Jump to content


Photo

Guan Sheng 12" Dob

This topic has been archived. This means that you cannot reply to this topic.
15 replies to this topic

#1 asaint

asaint

    Vanguard

  • *****
  • Posts: 2094
  • Joined: 25 Apr 2003

Posted 24 March 2008 - 12:05 PM

Guan Sheng 12" Dob

#2 Fireball

Fireball

    Viking 1

  • -----
  • Posts: 533
  • Joined: 24 Mar 2006

Posted 24 March 2008 - 02:31 PM

Thanks for this great review.
You mentioned something interesting
"Flocking paper was easily applied to the upper tube to reduce scattered light, and a ring baffle above the mirror to reduce stray light and help direct air from the fan across the mirror face."
How does such a ring baffle look like and where exactly did you place it ?
Clear skies !

#3 proud uncle

proud uncle

    Surveyor 1

  • *****
  • Posts: 1717
  • Joined: 22 Jan 2007

Posted 24 March 2008 - 02:59 PM

Thanks for a great, informative review! I have a GSO 10" Dob. So far, no problems with collimation springs. My primary collimation bolts can be turned by hand.

I like the thought about moving the Teflon pads inward. I would need to do some measuring to find the "sweet diameter", avoiding the recessed bolt ridges.

I have the same question as Fireball regarding the ring baffle.

#4 Brian Gibson

Brian Gibson

    Vostok 1

  • *****
  • Posts: 104
  • Joined: 17 Oct 2007

Posted 24 March 2008 - 06:33 PM

Great review. In Canada these were imported as Sky Mentor Dob's and I bought the same 12" model a couple of years ago for less than $1000 CDN. I agree that it is a great value and delivers amazing deep sky views away from city lights. I have made several improvements as well including the Bob's Knobs mentioned (primary, secondary and replacement springs), a dual speed Crayford focuser and I installed a pair of aluminum handles on the sides of the tube to make lifting it easier. These are handy when moving the tube while viewing. The picture below shows one of the handles and a black vinyl coated magnetic weight (available from hobby supply stores) that I use to help balance the tube. Very handy!

Brian Gibson

Attached Files



#5 goober

goober

    Sputnik

  • -----
  • Posts: 33
  • Joined: 31 Dec 2007

Posted 24 March 2008 - 07:34 PM

Good, thorough review. I owned one for 10 months and fully agree with your conclusions. I found the primary springs undernourished, and the azimuth stiction very annoying. I replaced the springs (better) and tried a few things to fix the stiction (didn't help much). I found the optics to be okay, not as good as an 8" Starhopper I owned at the same time.

I ended up selling the scope - too big and couldn't transport it to dark skies.

For an affordable big scope though, it's a good buy. Especially if you don't mind tinkering and tricking it up.

#6 MtnGoat

MtnGoat

    Surveyor 1

  • *****
  • Posts: 1939
  • Joined: 18 Feb 2007

Posted 24 March 2008 - 08:23 PM

The Zhumell version now comes with roller bearings for AZ motion, as well as 10:1 crayford.

#7 Brian Gibson

Brian Gibson

    Vostok 1

  • *****
  • Posts: 104
  • Joined: 17 Oct 2007

Posted 24 March 2008 - 08:52 PM

My 12" Sky Mentor actually had a roller bearing disc for the AZ motion but it had created small indentations in the base plate during shipping which made the bearings catch. I also found it was too loose so I replaced the bearings with teflon pads that provide a much smoother feel and a bit more friction.

Brian Gibson

#8 panhard

panhard

    It's All Good

  • *****
  • Moderators
  • Posts: 13669
  • Joined: 20 Jan 2008

Posted 24 March 2008 - 10:17 PM

Thank you for the honest and very accurate review.

#9 tannehill

tannehill

    Vostok 1

  • -----
  • Posts: 137
  • Joined: 26 Sep 2005

Posted 26 March 2008 - 10:05 AM

Thank you.

The ring baffle is just some foam self-adhesive weather stripping material. In my scope it served two purposes: to deflect air flow inward toward the mirror (from fan below), and as a light baffle. The first reason is most important for me, since I didnt' really have stray light coming up from below.

More details on iceinspace.com.au and check under the projects and articles section, Equipment, article titled GSO Dob Fan

Pic below.

Cheers,

Scott



Thanks for a great, informative review! I have a GSO 10" Dob. So far, no problems with collimation springs. My primary collimation bolts can be turned by hand.

I like the thought about moving the Teflon pads inward. I would need to do some measuring to find the "sweet diameter", avoiding the recessed bolt ridges.

I have the same question as Fireball regarding the ring baffle.

Attached Files



#10 WasHighPingDuck

WasHighPingDuck

    Lift Off

  • -----
  • Posts: 1
  • Joined: 05 Feb 2007

Posted 28 March 2008 - 09:26 AM

Hello Scott,

I enjoyed your review and I too purchased a 12" GSO last spring. Mine was an Antares brand and I did not experience most of the issues you mention however it is a white tube model so it may be a slightly later built unit.

Mine moves in both axes quite smoothly however I did counter sink the Teflon slider staples when it arrived and I apply a small amount of paste floor wax to both axes before set-up. They don’t sell it anymore but if you check your basement I bet you have some.

I can detect no lateral movement in the alt bearings when I move up or down. Are the long bolts that attach the trunnions to your optical tube tight? Mine were not and you have to remove the mirror cell or spider to reach the nuts. If they are not tight and at the same distance on the tube the bearings will walk on the mount.

My springs were proper size and tension on the secondary holder and the alt bearings. However the fender washer was installed upside down, dimples toward the spring and away from the secondary. It won’t stay collimated like that.

