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Skywatcher Stargate 500p - opinions/advice?

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#1 Cathal

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Posted 01 January 2019 - 09:18 AM

Hi everyone.

 

I'm due to receive some cash in a few weeks time, and I've decided to add another scope to my quiver. I've had a hankering for a very long time to have a large driven Dob to use, and it would appear that the best bang for my buck at the moment would be the Skywatcher 500p.

 

If anyone has advice, experience, or opinions on this particular scope, please do post below. I would certainly welcome any caveats seen by current users.

 

I have seen the videos by Adriano of the scope setup and use, and it does look good. I've also perused pretty much all threads here and on StarGazersLounge about the scope.

 

  • I'll be getting a Catsperch Pro or equivalent as I did a mockup of where the eyepiece would be for various altitudes, and I'm above flat foot standing at ~60 degrees or so. 
  • There's budget room for a Paracorr and some more Ethos eyepieces. I have a 31mm 82 degree Luminos, but I suspect an ES100 25mm would be my best match for low power, and an Ethos 8mm as best usable high power. I already have an E13.
  • I'm physically reasonably strong, easily benching 250lb or so last time I checked.
  • I've already had an opportunity to lift the 500p in its packing boxes (thanks to the wonderful staff at Teleskop Express) and I had no problem moving those around, so I'm not expecting much difficulty in lifting the mirror box around when setting up and taking down.
  • I have a mid-size SUV for transport over longer distance if I need to, and I've checked that the 500p should fit in the back without too much trouble. I'm not planning on too much transport of the scope though, either my house of the girlfriend's house have good stargazing from the backyard lawns.
  • I like tinkering and hacking, and I'm not averse to fixing and improving if necessary, seems a few people have had to tinker a bit with this scope after construction.
  • I have a CPC800 so I'm aware of image scale for EAA at a focal length of 2m.
  • My largest scope is currently a Skywatcher Flextube 300p, and I can lift and walk around with that reasonably easily when fully assembled.
  • I also know I have to put together an anti-dew system for the secondary at least - currently in design phase with either PWM controllers for LEDs and power resistors, or an Arduino setup. 
  • I do not have storage indoors or outdoors to hold the fully assembled scope, but I would only have to put the trusses and top ring on each time of use.
  • I have no expectation of astrophotography as such with this scope, but I am expecting to do some planetary video captures with the RS224.
  • May also get an atmospheric dispersion corrector for planetwatching in this scope.
  • I've already built and tested bluetooth/WiFi<->serial cables that will allow remote operation of the scope from the eyepiece via android device and SkySafari, tested with my AVX and CPC800.

Living in Ireland at the moment means not a lot of stargazing opportunity compared to those in sunnier climates, and the humidity here tends to average at 100% for significant amounts of the day and the year, hence the anti-dew expectation. 

 

 

 

a


Edited by Cathal, 01 January 2019 - 09:20 AM.

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#2 PETER DREW

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Posted 01 January 2019 - 09:39 AM

Go for it! I have a 20" F3.5 Dobsonian (not the Skywatcher) and it's a great scope for planetary, lunar and DSO's.

#3 spencerj

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Posted 01 January 2019 - 10:34 AM

You have certainly thought it through and it seems to be what you want. You have a plan for storage and transport. Congrats. Make the most out of the clear skies you do get.

#4 niallk

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Posted 01 January 2019 - 11:54 AM

Best of luck with your purchase Cathal - hope our skies clear up in 2019! (I've a 15" in Cork and hoping this incessant cloud cover shifts...)

Have you considered David Lukehurst? If go-to/tracking wasn't deal breaker, you could get an ultra-portable model: 20" with a standard mirror, or an 18" with >0.95 strehl for ~similar money.

#5 Arctic eye

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Posted 02 January 2019 - 05:36 PM

Hello Cathal.

 

I purchased a 500P for Lunar & planetary work more than two years ago, after doing succesfull imaging with a 400P flextube for several years before. It was good for DSO and up to medium power L&P, but not for high resolution imaging. The problem was with the main mirror, bad print-through, so eventually I got a new mirror for warranty, couple of months ago. The new mirror was even worse, and now I'm back with the 400P while rebuilding the 500P with custom made optics from elsewhere. 

