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 have a 90mm short-tube refractor, which I bought second-hand as a basic “grab and go” travel telescope, but I have begun to find 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 AltAz 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 could 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 eyepiece port. The two ports are selected by using a built-in flip-mirror, with a lever on the back of the telescope.
Unboxing 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 - I guess you could mount a dovetail bar onto the OTA if you wanted to, but the camera mount works for my needs.
My first impression was that the supplied tripod and AltAz mount are really too flimsy (and not really tall enough) to be of much use with this telescope. 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 couldn’t really stop the OTA from “drooping” whenever you try to point it at a target. I immediately put the OTA onto an EQ2 mount, and it was much better. (Having an even shorter tube than my 90mm short-tube refractor, it was even more stable than the refractor on this mount.)
Seeing conditions were awful last night, but I took it outside anyway - of course! (I suspect pretty well all new telescopes go outside on their first night - whatever the weather conditions!)
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, 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.
Using two eyepieces (one in each port, the straight-through port having the 45° prism) meant that I could 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.
More visual tests to follow tonight ...
In Summary - The Good:
- The price
- Optics of the OTA - while seeing conditions were terrible last night, what I could see was beautifully defined and pin-sharp - MUCH nicer than my second-hand 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 metal (aluminum apparently, as it’s not magnetic). The focuser thumbwheel knob itself feels a bit “plasticky”, but the focusing action is very precise and smooth, allowing pin-sharp focusing. [EDIT: After looking more closely, I think that the telescope back is in fact some sort of plastic / resin. It feels very solid and tough, not “plasticky”, and gives a nice solid “metallic” sound when you tap it - I have no complaints with the build quality.]
- 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 really too flimsy, and not tall enough, to be of much use. I put the telescope onto an EQ2 mount that I already own, and it was MUCH better! I have a few lightweight portable tripods and table-top mounts (doesn't everybody?), so I’ll see which one works best 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!)
I bought it on 20 June, and it arrived yesterday (2 July) - not bad!