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Equipment Discussions >> Mounts

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Starhawk
Space Ranger
*****

Reged: 09/16/08

Loc: Tucson, Arizona
Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #5841598 - 05/05/13 12:41 PM

I agree. The running average appears to be the way to go in my mind.

I'm wondering if a purpose made attachment mounting and a carefully installed shaft adapter might not do the trick. While quickness is met by a thin aluminum plate and some circuit board standoffs, I would not be surprised if they were the main error source.

-Rich


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orlyandico
Postmaster
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Reged: 08/10/09

Loc: Singapore
Re: Encoder-based PE Correction on the cheap new [Re: Starhawk]
      #5842680 - 05/05/13 11:35 PM

They probably are...

The choice of encoder shaft mount (Heidenhain claims their blind shaft is self centering) also might have something to do with it.


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orlyandico
Postmaster
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Reged: 08/10/09

Loc: Singapore
Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #5845356 - 05/07/13 11:26 AM

I wrote some nice Processing sketches to graph the measured periodic error in real time (pulling data off the serial port).

Without correction (this is the AP600, west-heavy, so about 15" p-p):


and with correction (misleading, because the mount is actually following the curve of the encoder's error):


much nicer than capturing the data with Putty and graphing it after the fact in Excel!

I did some more simulation and analysis, and realized that I cannot get rid of the 24-hour periodic error with low-pass filters (learned a lot about IIR filters though! - the smooth curve in the Processing sketches are actually 3rd order Butterworth lowpass filtered)

So I need to have some better shafts made. I goofed in my previous measurements (didn't have a digital caliper then) - measured the encoder shaft ID at 0.992" when it actually is (exactly) 1.000". So my shafts are undersize by 0.008" and the encoder wobbles on it.

I mean I had noticed this wobble before, but I didn't realize it contributed so much error.

Now hoping the person who made my original shafts would be willing to make more..


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Starhawk
Space Ranger
*****

Reged: 09/16/08

Loc: Tucson, Arizona
Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #5845495 - 05/07/13 12:53 PM

Interesting- if all else fails, 1.000" should be an off-the-shelf size. You could probably just buy some drill rod and use it.

-Rich


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orlyandico
Postmaster
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Reged: 08/10/09

Loc: Singapore
Re: Encoder-based PE Correction on the cheap new [Re: Starhawk]
      #5845538 - 05/07/13 01:08 PM

A simple rod / shaft won't do.. my existing shafts are like that, and I realized that there must be some mechanism to ensure that they are trued up to the polar shaft.

This is because the threads aren't that tight, so the shafts wobble a little bit. My revised shaft design (for the 1.000" ID, if ever I get this working I'll need a revised design with a 12mm shaft for the Heidenhain encoder) has a collar to ensure it's tight:



(the shaft I want made is the lower one)


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vdb
sage


Reged: 12/08/09

Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #5845795 - 05/07/13 02:36 PM

Ah, so my advise was correct then, it was my suspicion from the beginning, the physical mounting of the encoder is also very important (not the only source of error but an important one)

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Starhawk
Space Ranger
*****

Reged: 09/16/08

Loc: Tucson, Arizona
Re: Encoder-based PE Correction on the cheap new [Re: vdb]
      #5845858 - 05/07/13 03:02 PM

Suggestion: If you can get a machinist to help, the answer might be to hand him the mount and just say, "This is what I am trying to do" and see what he says.

There are ways of causing this to true up using an expanding shaft. Imagine if you used the far end of the RA pass-through to cause the shaft to line up. Something like a tapered interference at the far end would cause the shaft to true up if the bottom end was tapered. So, what you would have, in essence, would be two tapers pulled into either end of the cylinder section with a long bolt holding them together.

Alternatively, you could just shim the shaft with something like aluminum air conditioning tape. You would want to have it at three equally spaced points around the shaft. That might get your current shaft working so you can make progress without having to wait for a major refit.

-Rich


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neilson
professor emeritus


Reged: 08/22/10

Re: Encoder-based PE Correction on the cheap new [Re: Starhawk]
      #5846303 - 05/07/13 06:32 PM

Or some plumbers white teflon tape for pipe threads

neilson


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orlyandico
Postmaster
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Reged: 08/10/09

Loc: Singapore
Re: Encoder-based PE Correction on the cheap new [Re: neilson]
      #5846844 - 05/07/13 10:59 PM

I've tried shimming with masking tape. Too thick. I would imagine the aluminum AC tape is even thicker.

I have some ultra-thin teflon sheet from my dead-end AP600 declination bushing experiment. I could try that..


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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]
      #5857394 - 05/13/13 12:30 PM

Hi Orlando, have you already tried shimming the
encoder???

Great results so far, very impressive!


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orlyandico
Postmaster
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Reged: 08/10/09

Loc: Singapore
Re: Encoder-based PE Correction on the cheap new [Re: Mert]
      #5857556 - 05/13/13 01:35 PM

Traveling, so no shimming. Have ordered some new shafts. Hopefully will receive them in a couple weeks.

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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 [Re: orlyandico]
      #5857978 - 05/13/13 05:13 PM

I see, one more question though!

