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Equipment Discussions >> Mounts

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Pinbout
Postmaster
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Reged: 02/22/10

Loc: nj
Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #5774797 - 04/03/13 07:45 AM

Quote:

(and.. adding a $2200 TDM to a base iEQ45 results in $3700 - which is still well over $1000 less than the Renishaw-equipped iEQ45)





last year when I was speaking to the same guy at NEAF [i think he's the president], he said they're suppose to come out with a gt this year thats a lot cheaper. They're gonna make their own encoder, what kind I don't know but I'm sure gonna bug him about it this year at NEAF so do you have any specific question I should ask him that's not too over my head?


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cn register 5
scholastic sledgehammer


Reged: 12/26/12

Re: Encoder-based PE Correction on the cheap new [Re: Pinbout]
      #5774855 - 04/03/13 08:28 AM

There's a program called Star Guiding Emulator that puts a dot on a PC screen and moves it at the correct rate. You can use it to train PEC and practice imaging indoors.

http://sweiller.free.fr/SGE/SGE-star-guiding-emulator.htm

Chris


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Starhawk
Space Ranger
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Reged: 09/16/08

Loc: Tucson, Arizona
Re: Encoder-based PE Correction on the cheap new [Re: cn register 5]
      #5775075 - 04/03/13 09:48 AM

One of the biggest benefits of Orly's encoder system is it fixes a pec problem peculiar to the CGEM- the 8/3 resonance in the drive motors whic makes it impossible to get a true pec loop in one worm rotation.

The other is it's a bolt on and forget setup- no looking for a guide star and training. Just turn it on.

So, for those of us who have to flee city lights, this makes a lot of sense.

-Rich


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orlyandico
Post Laureate
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Reged: 08/10/09

Loc: Singapore
Re: Encoder-based PE Correction on the cheap new [Re: Starhawk]
      #5775109 - 04/03/13 10:01 AM

Well I'm kinda stuck right now, so...

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Alph
Carpal Tunnel


Reged: 11/23/06

Loc: Melmac
Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #5775387 - 04/03/13 11:49 AM

Quote:

I have a whole lot more respect for the Germans who built that thing now (you know the old saying.. the ignorant sometimes attempt really difficult problems because they don't know how hard the problem is.. well that was me).




The Telescope Drive Master, or TDM, is the product of Hungarian amateur astronomers, Istvan Papp & Attila Madai of MDA Telescoop and it is marketed and sold by Meade Europe.


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Alph
Carpal Tunnel


Reged: 11/23/06

Loc: Melmac
Re: Encoder-based PE Correction on the cheap new [Re: freestar8n]
      #5775483 - 04/03/13 12:44 PM

Quote:

but would only apply to a mount manufacturer...




Many mount owners would like to know if their mounts perform to specifications. I have noticed that many folks struggle with periodic error measurements and analysis, and the results are quite often inconclusive. Flexure, seeing, and other unknown factors obscure the real PE graph. Other mounts like Paramount ME/MX have over 1000 PEC table entries. The PMX control board issues PEC corrections every 0.25s. You can't measure PE with such a precision just by tracking a star.


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orlyandico
Post Laureate
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Reged: 08/10/09

Loc: Singapore
Re: Encoder-based PE Correction on the cheap new [Re: Alph]
      #5775641 - 04/03/13 01:53 PM

I have also wondered why the AP GTO CP3 has over 1000 PEC table entries.

When you look at the way PEMPro programs the PEC on a GTO CP3, there is a "minimum move" command of 10ms. Because the error is so low to begin with, and the minimum move causes a 10ms correction, probably only one in 20 PEC entries has something in it.

Oh, and I stand corrected about the TDM. I always thought they were Germans...

On a related note, I finally figured out what was causing the weird ADC readings on my box. It wasn't bad code or running out of memory - so maybe moving to the PIC32 was not such a great move - but noise with my rat's nest wiring. This never popped up when I was bit-banging the SPI because the speed was low, but now that I'm clocking the ADC at 2MHz it is an issue.

I rewired (and soldered) everything onto an Arduino shield and the ADC has settled down; it still (very rarely) glitches but I can work around that in code. The PIC32 is very fast - and actually has an onboard 1 Msps 8- or is it 16-channel ADC. So in theory one could implement the PEC reader with a PIC32 alone. But the ADC inputs aren't differential, and I don't feel like mucking around with op amps..

I also found that the Lissajous pattern of the encoder is almost perfectly circular (0.9995), that there is zero offset on both A and B channels, and 99.7% of the radius readings (3 sigma) fall within a +/- 5% range.