 

Just wanted you to know, in case you want to get serious with planetary imaging, that there is this possibility of bad luck with the optics. In that case it can happen that your 300P always outperforms the 500P in high resolution work. At least that's what happened to me, so I wouldn't recommend this scope for planetary imaging. But I am a high-res photographer to the core, so I have high demands. Other people who do more "general purpose" astronomy and DSO, seem to all be really happy with the scope. 

 

For DSO the light gathering is great, clear difference to 400P.

 



#6 IVM

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Posted 18 January 2019 - 06:06 PM

I've stuck the biggest secondary heater from Kendrick Dew Solutions of Canada on it, and it works great in our humid climate. I've used Kendrick heaters for years but was concerned about the peculiar shape of the back of this secondary. It turned out the big Kendrick sticks right over the main thick ridge on it, and the contact is pretty good. You'll love the scope. Just don't force anything during the initial assembly. If it looks like a part pre-assembled at the factory needs re-seating, re-seat it. The tolerances are awfully tight on this structure.



#7 Cathal

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Posted 21 January 2019 - 01:17 PM

Thank you all, for the pointers and feedback. I'm currently awaiting previous employer to pay me my redundancy monies, and when I get that I'll be placing the order. Then I'll have some fun with this expensive Lego kit!

 

I do have a requirement for this scope to be goto and driven. I currently use a manual 300p and while I can starhop just fine I do like the tracking and goto capabilities that I have on my other scopes. I find good tracking to be a help in teasing out fainter details. I have tried hand-guiding for video capture and that has worked reasonably well, but driven is just the right thing for me at this point. 

 

Cheers for the pointer for the print-through on the mirror. I'll be able to check that pretty well with a CCD and an artificial star upon build completion, and if there's an issue, I'll happily work with the vendor to get any issue rectified - one good thing about EU consumer protection laws. Reminds me - I must read up on Roddier and get familiar with that with my other scopes to get a baseline.

 

Good point as well on a secondary heater that is known-working for this scope - removes a possible headache for me as I know that dew is an issue for mirrors where I live based on personal experience - I've had both the primary and secondary dew over in my 8" Quattro during an evening's observing.



#8 Mokusatsu

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Posted 27 January 2019 - 11:09 PM

I've just bought one of these too!

I've devised a setup technique that is much easier for one person than the version in the manual, or the YouTube setup.

 

My setup process differs in a few ways:

- I start by placing the primary cradle directly onto the base, then I use a block of soft wood to temporarily hold the primary assembly up a few cm on the cable side when threading the cable through. This enables me to secure the cable ends directly onto the primary cradle in their final positions, adjust the cable fairly easily, and then remove the block and gently lower the primary onto the bottom, WITHOUT the wire then flying off and having to repeat the process a hundred times! Then I tighten the cable to ensure zero slip.

- I've marked my tubes with electrical tape to show exactly where the clamps are supposed to go. This means I am able to install the clamps BEFORE the secondary cage, which makes them a lot easier to manage singlehandedly than trying to wrestle them all while putting on the cage.

- Install the secondary cage onto one pole per side on the first two sides, then both poles simultaneously as you put in the third side. Tighten the third side up. Then twisting the top will easily align the other poles and they'll pop in. This is easier than trying to simultaneously wrestle two tube heads into the sockets at a time, which is a bit tricky when you're doing it one handed.
- To make it balance you'll need far more counterweights than are included in the box. I've added a 6kg plate from my gym set, and I plan to 3D print a collar to make it a little more secure. This is about right for a paracor plus ethos 21mm.

- Stock Skywatcher finderscopes are a bit crap. I plan to upgrade.

- I'm mulling over purchasing some "Black 2.0" paint (Google it!) to paint the light shield.

 

I've not yet done the extra brackets to prevent collimation drift with changing altitude, but I plan to.

 

Acting on the suggestion from the 20" skywatcher gallery here, I've ordered a gun bag from eBay to transport the tubes!

 

I plan to get out the table saw and make myself some nice custom boxes so I can feel better about throwing it in the back of the car and piling camping gear on top. The scope does not come with much in the way of protection like that and I'd feel really uneasy about the secondary mirror in particular if I wanted to drive it to a dark sky site.

 

My only issues so far, and I'm hoping I can get some answers here...

