THE GSO 680 200mm f/6 Dobsonian
by Antony McEwan, April 2004
This article is not the article it thought it was going to be! It thought it was going to be all about a new Orion ED80 APO refractor, but it's actually about a biggish 8" Dobsonian.... How? Well, it's a long story. After hearing so much good stuff about the, now famous, Orion ED80 Apo refractor, and impressed by the fact that it is now possible to own a quality colour-free refractor for £400 or less, I placed an order for one, thinking to document it's arrival and use in this text. But the delivery was delayed and something else took its place.
Having been using the Highland Astronomical Society's 10" and 12" reflectors at Culloden, I was beginning to appreciate just what a difference the extra inches of aperture make for revealing more detail in astronomical objects than smaller telescopes. I use my 127mm Maksutov primarily for looking at bright Messier objects and the Planets, and it yields fine, very sharp views, but the large reflectors just give up so much more detail with apparent ease. It seems aperture rules. So then I began to think about getting myself my very own big(gish) reflector for home use.
Newtonian reflectors work by gathering light onto a curved primary mirror at the bottom of a long tube. The curved primary then bounces the light back up the tube while narrowing the light path until it hits a smaller second (secondary) mirror which is positioned at 45 degrees, relative to the face of the primary. The secondary then bounces the light out through a hole in the side of the tube into the focuser, which holds the eyepiece and where it can be brought into focus by the lucky astronomer.
As I've said, I was inspired by the amount of detail visible, particularly on the Planets, when using the Society telescopes. With larger mirrors you can expect to increase the amount of useable magnification for your viewing, and so 'zoom in' to the image to see finer detail. To do this comfortably with most eyepieces the telescope needs to have a long focal length. Magnification given by any eyepiece is equal to the focal length of the telescope (in mm) divided by the focal length of the eyepiece (also in mm). As the focal length of the Newtonian reflector increases, another benefit is added: The diameter of the secondary mirror needed (the one that shunts the light path out the side to the waiting eyepiece), is reduced. This means less obstruction in the light path and therefore more contrast in the final image.
For these reasons, I wanted something with about a focal length of 1200mm and aperture of 8 inches, which would give a focal ratio of f/6. This would be long enough to provide reasonable magnifications with my selection of eyepieces, short enough to fit in my car, and not too heavy to transport. In the future I also intend to be able to mount this tube assembly on an EQ mount so that it can be used for tracking objects as well.
After scouring the internet, I had it down to two models: The Synta 8" f/6 Skyliner, and the GSO 680 8" f/6 Dobsonian. Both had 8" f/6 parabolic mirrors, 2" focusers, spring tension systems on the altitude bearings and four-vane secondary supports (spiders). I saw them both at Teleskop-Service in Germany, and as the Pound to Euro ratio is favourable, ordering either would mean a substantial saving on ordering from the UK suppliers, even including postage. I was helped through the selection process by Patrick Woitala at Teleskop Service, with the deciding factors being that the GSO 680's primary mirrors were thought to have a higher level of quality control and optical quality, and also that they are stocked and recommended by UK supplier, Bray Imaging, who provide many top-class instruments and eyepieces, etc. I was offered a choice of 'packages' to go for and ended up selecting the telescope packed with a 2" 28mm Kelner type eyepiece for wide-field viewing (42x and field of 1.33 degrees) and 9mm plossl for higher power (133x) views. Also included is an 8x50 finder on bracket and the ubiquitous Moon filter.
Delivery took place on Wednesday, 10th March, and the package arrived in two large, sturdy, impressive-looking boxes. The boxes were undamaged and well sealed. Looking at the boxes, one thing immediately struck me: The rocker box is flat-packed! I had half-expected the rocker box to be pre-assembled but hadn't checked. Previous struggles with flat pack furniture sprang to mind.
Once carefully opened, I laid the various components out and had a poke around at them. The rocker box was going to be built up from five pre-cut (and well finished) board sections, and all the various screws, bushes, washers and assembly tools were present and correct. The optical tube impressed me immediately with its stylish silver-grey colour. I imagined it would look pretty good with the all-black base and other components.
