Sky-Watcher 20" Stargate 500P Dobsonian

[CraziestOzzy]

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 :D

Link here.
(https://myastroshop.com.au/products/details.asp?id=MAS-041P)
...or here: (http://www.ozscopes.com.au/skywatcher-stargate-goto-20-collapsible-dobsonian-telescope.html)

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.

[sn1987a]

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 :D

[Kunama]

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 !!!!

[sn1987a]

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.:lol:

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. :D

[CraziestOzzy]

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.

[xstream]

You could always join Peters build list.
"When only the best will do". :D

www.sdmtelescopes.com.au

[Kunama]

or get into a bit of DIY ..... very rewarding.... http://www.iceinspace.com.au/forum/showthread.php?t=135679

(when I get my next one built this one will be available..... maybe....)

[glend]

The 'Seeing' conditions in your location are going to limit you far more than any marginal gains from a specialty, high priced mirror.

[JA]

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

[Camelopardalis]

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 ;)

[JA]

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

[Camelopardalis]

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...

[kittenshark]

Well if you ask nicely the retailer might star-test for you. ;)

[JA]

Sorry Cam, I missed that:D

Best
JA

[The Mekon]

[QUOTE=CraziestOzzy;1297730]G'day.
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.

[barx1963]

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

[bigjoe]

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

[sn1987a]

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 ....? :D

[Shiraz]

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/441194-mr-rohr%C2%B4s-test-reports-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.

[glend]

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.

[Shiraz]

good point - a top quality mirror would be of no value with some older eyes. Might be an additional factor that the OP could consider.

[CraziestOzzy]

Thanks for the replies people.
I have looked far and wide for some optical tests on the 20" mirror, didn't find anything as yet.
Figuring Stargate 20" is still a 'new' mirror.
I did find this video (not in English), that covers the earlier 18" version and if you are hard for time, go to 9:45 of the video to view the good stuff.

Video HERE. (https://www.youtube.com/watch?v=xu_ce46wUHw)

Seeing conditions up here (Townsville) are not that great for at least half of the year, when the humidity is high and upper wind shear strongest.
I do have close access to a mountain that is just shy of a 1000m above sea level and will be using this location a lot during the Winter for visual observing.

Reasoning for seeking a large aperture for visual observing, is to grab as much light as possible for viewing through a low magnification and wide field eye-piece.
The down side with this approach, is that high magnification eye-pieces will be next to useless if the atmospheric conditions are not ideal.
Which is why I will settle for low magnification and wide field viewing with the option of high magnification if the atmospheric conditions are right.

Also, having dabbled over the years with astronomy, I roughly know what I want (cough) and want to get my first ever big purchase right the first time, without spending extra money on future bigger aperture scopes.

I have starting looking around Brisbane area for the availability of the 20" and asking questions (nicely ;) ) and talking shop.
Seems the scopes are very limited in supply in Oz, so might have to wait a bit for one to be available.

Having said all that, thanks for your help...and when I do get my hands on one of these beasts, I will for sure place an unbiased and technical review of this scope...which is sorely lacking at the moment :hi:

[dannat]

pretty sure MAtt from telescopes-astronomy has an 18" demo model 4 sale if you're interested

[gaseous]

Have you seen the process involved in setting up the 18" or 20 Skywatcher? There's a YouTube video showing a chap (from Australia and on IIS I believe), setting one up, and the thought of going through that process in the dark every time gave me chills. Perhaps I'm a Luddite, but it seemed more hassle than a supposedly peaceful and relaxing hobby should warrant.

[CraziestOzzy]

Yeah mate, seen all available vids from Russian, Greek through to English.
Even had a look at the ones where they actually unpack the gear on delivery.
Fairly amazing in my view, is the ease of transport and setting up for each session.
Just crazy looking at the boxes that come shipped on delivery.
I would bet that there will be a few dry runs in the home, to have setting up down pat and would prefer setting up on location during daylight :thumbsup:
Great chance for Zen moments while setting up, with promise of a brew at the end of it...but that's me.

