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Old 26-02-2017, 11:18 AM
CraziestOzzy
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Sky-Watcher 20" Stargate 500P Dobsonian

G'day.
Joined this community to pose a discussion.
Sky Watcher have on the market their Stargate 500P Dobsonian
This Dobsonian has a 20 inch primary mirror with a relatively short focal length.
The secondary appears to be unusually large when compared with other Newtonians of similar aperture..

I have included two shops in Oz for the following links, so you know it's not blatant advertising

Link here.

...or here:

The 18 inch version the 450P, has been around for some time now and this beast is kind of new to our neck of the woods.
The 450P also has limited information regards quality of optics.
My biggest concern with this primary mirror for the 500P, is that there are no optical tests made with the mirrors or even made public.
Tests that are specific for each mirror.
I am aware that the primary is mass produced, but surely Sky Watcher could spare a few minutes and test each mirror and publish results for each mirror sold?
One would think that for almost 10,000 grand that a telescope manufacturer would at least provide such information?
I know that for the same price, I could get a custom made scope complete with optical specs from the USA, but the postage is a deal breaker.

I have looked hard and long on the internet for user reviews, seen a few but none regards optical quality to their primary or why the secondary in my opinion is unusually large.
I suspect that for such a short focal length combined with a larger aperture, some correction is needed at the eye piece end...yet again no information available.

Thought I would discuss your thoughts regards this 20" Dobsonian, before I confront Sky Watcher with some pertinent points and before I fork out the $$$ for such a beast.
As I live in Townsville, I have no way of seeing one of these scopes in the flesh, so any thoughts will be greatly appreciated.

Last edited by CraziestOzzy; 26-02-2017 at 11:19 AM. Reason: grammar
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  #2  
Old 26-02-2017, 11:57 AM
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sn1987a (Barry)
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A 20"f4 Zambuto mirror is about 9k plus shipping ( there's an 18f4 Quartz for sale on Zambutos site right now 6.5k US) . Secondhand 20 incher SDMs pop up on IIS occasionally with plenty of fruit for around 10k.

I would either buy a premium mirror and then worry about the scope, ie: build myself or buy a dob carcass sans mirror - or wait for a secondhand 20 incher with everything to pop up in the classifieds.

I have some dobs
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  #3  
Old 26-02-2017, 12:23 PM
Kunama
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As Barry mentioned, a top tier mirror is costly (actually over $10K landed or $12K if you want the quartz 20F4).

The Skywatcher secondary is listed as having a major axis of 136mm which translates to a minor axis of about 3.78" or 96mm which is not surprising for a 20" F4.0 scope.

I don't know anything about the quality of these mirror but can understand that they, like many astro manufacturers, don't want to publish any specs for fear of people, in places like the USA, then trying to handpick scopes.
In reality these are cheap scopes compared to custom ones.

@Barry, that Quartz 18F4 is very tempting !!!!
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Old 26-02-2017, 12:56 PM
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sn1987a (Barry)
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Yeah it won't last long, luckily I have too many and no money (spent it all on those Deloses) so it's safe from me.

The mirror is the main and by far the most expensive component of these scopes. If there's anything wrong with it the cost to fix is ruinous whereas all the other scope components are (by comparison) cheap and easy (ish) to fix.
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Old 27-02-2017, 07:10 AM
CraziestOzzy
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Thanks for your thoughts people.
I figured not revealing optical details from a Sky Watcher's mass produced mirror (that costs an arm and a leg to buy), was a deliberate act to prevent consumers from hand-picking the best scopes.
I will have to mull over this a bit more.
Will keep an eye of the classifieds here and dig deeper with the overseas custom makers.
Cheers.
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  #6  
Old 27-02-2017, 07:24 AM
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xstream (John)
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You could always join Peters build list.
"When only the best will do".

www.sdmtelescopes.com.au
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Old 27-02-2017, 07:53 AM
Kunama
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or get into a bit of DIY ..... very rewarding.... http://www.iceinspace.com.au/forum/s...d.php?t=135679

(when I get my next one built this one will be available..... maybe....)
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Old 27-02-2017, 08:09 AM
glend (Glen)
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The 'Seeing' conditions in your location are going to limit you far more than any marginal gains from a specialty, high priced mirror.

Last edited by glend; 27-02-2017 at 11:58 AM.
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Old 27-02-2017, 08:49 AM
JA
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Hello Ozzy,

Whilst it would be good to get an individual optical test on your device, I wouldn't expect it on such a commercial item. If it were a custom contracted item, then you could request that as part of any certification prior to purchase as part of the contract.

The best you could hope for is a written request to the manufacturer to part with some detailed test data. This may be difficult to get since commercial rivals would be happy to gobble this up and potentially use it against the manufacturer in product comparisons or to design/manufacture their gear to compete. Alternatively, if purchasing via a supplier, perhaps a savy supplier could do some of his own adhoc tests for you. Otherwise perhaps there are some basic tests of mirror figure, etc.. that you could perform at the supplier's premises.

Best
JA
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Old 27-02-2017, 11:07 AM
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Camelopardalis (Dunk)
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I wouldn't dismiss the SW 20" quite so quickly, all the SW Dobs I've looked through have given excellent views.