The collimation screws of the secondary are done up extrmely tight! I mean the allen key twists and distorts before the screws loosen (and yes I've tried the other way too), and the central philips head is done up so tightly the screw head stripped a little rather than turn. I've contacted my dealer and local Skywatcher Facebook page for advice.

Second issue... how the heck do you install the shroud? It has velcro that doesn't match up with its mating side anywhere, I'm not sure at all how it's supposed to go on and where the points are that I fix it to. If anyone has a video of putting on the shroud, please upload it!


Edited by Mokusatsu, 27 January 2019 - 11:15 PM.

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#9 Astro-Master

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Posted 28 January 2019 - 01:29 AM

Sounds like a PITA, I'm glad I bought a used Obsession, its easy to set up and a joy to use.



#10 Mokusatsu

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Posted 28 January 2019 - 02:25 AM

It's all a compromise between various factors. 

 

One thing about the 500p is it weighs a lot less than many other scopes of similar size. None of it is really a two man job.



#11 Cathal

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Posted 28 January 2019 - 03:18 AM

If I could find a driven Obsession 20" f4 for sale on this side of the Atlantic, it would certainly be on the list. As it is, that's a very rare item here in use let alone available on the used market. Importing a used Obsession over that distance is not really feasible especially with appropriate insurance.

I will be methodically removing and greasing every nut and bolt one by one on the 500p if/when I purchase it. I've had an unpleasant experience with the steel bolt through the secondary holder binding hard to the holder, and needing machining out. After that, given my rather humid climate, I'm resolved to prevent any different metals to be in unlubricated contact. It might be an interesting upgrade to the scope to have only stainless fixings when into aluminium. Though, stainless may then have some cold-welding Issues onwards.

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#12 Cathal

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Posted 15 February 2019 - 05:01 PM

500p has been ordered and paid for, will be shipped from the EU distributor to the vendor, where it'll spend a day being verified and checked as correct. Then it'll be shipped to me for (hopefully) the end of next week.

 

Then it'll be like Christmas for a 5 year old when the boxes get delivered!

 

I wonder how much cloud will be in those boxes..


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#13 niallk

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Posted 15 February 2019 - 07:39 PM

Congrats + enjoy! Will be interested to hear your reports on the scope. Here's to some clear dark nights in Ireland ;)

#14 Gaseous

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Posted 16 February 2019 - 05:43 PM

I’ve had the 20” Stargate Goto for about 6 months now here in Australia, and just thought I’d offer a few observations. Your plan to apply generous lubrication is a good one. They do supply a small tub of clear grease, but you may have your own preferred goo of choice.
I tend not to use the plastic light shield – most of my limited use with the 20” is from a dark site so I’ve not really found the need for it, and it’s just one more thing to fiddle with while setting up/breaking down. I also tend not to use the metal “guardrail” thingy that goes over the cable, for the same reason, and haven’t come to grief yet.
If you’re relatively robust then moving the individual bits shouldn’t be too problematic – they’re more awkward for your back than being outright super heavy. I’ve retained the large foam squares from the packing boxes to sit the base and primary mirror on while transporting them, and am in the process of getting a box made up for the upper cage/mirror assembly, as it’s an awkward shape to have knocking about loose in the back of a vehicle.
There are small screw/knobs at either end of the base which can be used (or at least I think that’s what they’re for) to secure the cable once you’ve detached it from the mirror rocker – with a bit of practice it means you don’t have to rethread the cable every time you use the scope, which for me was a blessing.
If you’ve got plenty of space then you can keep the truss rods connected in pairs with the tightening brackets, but if not, I’s suggest a bag as mentioned by others – a cheap tripod bag off Ebay can suffice. Putting the whole thing together, while certainly easier with two people, can be managed by one person with a small stepladder with no real dramas.
Be careful with the finderscope – while putting the secondary cage on in a vertical position, my finderscope fell out of the bracket and was only stopped from smashing into the primary by getting caught on a truss bracket. Unfortunately, the nature of the primary mirror cover means that you need to take it off before installing the truss poles as you can’t easily remove it once they’re in place. I’ve had to cut some notches out from the cover to allow its removal after the poles are in place.
I’d also suggest checking the tightness of the fixing screws at the back of the primary mirror – I was losing collimation badly until I realized the mirror was loose in its seating – I just assumed it would be pre-tightened in the factory, so it’s worth checking.
As mentioned by others, the supplied counterweights are a bit under par if you’ve got a large eyepiece + coma corrector in the focuser, and a small gym weight or two will sort this out.