[Troy]

That's one hard site to navigate where is the 18" demo?

[dannat]

it was on his newsletter, check the last one he sent out, also he had a skywatcher clearance sale list

[CraziestOzzy]

With the end to the 2017 NEAP, a post over on Cloudy Nights forums was revived.

LINK here
(https://www.cloudynights.com/topic/520481-skywatcher%E2%80%99s-20-goto-truss-dob/)
Some incite into the quality of the mirror, with mixed thoughts providing a balanced opinion to both sides of good and bad.
One thing for me to keep an eye out on with any future reviews that may hopefully surface, is the astigmatism from either primary or secondary due to the way they are both mounted to hard surface.
I believe that the design of the primary (while nothing new there), is sound as is also the mounting of the primary...but time will hopefully reveal some qualitative reviews.

A quirk I have noticed and one to make sure of...is to ensure that the telescope to be purchased is in fact a new model with the most recent hardware enhancements and corrections.
Seems to be some earlier production runs out there.

[gaseous]

It's amazing that these have been around for 18 months or so but there are very few (that I've found) reviews on the mirror/observing quality. A lot of conjecture due to the assembly method, a lot of detrimental comparisons to prestige mirror makers (?) but no real reviews by expert observers.

[CraziestOzzy]

Yeah, totally agree with you mate on all points.
I am hoping that with exposure (and recent availability) to the USA consumer, such voids will be filled shortly and watching the WWW closely.
Half way to saving the pennies on this 'scope, but will await for some unbiased reviews and actual real world testing, before I part with 10 grand.
Some reviews from the Netherlands and France...but they both bogged down with the usual Western bias against cheap Chinese manufacturing.

One thing is for certain should I ever buy this 'scope...I am going to ditch that flimsy thin plastic mirror protector and replace with a sturdier cover.
Having watched a few videos on the assembly of this 'scope, the supplied plastic cover will not seat and lock firmly and protect the primary against secondary dropping onto it or the truss tubes accidentally knocking against the side or face of the mirror.
One video actually shows large chunks of broken plastic on the sides of the mirror cover...so it is a safe bet the truss tubes will knock the mirror sides during assembly and require a more solid mirror protector.

[sn1987a]

5 people who want one get together chip in 2k each, do a review and put everyone else on iis out of their misery. That way if its a POS you've only blown 20% of your money and you'll live to fight another day.:P

[Rickster]

Hi all,
I just purchased one of these babies and will pick up this week.
I'll give you a summary after I give it a decent shakedown.

Rick

[gaseous]

Good man! Can't wait to hear your thoughts.

[electric]

Hi everyone!

My first post!

At the South Pacific Star Party, Serge from Astro Devices had his Skywatcher 18" dob set up next to Alex's 17.5" from Gondwana Telescopes. I got to use both scopes. BIG difference between the two.

Both sets of optics were really nice. The difference was the structures. The action of Alex's dob was silky smooth, needing just gentle effort to move. So nice to use.

Serge's dob is a backlash monster, so stiff and difficult to move, and it bounces!. It was impossible to manually track at high magnification. A real disappointment for all its flash appearance. With all the talk about these dobs from Skywatcher you would have thought the build quality would be up to scratch. But you get what you pay for, don't you.

[Steffen]

Alex, being an experienced dob user himself, pays extraordinary attention to the usability of his scopes. I don't think any mass-produced-to-a-price-point scope can ever match that.

That said, the bearing mechanics seem to have been the Achilles heel of SkyWatcher dobs all along, ultra-light or traditional. They just don't feel nice. The similarly priced GSO dobs in comparison are buttery smooth and easy to use, even though their optics aren't as highly regarded as SkyWatcher's.

