Comments on Sky&Telescope 12" Dob Review.

[markus.a.bergh]

Hello Everybody,

In this months Sky and Telescope there's a review of a 12" Skywatcher Dob. Nice review. I compared the review to my telescope - a 12" GSO and was wondering if anyone could shed some light (ha) on one of the big differences I observed.

According to the review, the telescope gave really great views at more than 200x magnification. That surprised me. I have great views up to about 75x to 125x using a Meade 5000 20 eyepiece/2x barlow combination. Higher magnifications - using budget GSO 7mm eyepice gives really crap views (more than 200x magnification). Jupiter at 200x is fuzzy and galaxies are too dark.

So why's there such a big difference?

I live/observe in a reasonably rural area, my telescope is properly collimated and the primary mirror (/optics etc) is pretty much the same quality (although its not a Pyrex mirror).

Maybe you need really really good quality eyepieces at high magnifications? Maybe the reviewer was in a really dark, desert like place? Or maybe the optics in this scope are just better - if so it would explain the $600 price difference.

Thanks for your help, Markus.

[mrsnipey]

Markus, I live less than 10km from Brisvegas. I too have a 12" and my favourite combination for viewing Saturn and Jupiter is my Baader Hyperion zoom on the 12mm setting with a 2x barlow (1500mm fl).
If my calculations are correct, thats 250x and the views are fantastic. Things start getting fuzzy on the 8mm setting though.

Having said that, most of my views of Jupiter this year have been pretty ordinary due to atmospheric conditions (especially the clouds!).

[markus.a.bergh]

I live in the Gold Coast hinterland so I should have darker skies than you do. I'm starting to think that you need good quality eyepieces for higher magnifications. Thanks for your reply.

[rmcpb (Rob)]

Make sure that your scope is fully cooled down and collimated before trying the higher magnification views.

[erick (Eric)]

Originally Posted by markus.a.bergh

I live in the Gold Coast hinterland so I should have darker skies than you do. I'm starting to think that you need good quality eyepieces for higher magnifications. Thanks for your reply.

I think that you are right. Best to see if you can borrow one from someone first - at a star party. If you have to buy now, unless you can spend a lot of money (eg. Televue Ethos or Pentax), I suggest that you look at Televue plossl eyepieces or the Vixen LV (which will give better eye relief than the plossl). Both these should give a better views than the GSO plossls.

Yes - agree with collimation, well-cooled mirrror and good seeing conditions as pre-requisites.

[markus.a.bergh]

I'v given up on cheap eyepices. The Nagle 13mm VI looks good and currently cheaper then a Pentax XW, but the eyereleif doesnt look too good on the nagler 13mm VI. Thanks for the help, Markus.

[erick (Eric)]

Great! Pick a couple of likely eyepieces then search for them on this site - lots of threads give people's experiences. Or start a new thread to ask specific questions about specific eyepieces.

[Geoff45 (Geoff)]

If a 12" can't take 200x magnification, then one (or more) of the following applies:
1. The optics (main mirror or diagonal) are faulty
2. The collimation is off
3. The seeing is bad
4. The eyepiece is a bad one.

I reckon that in most cases, it would be points 2 or 3.

BTW it is normal for galaxies to darken with higher mag--you are spreading the same amount of light over a wider area

[Starkler (Geoff)]

Originally Posted by ghsmith45

If a 12" can't take 200x magnification, then one (or more) of the following applies:
1. The optics (main mirror or diagonal) are faulty
2. The collimation is off
3. The seeing is bad
4. The eyepiece is a bad one.

5. Your mirrors haven't cooled to ambient. Good high power views depend on this.

It is more likely for the optics to be pinched by incorrect mounting than for the optics to be lemons. Check for nice round rings on an out of focus star test collapsing down to a point of light as you come to focus.

Do the rings look nice and solid or are they wavering and mushy? If not it indicates poor seeing or optics which haven't reached ambient temperature.
Experience will tell you which is the cause.

BTW it is normal for galaxies to darken with higher mag--you are spreading the same amount of light over a wider area

Very true.