My mirror was centre marked however I measured and found it to be off about a 1/8" and I had to remove it and apply another. The lock bolts on the mirror cell do affect collimation however like you I leave the laser on and snug them with that in mind and it secures dead centre. My Mirror clips were not over tight but I was unable to remove the mirror from the cell, I think it might be glued. I was trying to tighten a fan screw, as one was loose. This will also make washing the mirror difficult without getting the fan wet.

I had to shim my focuser straight in both axes using the laser collimator and a couple of plastic drawing triangles with the spider removed. A few home made aluminium shims and all was fine.

My biggest disappointment was there is little information on Antares site and there are dealers advertising this unit with a Pyrex mirror and an aluminium tube. I knew about the mirror before I purchased however the seamed steel tube will make modification more difficult and is heavier than I was told making transport to my dark site very hard.

One more thing, don’t trust your laser collimator to be straight, mine was off and any error in the collimator will be magnified by the length of the light path. An easy test is to loosen it slightly in the focuser and rotate it, if the dot moves on the primary the laser needs attention. I also improved mine considerable by taking it apart and covering the torpedo laser hole with a piece of aluminium tape, then I poked a tiny hole in the centre of the dimple in the tape with a small needle. Now mine has a very small bright dot and diffraction rings. How kewl is that? Build a stand with two vee blocks to rotate it on to line it up when your done.
Always be careful not to aim the laser light at your face or reflect it there. Laser light will seriously and permanently blind you!

Clear skies,
Dan

#11 tannehill

tannehill

    Vostok 1

  • -----
  • Posts: 137
  • Joined: 26 Sep 2005

Posted 29 March 2008 - 09:36 AM

Thank you for your experience and thoughts.

As best I can discern, the lateral shift is due to slight asymmetry of the trunnions, not any loose bolts or such. It's not a functional issue, really. Only if one installs encoders AND thinks about screwing down the alt tangent arm tightly. THEN it'd be an issue. But one should never really do that anyway, regardless of the presence or absence of lateral tube shift...

I never had any luck with paste wax, armor all, etc. Seems like after a few hours of use stiction worsens; I think it collects dust. Yes, I buff it so there is no residue. Listened to Dave Kriege's video, talked with him, talked to others. Can't say that I can reconcile our disparate experiences. There are veterans out there who support the lube-free approach. Yes, it helps, just not durably, and I've found that after several nights of use, if the surfaces are kept clean, that stiction and friction spontaneously improve...teflon wearing in, perhaps tiny particles of abraded-off Teflon helping to improve the motion, etc. This pattern is the same with my Obsessoin as it was with the GSO. Just, the larger alt trunnions of the Ob make for less prickly balance requirements, and I've not seen any ATM-trick that duplicates that. Guess that'd be hard, since it's the physics and lever-arm equivalent elements that make the larger trunnions more pleasant in this regard. On the GSO this meant balance had to be spot on. On the Ob, less critical.

Only have a Glatter laser, and tested it in a jig years back; dead-on. But I concur with your experience with laser collimators in general, they should be checked for this.

Regards, and Permissive Skies and a Clear Spouse...or is that the other way 'round?

Scott

#12 DarrylS

DarrylS

    Mariner 2

  • *****
  • Posts: 214
  • Joined: 02 Sep 2007

Posted 31 March 2008 - 09:55 AM

The picture below shows one of the handles and a black vinyl coated magnetic weight (available from hobby supply stores) that I use to help balance the tube. Very handy!

Brian Gibson

Hi Brian,

Which hobby store did you buy the weights from? I'm looking for some as well...

Darryl

#13 Brian Gibson

Brian Gibson

    Vostok 1

  • *****
  • Posts: 104
  • Joined: 17 Oct 2007

Posted 31 March 2008 - 05:39 PM

Hi Darryl,

I bought them at a Michael's craft supply store which I'm sure that you have in Ottawa. They come two to a package and weigh about 6 oz each. I don't recall the price but it was reasonable. They were designed as magnetic plant hangers and have a small hook on the front surface.

Brian

#14 DarrylS

DarrylS

    Mariner 2

  • *****
  • Posts: 214
  • Joined: 02 Sep 2007

Posted 31 March 2008 - 06:49 PM

Perfect -- thanks a ton Brian!!

Darryl

#15 skypilgrim

skypilgrim

    Apollo

  • *****
  • Posts: 1211
  • Joined: 25 Dec 2006

Posted 04 April 2008 - 01:22 PM

I'd also like to chime in that this was certainly one of the best reviews on this type of dob I've seen in years. Nice tips and photo's as well. :waytogo:

The one area that I'm still struggling with is altitude motion (on my old XT6). My current thinking is that the springs are too tight. I agree that wax and Armorall don't really work. Rather than buy the Scope Gripp as suggested I wonder if a couple of turnbuckles would do the trick? Should just be a matter of dialing in the correct amount of tension. Has anyone tried turnbuckles on these scopes? :question:

Sam

#16 tannehill

tannehill

    Vostok 1

  • -----
  • Posts: 137
  • Joined: 26 Sep 2005

Posted 09 April 2008 - 07:48 PM

good thought, never tried that. I have tried key rings of various diameters, but those aren't really easily adjustable obviously.

Thought of changing the separation between the alt teflon pads variable, since moving them apart increases friction and vice versa. Haven't thought long on it, but if a easy method to make this a quick twist of a knob type action might be useful. Then again, to go to that much trouble, I probably would just make larger alt trunnions and fix the problem at its source.

Cheers

Scott






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics