Guan Sheng 10" dobsonian solid tube

[Mountain_Wanderer]

Hi all,

I'm keen to get your thoughts on the Guan Sheng 10" dobsonian solid tube (GS-880 10" ASDX 250mm x 1250mm).

These babies are going for $649 at Andrews at the moment and seem to be great value for money.

I'm in the process of deciding on my first telescope and am mostly a beginner to astronomy.

Here's some text taken off the website:

"Bonus GSO 2" SV30mm eyepiece.

2" focusers and 2" to 1.25" adaptors.
Resolving power: 0.49 arc second.
Limiting magnitude: 14.5.

Also includes a right-angle correct image 8 x 50 finderscope, 2" dual-speed Crayford style microfocuser with 10:1 fine focusing, 2" to 1.25" adaptor, fan and premium GSO 9mm, 15mm and 25mm 1.25" Fully Multi-Coated Plossl eyepieces."

I'm keen to hear the comments from people who have managed to buy and use this scope.

Thanks
MW

[raymo]

If you're happy to go with a solid tube, great value for money.
raymo

[Mountain_Wanderer]

Thanks mate. Can anyone tell me what the focal ratio is on this scope? Is it good? What is a decent focal ration to accomidate the best of both worlds?

[astro744]

Originally Posted by Mountain_Wanderer

Thanks mate. Can anyone tell me what the focal ratio is on this scope? Is it good? What is a decent focal ration to accomidate the best of both worlds?

f6.3

I read this once in one of the old ATM books possibly by Albert G. Ingalls (Willman-Bell) many years ago. It is the best of both worlds, one being the rich field telescope (RFT) world and the other world being a telescope that is longer for greater power and being kinder on more simple eyepiece designs. Of course today coma correctors are more common as are much shorter and faster telescopes and of course higher quality more complex eyepieces.

Do not be too concerned about a particular f ratio. Anything from f5 to f6 is great for a typical Newtonian. f4.5 is OK but does benefit greatly from a coma corrector. f7 to f8 is good to but the optics get long in the larger diameters.

[WynneP (Peter)]

A 10" GSO dob is a great choice. I have a 12" one and I'm totally satisfied. From my experience, GSO optics are good and the various components are robust and effective. One would expect that since they are so cheap, there must be major compromises, but this doesn't appear to be the case. Also, a dob has considerable advantages in ease of setup and viewing - the eyepiece never gets int a difficult-to-view position, which I really appreciate.

Enjoy!

[Mountain_Wanderer]

Thanks guys. So what is the focal ratio for this 10" dob by GSO?

I notice that the 8" version might be a better focal ratio?

[Mountain_Wanderer]

So I should be aiming for a focal ratio around f6.3 ?

[N1 (Mirko)]

The focal length of 1250mm divided by the aperture of 250mm gives an f ratio of 5. This means that you will see some coma, especially in very wide angle eyepieces. Anything up to around 70° I find perfectly acceptable.

I have the exact same telescope (as do a few others on here) and I'm completely satisfied with it. The OTA is biggish and the whole system not exactly light weight but still manageable. The views are outstanding.

[Monstar (Johnny)]

Focal ratio=focal length/aperture=1250/250=f5

This if the same as the Heritage you were looking at, I think for basic visual it would mean it's easier on eyepieces and collimation.

If I had to choose this or the Heritage it would be this every time except the portability aspect, but I wouldn't care.

[doppler (Rick)]

And a 10"f5 will fit across the back seat of most small cars.

[astro744]

Originally Posted by Mountain_Wanderer

So I should be aiming for a focal ratio around f6.3 ?

I'll see if I can find the reference to f6.3 in some of my old books. I remember reading about it 25 years ago and I then started a 10.1" f6.3 project. 10.1" was the Pyrex blanks a company called Coulter used to sell at the time. The discussion that resulted in f6.3 was based around rich field telescopes. I seem to have a bit of deja vu here as I now remember looking for this reference once before and I think it's now not in the Ingalls ATM books but in another source I had borrowed from a public library once.

