Rainbow Astro RST-135

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Rainbow Astro RST-135

Post by rreed » Sun May 03, 2020 9:58 pm

What's the latest on the viability of having pinpoint stars on 5 min exposures using a RST-135 mount integrated with AISAir Pro across a wide variety of seeing conditions? I have the combo 120mm Guide Scope, FSQ85 and FSQ106 and a Canon DSLR. I read that post by Chen about needing more control over the guiding software. Just wonder if that's coming, or if not, there's an acceptable work around.

Also, for those who use the Rainbow, I see that there is a new altitude adjuster being shipped with the Rainbow RST-135 mount. I'm curious to know if it's wise to put an FSQ106 with a DSLR on this small mount or if the weight will be too much. I'm worried about losing power to the mount and having the FSQ slam into the pier/tripod.

And for that matter, would the Hobym Crux 140 be any better?

Many thanks!

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Re: Rainbow Astro RST-135

Post by W7AY » Mon May 04, 2020 12:14 am

Hi again, Rick,

I have for a number of years now, been able to test guiding with an indoor setup (thus no "seeing" problem). and can guide the RST-135 using ASIAIR to between 0.25 and 0.4 arc-second total RMS error. The test setup is basically simulating star movement on a celestial sphere from a laptop that is 8 meters from the mount.

To get those kinds of guiding accuracy, it requires me to use a sampling time (guide scope exposure time) of 0.5 seconds, and very high aggressiveness settings.

Given that, and the size of the Airy disk of your scope, it will determine how much away from a perfect star you get.

I can typically get halfway decent (0.5 arc-second type total RMS) guiding under the stars. Unfortunately, when seeing is poor, you need to lengthen the exposure time.

With its large native periodic error (but with a long period of 420 seconds), the RST-135 needs short sampling periods to work well. When I have to increase the exposure time to 3 seconds, the total error tends to go above 1 arc-second RMS.

The usual way one gets around this (M-Zero users have almost exactly the same problem) with PHD2 is to keep using a short exposure time, but introduce a LowPass filter profile to average out the seeing. At the same time, reduce Minimum Movement (MinMo) parameter to a very small value. Avalon actually recommends a zero MinMo for their mounts.

0.5-1 second exposures requires more sensitive guide camera and optics of course. I use a Borg 36ED with either a 174MM or 290MM for guiding.

Unfortunately, ASIAIR, although is a port of PHD2, does not expose either the Lowpass Profile, nor the MinMo parameter as user settable parameters. So we just have to suffer when seeing gets even moderately poor, or go look for some wider field object to take.

I have been able to ameliorate the lack of MinMo by selecting longer focal length guide scopes (forget the 120mm guide scopes; they simply don't work well).

Because of this particular shortcoming of the ASIAIR, I am trying to find other guiding solutions; I plan to check if the Lacerta M-GEN 3 has the proper guide parameters. Or use StellarMate just for the guiding portion. ASIAIR will still control the mount, just not the guiding, which will be done through the ST-4 port.

As I mentioned in the Cloudy Night RST-135 discussion just last week or two, you are not getting a premium mount with the RST-135 in terms of tracking. You are basically paying for portability and the lack of need for any counterweight, nor need to waste any time balancing the setup when you get outdoors. Those properties are worth it to me, and I have already ordered a second RST-135 in case the current one quits. I cannot see going back to my Takahashi EM-11. I am old and feeble (prime victim of COVID-19) and even the EM-11 is too heavy for me.

I don't have any experience with the Hobym mounts; but they look finely crafted. But if you look at some of the screen shots of PHD2 from RainbowAstro, even the upcoming RST-300 does not give better numbers. I think it goes with the territory with strain wave gears.

As to the FSQ106, I have no qualms with my Borg 90FL with its heavy F/4 reducer. Keep under 20 lbs payload and you should be good. For your own sanity, add a counterweight (unless you are on a fixed pier). if you go over that. With the RST-135, it is not a matter of torque -- the mount has plenty (perhaps even dangerously plenty), but the fact that the whole thing might topple over depending on the location of the center of mass relative to the tripod base :-).

I solved the backdrive problem by adding my own battery powered DC uninterruptable power supply using an Epic PWRgate and a small 7 Ah SLA battery just to feed the mount. Together with Horizon and Meridian limits, that should keep the mount out of trouble for quite a while in case of a power failure.

Clear skies,

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Re: Rainbow Astro RST-135

Post by ASIAIR@ZWO » Wed May 06, 2020 6:42 am

Thank you guys, we just wanna keep the app simple.
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