[ngcles]

Hi Markus & All,

Thanks for the comment on the review mate -- much appreciated. I'm not fishing for feedback, but sometimes (in fact a lot of times) when you write something you do wonder whether anyone reads it at all. Sometimes you hear something (positive or negative), very often nothing at all -- stony silence. It's a curious thing.

In addition to what others have suggested on what may or may not be a difference between your 'scope and others. You will notice from the article itself, the night I conducted the initial test was unusually good seeing. In fact it was probably the best seeing I've experienced from Sydney for many, many months, if not a year or more. So, high magnifications were much easier to achieve with a stable, pleasant-to-view image.

It doesn't happen that often that the atmosphere will co-operate to the extent that a 12" telescope will perform to near its resolution limits. I didn't check any very close doubles to quantify the seeing, but I'd reckon it was running at about 0.5-0.8 arc-secs. Eta Orionis which is pretty close (about 1.2"??) looked like you could drive a truck through the gap. Certainly Seeing like that does't happen every night -- it is rare to very rare in Sydney and probably in Australia.

And yes, I was using quality eyepieces. Televue plossls (12 YO vintage) and Naglers.


Best,

Les D

[Alchemy (Clive)]

interesting, i read the article(see more than one read it), and as Les has done a review on both the scopes.... the gso was a while back..... would Les have an opinion as to whether the Skywatcher 12 inch is an improvement on the GSO (given the direction of this thread that the underlying consideration).

clive.

[GrahamL]

hi markus

Seeing has been pretty woefull this year for the most part around our way ..has your secondary been dewing up constantly? ..mine has .

200+ just hasn't been possible lately I found

[erick (Eric)]

Originally Posted by Alchemy

interesting, i read the article(see more than one read it), and as Les has done a review on both the scopes.... the gso was a while back..... would Les have an opinion as to whether the Skywatcher 12 inch is an improvement on the GSO (given the direction of this thread that the underlying consideration).

clive.

Which brings to mind a related thread I started:-

http://www.iceinspace.com.au/forum/s...ad.php?t=32880

which didn't come to a conclusion.

ps. when was the GSO review please? Maybe before I started reading the mag?

[Alchemy (Clive)]

it took a bit of searching but its the NOV/DEC 2003 issue of Sky and Space page 64 .... hasnt been published for a little while , you may be able to order a back issue perhaps? a brief synopsis is Mirror good, cell good for value but needs improvement, secondary bigger than necessary....the rest of it pretty much standard for any review.

[erick (Eric)]

Yes Clive, that's before my time. Thanks. I would expect the scopes have moved on over 4+ years since that review.

[Alchemy (Clive)]

i think ultimately, a side by side test, maybee this could be done at a star party... i had a quick squiz at the mirror cell holder and it is different on the GSO than the one in the old review. for the 800 odd dollars i reckon the 12 inch gso is pretty good value. Going through the old mags check out these prices 2001 /2

10 inch gso dob 1199
nagler 31mm type 5 1430
panoptic 27mm 775
orion skyquest 10 inch 1999
nexstar 11 gps 7980
nexstar 8 3750
sbig st6 5560
televue 102 5399

things have improved somewhat

[Screwdriverone (Chris)]

Originally Posted by ngcles

Hi Markus & All,

Thanks for the comment on the review mate -- much appreciated. I'm not fishing for feedback, but sometimes (in fact a lot of times) when you write something you do wonder whether anyone reads it at all. Sometimes you hear something (positive or negative), very often nothing at all -- stony silence. Its a curious thing.

Best,

Les D

Hi Markus & Les,

Les,
On the topic of whether anyone may read the review, I for one, did. I thought it was an excellent article, gave an exhaustive test of the capabilities and features of the scope and was glad to see it performed admirably against your own one (great idea to compare the two) and thought if it could barely be shaded by yours on Saturn, then it must be a good thing. [One of my reasons for reading it is my desire for a 10-12" dob in the future and comparing the GSO and the Skywatcher 880-980 ones had me wondering if the Skywatcher had good views. Your article has helped me choose the SW collapsible 10 or 12 inch as the next weapon of choice ] A fine read! Well done.

