Who Makes the Best Dobs?

[sabana]

Orion, Webster, Celestron, Meade etc.

When I say make. I mean use the best materials for mirror cells, mirrors, wood, focussers?

Thanks

[umadog]

Wow... Well you do realise there is an order of magnitude price difference in those companies you mention? Manufacturers such as Webster (who you mention), Teeter, Starmaster, Starstructure, Lukehurst (UK), Sumerian (Dutch), and Obsession all make their products by hand and not in a large factory. All those manufacturers work with high quality materials and all do a good job, although they do make rather different products. For example, Starstructure are aluminium, Sumerian and Lukehurst make ultra-portables, Webster focus on larger faster scopes, etc. There are other manufacturers I've failed to mention. The point is that all of those are expensive and use good materials.

Orion (the US company), Celestron, and Meade use Chinese optics (which are now rather good) and mass-produce their scopes. The products are generally good and a real bargain. Their Dobsonian mounts are made of particleboard, which is heavy. The designs have quite a high centre of gravity which results in a tall and bulky rocker. The premium makers produce significantly lighter and more low-profile designs. This is one of their major advantages. If you're handy at DIY, you could do an equally good job yourself. It's not really fair to compare the premium outfits to the mass-produced. They make different products, it depends how much you're prepared to spend.

As for the components, the premium guys mostly use focusers from Feathertouch or Moonlite. There's nothing stopping people from upgrading their Meade or Orion scope with such a focuser. Webster make their own cell. Teeter outsources this to a 3rd party. I don't know what the others do.

Mirrors from the mass-produced scopes are made by Synta or GSO. These Chinese optical houses are now pretty good but the quality is variable. Most mirrors are fine, a small proportion are stunning, and a small proportion are dogs which probably shouldn't have left the factory. The premium guys source mirrors from other places. Some are flexible: Sumerian will use whatever you like, including GSO. Others are not so flexible: Webster's stock scopes come only with Zambuto or Kennedy mirrors, although they also work with Lockwood. Obessions come with Galaxy or OMI mirrors. When you go premium you're paying a lot more for the mirror. Generally speaking, this buys you piece of mind because you're paying an experienced optician to hand-make a quality mirror. Opinions are slightly divided as to whether you will notice the difference but recently Zambuto loaned a 10" for people to stick into their mass-produced scopes and report back. The reports were uniformly very, very, good. There was, apparently, a big difference. It is possible some of this could be ascribed to the test mirror having a new, clean, coating. But that probably can't explain all the differences which were seen. It is, of course, still possible to end up with a bad mirror even from a more expensive source. However, the odds of this happening are lower and you may be able to get it corrected for free. With a GSO mirror, the best you're likely to get is a refund.

Finally, the other big factor to consider is waiting time. With Meade you can walk into a shop and buy a scope. With Obsession your looking at a two or three month wait. With other companies it may be more. I've heard of people waiting about three years for a telescope from some of the these guys. The long wait times are generally for larger mirror sizes, however.

[sunshine185]

Great write-up umadog, you certainly know your stuff. Best morning read i have had in a long time.

[Beulah]

Great write-up umadog, you certainly know your stuff. Best morning read i have had in a long time.

1+

Learn something new every day.

Thanks Umadog.

[swamp thing]

+2

I think Umadogs pretty much covered it.

Regards Steve

[Charon]

Though not a Dob owner I enjoyed the read, food for thought for a future purchase. Thanks umadog.

[Quark]

Thanks for that Umadog, great piece.

[Black Knight]

Great write-up umadog, you certainly know your stuff. Best morning read i have had in a long time.

Quoted again for the truth! Really interesting read Umadog!

[Moonshane]

+ (how many are we up to??)

umadog covered it well.

I personally went down the hybrid route. I bought a mirror, spider / secondary, tube/rings and a focuser and then made the rest myself. the result is a 16" dob which works brilliantly and is personally made for me withbthe focuser at a nice height and just about able to get into a small hatchback car.

[johninderby]

It's well to remember that no company makes a perfect dob. Even Obsession economises on some of the parts used to keep costs down. Hence the popularity of more accurately made CNC side bearings to replace the Obsession cast aluminum side bearings for one example.