That being said, the views are spectacular! Enjoy.


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#15 Cathal

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Posted 27 February 2019 - 08:50 AM

It arrived laugh.gif

500p_on_the_lawn.png

 

Many thanks to all for the advice, definitely useful things to know.

I found that I had a few issues with it, but thankfully nothing particularly show-stopping.

  • I had to put on an extra 5kg of counterweight, and I'll investigate a better way of putting weights on the rear, but that will come later. Maybe a short counterweight bar and a real removable counterweight similar to a short CG5 bar. I needed more weight to balance even before the Paracorr and ES100-25 went in, and currently have the three supplied plates and a 5kg disk, and it's balanced. I take the weights off to make carrying a little easier when setting up and taking down.
  • The primary mirror does suffer from the flop even after tightening of the ring-nut of the mirror support, rectified with some M4 threaded bar and some repair washers and nuts to keep the mirror kinematically restrained to the mirror collimation plate. Before this, the laser return would disappear off the Hotech target. Now it remains in the hole from 0 to 90 degrees altitude. Collimation is generally repeatable once everything is seated and tightened.
  • The lower altitude stop is definitely in the wrong place from the factory, so I drilled and tapped a new location for the stop that gives me exactly zero degrees altitude and no falling off the runners.
  • Getting a RACI finder is an absolute necessity with being on a ladder or high chair when at the eyepiece. 
  • The 2" eyepiece adapter won't allow the Paracorr into the focuser tube, as it's threaded for 2" filters. The focuser tube to eyepiece adapter ring hasto be used instead, and that doesn't have a compression ring. As a result I've had to order one of these to provide an appropriate Paracorr-focuser interface. Skywatcher should really have done better with this one.
  • The shroud is pretty good, could do with some velcro straps on the top to keep in place on the UTA. I have to use the lightshield as I'm in a Bortle 5 area with some backyard lights from neighbours, and yes, it's a little fiddly to remove and put on each time but not that onerous a job.
  • The goto is pretty poor. My old ETX-70 was more likely to get things in the finderscope range. It appears that my gotos are usually in the region of a degree out, and I'm a little mystified. Clutches are tight, alignment done in a short space of time with a crosshairs eyepiece, drive cable pretty much as tight as I can get is (if it's not really tight I get slippage on the drive wheel). My only thought is that there's a very slight difference between the expected diameter of the cable drive wheel and the actual, and that is making a little mess of things. I would have expected either there to be a method of verifying that or a way to calibrate that .
  • My handset has a dicky button. A minor inconvenience but the vendor is making good on that.

First light was a tour of all the standard winter items. M42 was *stunning* with easy E and F Trapezium stars, actual colour seen in the core of the nebula. I didn't get long with gaps in the cloud and high cloud coming in, I got a quick set of views of Castor, M1, M35, and finishing the winter object tour with the Eskimo. Then on to Ursa Major as it climbed over the house and out of one of the LP domes. M81/M82 in the same field of view was very pretty, and I could *just* make out M51 spiral arms through the LP and incoming cirrus. 

 

I have the space to keep the truss poles assembled with one of the clamps keeping the pairs together - makes one-person assembly quite easy. I can now assemble completely inside 10 min without the use of a stepladder. I got a hand-truck that I can compression-strap the scope to the base and the truss poles to the handles of the truck, and I can then use the OTA as a handle to wheel the scope to wherever I want to set up. This is going to be useful when I use the scope with the Astrosoc's student viewing in the local university, as their setup spot is across a large lawn that I do not want to drag across two 30kg awkward pieces by hand, and I can't drive to it.

 

Last night I set up on the lawn, and while waiting for darkness I put the camera on my Samsung S8 phone to the eyepiece and snapped this:

m42_s8_afocal.png

Interest was piqued by this for the possibility of EAA with this big scope and I stuck the ASI224 in for a quick look. There was a little bit of wind to shake the scope (with shroud on), and the background was still a bit bright. So, gain of 399, exposures of 1/8sec (0.125s) and using Sharpcap stacking for ~45sec gave this image without an IR-cut filter:

ASI224_core_small.jpg

I'll definitely try my hand at lucky-imaging some of the brighter nebula at some stage soon, weather permitting.