The f6.3 on a 10.1" also suited me in a Dob configuration as the height of the eyepiece off the ground was eye level at zenith for me meaning no step ladder. This was with the eyepiece at 90 deg to the side of the tube. I have since rotated the eyepiece 30 deg. so that lower viewing positions are more comfortable.

I also had my mirror professionally figured as I was never really happy with my own results due to rushing the project during figuring (a big no no in mirror making). After being corrected my 10.1" is now f6.4 and gives remarkable images of the highest contrast.

You wont get f6.3 commercially but can get f6 in an 8" Dob. However if you want 10" for brighter images, I wouldn't let f ratio be the deciding factor since a 10" f5 will perform very well and with good eyepieces even the extra coma will not likely bother you at all and unless you do use high quality eyepieces you're likely to see other aberration first at f5 or f6.

The difference in brightness is a function of surface area. Just square the radius or diameters as its the ratio your are after. e.g. an 8" is 8x8/6x6=64/36=1.78 times brighter than a 6". A 10" is 10x10/8x8=100/64=1.56 times brighter than an 8". A 10" is 100/36=2.78 times brighter than a 6".

Choose f5 or f6 and you will be happy. Best advice is to buy the biggest mirror you can afford to purchase and also afford to move around physically.

[209herschel (Herschel)]

Hi there, I bought this same scope 3 weeks ago and I'm very happy with it. There is a learning curve if you're new to this - as I still am. I'm in the innerwest of sydney and seeing conditions are not great and this is something I really didn't pay attention to. There are differences in seeing from night to night. I'm still to get to a dark sky but I'm sure it'll make a huge difference.

It's pretty easy to move around - I take the OTA off the mount, put the mount outside and carry the tube onto it. I also bought a dolly from Bunnings for $50 and I leave it on that. I roll it across the yard easily. The 30mm superview is great and I've bought a barlow. Best view of Saturn so far last night - made out the Cassini division and shades on it. The 30mm is great is viewing clusters.

All in all, an excellent buy. It's a pretty big package from the store if you're picking up, in two parts when you buy it. You need to be able to put the seats down. Having said that, out of the box and wrapping, it can fit horizontally in the bute area of medium suv.

Comes with 2 year warranty and it's the same as the Bintel 10" which doesn't have the 30mm superview. Doesn't come with instructions to assemble mount so ask them to print a copy. Final thing, mine had a 6mm in the box instead of the 30mm superview. I sent it back and they sent the 30mm immediately. They had a different deal in the last shipment so you might want to check you have the 30mm because it's my favourite.

Best of luck.

[Mountain_Wanderer]

I'm tossing up between an 8 or a 10 inch based on portability.

Just home much bulkier and heavier is a 10 inch compared to an 8 inch?

[Monstar (Johnny)]

Lol if you bring SW flex tubes into the equation as well you'll be back where you were in Sept 2010

http://www.iceinspace.com.au/forum/s...ad.php?t=65915

[Mountain_Wanderer]

yeah i remember back then. in the end i choose no scope at all and never got anything.

[Monstar (Johnny)]

I don't know if you have or not (I didn't) but I really think it would help you to get to a club of some sort to get the "feel" for some different kinds of/size of scopes.

[doppler (Rick)]

Originally Posted by Mountain_Wanderer

I'm tossing up between an 8 or a 10 inch based on portability.

Just home much bulkier and heavier is a 10 inch compared to an 8 inch?

An 8" F6 has the same tube length as a 10' F5 just a bit fatter tube. I think the astronomers rule is, what ever you buy you always wish you got the bigger one.

[peter_4059 (Peter)]

The 10" f5 GSO is a fine first scope. I'm still using mine.

[N1 (Mirko)]

To complicate matters, consider this recent posting in the classifieds:

http://www.iceinspace.com.au/forum/s...d.php?t=122820

depending on where you are, and whether 8" f/6 is still an option for you, this one would represent excellent value. Bob's knobs mods and all.