I hardly think that the market for the 20" mirror is sufficient to justify calling it "mass market" and that is reflected in the price, it's not like it just rolls off the line the same as all the 8", 10" and 12" mirrors...or maybe I misunderstand the manufacturing process
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Old 27-02-2017, 01:18 PM
JA
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Quote:
Originally Posted by Camelopardalis View Post
I hardly think that the market for the 20" mirror is sufficient to justify calling it "mass market" and that is reflected in the price, it's not like it just rolls off the line the same as all the 8", 10" and 12" mirrors...or maybe I misunderstand the manufacturing process
Hi Cam,

I'm not sure if that's a comment for me, regarding my "commercial item", characterisation, but if so, then I'd still suggest the same, as it's a commercial item forming part of a vendor/manufacturer's standard albeit, low volume products and not a commissioned or contract manufactured/one-off item. Much like Canon or Nikon's $10,000-$20,000+ telephoto lenses, which whilst they may be tested during manufacture, come as a final product to the purchaser, without any objective test report on optical parameters for that given lens. You can of course go online and get the generic MTF data for that model of lens as a start, but nothing specific to your individual copy of the lens. "Thanks for buying Canon / Nikon" is about it.

Best
JA
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Old 27-02-2017, 02:54 PM
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Camelopardalis (Dunk)
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Quote:
Originally Posted by JA View Post
Hi Cam,
I'm not sure if that's a comment for me, regarding my "commercial item"...
Nah, the OP mentioned mass produced, but my comment was more general

In an ideal world, we'd get to star test, bench test, whatever test our next new scope, but the reality is we rarely get to see how it really performs until we've been out under the stars with it a few times...
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Old 27-02-2017, 02:55 PM
kittenshark (Cheryl-Ann Tan)
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Well if you ask nicely the retailer might star-test for you.
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Old 28-02-2017, 08:48 AM
JA
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Quote:
Originally Posted by Camelopardalis View Post
Nah, the OP mentioned mass produced, but my comment was more general
Sorry Cam, I missed that

Best
JA
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Old 28-02-2017, 06:57 PM
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The Mekon (John Briggs)
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[QUOTE=CraziestOzzy;1297730]G'day.
Quote:
One would think that for almost 10,000 grand that a telescope manufacturer would at least provide such information?
I know that for the same price, I could get a custom made scope complete with optical specs from the USA, but the postage is a deal breaker.
As others have pointed out, your expectations of the above are a little high unless you are talking pre-owned scopes.

A 20" SDM will cost more than twice the skywatcher price.

I never received a test certificate before buying my 18" SDm, but then again, the mirror maker has an excellent reputation.

From all accounts the skywatcher mirrors though mass produced perform well. If you want to check out before buying, the head off to a few southern star parties, you may not see a 20", but there are bound to be other skywatcher reflectors on the field to give you some idea.
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Old 28-02-2017, 09:15 PM
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barx1963 (Malcolm)
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On a side note I noticed an ad the latest US Sky & Tel saying "something big is coming" from Skywatcher at NEAF in April. Maybe a 22" version, or even a 25"? The picture that accompanies it certainly indicates a big dob!!

Malcolm
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Old 28-02-2017, 09:23 PM
bigjoe (JOSEPH)
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Quote:
Originally Posted by glend View Post
The 'Seeing' conditions in your location are going to limit you far more than any marginal gains from a specialty, high priced mirror.
ABSOLUTELY TRUE GLEN!

Yet how few take this into consideration when purchasing.
No good getting Lockwood , Zambutto mirrors if your seeing is never better than 2" or so; your average SW mirror's resolution would be better , far better even than required in those conditions
Spending money unnecessarily then , unless its just for" bragging rights"
bigjoe
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Old 28-02-2017, 09:47 PM
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sn1987a (Barry)
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Buy a secondhand premium 20". If it's not what you'd hoped for then sell it and get most of your bucks back. Alternatively buy the new 20" Skywatcher. If it's not what you'd hoped for then sell the secondhand Skywatcher and get ....?
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Old 28-02-2017, 09:53 PM
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Shiraz (Ray)
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Quote:
Originally Posted by bigjoe View Post
ABSOLUTELY TRUE GLEN!

Yet how few take this into consideration when purchasing.
No good getting Lockwood , Zambutto mirrors if your seeing is never better than 2" or so; your average SW mirror's resolution would be better , far better even than required in those conditions
Spending money unnecessarily then , unless its just for" bragging rights"
bigjoe
I think that this is true for DSO imaging, but for visual use or high speed planetary imaging, even 2" seeing can occasionally stabilise for short bursts to give a diffraction limited clear view and the extra capability of a really good mirror may show through.

having said that, I cannot recall ever reading a negative review of a Skywatcher mirror, including interferometer testing (eg http://www.cloudynights.com/topic/44...of-my-mirrors/). If there was anything sub-par about them, it should have surfaced by now. There seems to be an understanding that Chinese mirrors are a bit hit or miss, but I suspect that this is one of the many self-perpetuating "truths" that persist on the internet, long after the factual basis for the idea has faded into history..

the other point in favour of the big Skywatchers is the undercut conical mirror structure, which is dead easy to mount and should come to thermal equilibrium relatively quickly - forget about mirror quality if the reflecting surface is warmer than the air or the edge supports are generating astigmatism.

Last edited by Shiraz; 28-02-2017 at 10:15 PM.
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Old 28-02-2017, 11:41 PM
glend (Glen)
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My comments were based on the OPs location, Townsville. Coastal tropic locations are not known for great seeing. I owned a very nice 16" dob for several years and it was at its best at a inland dark site location. I might add that the age of your eyes is a big factor in what you can get out of premium mirrors. According to my eye specialist, everyone's eyes lose clarity as they age, some get cataracts but most others have a subtle decrease in clarity and yellowing of the cornea that happens so slowly people don't know its happening. Year on year deteriouration affects most of us. Some of us migrate to imaging as visual acuity lessens.

Last edited by glend; 01-03-2017 at 04:08 AM.
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