 

So I then broke out the ASI294MC-Pro and stuck it in, focused, cooled it, and started capturing on a few objects. 1.7 sec exposures and 400 gain, 2x bin, captured and live-stacked in SharpCap. Pics are cleaned up a bit in APP.

 

4m30s on M1:

M1_asi294.jpg

 

3 min on the Flame:

Flame_asi294.jpg

 

 

Last pic, as the cirrus came in, rendering the sky about mag 1 only (Plough pretty much on verge of invisible) 12 min on M51:

 

M51__Stack_small.jpg

Couldn't believe how well this one turned out given that SharpCap could only see ~10-12 stars to align on.

 

Had to give up as I could no longer see any stars overhead nor could I see any stars in the finderscope. With the poor Goto I wouldn't have been able to point at anything, even if I could then see through the clouds.

 

Generally the drive was wandering about the place, up to 100 pixels in a 5 min period. Fine for visual and EAA and not so good for planet capture. I'll definitely be aiming to get more time on any object now that I know I can do this method of capture. Mag 19 stars with stacking of ~2sec exposures? That's crazy..

 

It does appear that the optics are actually fairly good once thermally stable, but I'll have another opportunity to ascertain that in the near future.


Edited by Cathal, 27 February 2019 - 09:01 AM.

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#16 IVM

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Posted 27 February 2019 - 10:54 AM

Excellent!!

 

I am gravitating now toward aligning on stars in the area I intend to observe in (a quadrant of the sky). During the last session everything was in the center at 360x this way, really a remarkable performance for a transportable (and, in parts, hand-portable!) telescope this size. Make certain there is no slippage of the cable whatsoever. I had this problem in the beginning, and it was caused by some grazing in the altitude bearing. All tolerances on this thing are super-tight. Also, if you set up on grass, you may have an issue with the ground giving as the telescope moves.



#17 niallk

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Posted 28 February 2019 - 03:10 AM

Congrats Cathal - great first light report!

Clear skies,
-Niall

#18 Mokusatsu

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Posted 12 March 2019 - 06:34 AM

An update on mine. I had it out for the first clear night last night under a darkish rural sky, after resolving early collimation difficulties caused by seized up secondary screws that were overtightened at the factory. Now that it's properly set up I can speak of performance.

I've tinkered with it in a few ways.

 

One of the first mods was to change the secondary collimation screws to M4x50mm knob screws I bought from eBay quite cheaply. No more stuffing about with an allen key in the dark, it made collimation much faster. (Though, I do admit, collimation more easily gets knocked out. Overall I find it better this way.)

After mastering collimation, via an extra long cheshire with eyepiece, then standard laser, and finalising with barlowed laser, star images are excellent with a paracorr and my 21mm ethos and 13mm ethos (154x) eyepieces. They're fuzzy with my 6mm ethos eyepiece though (333x). I don't know what the maximum useable magnification is, but it's clearly losing it before 333x. If I ever get a chance to try out someone's 10mm ethos and it's ok, I'll buy one.

Views of Eta Carina, The Tarantula, M42 and Omega Centauri were absolutely spectacular.

 

With a 2" UHC filter and the 21mm Ethos I went hunting for the horsehead nebula. Although I was able to see the flame nebula quite well, and could discern some faint nebulosity around where the horsehead is supposed to be, I couldn't make out the horsehead. I then changed to the 13mm ethos and put on my 1.25" Hydrogen Beta... and still couldn't make it out. HB is a very narrow filter, everything is very dark. I may well have been staring right at it and not seen much, but visually it eluded me. I gave up my hunt when fog rolled in and every star aquired a reflection nebula.

Last night I had to use the scope in full manual mode. It was electrically dead on arrival. Turns out the soldering for the power socket had failed, possibly because of accidental turning when I was screwing on the plug. I fixed that today, but took the opportunity to swap the power socket for a standard 2.1mm DC power socket, meaning I can use standard 12V power cables, plus it just plugs in without needing to screw it in. If someone trips over the cable it would pull out, as opposed to fail catastrophically the way I imagine the screw in connector would. Anyway, that was an easy repair and IMHO an upgrade. (You'd want a 12mm x 2.1mm chassis mount female DC plug, I could only find 13mm ones so I drilled out the hole slightly. An alternative option would have been to solder the original one back in and then drill another hole in for a 2.1mm jack (which come in 13mm and some smaller sizes) and then solder some wires to the connector of the original plug, thus giving you the option of using the original plug or a 2.1mm jack. With hindsight that would have been better, and if I could do today over I'd go for a two socket install.) 

