Optical quality of GSO dobsonians

[Argonavis]

I visited a local telescope vendor on Friday and inspected his rebadged GSO dobsonians. The price of these are amazing - for the 12 inch model about $1395. The altitude and azimuth movements were very smooth, which makes it very usable at higher powers. The bearing are friction adjustable. The focuser movement was also very smooth and impressive. Overall I am very impressed with the quality offered for the price. The only thing I don't like is the rocker box made from cheap melamite materials rather than something more durable like plywood.

The owner assured me that the mirrors are pyrex (unlike other GSO dobs, which are BK7 glass) and that the figure is 1/12 wave. Presumably this means that it is 1/12th wave Peak to Valley, which translates as 1/6 wave deviation from the ideal parabolodial. This squeeks inside the 1/4 wave Rayleigh criterion, so they are correct in calling them diffraction limited. He told me that this is very good for a machine produced optic. I would tend to agree, although machine produced optics tend to suffer micro-ripple surfaces. This would not be detectable using a raw wavefront error rating. Maybe these are very smooth, as he assured me (continuing his sales spiel) that one of his sales agents reports extensive details on Jupiter using the 8inch model (How many bands? really?).

I was wondering what experience people had had with these Guan Sheng Skywatcher/BT/Southern Cross telescopes? I would love to test one against my 12inch f7 of known very high quality, but I don't have a spare $1.5k just to satisy my curiosity.

[iceman]

ditto AN, I guess it just depends what you're comparing it to. All of the GSO optics in the 10" and 12" models are f/5, which would make comparing against your f/7 quite difficult.

Quite funny hearing about the sales spiel.. 2 bands on jupiter, i bet :)

I guess when you look at some of the imaging done with these optics, both planetary and deep sky stuff, and compare against a much more expensive Meade or Celestron SCT, they're producing good quality stuff.

Though again, I don't know that anyone has ever done a side by side of a GS scope versus a custom made scope with a premium mirror.

[[1ponders]]

Bill, both Nightshift and jjjnettie have the GSO Dobs. I know NS has the 12" and I'm pretty sure Jeanette does as well. Maybe you could bring your f/7 up to Ron's and compare.

[Argonavis]

[QUOTE=iceman]ditto AN, I guess it just depends what you're comparing it to. All of the GSO optics in the 10" and 12" models are f/5, which would make comparing against your f/7 quite difficult.[QUOTE]

I don't see why. Provided magnification is held constant, there should be no difference in optical quality between an f5 and an f7. Granted, getting a diffraction limited f5 is a bigger ask for the optician, but if I am assured that the f5 is diffraction limited at 1/12th wave, they should be directly comparable, ie what you see in the eyepiece should represent the optical quality (subject to seeing).

[Argonavis]

at f7, its a bit of a beast to move, especially as I traded in my station wagon for a sedan. It doesn't even fit in my trailer!!

[[1ponders]]

That could be a problem. :confuse3:

[slice of heaven]

I have both a 12" F5 gso and a 12.5" F5 Parks.
I've only really deliberately compared them side by side a few times (Deep Impact was one time)
'Optically' the Parks is a step above the GS fractionally. The view seems crisper and the contrast seems marginally better(even though the secondary is nearly 20mm larger). There's slightly less coma on the Parks.
I'm still happy to use the GS scope after the kids have finished, rather than drag the Parks out as well,as the GS gives great views. It might lack a little but unless I really want to get the most detail possible out of an object or an event, I'm really not that fussed (optically)over which one I use.

[Argonavis]

Thank you Slice - this is an interesting endorsement. They certainly appear to be excellent value for money.

[janoskiss]

How much is the Parks worth, Slice?

[Argonavis]

"Compare", as the post-modern educationalist would remin us, can be a damaging place to be. Often people are happy with what they have and it may be best that they do not get the opportunity to look through something better. Aperture fever can result, for which there is no cure but more aperture, in a never ending obsession.