With respect to Markus' question re the GSO 12" and the Skywatcher mentioned in Les' AS&T article, I think that mirror ambient is probably one of the issues here as well as collimation. I have looked through an 8 inch GSO (AlexK13's from Bintel) after minimal ambient cooling and noticed a difference between the views then and later on in the night. Things just seemed to "settle down" as time went by.

As magnification increases, tolerance for miscollimation issues decreases dramatically, something I found out on my Skywatcher 5 inch Newtonian which really had me in a twist for a while! Make sure your collimation is pretty much spot on and then everything should start to improve a LOT! When my scope was out of collimation, NOTHING seemed sharp, I couldnt focus on Jupiter at all with anything over 100X mag and almost threw away all my non ED eyepieces. When I had the collimation sorted out, every eyepiece now gives great views with respect to their magnifications.

Summing up (another one of my essays ) Collimate first, make sure your scope is cooled down, and then you should see a marked improvement over what you currently see now.

Cheers

Chris

[ngcles]

Hi All,

Yep, it was the Nov/Dec 2003 issue of Sky & Space where I did the review on the GSO dob. I've got a copy of it lying around somewhere -- probably filed out in the shed somewhere inaccessible so I'll go from memory as I really don't know how long it would take me to find it.

I'll say this up front -- I'm not going to cast a definitive vote for one over the other. That would be decidedly unfair on both telescopes as they were nearly 5 years apart and I’m going on memory for one of them. I haven't carefully examined a GSO for a while and things might well have changed from the one I looked at — which was a good ‘scope as I said. There is no way that I could do a comparison/evaluation and make a value judgement on that basis. What I’ve written below is a few notes on (my memory) of the two reviews 5 years apart and should be strictly viewed in that context. More importantly it would be unfair on both magazines to cast a vote either way. Again, please bear in mind I'm going on memory — the GSO test was five years ago.

The Skywatcher is a few hunderd dollars more expensive than the GSO. The exact extent of the difference depends on where and when you shop, so shop around for the best deal on either unit.

The execution of both 'scopes in many cases is basically pretty similar. The GSO I tested back then had (I think) a rack and peanut focuser which works fine but a crayford is definitely better. GSO now offer that and it comes with reduction gearing for micro-focuser. having said that, the standard focuser (no microfocuser) on the skywatcher I tested was very good and pleasant to use, but did have a wobbly set-screw -- which I wouldn't expect on every one. I assume it was in all likelihood a simple one-off manufacturing fault.

The paintwork/finish of the tube on the skywatcher was certainly of a high standard and one thing I didn't include (couldn't fit in the space) in the article was the "seamless" tube — the tube has no folded seam like other Newtonian tubes but is instead welded — and I couldn’t pick the weld. Very nice workmanship. The matte black on the inside of the skywatcher was definitely a better finish. But, if I owned either, I'd be installing German black cotton velvet anyway.

Leaving aside colours, the dob base/mount on both is basically similar. The edges/trim of the skywatcher just looked/felt a bit tighter and the GSO I looked at 5 years ago just not quite as well sealed at the joints. Of course that can be fixed with a bit of silicone goo. As I remember, the execution of the altitude bearing is a bit different -- the GSO is a more traditional approach. Neither are bad at all. The Skywatcher's base has to be a bit wider/bigger to accommodate the trunnions inside the side-tines of the mount. The skywatcher’s design also provides, in effect, a clutch on the altitude bearing that is different to the GSO spring system.

The lazy-susan flat bearing making the azimuth bearing in both cases is functionally, if not essentially identical. In the case of the Skywatcher it is sandwiched between thin metal plates. In the GSO one I tested (again, 5 years ago) that didn't have the plates or similar. The bearing had been packed immobile and apparently under a little pressure (for who knows how long) between the groundboard and the bottom of the rocker. As a consequence, the rollers had left little dents in the formica on the bottom of the rocker so that when it was rotated in azimuth, it sounded (and felt a little) like a train going over the joins in the track -- gadunk, gadunk, gadunk, gadunk, gadunk, gadunk, ... etc. I can’t remember whether I mentioned that in the review and again, this was apparently a one-off fault or at least very uncommon. It could easily be cured in any event by sandwiching the lazy-susan bearing between a couple of old-fashioned LP records you can buy at any op-shop for 50c each (and already have pre-drilled centre holes).