Because the spider veins line up with the focuser, you can't use a filter slide with this scope. I'm considering moving the focuser plate just enough to clear the spider. This will require precisely drilled and tapped new holes and cutting a slot in the secondary plate, but I'm quite keen to use a filter slide so it'll have to do. This doesn't seem very difficult to do, but of course it would be better if Skywatcher started offsetting the spiders so the focuser is lined up with a gap rather than a spider vein.

Tracking hasn't been that impressive on previous trials and goto doesn't even put objects within the field of view of my 21mm eyepiece. I'm hoping to improve on this by lubing up the gears better and tightening the cable more. I'd have liked to check that out last night, but due to the soldering fail I was star hopping old school style.

Putting the weights on is really awkward. I've purchased an M10 threaded rod and a M10 knob, which will replace the current counterweight attachment.

Another issue is the finderscope can't be adjusted enough to align with the scope. It's too off axis to correct. So I bought a Celestron Star Finder Pro and was disappointed to find that this too didn't have the adjustment range to compensate. Do I try a Telrad next?

Anyway, despite the problems I have to say the views through the 21mm and 13mm eyepiece were stunning, and the other guy who was stargazing with me said the views were the most stunning he'd ever seen in his life, so the optics got my fellow stargazer's tick of approval. (My opinion on star sharpness is less important, I suffer astigmatism and stars are always a bit fuzzy for me, even with a Dioptrix.)

 

PS: Does anyone have any inside information about whether Skywatcher plan to issue a firmware update that activates autoguiding any time soon? 


Edited by Mokusatsu, 12 March 2019 - 06:37 AM.

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#19 AlienRatDog

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Posted 12 March 2019 - 09:30 PM

Yikes, that sounds like a lot of tinkering for a telescope one is paying $7500 for.

#20 Cathal

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Posted 13 March 2019 - 03:05 AM

Yikes, that sounds like a lot of tinkering for a telescope one is paying $7500 for.

View in context of alternates of similar setup and capability (20", portable, storable, and tracking/goto) costing well north of 10k.

I was aware of some tinkering required, but that's part of my enjoyment with things anyway. For me, I'll take some tinkering for a price reduction of that amount - if I can reach the appropriate performance at the end of the day.

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#21 Arctic eye

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Posted 13 March 2019 - 05:33 AM

A lot of tinkering and bad optics... would’t bought mine if I had known. 7 kEur down the drain. But if one likes finetuning and only uses low powers (8mm EP or longer) it’s fine. Unfortunately what I do is high-res, so custom optics rebuild it is now.

#22 AlienRatDog

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Posted 13 March 2019 - 06:20 AM

View in context of alternates of similar setup and capability (20", portable, storable, and tracking/goto) costing well north of 10k.

I was aware of some tinkering required, but that's part of my enjoyment with things anyway. For me, I'll take some tinkering for a price reduction of that amount - if I can reach the appropriate performance at the end of the day.

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I understand if the performance was not as good as good or refined as a premium dob, but it should at least work to its own specification. Goto not working, parts not machined within spec, cases of bad optics (isn’t diffraction limited advertised)? It’s like the car analalogy of comparing a BMW and a Ford Focus, the the BMW is a better machine but the Focus should work within spec out of the box. Sure you may modify it (fancy exhaust, tune the engine) but you wouldn’t have to replace the alternator or rebuild the engine when it’s fresh off the lot and you just paid full price for.
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#23 Astro-Master

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Posted 22 March 2019 - 11:05 AM

Sounds to me like Sky Watcher needs to do some tinkering and tweaking on their scopes before they sell them.



