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Hi Peter,
I have a 20" model as you know, it is about 4 years old. The support from PWI has been very good, they are very knowledgeable and treat advanced amateurs as such, I guess they know their clientele is generally more versed in OTAs than your average Tasco customer. I had several problems to overcome and they did their best to help me fix it as quickly as possible.
The first issue was a slight wedge in the corrector lenses causing dispersion so that every star was a little rainbow. The effect was small but I was able to measure it, easily with a one shot color camera. The dispersion looked like what you get when shooting low in the sky, but was present at zenith and rotated with the lens. They eventually sent me two replacement corrector lens sets with no complaints. The original lens set was from China and the wedge was detectable by observing the orbit of a laser spot as an individual lens was rotated in a v-block. They evaluated all their stock and determined that they ranged from bad to barely ok, they sent me the best one of the lot, it was better but not perfect. And so they went to a German supplier, I had to wait a bit for it, but then wedge gone, dispersion effect gone.
The next problem was the ghost reflections off anodized Al of the inside walls of the corrector cell. This took some detective work to track down, and I was actually lucky to have the spare lens cell assemblies here in Adelaide (the scope was installed in Ark by then) to play around with. Shining a collimated light beam (torch + 66mm refractor) thru the cell onto a screen in a darkened room revealed the problem. The solution was to take the cell apart and flock the interior walls of the cell. I explained the problem to PWI and they started making baffles, they offered me some but I had already flocked the tube so didn't need them.
While flocking the lens cell I flocked everything else that was anodized Al in there that I could (focuser, rotator, the OTA behind the lens cell I believe). This made a tremendous difference in reducing to the flat "hot spot".
The scope thermals are quite predictable. I open the slit just before or at sunset. The fans are set to operate if the primary versus ambient temp is greater than 1 deg. The fans will run for about 3-4 hours generally, unless the temp is dropping due to weather changing in addition to normal night time drop (then they will run all night). I only have the three fans on the back face. I have thought about installing fans on the sides to blow across the mirror but haven't inquired about it yet, too many other bigger problems lately...
The first focus run is always different from the last one from the previous night, and normally a second autofocus about 1-1.5h later will be the biggest shift of the night, autofocus every 2-3 hours after that will make only small adjustments. If I am late opening the dome the images for the first 0.5-1 hour will have extra aberrations off axis, and best focus will also be compromised.
I think most of these issues would be minimized if the mirrors were fabricated from zerodur...but then I wouldn't have been able to afford a 20"...so the trade off is acceptable with modern autofocus software.
Collimating is actually really easy, it's just three screws on the secondary. For the longest time I thought collimation was really hard and I played with secondary centration, camera tilt, etc. None of this really helped. It turned out that the mirrors, and corrector lenses had some residual astigmatism. So no matter what I did to collimate or tilt the camera, I got funny star shapes in the corners of the big chip. The center was always OK. So finally on the last night of my last trip I finally figured it out...rotating the lens cell in the baffle tube about 30 deg at a time I found a position where the lens + mirror astigmatism canceled, and stars were good all the way to the corners. Now remember in less than good seeing or with a smaller CCD no one would ever notice this effect, but now that I have spent untold hours fretting about this, I can spot it in some CDK images posted on the web.
The tube keeps it's collimation at different altitudes very well (I don't think I have detected an effect). This could be due to very good mechanics, the mirror being epoxied to the back plate, or a lack of sensitivity to collimation in the optical design, probably a combination.
The stock focuser is very sturdy however uses up too much backfocus when using an external rotator + MMOAG. I would recommend getting the integrated focuser + rotater (IRF90) since I think it is stiffer than the Bellerophon + Optec, and allows you to focus using multiple stars and a few exposures instead of a single star and many exposures. This would help reduce RBI from the bright focus star. I was going to upgrade to this unit in the near future, til my camera went kaput.
Get the optional shroud too, I take it for granted now, but it really helps keep the dust out and probably reduces the occurrence of dew on the secondary (I don't have that problem in Ark but near the coast you might). You can also get a dew prevention system (heater + sensor + software), I have the sensor + heater but haven't bothered to write software for it since dew has been a non problem.
As for site selection, well at 3m of focal length in poor seeing you will be binning and choosing larger targets such as nebula. The gain in aperture is really important in dark skies, but in suburban light pollution you will hit the sky glow wall of diminishing returns pretty fast.
Best
EB
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