On the sketches you posted a while ago on the shaft you
would like to make, did you check if the end of your
RA-axis where the polar scope is screwed into, is trued
up???
If not, screwing your new axis flat against that RA-axis
end which wouldn't be trued up wouldn't make much of an
improvement I am afraid!
Maybe just rotating the RA-axis with a little feeler gauge
against the flat end will show??

I hope to read more on your project, this is very
interesting all together!!!

Edited by Mert (05/13/13 05:17 PM)


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mattflastro
Vendor - Astrovideo Systems


Reged: 07/31/09

Loc: Brevard County , FL
Re: Encoder-based PE Correction on the cheap new [Re: Mert]
      #5868545 - 05/18/13 12:24 AM

Quote:

I see, one more question though!

On the sketches you posted a while ago on the shaft you
would like to make, did you check if the end of your
RA-axis where the polar scope is screwed into, is trued
up???
If not, screwing your new axis flat against that RA-axis
end which wouldn't be trued up wouldn't make much of an
improvement I am afraid!
Maybe just rotating the RA-axis with a little feeler gauge
against the flat end will show??

I hope to read more on your project, this is very
interesting all together!!!



you need an encoder mount similar to a push pull lens or mirror cell . No amount of shimming would make this mess true with the sub 1ppm accuracy you need . Scope makers know they couldn't build the telescope concentric and collimated and that's why there are collimation adjustments. Imagine you didn't have secondary mirror collimation adjustments and had to machine or shim the mirror holder until it was perfect. Adding collimation screws to your encoder isn't that big of a deal compared to all the IIRF , Bessel, etc FFTs you went thru . For a few pesos you could get some nice differential action submicron accuracy actuator screws like ther ones used in optical kinematic mounts:
Differential screws

The one from the link has a precision of 25microns PER REVOLUTION. That's 1 micron per 18degrees of rotation . Try that with shims or machining .

Edited by mattflastro (05/18/13 08:07 PM)


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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: mattflastro]
      #5889034 - 05/28/13 01:59 PM

Hi Orlando,

By any chance have you had any chance to test something
more, since I'm very ( read VERY ) interested in your
project!
I hope to read more on your progress, keep up
the good work Orlando!


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orlyandico
Postmaster
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Reged: 08/10/09

Loc: Singapore
Re: Encoder-based PE Correction on the cheap new [Re: Mert]
      #5890100 - 05/29/13 01:04 AM

Still waiting for my new shafts Mert.

I have been traveling for the past 2 weeks.

The new shafts are slowly making their way to me in Singapore from the US. I hopefully should have them by Saturday.

Matt: am not too interested in the collimation-style adjustment. More opportunity for user error and quite laborious to adjust. And the TDM manages to do it without adjustments, so it should be possible.

My new shafts are 0.998" (instead of 0.992") for the 1" ID encoder shaft. Hopefully much less wobbling so no more gross periodic error.


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Edmond S.
member


Reged: 07/05/10

Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #5890518 - 05/29/13 10:58 AM

Hi Orlando,

I have been following this post for a while. This is indeed a very interesting project.

Lately I start to investigate my CGEM performance and getting more intrigued by your solution. Keep us posted how it turns out.

another thing: is it possible to use PECPrep to figure out the fundamental PE and the 8/3 PE, create/generate the combined correction on the fly and send it to the mount ? Sort of like a PE from the computer, but takes 24min to generate profile.

Edmond S.


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orlyandico
Postmaster
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Reged: 08/10/09

Loc: Singapore
Re: Encoder-based PE Correction on the cheap new [Re: Edmond S.]
      #5890584 - 05/29/13 11:45 AM

Hi Edmond, no it is not possible.

You see the non-integer 8/3 cycle only repeats every 24 minutes. Since the PEC indexer on the CGEM repeats every worm turn, you don't know where you are in the 24-minute cycle.

Some of the Meade mounts have a 3-cycle indexer so they can figure out where they are.

Also the CGEM only has an 88-cell PEC. So even if it were possible to do 24 minutes, you would only have 88 cells, so that's only (88 / 24 minutes) = 3.67 corrections per minute (or 1 PEC correction every 16 seconds).


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Starhawk
Space Ranger
*****

Reged: 09/16/08

Loc: Tucson, Arizona
Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #5891232 - 05/29/13 07:56 PM

I'm still thinking about this as well.

-Rich


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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]
      #5891992 - 05/30/13 09:00 AM

Hope to see an image of the new shafts Orlando, CNC-ing
is fun!!!


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orlyandico
Postmaster
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Reged: 08/10/09

Loc: Singapore
Re: Encoder-based PE Correction on the cheap new [Re: Mert]
      #5910748 - 06/09/13 01:29 AM

small update. Installed the new shafts. Much more sturdy. Very tight fit, had to sand down the shafts a bit.

bad news: huge error still there.

I will be attaching an indexer to the shaft (a 128ppr $10 absolute encoder) and will try to apply PEC to the huge error. Inelegant solution, but I need to get this working (for academic reasons - I want to use it as a project for my astronomy postgraduate course).

Seems that an absolute encoder is a better choice, am still checking if the (5000ppr, 25-bit resolution) Heidenhain ECN425 is cost-effective. But it's instructive to note that this encoder only has a +/- 10" accuracy. I expect this to be true of any 5000ppr sine/cosine encoder. Makes me wonder (further) how the TDM does it, since they use the same darn encoder technology (ERN180/480).


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