#24 IVM

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Posted 22 March 2019 - 03:07 PM

Don't start me on that, Master... I did think that the super-big professional scientific equipment company to remain unnamed here, which sold me (at work) a super-big piece of professional scientific optical-mechanical equipment, to remain unnamed here, for a super-big professional scientific price, to remain unnamed here, needed to do some tinkering and tweaking before they sold them... or, at least, more successfully so, when the stuff was being professionally installed, or at the very least, when it was returned to their factory for tweaking, than they actually did it. The Sky-Watcher, at least, I was able to put in a nice working condition without too much trouble (a couple of evenings of assembly and disassembly for diagnostics, which was a good exercise anyway, as I was afterwards able to assemble and disassemble the thing in complete dark, plus a couple minutes of actual tweaking work). The said super-professional piece - never to the full extent.



#25 Cathal

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Posted 01 April 2019 - 01:25 PM

Just some updates on my ownership of the Stargate 500p. Warning, long post ahead.

 

tl;dr.:Overall, I'm reasonably impressed with the sights but there are big problems that need to be resolved before this scope is properly useful.

 

Highlights so far:

  • I've bagged my first definite viewing of the Pup, at an altitude of about 10 degrees. 100% visible, just outside one of the secondary spikes from Sirius. I thought I had sighted the Pup through my 12" previously, but this time in this scope it was absolutely certain.
  • Definite spiral structure in M51 for my first time visually (not the easiest from a yellow LP location.)

(though, these should not be difficult with a 20" mirror..)

 

I also got a good look at Jupiter during fairly steady seeing in the early morning and I was fairly underwhelmed by what I saw, as I got better contrast previously with the CPC800 I recently restored to usefulness. May be related to one of the issues below.. 

 

I added a secondary dew heater from Kendricks, and this works very well. It has been the saviour of three observing sessions so far, that each ended up ending after dew formed on the primary - even inside the shroud.

 

The shroud does actually work well, once I figured out how to mount it and set it up. I'd have preferred velcro closure straps to keep it on the UTA, and a better way to keep the bottom end away from the primary and below the collimation plate. However it does now work.

 

Unfortunately, I may have run into some of the design and assembly flaws that others have hit with this model of telescope. Some are due to poor quality control by Synta at the factory, some are due to poor engineering decisions by Synta when actually designing the scope. I've certainly had opportunity and motive to practice some engineering troubleshooting with the scope, given three main issues that I'm currently having:

  1. Goto performance is not good.
  2. Collimation change with altitude is present
  3. Possible print-through of the mirror construction.

Starting in reverse order..

Optical quality issues.. After some evenings where there was better than usual seeing for my location I tried some careful startesting after careful collimation with my combination Cheshire and my Hotech laser. Previous star tests suggested that there was a lot of undercorrection and a hint of astigmatism, both of which I put down to the mirror not being fully thermally equilibrated. Last week I got the scope set up in the shadows before sunset and about three hours later I took an opportunity to take some 2 minute video captures of Regulus, defocused the correct amount inside and outside to utilise with WinRoddier. Here's my WinRoddier screen after analysis:

https://photos.app.g...Ci8qGthwDvtU629 The orientation of the mirror cell vanes were in line with the secondary vanes, but it's fairly easy to see that there are abberations and shadows that are spaced at 12 radii equally spaced around the mirror, as well as a fairly good indication that there is a large zone at about 70%, as well as strong hints of undercorrection overall. While I know that this scope is sold as a premium scope, I'd have honestly expected better. I'm not confident of the numbers for the astigmatism or spherical abberations being present, but there's certainly something up with the optics based on this analysis. The print-through is definitely not as pronounced as both of the mirrors that Artic Eye received, but should not be present in any way on the front surface of a telescope mirror. I've to try and set up an artificial star across a few football fields to try and get a better set of videos to get cleaner intra and extra images, to be less affected by any seeing and atmospherics. I'll also try to re-mount the collimation plate during one of the tests to allow the primary mirror to rotate, to see if the astigmatism I'm seeing is in the primary or secondary.

 

Collimation issues.