[Rickster]

Hi All,

Just a quick update on my 20” Stargate goto that I constructed last weekend.
At this stage I have set it up inside to check operation and familiarize myself with this beast.
My first impressions of construction, quality etc are all 100% positive.
Contrary to some of the 2nd party negative comments about smoothness of operation etc, I haven’t experienced these issues.
Once the scope was balanced, I did need to add weight to do this correctly and I also use a binoviewer which is quite heavy, the manual operation was fine. Not sure why others may have experienced problems with this? If I had problems as described in previous posts below, I’d be taking it back for a refund!
I also put the light shroud on which I did easily on my own. I’ve heard a few comments about issues with this but I didn’t experience any.
Once manual operation was correct, I then engaged the goto drive motors and ran it through its paces. Very smooth and very impressive.
So in summary, more than happy with my purchase!
Next step is to point it at the heavens.
Cheers
Rick

[gaseous]

Good to hear Rick! Now that you've done it a few times, what's the average time to set it up? Looking forward to your first light report.

[electric]

Rick, great to read your experience is a good one.

If you are referring to my post, my experience was not with the goto model, but the push-to.

At SPSP, Skywatcher had a 20" goto. Obviously not your scope as it sat unattended and unused on the Saturday night, laid up with major mirror issues. I am surprised that this has not been brought up at all here. The Skywatcher flagship scope sits idle, much like the Australian Navy's flagship... :lol:

I sincerely hope your mirror is a good one :) No one drops this amount of money for anything but a good experience :thumbsup:

[AstroJunk]

But in the few nights in any decade when the seeing is perfect, I know what quality telescope mirror I want to be using and I say this from experience. To have the view of a lifetime, you have to have a mirror that is figured to perfection and anything less is settling for a lifetime of mediocrity.

BTW I have seen through a number of truly fabulous Skywatcher mirrors, but have yet to use a big un. I expect that they are pretty good though, and look forward to getting some eyepiece time with one :)

[Rickster]

Hi gaseous,

I'd say once familiar with the setup, it should take an average of 20 mins to complete and be ready for viewing. Less if you leave the mirror cradle on the base.
I did this on my own, but would be easier with two people. The base and mirror cradle are quite heavy.

Cheers

[Rickster]

Hi electric,

Yes, as I stated in my previous post, I haven't experienced the issues that you mention. With the clutches disengaged the operation is smooth in manual mode. Anything less and I would be asking for a refund!
As far as the mirror is concerned, I have only heard good reports but I'm yet to view so I cant comment as yet.

Cheers

[Rickster]

Set up my rig for a brief viewing in a light polluted limited viewing window area last Friday night.
After collimation was complete I setup goto and pointed to some DSOs. I could only view some 6 to 7 magnitude targets but the view was amazing.
Next I pointed to Jupiter and Saturn but for some reason I couldn't get sharp images and also couldn't get my binoviewers to work. I need to look into this further. Not sure if the viewing conditions were bad but planets were disappointing. Next time I will setup my 14" dob also and compare. With the 14" I get great views of planets and binoviewers work a treat.
Surely a 20" mirror should give me more detail in planetary views? Any ideas?

Cheers

Rick

[gaseous]

I've heard binoviewers can be ornery critters depending on your available focusser travel. In a skywatcher dob you can adjust the poles to get the necessary in-travel, but not sure what you could do with the stargate if you're struggling to reach focus.

Viewing-wise, the seeing may have been a bit poor, resulting in sub-optimal planetary viewing, although when I bought my 16" dob I was told the planets wouldn't be as good as in my 8" dob. I didn't really delve too deeply into the whys and hows, but I'm sure there's a reason why this may be the case - maybe the larger aperture creates an image that's too bright?

[raymo]

The larger a scope's aperture, the more it is affected by poor seeing.
On nights of good seeing or better, your 20" will blow your 14" into the weeds,
but on fair to average nights you won't see much difference [on the
planets]. You could make up a matt black cardboard aperture stop, say 16" and
see if it helps on nights of poor seeing.
raymo

[Rickster]

Thanks for the info,

I think the viewing conditions were poor but the DSOs were impressive. Cant wait for good seeing conditions!
I'll give the cardboard aperture stop a try if I cant get better results.