Premium mirrors do cost a lot. You just can't ignore the amount of skilled hand labour it takes to make one. So if it's a Zambuto, Kennedy, Galaxy or OO UK mirror it's going to hurt your bank account.

I don't know if things have changed recently but GSO was using a random sample method of quality control on their mirrors. So while a few of each batch would be tested most were never tested before they went into the scope you might buy. Skywatcher apparently did at least do some sort of test on every mirror.

John

[Moonshane]

whilst there is definitely a difference between average and excellent quality mirrors, in the UK our seeing means that often the differences are not apparent. also, larger mirrors are often used only to view low brightness objects at lower powers and there again mirror quality matters a little less.

for me the major differences between companies is the build quality of the mount.

[Steve]

Good post Umadog, bookmarked for reference

[rwilkey]

+ me Umadog, a great piece and expertly written, I learnt a lot.

[Rich MedRev]

Excellent post umadog,

I can also vouch that a good mirror cell and the means to fine tune the collimation make a difference, particularly with faster mirrors. I myself have a 10" dob and the improvement for collimation once i replaced the original crude 3 point cell with a well engineered 9 point cell was very noticable and my mirror (F6.4) is by no means fast.

[umadog]

whilst there is definitely a difference between average and excellent quality mirrors, in the UK our seeing means that often the differences are not apparent. also, larger mirrors are often used only to view low brightness objects at lower powers and there again mirror quality matters a little less.

18 months ago I'd have agreed completely, but now I'm not so sure. There are a few reasons for that.

The first reason isn't a seeing-limited point (which I know is what you're getting at, Moonshane). Mirrors become more expensive to make as they become larger, faster, and thinner. To make a larger aperture practical the trend recently has gone towards faster optics. Making fast glass well in larger apertures is tough. Mirror thickness influences both weight and cool-down time (which is proportional to the square of the thickness). What isn't obvious on a first pass is, for example, that Obsession installs 2" glass and Webster (along with many other guys) install 1.5" glass. The thicker mirrors are easier to make and easier to mount and that allows Obsession to charge lower prices and make more telescopes. They provide different reasons on their website, but that's marketing for you. So in these respects, paying more for a better mirror does gain you something: a more compact scope, with a wider field of view, that cools faster. If such a mirror isn't properly made you'll be sorry. Of course, you'll be needing a coma corrector with this stuff...

The second reason for paying more is optical, and relates more closely to Moonshane's point. It's true that under most circumstances scopes about 10" and up are seeing limited in our skies. However, whether mirrors of different quality can be distinguished under poor seeing depends on what you mean by "average" and "excellent" quality. For reference, here is how Zambuto describes it: Zambuto Optical Company - The Seven Criteria What matters is that some things (such as surface roughness or a turned-edge, IIRC) can scatter light very far out of the Airy disk. All over the image, in fact. This scattered, out of focus, light makes the sky background brighter, masks subtle details on nebulae, and impacts planetary detail. Depending on how you quantify the mirror quality, it could look good on paper but still exhibit issues of this sort. You'll notice stuff like that under typical UK seeing. Note, though, that there are mass-produced mirrors out there which perform like premium glass.

Thirdly, are these larger mirrors used mainly for low-brightness stuff? Speaking for myself, the answer is no. I've had an 18" f/4 Zambuto since July and wasn't planning on it being a planetary scope. Who wants a large aperture f/4 for planets, right? First dark sky trip and it's 03:30. Jupiter rose at midnight but I avoided it for dark adaptation reasons. Finally, I decide to grab a look before bed. The view easily stole the night. I had never seen detail like that before. Nowadays, half the times I use that scope it's to see Jupiter from the back garden. I could set up the 10" in 5 minutes but I don't, because the 18" kicks its ass on planets. I can't tell you why the views are better but there are certainly a few factors. The big scope has tracking, which definitely helps. The larger aperture brings out the colours a lot more: the reds are redder and the purple streaks on the equator come out nicely. I also get the clear impression that contrast is higher, which one would expect from the better mirror. Whether that's true is hard to judge objectively. I do know, however, that other people are also seeing the detail (un-prompted). It's not a subtle difference. Whether a smaller mirror from the same manufacturer would look the same, I don't know. However, I know someone with a 10" Zambuto in Oxfordshire and he's getting an 18" next year so hopefully an answer to that question will be had. I also think that an equatorial platform, cooling fan, and accurate collimation will allow anyone to get more out of their GSO or Synta mirror. Masking the edge if needed or adding a coma corrector at f/5 and below can also help.