I already knew there was mirror flop with altitude present in this example, so I spent quite a bit of time trying to mitigate any sources of movement in the mirror support and collimation setup. My first attempt was to use M3 threaded bar and repair washers to clamp the mirror to the collimation plate and this appeared to be successful. I saw in star testing that there was a possibility of astigmatism or a trefoil pattern so I removed these clamps and went digging further into the issue. I went as far as removing the mirror from the collimation plate in order to grease and tighten properly the central aluminium shaft (which has really poorly machined threads) and the round "nuts" that hold the mirror to the collimation plate. I removed the felt washers as they appeared to perform no useful purpose other than to allow movement. I also noted that the grub screws that are supposed to stop any loosening of the round "nut" on the outside were unable to bear upon the collimation plate as the threads were fouled from the factory by either paint or other matter - to the point where an apparent tightened grub screw was still wholly within the threads, so I re-tapped those and I could get the grub screws to protrude as expected to the inside of that "nut". Upon reassembly, there was no flexure visible between the mirror aluminium shaft and the steel "nuts". I thought that this would have solved the mirror movement issue, but these videos show the current state..

https://photos.app.g...vDWRKwQMrcF3e28

There's no flexure between the collimation plate and the telescope structure to be seen, but a lot of mirror movement. 

All of the movement is here, between the mirror glass and the glued-in aluminium central shaft:

https://photos.app.g...B678LCYfPszfKt5

This is finger pressure on the rear glass plate to cause this movement. It looks as though there's no way with the design and construction of this mirror mounting system to mitigate against this type of movement. It looks as though the flexible material that Synta are using to mount the aluminium shaft to the mirror glass is *seriously* underperforming from a stiffness and support point of view. I'm currently reviewing ways of immobilising the mirror to the collimation plate so that collimation is no longer altitude-dependent. Either clamping the rear of the mirror plate to the collimation plate equally around the circumference, or shimming the central shaft collar where it bears on the ground section of the mirror glass, but I am awaiting vendor feedback before I proceed with any of this.

This video shows the range of movement of the returned laser spot with the same finger pressure as above on the mirror rear plate: https://photos.app.g...wqqErniCc9sQTu7 and this video shows the change in return with altitude. https://photos.app.g...AJFWHAcrdvL3Yw7  The return moves about 7mm at the focal plane between the horizon and vertical, which is to the edge of an 8mm Ethos from the center of the FOV. Of course, using a Paracorr minimises the coma resulting from this, but that's not really the point.

 

Goto performance.

My experience so far with the goto and tracking has left me underwhelmed to be frank. My old ETX-70 performed better on both. I could do a two-star alignment on e.g. Sirius first then Capella, and then using the Goto to return to Sirius shows it about a degree away from where the scope ends up. At least that error is consistent between gotos. There's no apparent slipping of the altitude cable, the scope is pretty much balanced after an additional 5kg onto the weight bolt, and I'm using a crosshair eyepiece to do the alignment. My average gotos are ending up near edge of the finderscope FOV, requiring some starhopping to get to the desired location. At least then I can re-sync the Synscan handset but I suspect that the re-sync is not working as well as I would hope. Plus, to perform the re-sync one must exit completely from that menu in the handset which does get a little tiring to re-scroll through the star name list from the beginning as slowly as the Synscan handset does. (as an aside, that is one thing that the SkyAlign handset from Celestron gets right - starting in a list where one left it as well as a fast scroll through the entries). I'm at a loss really here how to quantify any non-orthogonality of the axes if present, as the alignment process appears to have no way to account for any deviation from perfect. There is an entry in the handset parameters for a correction factor, but this is not documented anywhere I can see, nor does it appear to be set by any calibration process available. I also physically measured the structure and found that there was a deviation from the expected. The rear of the azimuth plate guides where the side bearings sit is about 5mm wider than the front, and the scope can move between the bearing wheels. That would generate an azimuth error and I'm pretty much seeing only altitude errors so I'm not confident that this is a cause of my issues.

The tracking itself is less than ideal, as I'm apparently getting a binding in the altitude drive that causes a jump every so often in what is seen, from maybe once a minute to every 3-5 seconds, depending on where the scope is pointing. This does get a bit annoying when trying to tease out detail on faint visual objects, and is a real pain when trying to stack frames for EAA. I'm also getting things wandering from the FOV eventually.

 

I am working with the vendor to see if there is any resolution possible with these issues, and they have been very good to work with so far. I may be looking at requesting a replacement of the drive base as well as a replacement mirror to get this scope up to an acceptable standard. If I can't get the scope up to standard, it will be returned for refund and I'll have to look elsewhere for the quality of scope I want.


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