So on startup I take several thousand readings (two slots worth @ 17 seconds/slot) and calculate the average radius. From this I can set a low (0.95X) and high (1.05X) bound for acceptable radius, which makes it trivial to throw out bad data.

I haven't implemented the algorithm described by Kavanagh yet, but now that the ADC issues have been sorted out that's next on my list.


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tjugo
scholastic sledgehammer


Reged: 11/06/07

Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #5775794 - 04/03/13 03:16 PM

Hi Orly,

Keep it going! This is a nice project, I hope you get to your goals 2 arcsec p-p under US$500. This will be a very nice addition to EQMOD.

Cheers,

Jose


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vdb
sage


Reged: 12/08/09

Re: Encoder-based PE Correction on the cheap new [Re: Pinbout]
      #5775848 - 04/03/13 03:44 PM

Quote:


last year when I was speaking to the same guy at NEAF [i think he's the president], he said they're suppose to come out with a gt this year thats a lot cheaper. They're gonna make their own encoder, what kind I don't know but I'm sure gonna bug him about it this year at NEAF so do you have any specific question I should ask him that's not too over my head?




Just ask how much it costs and when it ships


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OzAndrewJ
scholastic sledgehammer
*****

Reged: 11/30/10

Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #5775873 - 04/03/13 03:58 PM

Gday Orlando

Quote:

Rigidly affix a laser pointer to the mount, so that the dot shines on a curved surface (like the inside of a dome, or a curved piece of white-painted board) some distance away.

Run the mount while taking a movie of the laser dot.




I tried this once, but i used a tape measure on a fence over the road
and took webcam snaps every second.
Didnt work well, so i did some numbers.
One arcsec error at 100metres is approx 0.5mm,
and my fence was only 30m away.
Considering tracking is at 15 arcsec/sec and you also have
camera timing issues etc, you have no hope of getting
any real accuracy with this mechanism.
( with a laser, you also have the problem of getting a correct centroid )

Andrew Johansen Melbourne Australia


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orlyandico
Post Laureate
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Reged: 08/10/09

Loc: Singapore
Re: Encoder-based PE Correction on the cheap new [Re: OzAndrewJ]
      #5776798 - 04/03/13 11:46 PM

Thanks Andrew. So not such a great idea after all..

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freestar8n
Post Laureate
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Reged: 10/12/07

Re: Encoder-based PE Correction on the cheap new [Re: Alph]
      #5777016 - 04/04/13 05:20 AM

Quote:

Many mount owners would like to know if their mounts perform to specifications. I have noticed that many folks struggle with periodic error measurements and analysis, and the results are quite often inconclusive. Flexure, seeing, and other unknown factors obscure the real PE graph. Other mounts like Paramount ME/MX have over 1000 PEC table entries. The PMX control board issues PEC corrections every 0.25s. You can't measure PE with such a precision just by tracking a star.




As a first step I would use MetaGuide with a bright star above the equator at long focal length to record the centroid every 0.5 seconds with a good timestamp. MG calculates the centroid differently and it should have benefit in this application - but it does help to have good seeing. It also helps to make sure the system time is kept accurate. If anyone is interested in studying the logs I'd be happy to look at them.

I'm still not sure what the application is other than unguided imaging - where the amplitude of PE must be kept to a minimum. But if something like flexure is involved, and it isn't very repeatable, then guiding would be needed anyway.

So in a thread like this one I think it's important to distinguish the possible applications that make sense and those that may not.

a) Orly's goal is unguided imaging at low cost with a CGEM. I think this could work pretty well, especially for widish refractor imaging - and it would be relatively painless once the mount is well polar aligned.

b) Reducing the inherent PE to about 4" so it then acts like a high end mount and will guide better. I'm not optimistic this will be much different from just autoguiding well using OAG without encoders. I haven't worked with a cgem, but I have seen decent images with them that show they are capable of guiding pretty well - and the central issues of gearbox terms and non-integer PE are common to a lot of mounts, including the cge, g11, rcx400 - etc.

c) As a tool for users or manufacturers to measure PE without a star. This also doesn't make a lot of sense to me because manufacturers can afford a high res encoder test stand, and users can use MetaGuide to record accurate logs with a real star. How often would they need to do it, anyway?

So I'm very interested in seeing how far Orly gets with improving the unguided imaging at low cost. That part makes sense to me, and it's an interesting project - especially when working within cost constraints.

Frank


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orlyandico
Post Laureate
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Reged: 08/10/09

Loc: Singapore
Re: Encoder-based PE Correction on the cheap new [Re: freestar8n]
      #5777368 - 04/04/13 10:53 AM

Sadly, after all the added effort I've done over the past few days, this is the best I can get:



That's +5/-7 arc-seconds.