[[1ponders]]

Mmmmmmm...Obsession...Algghhhhh. True I think nightshift is a bit hooked on aperture atm. He's gone up from an ETX90 to a 12" GSO. Shouldn't be too hard to convince him to go for a 16" or 20" truss Dob

[Argonavis]

I am setting up a chapter of Aperture Anonymous, with a 12 point plan, and a committment to seeing the Universe as it really is....in a 100mm refractor

[Argonavis]

I remember comparing the views through Volan's then 8 inch reflector and my then 13.1 inch reflector, and being very surprised to find that many DSO's looked substantially the same through either telescope. The larger scope only comes into it own when the seeing is good, which is probably why Volans purchased it from me some time after.

[slice of heaven]

That is where the GS seems to reign supreme...value for money.

[asimov]

I've Never had the opportunity to compared my Parks 12.5" to another breed reflector of the same size. Pity.

[slice of heaven]

It cost me a good days pay over 10yrs ago (second hand)

I don't think their competitively priced these days to even consider IMO, their quality throughout IS outstanding though.

[ausastronomer]

AN, there can be a difference between an F5 and an F7 newtonian scope given equal optical quality, particularly if the F5 scope is a tube scope and not a truss scope.

With a tubed scope you need a larger secondary mirror due to the necessity of the tube to have a larger diameter than the Upper Cage Assembly on a truss scope. This is necessary to alleviate tube currents in the tubed scope which are not an issue in the truss scope. In addition to this an F7 scope regardless of design will always have a smaller secondary than an F5 scope. The larger secondary on the F5 scope can lead to a loss of contrast, particularly if it is over 20% by diameter.

An F7 scope will have a much larger diffraction limited field of view. This means off axis aberrations will be reduced and images better quality off axis. Notably coma and field curvature are reduced in the F7 scope.

Its also a lot easier to grind say a 1/10th wave F7 mirror than it is to grind a 1/10th wave F5 mirror. In addition to this, as the mirror gets larger it also gets harder to grind an outstanding one, due to the steeper radius of curvature and greater parabolisation required to correct for spherical aberration. Hence the likelihood of a great F5 mirror under mass production is slightly less than the likelihood of a great F7 mirror.

The above having been said, with the exceptional quality of fast mirrors from premium mirror makers like Mark Suchting, Carl Zambuto, Bob Royce etc, you would be hard pressed to tell the difference at the eyepiece, using premium eyepieces. Instruments certainly could detect the difference. This is particularly so with the F5 scope being a truss design with the Central Obstruction under 20%.

There is some worthwhile reading about the effects of F-Ratio and Central Obstruction on Bob Royce's website:-

http://www.rfroyce.com/mirror_performance.htm

http://www.rfroyce.com/optimum.htm

The main difference with mass produced scopes like the 12" GSO dobs is that the quality of the mirror can vary a lot more than it will from a premium mirror maker. I have used several samples of the 12" GSO dobs and they have all been good to very good, certainly no lemons. I am yet to see a 12" GSO dob that was outstanding optically.

I have seen a large number of 10" dobs and they have ranged in quality from good to very good to outstanding. A couple of the 8" scopes I have used have also been excellent optically. My own 10"/F5 GSO dob has an outstanding mirror in it. A fellow AS of NSW member and very experienced mirror maker (David Collis Bird) commented to me a few years ago how good the mirror in it was. He said, "I can't believe they can make a mirror this good for this amount of money, it is better than 1/8th wave and would have a strehl over .96" Several of the guys on the forum have viewed Jupiter and Saturn through it at high power as recently as last Saturday and could attest to its quality. The problem however is how many do they make this good ? Probably 1 in 20 at best, with a premium mirror maker they are all this good or a tad better. Given that all the GSO scopes are as a worst case scenario good and ranging to well above good, they represent excellent value for money.

Mark Suchting is a forum member under the name of Lambda20 and could probably explain all this a lot better than I have.

CS-John B

[Argonavis]

John - excellent point. I did mean on-axis images, but didn't make that clear. Off-axis, things do get hairy. And as a telescope maker, we have a limited choice of semi-major axis diameter diagonals to choose from (unless you make your own, which is unusual). So the diagonal size used for a f5 is probably (but not necessarily) the same as for a f7.

Impressed with your 10inch GSO, I am green. I think 10inch f5 is the ideal scope, considering light gathering, conpactness and portability.

[AstroJunk]

And boy did it cost me looking through your 25" ;)