Cheapest SW RA motor, understanding operation

[themos]

Hello, I'd like to know if anyone has a good understanding of how this

Skywatcher - RA Economy Motor Drive for EQ1

works at the electronic level. It seems to be a PWM dc motor with a pot controlling the duty cycle. I've measured the voltage across the motor terminals and I get an AC component (about 9V) and a DC component (1.5-5 V depending on pot setting) which is compatible with that idea (I think!). Has anyone been able to derive the schematic for the controlling circuit? I'd like to throw an Arduino in there to do the speed regulation...

[jgs001]

I've no clue, but I'd be interested in the results... The mechanism seems to be a little flaky at times in speed control, but does work... In tests, with a 50mm lens, I can get about 2.5 minute exposures, and at 24mm, 5 minutes is possible.

[themos]

Yes, the poor man's astrotrac... at 55mm, the 450D gives about 20" per pixel, and requiring drift less than 20" in 2 minutes requires polar alignment better than 40 arc-minutes, I've calculated, which isn't too stringent. It would be interesting to know if we are going to be polar-alignment-error limited or RA rate-error limited...

[jgs001]

not sure I'd be able to tell... my PA is rather slapdash... as it's with a finder (not aligned) stuck on the mount, but a helluva lot better than nothing.

[themos]

I've got a tabletop tripod and a camera bracket for it - vi(v)a astroboot! - so I will have a play with PA procedures.

[themos]

First results are encouraging @55mm/450D. I was shooting untethered so exposures were only up to 30 seconds. My PA procedure was based on thinking of the camera+lens as a polarscope:

1. Align camera with RA axis (the equivalent of adjusting the reticule of the polarscope)

2. Align mount with Polaris.

To do 1, I used the brightest star, Arcturus. During a 15 second exposure, I slowly rotated in RA by 180 degrees. This produces an arc of light in the image. Adjust declination to make the arc as small as possible (that will be our declination=90) and then adjust az/alt to bring the star back to the centre. With counterweight down, your camera is now pointing directly over or under the RA axis. I can't think of a way of mechanically taking that error out so we must allow for it, somehow. If we had a movable reticule on the LiveView display... Anyway, once you get the smallest arc you're happy with ( I got a squashed half-ellipse) note where the centre of that ellipse is and that's your RA axis.

To do 2, just adjust azimuth and altitude of the mount until Polaris is there (and possibly offset by half-a-degree towards Kochab, you need to work out how many pixels that is for your setup)

Then point at your favourite object, tighten the clutches, start the motor and start shooting. The first few subs will be bad because the drive hasn't taken up the slack yet, so be prepared for lots of star trails at first. It takes a few minutes for the slack to be taken up in my EQ1. But after that, I got some brilliant exposures with no discernible trailing (viewed at 100%). You also need to play with the speed control potentiometer of the drive which must match the sidereal rate.

Next time, I'll use ExtraWebcam to convert my 450D to a webcam and use EQAlign, tethered, to fine tune PA. But for untethered, I'm happy that I can get reasonable PA fairly quickly.

[Psychobilly]

Tetherd you could overlay AlsReticle over the display..

IIRC Ken - Merlin66 posted a linked to it a while back...

The rate setting on the cheap drive was suppsoed to be quite temperature sensitive... so they did a more expensive quartz oscillator based one...

Peter...

[jgs001]

I used a 66/700mm refractor with a 5mm ortho and a barlow to get about the right tracking speed. Not very accurate but it seems to have worked. I marked the spindle for the pot to show the approximatly correct setting. I'm not expecting HEQ5 performance out of it, but given the difference between 8 seconds on a static tripod and 2 minutes on the EQ1...

[themos]

AlsReticle, noted. I am thinking, if tethered, EQAlign should work to help me drift align. I tried it indoors so the software (ExtraWebcam + EQAlign) works now (you have to get the latest ExtraWebcam version with the WDM driver).