I notice that there is a slow drift in the encoder values (it has its own periodic error!) both in the interpolator, and in-between slots, i.e. the distance between slots is not constant. The easiest way to characterize the periodic error of the encoder + interpolator would be a precision rotational reference. Since I don't have that, I'll probably read out the encoder while guiding. But the science project ceases to be fun when it's like a job.

Need to take a break from this...


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Raginar
Post Laureate
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Reged: 10/19/10

Loc: Rapid CIty, SD
Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #5777556 - 04/04/13 12:23 PM

Well, hopefully something will come of your expenditures (both time and treasure). It's a cool idea .

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Starhawk
Space Ranger
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Reged: 09/16/08

Loc: Tucson, Arizona
Re: Encoder-based PE Correction on the cheap new [Re: Raginar]
      #5778798 - 04/04/13 11:31 PM

What is your control loop doing?

-Rich


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orlyandico
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Reged: 08/10/09

Loc: Singapore
Re: Encoder-based PE Correction on the cheap new [Re: Starhawk]
      #5778812 - 04/04/13 11:42 PM

I don't think the problem is the control loop (which I've dialed back a great deal).

The problem is that the encoder + interpolator has its own periodic error. So the box thinks it is tracking perfectly but in reality the drive is slowly wandering up and down.

What I have seen is that PHD very rarely (attempts) to issue guide commands while the box is active; the track is stable for 2-3 minutes at a time ( < 2" ) and during those periods PHD does nothing.

I really need to characterize the error on the encoder + interpolator, but my last attempt (ganging the CGEM and Mach1 together with a huge dovetail) didn't work too well...

Now how do we reconcile this with the periodic error being measured accurately?

Simple. If not correcting, the PE of the mount swamps the PE of the encoder. The encoder's PE is only about 8" p-p or less. But if we are correcting, what happens is the mount PE disappears and the mount gets slaved to the encoder's PE.

BTW here is the tracking log for the above graph -
https://dl.dropbox.com/u/63497702/EncoderData/data-20130404221247.csv

The columns are

Time elapsed (milliseconds)
A encoder raw count
B encoder raw count
Interpolated Theta (range 0.. 2591)
Grating Slot # (gross angle)
Total Angle (gross angle + Theta)
Expected Angle
Error
Correction (millisecond pulse length)

All of the angles are in deci-arcseconds, so an error of 10 is actually 1"


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Mert
Post Laureate
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Reged: 08/31/05

Loc: Spain, Pamplona
Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #5778824 - 04/04/13 11:52 PM

Is the encoder adjustably mounted onto the aluminum
plate?
You might think putting an O-ring under each mount point
so as to put a bit more preasure on 1 side, adjust the
tilt a bit????
Then log and see how the output behaves over 1 turn
of the RA-axis adjusting and see how that influences??


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orlyandico
Post Laureate
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Reged: 08/10/09

Loc: Singapore
Re: Encoder-based PE Correction on the cheap new [Re: Mert]
      #5778827 - 04/04/13 11:59 PM

Mert, I don't believe that encoder swash is an issue (read the Renishaw article "The accuracy of angle encoders"). Decentering of the encoder shaft would show up as swash.

That sort of periodic error would only manifest over a significant fraction of the encoder's rotation, not a few degrees.


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Mert
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Reged: 08/31/05

Loc: Spain, Pamplona
Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #5779378 - 04/05/13 10:55 AM

Yes, that's exactly what I was wandering about!!!
Then, how come that your significant drive signals leak
through where before it was a very nice flat curve??
Really interesting!!!


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Starhawk
Space Ranger
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Reged: 09/16/08

Loc: Tucson, Arizona
Re: Encoder-based PE Correction on the cheap new [Re: Mert]
      #5779742 - 04/05/13 02:26 PM

You'd need to see the encoder wobble for the error to amount to much over this time scale.

I am struck by three things:

(1) the high frequency movement is still there and is also expressed in the pempro plot. You aren't apparently responding to that.

(2) the larger scale oscillation seems to be at similar frequency to the pempro plot. It really looks like the correction isn't very effective.

(3). Three rotations of the worm should be observed before concluding you are ahead or behind. Remember you need the whole 8/3 resonance. It is plausible to be as big as you're seeing- otherwise no one would care.

Try this: record input to the Arduino and just record it's control outputs without sending them to the drive. Then plot them against the encoder plot to see what the controller is doing or not, and if it has a bias.

And don't give up. You are very close.

-Rich

Edited by Starhawk (04/05/13 02:34 PM)


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