I also checked that all the accessories were present: 2" 28mm Kelner, 8x50 finder and bracket, 1.25" 9mm Plossl, Moon filter.....all there. All were individually boxed and well protected within a larger substantial cardboard box as well. Good packaging!
I won't bore you with too much about the assembly, but suffice it to say that even I managed it fine. It took about two and a half hours all in, but that included coffee breaks and text messages to friends to keep updating them. All the holes for the screws and bolts were smooth and everything fitted together very easily. I followed the instructions in this, and did not add any wood glue to further strengthen the assembly. I convinced myself I could always do that later if needed.
Looking over the various bits and pieces, I was struck by several items that I thought were better than I was expecting on a telescope at this price. The focuser is particularly well adjusted and very smooth. It is a 2" focuser with a smaller slot-in 1.25" adapter. The adapter actually has a compression ring to hold the eyepiece in place, instead of the more usual single set-screw. Compression rings don't mark your eyepiece barrels, whereas the set-screws can. The focuser also comes with two ultra-small adjustment screws which allow you to adjust out any wobble in the draw-tube as it racks in and out. These were perfectly adjusted and there was no play at all in the focuser.
Sadly the focuser is 'lubricated' with the standard thick grease/sludge that most cheaper telescope manufacturers seem to be using these days, so at some point soon I shall simply remove the draw-tube, rack and pinion and apply some nice smooth silicon grease.
The primary mirror is housed in a three-point support cell, with easy to get at collimation screws and locking screws to lock the collimation once it's right. This should help prevent slight knocks putting the scope out of alignment. The cell is also completely exposed to the air. I think this should make the mirror cool down to ambient air temperature more quickly, which is a good thing, but wonder if it may expose the mirror to more dust or other contaminants than usual. Not sure if this is a 'nice touch' or not - I may place some mesh or filter material over the rear end of the tube just in case.
Having not owned a reflector for many years, I knew that collimation, or aligning of all the mirrors and the focusser, could be tricky. In actual fact it was worse. By following instructions found on the net and using a borrowed laser collimator, I was able to get the mirrors in alignment o.k., but then checking it by eye by looking into the focuser without an eyepiece in place, showed what looked like an optical path that was offset to one side. Very disturbing. I was able to adjust this so that the collimation appeared correct by eye, but then when it was checked using the laser, it was all off. Some people never buy reflectors because of their fear of collimation……
Eventually, I arranged to get some tuition on the matter from Rob Nuttall. I took the telescope over to his one evening and we used his laser collimator…..and got the same results! I was a little relieved to see that even Rob was a bit nonplussed by this (it wasn’t just me), and much chin scratching and head-shaking ensued before we realised that the secondary mirror had to be moved 'up' the tube, away from the primary, to align it properly with the focuser while maintaining correct mirror alignment. Sound confusing? We both agreed that if this was a first telescope for someone, then they’d be well disgruntled by now, and could possibly be put off for a long time. Eventually we worked out how to actually achieve this movement and from then on it was just a matter of adjustment, check, laser, adjustment, check, etc. Only took a few hours. Once in the swing of things, the actual adjustments were quite easy to make and all the screws and bolts moved easily and locked in place well. It could have been a lot simpler if the position of the secondary relative to the focuser had been checked prior to despatch though.