Maybe I can hack a quartz-driven rate control myself. Anyone have any pointers?

Looking forward to trying 2-min subs...

[themos]

Tried out EQAlign+ExtraWebcam last night on the EQ1. Mixed results: the drift is very slow at that focal length (55mm) so you need to let EQAlign follow the star for 7 minutes at least. Once you finish the calibration stage, EQAlign knows which way is West and how many arc-seconds per pixel. I then followed Deneb (high in the East) for a while and EQALign was able to tell me if my RA motor was going too fast and what the declination drift was doing. I adjusted the RA rate while looking at the graph. The indicated PA error was about 30 arc-minutes. In the end, I settled for an indicated drift of about a pixel in 3 minutes. But when I took 2-min exposures, there was obvious star trailing so I will have to revisit the procedure.

[themos]

Here's two candidate PA procedures that I've found, after rough alignment:

1. Take a long bulb sub of a star (near equator) with 5 seconds of sidereal rate, then 30 seconds with no tracking and then 30 seconds of double sidereal. That gives you a wedge of a star trail and from the wedge angle you can deduce how much and which direction to move the mount. Explained here Polar Align - No Drift

2. Take a sub around Polaris, then rotate mount in RA by 30 degrees and take another one. If you know your field stars and note down their pixel coordinates, this program (DOS!) will give you the pixel coordinate of the NCP and the pixel coordinates of your calibration stars if you were aligned. Explained here: ALIGN – A Polar Alignment Utility

I quite like the sound of the second one. Has anybody heard of a more up-to-date version?

[themos]

Two things:

One: I will be looking to set up an Arduino oscilloscope to investigate that EQ1 RA motor signals. One could then rig up a USB controlled Arduino to replicate them with the addition of RA guiding signals (with ASCOM interface, natch). The guidescope (a camera lens with a webcam) could then replace the counterweight at the end of the couterweight bar, needing only some mounting contraption.

Two: I've been thinking about that ALIGN utility and I'm pretty convinced it will result in a (relatively) quick PA procedure. Maybe I can reproduce the geometric calculations in a spreadsheet and then we will all be able to use it. I think that if the second of the two captures is taken with the counter-weight down, it means that x and y pixel displacements translate directly to azimuth and altitude mount corrections.

So, with good PA and RA guiding, I could have half-an-Astrotrac, maybe.

[themos]

I have had some success with this PA method. I implemented it in a Google spreadsheet (see details in http://stargazerslounge.com/diy-astronomer/108643-help-me-test-my-polar-alignment-procedure.html). Achieved better than 10 arc minutes PA error.

For the RA rate, I hooked up ExtraWebcam and EQAlign and monitored the RA drift in the graph as I twiddled the pot on the drive.

I ended up with usable 4 minute subs @ 55mm focal length.

Ok, the astrotrac can do 5 minutes @ 400mm so we have some catching up to do!

[themos]

OK, update: I haven't made any progress deciphering the EQ1 RA motor electronics. I will be going to Greece for 2 weeks and I'll be taking the tabletop EQ1 mount with this RA motor with me. I will try to image widefield with 55mm and 135mm lenses on the 450D and the CLS Astronomik filter. They give 23x15 degrees and 9x6 degrees, respectively. I will do PA with my method of taking two exposures and working out the alt/az shift from that. The rate adjustment I will do with EQAlign and ExtraWebcam running on the laptop. Target will be Ophiuchus/Scorpio/Sagittarius area. Wish me luck!

[themos]

Aaargh, while I was cleaning and adjusting the worm on the EQ1 last night, I over tightened the grub screw that presses down on the screw-in bearing of the worm (to stop the bearing from unscrewing and thus letting the worm wiggle along its axis) and some of the metal of the worm holder broke! I superglued it back together but I may have ruined my chance of trying this mount in lower latitudes. Suddenly, the AstroTrac feels like good value for money...