Review: TwinStar 90 mm / 1,200 mm (f/13.3) Maksutov
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!
Update after a couple of weeks’ usage (and some photos)
Well, I’ve been using this telescope for a couple of weeks now, and I’m still VERY happy with it. It has well and truly met and exceeded my expectations - which is to say, it has out-performed what I should probably have reasonably expected for the price!
Firstly - a correction: I said in my first post that the telescope rear cell was made of metal - after looking more closely and tapping / knocking it repeatedly, I think that the telescope back is in fact some sort of plastic / resin. It feels very solid and tough, not at all “plasticky”, and gives a nice solid “metallic” sound when you tap it. The two ports are both metal, so there's no concern about the ports wearing out from frequent changing of eyepieces and other accessories. It looks like it will be very durable, and I have no complaints with the build quality.
As noted in my first post, I have replaced the original 8x21 finderscope with a Red Dot Finder (RDF) - the attached photos show that the supplied finderscope is 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). The RDF sits at a much more comfortable position, and is much easier to use for targeting night sky objects.
I have been using the telescope on an EQ2 mount, but I have tightened up the threads on all the knobs of the supplied Alt-Az mount, and 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. I was having a lot of trouble targeting stars and planets with the original finderscope and “sloppy” threads on the mount knobs, but with everything tightened up and using the RDF, I find that I can now target and hold stars and planets pretty well - although the lack of slow-motion controls makes it pretty awkward to track an object, especially at higher magnifications. I am prepared to revise my previous assessment - the supplied Alt-Az tripod is just about adequate as a lightweight travel mount (but the EQ2 is MUCH better!) - and of course, the Alt-Az mount is a lot simpler to use for terrestrial use of stationary targets (with the supplied 45° erect-image prism on the rear port).
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!)
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 am awaiting delivery of a threaded T-Adapter which will allow me to mount my DSLR body onto the straight-through rear port for prime focus terrestrial and astronomical photography - it will be interesting to see whether the “image shift” and “focus shift” issues are a major problem if I use the 90° port for viewfinding and then switch to the back port (and re-focus) for imaging. I hope to post a few sample shots in the next few days.
(P.S.: Yes, I just realised that in the photos with the supplied finderscope mounted, it is installed backwards! Rest assured - that isn't the reason for the problems I was having with the finderscope; I just mounted it back on to take some pictures of the "as supplied" kit, and didn't pay any attention to which way it was facing! )
Hi Julian.
An interesting read as I'm in the process of putting together an almost identical setup!
I think I read somewhere that you bagged an Astro Pete's EQ2 with the 1/4" bolt head. Can I ask, are you happy the single bolt is enough to hold the weight of the combined tube and DSLR securely or would you prefer something with a bit more resistance to azimuth rotation?
The reason I ask is that I'm about to butcher the other style of eq2 head that Astro Pete had and it might well go wrong!
Hi Julian.
An interesting read as I'm in the process of putting together an almost identical setup!
I think I read somewhere that you bagged an Astro Pete's EQ2 with the 1/4" bolt head. Can I ask, are you happy the single bolt is enough to hold the weight of the combined tube and DSLR securely or would you prefer something with a bit more resistance to azimuth rotation?
The reason I ask is that I'm about to butcher the other style of eq2 head that Astro Pete had and it might well go wrong!
Thanks
Chris
Yes, I'm using the 90mm Mak on an EQ2 mount that I got at a bargain price from Astro Pete's a few months back.
The EQ2 has a standard two-bolt arrangement on the mount itself (I think it's mainly intended for attaching a dovetail or a set of rings), but I am using the camera adapter plate which fits to the standard telescope mounting with two bolts (all of which came with the EQ2 mount and tripod). There was no need to "butcher" anything - the set-up allows either standard telescope brackets or 1/4" camera fittings to be used, just by removing / installing the camera adapter plate.
The camera adapter has a cork facing to prevent slipping / rotation of the telescope, and the telescope mounting base has an "embossed" diamond pattern on the underside, which "bites" into the cork very nicely with only moderate tension on the 1/4" screw. I am not having any problems with the telescope slipping on the mount.