Well, I’ve had the telescope about three weeks now and can’t honestly say I’ve had an ideal night to try it out for an extended test. Instead, I’ve had a few snatched chances of a half hour here, or an hour there, etc. During the first of these sessions, a problem arose. Most of my standard eyepieces are Televue plossls. Great eyepieces, real workhorses. The Dobsonian would not quite achieve focus with them. There wasn’t quite enough out-travel in the focuser for them to reach focus point. To get them to focus, I had to pull them slightly out of the focuser by about 2-3mm, and then lock them securely in place to avoid them dropping out. That solved the problem but it annoyed me enough to email Patrick at Teleskop Service to ask what that was all about. According to him, the focuser is biased towards a short amount of out-travel to allow certain Erfle type eyepieces, and the famous Televue Naglers, to come to focus more easily. This sort of made sense, as they would be wide field eyepieces, which would allow the user to view an object for longer before having to nudge the scope along to track the object as it moved across the sky. I explained that I didn’t own any Naglers but I wasn’t offered a free set to compensate. Instead I am awaiting a 2” extension tube which should attach to the drawtube and allow an extra 20mm of travel to fix the problem. Kudos to Patrick and his company for very quick email responses!
Of my other eyepieces, the Pentax XLs come to focus no problem, but they have more lenses in them than plossls, and so are more similar in design to Naglers. My orthoscopic is unaffected, and even the Plossls work o.k. when used with a barlow. The 2” 28mm Kelner supplied with the scope also needs to be pulled out a little bit. Overall it is not a great problem.
The actual views I’ve had through the telescope are fantastic! Going from a 5” Maksutov to this 8” biggy was definitely a good move. Unfortunately I haven’t had the telescope at a dark sky site yet, so all viewing has been done from my light polluted back garden and I haven’t had a chance to try some views of any deep sky objects yet. The few views I’ve had have been of the Moon, Saturn, M45, M44, Castor, Polaris, and some unknown stars that peeped through the murky orange glow of Cromarty’s night-time glare. Even without detailed tests, the lunar views have been amazing. After about an hour to cool down, and with the half Moon high in the sky in steady seeing, the clarity and sharpness of the image was outstanding. I cruised the terminator with many eyepieces, ranging from magnifications of 42x to 266x, at which point the view was still sharp and clear.
Just for laughs I stuck my 2.5mm Lanthanum into the focuser (focused no problem) and had some good views at 480x, though the image was definitely not so sharp or bright at this magnification.
Other practical or mechanical features that I noticed were that the spring tensioners attached to the altitude bearings actually work very well. They apply enough tension to the scope so that the tube stays pointing to the altitude you set it at without slipping in either direction, and changing from a light to a heavy eyepiece presents no balance problem at all. Very nice. Also the azimuth motion is very good. I had concerns about that, as after I’d assembled the base, it seemed that it would be a bit sticky to rotate, but once actually looking through the eyepiece, the motion seems plenty smooth enough. Maybe a little room for improvement but nothing extreme. The supplied 8x50 finder is good quality too, and is focussable by means of a rotating threaded ring that moves the objective cell up or down the tube until it reaches the desired focus position. Then you just tighten the mini-dew shield against it, and it holds the cell in the correct position. Views through the finder are clear and sharp.
I naturally hope to get some dark sky viewing as soon as possible, and will post an update to this report once that is done. I expect great things once away from the nasty street lights but I guess that for now I’ll just have to wait and see……
Conclusion - so far
As a package, I think this telescope takes some beating. It’s got a lot of usable aperture, which seems to be of very good quality. There are several nice touches included: The compression-ring 1.25” adapter, spring tensioners (which are really effective), good finder, inclusion of a reasonable 2” eyepiece and ease of assembly are all good points, but I was disappointed by the fact that collimation was made so daunting by the simple fact that the secondary was in the wrong position. Not so bad for someone who’s owned reflectors before but not good for a beginner. Once collimation is done, it’s a very rewarding telescope and can be set up quite quickly, as long as you allow a reasonable time for the optics to cool to the ambient air temperature before viewing. My one was supplied with BK7 glass, but Pyrex is available at extra cost, and would cool down quicker. On the night when I viewed the moon at 480x, it had cooled for about an hour and the outside temperature was 5 or 6 degrees Celcius.
The current price for the package I bought is 428 Euros, which works out as*….not a lot really. Great value, I’d say!
*(At the time this article was written, 428 Euros was about £290.)
Click here to read the update to this review.