mirror, so that the centre of the image shifts slightly to one side as you swap between the two ports. It is quite repeatable, so that when you flip back again, the image is right where you left it - my guess is that the flip-mirror pivot is not perfectly aligned perpendicular to the OTA axis, rather than slop / play of the flip mechanism. You might find your target is slightly out of view if you have a very wide-angle eyepiece in one port, and a very short focal-length eyepiece in the other, but I have no problems flipping between a 25mm and a 10mm, for example. It's not a big deal, but it may mean that the 90° port is not as perfectly collimated as the straight-through port. (Is this why most Maks on the market these days don't have a built-in flip-mirror any more?) It hasn't caused me any real grief, but I mention it in case it is important to a potential buyer.  

I bought a threaded T-Adapter which allows me to mount my DSLR body onto the straight-through rear port for prime focus terrestrial and astronomical photography. The wider field of view with the DSLR sensor compared to a typical eyepiece means that the image-shift is detectable, but not a problem.  I have also bought a threaded "Visual Back" that lets me fit my planetary webcam straight into the back port, without needing to use the supplied 45° prism. The webcam has a much smaller sensor (and field of view) than the DSLR, so the image-shift issue needs to be managed with this set-up.

The “focus shift” is a bigger issue, as it makes it hard to use the vertical port as a view-finder and then flip to the rear port for photography - until I tried mounting the eyepiece in a diagonal to increase the length of its light more or less match the DSLR in the rear port. I found that I can focus the camera / webcam accurately, and use a 25 mm or 40 mm Plossl in a diagonal in the top port, and while the eyepiece focus is not perfect, it is quite adequate for view-finding and targeting.

In Summary - The Good:

  • The price
  • Optics of the OTA - everything I have pointed it at is beautifully defined and pin-sharp - MUCH nicer than my 90mm short-tube refractor (and no detectable CA, of course!)
  • Build quality of the OTA - it feels nice and solid, with a good “weighty” feel. The tube and back (including the finder-scope dovetail) are made of some sort of plastic / resin, but it feels very solid and tough, not “plasticky”, and gives a nice solid “metallic” sound when you tap it. The focuser thumb-wheel knob itself feels a bit “plasticky”, but the focusing mechanism and action is very precise and smooth, allowing pin-sharp focusing.
  • Two ports - one for visual, and one for photography.
  • Did I mention the price?

And the Not-So-Good:

  • The supplied Kellner eyepieces are OK, but not exactly great. They don’t have any rubber eyecups, so I found I was getting a lot of stray light in my light-polluted backyard. The focal lengths (9mm & 20mm) make a good “starter pair”, and a first-time user would have no reason to complain, but I really enjoyed the view a lot more with my other Plossl eyepieces.
  • The 8x21 finder scope is pretty useless - small field of view, dim, and mounted much too close to the body and the main eyepiece port to be comfortably accessible. However, it installs in a dovetail mount, so I put in  a red-dot zero-magnification finder (which has a much longer stand-off than the supplied finder scope mounting bracket), and it worked MUCH better! (One thing to note is that the dovetail mount on the telescope back seems to be a little bit smaller than the “standard” mount, so I had to file the foot of the red-dot mount a little bit to get it to fit in, but once done, it sits nice and secure.)
  • The tripod, which is somewhat flimsy for astronomical use. I put the telescope onto an EQ2 mount that I already own, and it was MUCH better! I’ll keep the supplied mount as a compact travel mount for the OTA, but I’ll use the EQ2 at home.

Shopping for one:

If you’re interested in tracking one down for yourself, search eBay for “90mm Cassegrain”, and make sure you set “Item Location” to “Worldwide”, not just “on eBay Australia”. You’ll find several sellers (mostly in the USA) with the same OTA, but some of them sell it on the basic AltAz tripod, and others have it on an iOptron “Cube” GoTo mount. (There are some other OTAs with different brand names, which look suspiciously like they come from the same production line, too.) The sellers are asking different prices and shipping fees - some have a “Buy It Now” price, while others have a “No Reserve Price” auction format - sort them by total price until you find the best deal!

I was watching the listings for a while, and I noticed that the “No Reserve Auction” items listed by the seller I bought from seemed to always get close to the closing time with no bids, so I put in a bid of US$150 with a few hours to go, and got it for the starting price as the only bidder. (As well as Chinese telescopes, he also sells office chairs and glow sticks!)