The mirror cells were again very similar in design except that the skywatcher is all metal and more robust. The collimation adjustments on the skywatcher were very fuss-free and no tool. There were quite a few plastic components on the GSO in the cells. Both were 9 point cells. The spider vanes on the GSO I looked at were somewhat thicker (though not too bad) and would produce more diffraction effects over the skywatcher. Has this changed? Dunno. Is there still plastic in the GSO cells? Dunno.

Some GSO's have pyrex mirrors while others are float-glass (plate glass) substrate — you get the choice depending on the model. Pyrex is a decidedly better material because of the very low coefficient of expansion, but float-glass is cheaper. The pyrex mirror will hold a better figure in all temperatures. The Skywatchers are all pyrex mirrors.

I’m pretty sure the GSO I tested was a pyrex mirror. The optics were very good and lost little to my own 12”. I was very, very impressed by the smoothness and figure on the Skywatcher’s optics, though for purely visual use, it could easily accommodate a smaller secondary but still have a good-sized 100% illuminated field.

Bottom line is the Skywatcher is a very good ‘scope out of the box as I said in the review. Does this justify the price difference? That’s up to you. Is it good value for $1300 (again, shop around they can be got cheaper)? I reckon so.

Put in context, back in 1998, I made (assembled) a 12” Newtonian tube assembly for my Samson GEM. The mirror is a 50mm thick 307mm diameter f/5.3 pyrex Deep Sky Optics mirror by Mark Suchting (Satchmo). It is an essentially flawless mirror. In its aperture class I’ve not seen one better — period. Mark actually made it for a project for himself and then changed his mind and sold it to me. So, it is not only a Suchting mirror, it was one made for the maker himself— I think you get the picture. The tube is a 70/30 split tube of 1mm thick sheet aluminium which is fully baffled (9 baffles) and is cleverly reinforced. It was engineered by Gary Mitchell. The tube is so solid, you can sit on it (I weigh 100kg) without deformation. The cells are Novak, the focuser is a JMI NGF-2. Despite all that it weighs in at just 19kg! Frankly, it is as good as a visual-use 12” Newtonian can get, simple as that. With a larger secondary it would be a brilliant astrograph too. But, in 1998 that tube assembly alone cost me a total of nearly $2,600- to assemble myself.

How much are the skywatcher (or indeed GSOs) by comparison for a complete telescope with eyepieces?

Thanks very much for the comments on the S&T test report -- much appreciated. Glad it was of value to someone.


Best,

Les D

[erick (Eric)]

Many thanks Les. That is very useful information. Thanks for the time you put into passing it onto us.

Eric

[Satchmo]

Originally Posted by ngcles

The mirror is a 50mm thick 307mm diameter f/5.3 pyrex Deep Sky Optics mirror by Mark Suchting (Satchmo). It is an essentially flawless mirror. In its aperture class I’ve not seen one better — period. Mark actually made it for a project for himself and then changed his mind and sold it to me. So, it is not only a Suchting mirror, it was one made for the maker himself— I think you get the picture.

Hi Les, My records show that you ordered a 2" thick specifically 54" custom focal length mirror and received one, after a number of months lead-time. I do not have a different standard of mirror for myself vs anyone else, and never have had. Please PM if you want to debate that further.

If your purpose was to spell out to the public, that 'by comparison' with your own premium mirror, that you thought your mass-produced optic review telescope was optically really , really , really ,really good, then you probably succeeded.

As to the rest of your optical review and comparison with a review telescope its hard to comment other than the obvious missing comparative technical details of the 'experiment' , like coatings and relative thermo-mechanical conditions , my colleagues and I are asking the same question: What exactly is your point on the basis of a sample of one , with specific reference to the average person who will buy such a mass -produced telescope on the street ?

Regards
Satchmo