Scope review

My largest scope up to buying this one was a 12″ manual Dob, with some lovely views and memories from it. I had been hankering after an upgrade for some years though with the intent of getting a scope that would satisfy my aperture fever.. I wanted the largest aperture I could handle by myself, without needing to have it stored assembled. I’ve not got the room to keep a really large scope fully assembled so I needed to make sure I could assemble and disassemble whatever scope I got in a fairly short space of time given Irish weather, and without too much bother and faff, otherwise I’d just get tired of having to build each observing time.

I did my research on the scopes available, and this particular model started to filter up to the top of my shortlists. 20″ aperture, truss construction with no single piece heavier than about 30kg, generally reasonable to good optics based on other owner experiences, and a goto system utilising the well known Synscan handset, and the support of a large brand and a known-good dealer network.
I also knew that I would probably have some engineering to do myself with the scope, but I’m happy with that as I’m a bit of an old-school hardware hacker and tinkerer at heart. Fettling something does not phase me at all.

This particular scope arrived, and I spent a good while with the first assembly, figuring out what needed to go where, and what snafus were presenting, The manual is abysmal really, but the scope is not difficult to figure out to someone any way mechanically minded. I did need to add more counterweight but that was to be expected with a Paracorr and 100-degree eyepieces in the focuser. The focuser could do with an upgrade to a compression ring fitting at minimum but it does suffice for the purpose. The focuser movement is smooth and clean which is really useful!
The truss construction with the ball and sockets construction method is well thought out, and means that collimation is fairly repeatable upon each assembly.
The shroud is a lovely addition, though it did take me a little bit of time to realise that the velcro seam runs under the “tube” with the hole in the shroud near one edge actually goes over the focuser and finderscope. There are improvements that could be made to the shroud but it’s pretty good when tightened at the top and bottom over the truss attachment bolts. When tightened properly it does not come into the light cone and also provides a lot of stray light protection if one is observing in a not-dark location. The shroud also prevents body heat and exhaled breath from reaching through the light cone and affecting the views and optics.
The goto system is reasonable, once aligned and synced on something in the neighbourhood the gotos are then fairly good. It’s a good idea to sync on a bright star near where the desired observing object lies, to get the best chance of getting that object in the eyepiece.
There is an altitude stop that prevents the tube from getting to within 11 degrees or so of horizontal. I moved that stop to allow my scope to get to the horizon. There’s no apparent issue with that once I had the correct counterweights for my setup.
I’m 5’10” tall, and I need a step or two once my target reaches above 37 degrees altitude, but I was expecting this for a scope of this size. I could not afford a scope with f/ratio of under 3 at this point. Two steps up a stepladder is all I need to get observing at the zenith, so it’s not too bad at all. I may invest in a good Catsperch chair at some point in the future. I’ll have to see how I go with a full night of being on a ladder.
If there’s a breeze blowing and the shroud is on the scope, then there will be a bit of movement seen at the eyepiece. In general though the views are steady, and there’s no loss of target when swapping eyepieces out which is a real boon for me after using the 12″ where I’d have to re-find the target again.

As for the optical quality, I’ve not seen any issue yet that I’m worried about. My seeing is generally in the 2-4 arcsec range on average nights, under an Atlantic jetstream. I’ve had a few evenings out under mediocre skies so far, but I’ve been able to get E and F in the Trapezium pretty much with ease for most of time. Stars are scintillating balls of mush, but a ball of mush that is seeing-caused. No astigmatism showing up so far, no real roughness apparent based on my testing at this stage. When the mirror is cooling down it does show an undercorrection but this appears to lessen as thermal equilibrium is reached. I haven’t had a long enough session yet with the scope to be able to determine the finer optical quality, but that will be done in time. So far, I’m happy enough with the performance – it’s certainly the case that I’m being atmosphere-limited, and that’s a nice place to be.

One thing that is really of interest with this scope, is that it appears to be exceptionally suitable for Electronically Assisted Astronomy, and for Lucky Imaging of DSOs. The f/ratio of under 4 and the aperture of a half-metre means that deep exposures can be obtained in a very short space of time.. I got to about mag 19 with about 4 min worth of 1.7 second subs with an ASI294MC-Pro, and seeing that result had my jaw hit the floor. The tracking is probably not good enough for multiple tens of seconds of sub lengths, but a low noise camera and a lot of disk space will work wonders..

The known issue of mirror flop causing collimation skew when changing altitude is fairly easily resolved, and I have also added a Kendricks secondary dew heater as I’ve had the secondary mirror dew up when not using the shroud. I may yet add a boundary layer set of fans and fans blowing at the rear of the primary to aid in thermal equilibrium

If you are in the market for a transportable half-metre telescope that one reasonably fit person can assemble and disassemble with relative ease, that provides lovely eyepiece images, and that does not cost the earth, then the Skywatcher Stargate 500p is a scope that should be high on your list.

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