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10" f/5 Guan Sheng Dobsonian Reflector
Submitted: Tuesday, 21st February 2006 by Darren Wong

  • type : newtonian reflector, parabolic mirror
  • clear aperture : 250mm (3")
  • focal length :  1250mm (f/5)
  • central obstruction : 23% (approximately)
  • eyepieces : 1.25" 26mm FMC GS Plossl and 9mm FMC GS Plossl
  • other included accessories : free binoculars 10x30 and generic moon filter (green)
  • finder : 8x50mm (nice spring loaded)
  • mount : dobsonian mount

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10" f/5 GS Dob

My first foray into large scope country. I had planned to get this scope in the near future but did not expect to get it so soon. Due to a stroke of luck, my research poster won me some money at a recent BioNano conference for my work on resilin and hence this scope. I have had experience with such a scope as my close observing buddy David has one and I have used it rather extensively from SAS's dark sky site out at Leyburn.

My first look through a large scope when I first came back to oz was through a 10" GS dob (David’s scope). I remember fondly the views I had of 47 Tucana, the brilliant southern globular, which was fully resolved into a rich swarm of stars all the way down to the core, and this from a suburban Brisbane site close to the city! From dark skies, some of the component stars actually show color as well. Also the view’s of Jupiter's belts (too numerous to count) and all the festoons and detail and Saturn's belts and rings (A, B, C components all easy as was Cassini's Division and Encke Division). I have even seen filamentous detail in M1 the Crab Nebula in Taurus from Leyburn and have sketched several objects through his scope. But this is getting ahead of myself. Now to the very beginning….

I had known all along from the first time I used David's scope that a 10" f/5 solid tube dob would be the scope of scopes for me. It has sufficient aperture to go deep into Abell and Hickson's territory and yet portable enough to fit in a small car without the tube being converted to a Truss. Also the weight of the OTA and mount is not overwhelmingly heavy and one person can easily carry and set up the whole scope in two sittings. Also a 10” scope collects 56% more light than an 8” scope and a 10” has been said to be the smallest of the great dobs. So it was a logical choice for a visual astronomer like me. The die was cast and the order placed on the 19th of December, 2005 from Andrew's Communication down in NSW. The scope arrived at my doorstep 2 days later in two boxes, one housing the OTA and the other flat box housing the mount.

When the courier called me at the office to inform me that the package had been delivered, I rushed home to get the boxes into the house. I was surprised to find that the smaller flat box containing the mount to be as heavy if not heavier than the box housing the OTA. Without further ado I began to open the boxes and low and behold...a nice noble white shining tube, a dob to call my own! I then began to try and carefully unpack the various parts without damaging the contents in my highly excitable state of mind. I made a mental checklist of the various parts that came with the dob, the nice spring loaded fully multicoated 8x50mm dovetail finder (that has nice eye relief, a wide FOV and is very intuitive to align), the two 1.25" GS FMC (yes fully multicoated...NICE!) in 26mm and 9mm focal lengths, the free generic green moon filter and the 10x30 roof prism binoculars.

When I finally spied the instruction leaflet, I found it to be totally inadequate and spartan. All that was provided was a very small diagram detailing the parts and where they went. The printing was so small that one could not distinguish one screw part from the other. But using my knowledge of the scope, I managed to get everything assembled in about 20 minutes (maybe faster I was not timing myself). Everything to aid in the assembly was provided excluding a Philip's head screwdriver. An allen wrench was provided and most of the screws worked with this. Throughout all these steps, I was meticulously taking pictures as well. So I guess one could put the scope together much quicker. When the scope was assembled, I proceeded to check the collimation. It was nice to note that a centre doughnut was present, saving one the frustrations of having to self centre spot the primary mirror (which involves removing the primary mirror mount from the tube and marking the centre with a sharpie...not something for the faint of heart). Collimation was pretty spot on and I especially like the finger collimatable collimating knobs and tightening screws.

Surprisingly the weather remained clear for a week and a half after the dob arrived. The weather gods must have been on holidays too. Turns out that I got the dob at just the right time as I had heaps of opportunities to test it out under Brisbane's suburban skies. During summer the limiting magnitude does not penetrate as deeply as the transparent winter skies. I estimated the magnitude limit from my courtyard to be approximately 5.3-5.4. As my current ground level courtyard (the scope will be too cumbersome to carry up and down flights of stairs but it should be doable) only gives access to stars at zenith and near zenith, stars to the south where all the celestial goodies are, and to the north are inaccessible. I made do with M42, the Orion Nebula and the surrounds. M42 was magnificient in this scope. The added aperture made the wings stand out beautifully and I found myself mesmerised by the views even without the aid of a filter. The trap easily stood out at medium magnifications and the E and F component were ridiculously easy. Also at higher power, the cirrus cloud effect became very pronounced in M42. I also looked at M78, the reflection nebula in Orion. This one was easy as well although the views were washed out due to light pollution. The DGM Optic VHT filter made it stand out better. I also looked at the globular in Lepus and this was also resolved at higher magnification. I then surfed around and looked at open clusters (i.e M41, etc.) in Canis Major and checked out NGC 2438 in M46. Its amazing how much power you can use on such large an aperture without the image dimming. 2438 looked like a fainter version of M57, the Ring Nebula at 208x.

At the end of the week I had a chance to try out the dob on the whole sky from Andrew's place at Parkinson's, and to pit the dob against a larger scope, namely Andrew's incredible 12.5" f/6 Truss dob. This would also give me a chance to try to fit the dob in my Hyundai Excel Sprint. Fits in nicely once I remove the back cover and put down one of the top of the back seats :) I was again reminded of how easy it was to move the dob. It was not heavy at all. On getting there, we checked out all the usual suspects. The standouts were 47 Tucana (brilliantly resolved to the core with diamonds on black velvety effect...even at low magnifications), M42 again (rusty colors were visible in the wings and we confirmed this in Andrew's scope as well!), planetaries and their inner detail shone through with Andrew's newly acquired OIII filter (NGC 2392, the Eskimo Nebula showed its triangular face and fur parka; NGC 3242, the Ghost of Jupiter showed clearly its CBS eye detail and NGC 2438 showed its ghostly ring visage), the delicate swirls of the Tarantula Nebula with and without filters and its surrounds (its great to be able to surf around without worrying about lock knobs as on my EQ mount...look at all that nebulosity around the Tarantula) and not forgetting Eta Carina and all those dust lanes....goodness! Then there was Saturn. There were so many moons hovering around Saturn and Saturn itself presented so much detail. The banding on the planet stood out vividly as did the pole darkening’s. No more subdued shades! The rings showed abundant detail from Cassini's Division going all round the planet to the delicate Crepe Ring and Encke's Division. The shadow of the planet’s disc on the rings made Saturn stand out in 3D.

If I had to summarise the scope in two words, it would be BRILLIANTLY BREATHTAKING. Oh and did I mention that the 10" held its own against larger competition? All the detail that was present in Andrew's scope was also present in mine. Star test revealed a very well figured primary mirror. Patterns on both sides of focus showed similar patterns reminiscent of the excellent mirror in my Celestron/Vixen C6. And did I mention the nice compression ring in the focuser?

If there were any gripes, it would have to be the very tight tension springs and the lack of smoothness in the azimuth bearings, but these can be easily remedied using milk jug washers or some ezy sliders.

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OTA in the box

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Base in the box
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Assembling the base

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Almost done!
Review by Darren Wong (dhumpie). Discuss this review on the IceInSpace Forum.
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