[Moonshane]

super post Umadog. a lot of info in their and I certainly learned something from it.

to be fair I actually tend to use my larger scope too nowadays because even when the seeing is poor, I can get better contrast with the 16" (1/8PV 0.984 strehl) masked off to 6.7" than I get with my 6" f11 (1/6PV).

[umadog]

Interesting. Do you find stopping it down makes a big difference? Have you tried masking the edge or have you only experimented with stopping down? I've not tried either, I must admit.

Have a read of the Zambuto link too. That makes for interesting reading. It highlights why you shouldn't put too much faith in a Strehl ratio and why not all 1/8th wave mirrors are made equal. There're a lot of ways to be 1/8th wave...

[sabana]

Thans for the detailed response Umadog. The mass produced market serves a purpose. But I often wonder about quality control. Especially with the mirror and materials used on the cells etc. I know a lot of mirrors are ground from Pyrex. And a lot from sub standard glass. This leads to an inferior image (but by how much, hmmm this very difficult to quantify).

[umadog]

I don't have much of a sample to go on, but I think the cells are generally ok. GSO and Synta mirrors are a little on the thick side so they're easier to support. I had an XX12 and never noticed any astigmatism during star testing. I've not heard of people going out of their way to upgrade the cell.

The mirrors... That's probably more of a lottery with a mass-produced scope but keep in mind that there are bad $400 Chinese mirrors and there are bad $15,000 premium mirrors. If you set up your scope next to a similar one with a known good mirrors and compare planetary views you will soon develop an appreciation for how good your mirror is. Sure, it's hard to quantify but you'll know if the views are the same or significantly different. Also, competently done bench tests don't lie and you can have a mirror re-figured should you find that to be worth your while.

[sabana]

Umadog. Is there any sort of conclusion to this question? Or are the manufacturing of Dobs for consumers more of a personal choice on the part of the buyer?

re budget versus aesthetics. Thanks

[Moonshane]

Interesting. Do you find stopping it down makes a big difference? Have you tried masking the edge or have you only experimented with stopping down? I've not tried either, I must admit.

Have a read of the Zambuto link too. That makes for interesting reading. It highlights why you shouldn't put too much faith in a Strehl ratio and why not all 1/8th wave mirrors are made equal. There're a lot of ways to be 1/8th wave...

it really does seem to improve contrast and reduce brightness and the sharpness of the image seems enhanced. this is possibly most obvious on doubles which become tighter and cleaner looking. this translates to better planetary / lunar images too when the seeing is not 'amazing'. when the seeing is amazing then aperture wins for sure.

I know what you mean about optical quality / measuring and will read the report but to be honest I rely on what I can see and don't sweat too much about how it should look.

[umadog]

Yeah, that's what I'm getting at. I too rely on what I can see. So far I've found that the larger aperture gives me smaller star images so I don't stop it down.

Sabana, I don't really know what the conclusion is. I think it depends on what you're after and what you're prepared to spend. It's always true that you get what you pay for. However, it's also true that with the premium stuff you're paying exponentially more for what, ultimately, are smaller gains. It's certainly more than just aesthetics, though.

[cyg]

A lot of information thanks

[johninderby]

A very good thread.

If you do get a dob with premium optics it makes for a great all-rounder that seems to do everything well. Increased contrast makes such a difference in planetary observing in particular.

John

[Beulah]

Also, competently done bench tests don't lie and you can have a mirror re-figured should you find that to be worth your while.

Excellent idea - I might do that the next time my Synta mirror is up for a recoat.