I was playing with a compact digital camera on one of those universal eye-piece-adapter brackets last night, and the extra weight at the back of the scope didn't cause any problems. I haven't used it with the heavier DSLR body yet (I'm still waiting for the T-Adapter to arrive), but I think it will probably be fine. The OTA is a bit "nose-heavy" relative to the mounting point, due to the length of the projecting tube (and its thick front meniscus corrector), and the mount is located near the back of the OTA. With a T-Adapter and DSLR body, the whole set-up will probably be fairly neutral, or perhaps slightly "tail-heavy", but no worse than its "nose-heavy" configuration with only standard eyepieces.
I'll report back when the T-Adapter comes and I get a chance to try it out.
Thanks Julian. I chose this top because I had thought of putting a refractor on it, but it hasn't turned out that way! I'd try for a swap but it's a 230km round trip!
Thanks Julian. I chose this top because I had thought of putting a refractor on it, but it hasn't turned out that way! I'd try for a swap but it's a 230km round trip!
Why not contact Astro Pete and see if he can send you the camera adapter (my EQ2 came with one supplied, and he may have some spare ones lying around), or you can pick one up at BinTel or other telescope outlets:
Thanks Suzy & Brian - I'm happy for the Moderators to copy this to the Reviews section if they think it's worthy.
(It might be best to make an amalgamation of a couple of posts into a single review to keep it simple.)
For what it's worth - I got the T-Adapter today, and fitted my Pentax DSLR body to the rear threaded port - see attached photos. (It fits perfectly, so I can confirm the rear thread is exactly the same as a Celestron C90 Mak.) As I suspected, it's actually beautifully balanced with the camera body acting as a counter-weight for the overhanging tube and corrector plate, so it has virtually no tendency to rotate about either axis, even when the friction brakes are completely released.
The camera reaches focus nicely over a wide range from about 5 metres to infinity, but it's pretty dim viewing through the camera viewfinder, so I'll have to get focus at infinity on a very bright star or the Moon, and then shift the scope to whatever target I want to photograph. The camera's field of view is pretty similar to what I see with a 40 mm eyepiece in the right-angle port, so I'll aim roughly using the RDF, check the field of view in the 40 mm, then flip the mirror to the rear port and take my shots.
Unfortunately, there's quite a bit of focus-shift between an eyepiece in the right-angle port and the camera mounted on the straight-through, so if I set the focus for the camera, the eyepiece in the other port is pretty blurry, but will hopefully be adequate to check the framing. I will do some experiments with an extension tube to see if I can get "reasonable" focus in the 40 mm eyepiece when the focus is set properly for the camera.
The sideways image shift between the two ports that I mentioned is negligible with the field of view of the camera and the 40 mm eyepiece, so at least that won't be something I need to deal with.
I wanted to get a few trial shots tonight with both the DSLR and a 3 Mp "digital eyepiece", but it's foggy tonight - in Brisbane! Oh well, tomorrow night, hopefully.
So I had a tinker with the various bits and bobs I have, to see if I could extend the light path of the right-angle port to get an eyepiece to be more-or-less par-focal with the DSLR body on the straight-through port.
First I tried my "Shorty Barlow" tube (with the Barlow lens element removed), but it was too short.
Then I tried a 45-degree "erect image" diagonal - but it was too long.
Then I tried a simple 90-degree star diagonal - and it was just right!
It's not very elegant - but where there's a will, there's a way!
The eyepiece focus is not quite spot-on when the camera is pre-focussed, but it is perfectly adequate for target-finding and framing, using an eyepiece in the right-angle port, and then flipping to the pre-focussed camera on the back port. (I imagine I could improve things somewhat with an adjustable extension tube for the eyepiece, but this set-up will do for now.)
The field of view in a 25mm Plossl is about the same as the camera gives, and my 40mm Plossl gives a slightly wider view than the camera, so is probably the best choice for target finding.
Skies in Brisbane today are crystal-clear, so I'll have a go at some night shots tonight (as long as the fog stays away!)
Rest assured - that isn't the reason for the problems I was having with the finderscope; I just mounted it back on to take some pictures of the "as supplied" kit, and didn't pay any attention to which way it was facing! 
)
