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:
- Goto performance is not good.
- Collimation change with altitude is present
- 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.
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..
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:
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.
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.