In regards to my binoviewers, I use a 2x Barlow and it works great with my f4.5 14" Dob. In fact I now mainly use the binoviewers with this telescope because they are the perfect for most of my observing.
It seemed with the 20" that I could get focus but the image was blurry, I need to setup again and try and work out what's going on. Unfortunately, weather is iffy in Adelaide at this time. I guess it is winter.....
There's a youtube vid where they setup binoviewers with the 20" Stargate. They're different type to mine but need to find out what they use to make it work.

Cheers

Rick

[AstroJunk]

As Raymo says; Seeing.

I have a top draw 20" with a superb mirror that on a good night can split Sirius wide open and give planetary views like a Go/Peach image. On a bad night, I might as well be using a $30 scope from toys'r'us!!!

When that stunning night comes, you will know just how well that telescope can perform :thumbsup:

[Rickster]

Thanks all for the info, I didn't realise images would be worse on a bad viewing night with a larger mirror. I just expected them to be better than a smaller telescope on a bad night or any night.
Looking forward to some good viewing nights :thumbsup:

Cheers

[sopticals]

Always find larger apertures give more detailed views regardless of seeing. Comparing 25" and 33" to 14" (and less), is like comparing 18 mega pixel visual images to 12 (and less) mega pixel ones.

Stephen.

[tonybarry]

Fellow club member Ted Dobosz recently circulated this article about aperture and seeing:-

http://www.skyandtelescope.com/astronomy-equipment/beating-the-seeing/

It contains many gems of wisdom. A take-home message might be that aperture is always going to help, where it can. But it will not help where it can't ... if the seeing is bad, then it's bad for every telescope.

Regards,
Tony Barry
WSAAG

[Steffen]

I cannot agree without a couple of important caveats.

A larger instrument will obviously show details that are too dim to be seen in a smaller one. A lot of detail in faint fuzzies can only be seen from certain apertures up. This is simply a matter of light gathering.

With bright objects (moon, planets, many double stars) on the other hand I find that smaller scopes outperform larger ones quite regularly. For example, my puny 6" Mak routinely throws up better views of the planets than our club 30". I believe that is because 6" scopes can be made with excellent optics while still being affordable for the average amateur. Large telescopes cannot. And no, I don't consider a $20k SDM an affordable telescope for an amateur.

This is why I believe amateurs with finite budgets need to find their own sweet spot on the aperture scale between the opposing slopes of optical quality and light gathering power.

[Rickster]

The main reason I purchased the 20" was to view the faint fuzzies and obviously to get better views of the not so faint DSOs. I assumed I would also get more detail from planetary objects also but this is all interesting as there seems to be different takes on the subject.
I do remember going to Ian Bedfords Stockport SA observatory many years ago and seeing amazing views of Jupiter through his 30" Newtonian Cassergrain. I've never directly viewed such great views of Jupiter through a smaller telescope.

Rick

[geolindon]

Thanks for this thread.
Rick thanks for sharing your experiences, Patrick drew my attention to your post re 20 mins set up time. I am considering a non go to version and wonder what if any portion of that 20 mins is setting up the go to?
regards, L

[Rickster]

Hi,

My advice would be to buy the goto version. I actually got mine for the price of a non goto unit including delivery to Adelaide from Skywatcher in NSW.

Just a few points to consider before you purchase this unit -

1. Setup is around 20 mins with mirror cradle on base and this includes setting up goto. Goto is quick and easy to setup once you are familiar with it.
2. I modified the Mirror cover (which is flimsy), so that it stays on while I set up. Last thing I take off.
3. Mirror is very exposed so I have fixed posts around so its protected when fitting the light shroud.
4. You will need a trolley setup to move it around unless you have another person to help.

Hope this information is of some use.

Cheers

Rick

[dave brock]

Any updates on this scope Rick?

Dave