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Equipment Discussions >> Mounts

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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: freestar8n]
      #5772236 - 04/02/13 07:10 AM

Am not sure if a CGEM is comparable to a CGE or CGE Pro though.... Those mounts are in a higher class. And when you go higher e.g. AP, these don't need tight guiding anymore...

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Raginar
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Reged: 10/19/10

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Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #5772399 - 04/02/13 09:15 AM

Orly,

Have you tried taking a picture with what you have? Perhaps you've got it 'good enough' for AP just not the perfection your were looking for?


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orlyandico
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Loc: Singapore
Re: Encoder-based PE Correction on the cheap new [Re: Raginar]
      #5772518 - 04/02/13 10:22 AM

hi Chris,

No I haven't tried taking a picture.... but I kind of determined that I want perfection. Besides this would cost $500. I don't think people would pay $500 to take a CGEM to +/- 4" PE. But they might if it was +/- 2" or better.


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RogerRZ
Whatta you lookin' at?
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Reged: 01/09/06

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Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #5772556 - 04/02/13 10:40 AM

Quote:

hi Chris,

No I haven't tried taking a picture.... but I kind of determined that I want perfection. Besides this would cost $500. I don't think people would pay $500 to take a CGEM to +/- 4" PE. But they might if it was +/- 2" or better.




Yes, he would...


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EFT
Vendor - Deep Space Products
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Reged: 05/07/07

Loc: Phoenix, AZ
Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #5772571 - 04/02/13 10:49 AM

Quote:

hi Chris,

No I haven't tried taking a picture.... but I kind of determined that I want perfection. Besides this would cost $500. I don't think people would pay $500 to take a CGEM to +/- 4" PE. But they might if it was +/- 2" or better.




Oh yes they would and I suspect that the well tuned mount might do better in the first place since it would be starting with lower and smoother PE and random error. If it can be done for $500, that would be great.


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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: EFT]
      #5772603 - 04/02/13 11:04 AM

I think $500 is optimistic. It can be done for $500 if you assemble everything yourself (as the encoder is $325).

If it's a partial kit or something, someone would have to do the integration and that drives up cost because labor ain't free.


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Alph
Carpal Tunnel


Reged: 11/23/06

Loc: Melmac
Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #5772933 - 04/02/13 01:17 PM

Quote:

I don't think people would pay $500 to take a CGEM to +/- 4" PE. But they might if it was +/- 2" or better.




I donít view this project as limited to the CGEM only. I personally would be interested in a simple device that could be used to accurately measure PE of a mount in-house. To limit the cost and the hassle of building electronics board, the device should be operated/controlled from PC. What would it take to make the output from an encoder readable by PC? The data feed from an encoder could be consumed e.g by Pempro. That would be a very nice product.


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Starhawk
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Reged: 09/16/08

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Re: Encoder-based PE Correction on the cheap new [Re: Alph]
      #5773006 - 04/02/13 01:56 PM

It would take an electronics box if you want to be inexpensive.

Otherwise, there are evaluation board setups which use software like Labview to use a PC. So, besides losing portability, that approach would also be quite expensive and entail a new software suite.

The good news is that stuff is all off the shelf, now- just go buy it.

-Rich


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Mert
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Re: Encoder-based PE Correction on the cheap new [Re: Starhawk]
      #5773067 - 04/02/13 02:26 PM

I have another question for you Orlando:

On the drift item you have demonstrated, did you check
if the sum of all the corrections done over 1 worm cycle
adds up to 0 aprox???


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orlyandico
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Loc: Singapore
Re: Encoder-based PE Correction on the cheap new [Re: Mert]
      #5774517 - 04/03/13 12:21 AM

Alph, the box as it is today can measure PE when attached to a PC via USB.

Now as we've seen, there is a good but not perfect correspondence between the PE measured by the box, and by PEMPro. I would not consider PEMPro to be the "reference" because it is affected by seeing, while the box is not.

I moved the whole box over to the PICMX32, since I will need more I/O pins going forward and the PICMX costs about the same as an Arduino Mega2560. Discovered that MPIDE and the Arduino IDE aren't 100% identical, and had to re-write some code.

Hopefully this will solve the timing issue (the Max32 has a TCXO) and the 128K of RAM will allow me to do the statistical correction in Kavanagh's article. The only challenge is I have introduced a bug in the code... the ADC reads out properly during calibration but in the main loop outputs -1.....

Mert, yes I did check and the average error over 1 cycle was close to zero (it was 0.05").


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Pinbout
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Reged: 02/22/10

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Re: Encoder-based PE Correction on the cheap new [Re: EFT]
      #5774543 - 04/03/13 12:36 AM

Quote:

Quote:

hi Chris,

No I haven't tried taking a picture.... but I kind of determined that I want perfection. Besides this would cost $500. I don't think people would pay $500 to take a CGEM to +/- 4" PE. But they might if it was +/- 2" or better.




Oh yes they would and I suspect that the well tuned mount might do better in the first place since it would be starting with lower and smoother PE and random error. If it can be done for $500, that would be great.




that's cheaper than Explorer Scientific's high end encoder TDM $1799.

iOptron suppose to have a IEQ45 gt version that's a lot cheaper than their first version which with a 3rd encoder that made the mount cost $5k

Edited by Pinbout (04/03/13 12:55 AM)


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Alph
Carpal Tunnel


Reged: 11/23/06

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Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #5774570 - 04/03/13 12:57 AM

Quote:

Alph, the box as it is today can measure PE when attached to a PC via USB.



Yes, I am aware of it. Let me rephrase my question. What are the bare minimum hardware requirements to get the output from the encoder to the PC? I see a lot of potential in having just that.

Quote:

Now as we've seen, there is a good but not perfect correspondence between the PE measured by the box, and by PEMPro. I would not consider PEMPro to be the "reference" because it is affected by seeing, while the box is not.




Sure. However, you still need to sync up with the worm index and upload PEC to the mount and thatís where PEMPro comes in.


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orlyandico
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Re: Encoder-based PE Correction on the cheap new [Re: Alph]
      #5774625 - 04/03/13 02:09 AM

The bare minimum is what I have right now:

1) an Arduino of some sort (an Uno will work fine if reading the PEC is all that's required) - this is about $30, and connects to the PC via USB

2) an Arduino daughterboard with an AD converter on it (total cost $10)

3) the encoder itself, that needs to be wired up to the Arduino

The vast majority of the cost would be fabricating the mount-specific adapter, and the encoder itself.

As for sync'ing the worm index and uploading PEC... that's a job best left to PEMPro or something. It might be possible to produce a PE file from the encoder that PEMPro can understand.

As for the TDM.. it's $1800 but you need to budget another $400 for the mount-specific adapter.

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). I still think the price can be less.

The last iEQ45 I checked that had a Renishaw encoder was $4800. Which is idiotic. Nobody will buy a $4800 China mount with 1/2 the capacity of a Mach1 when you can get a Mach1 used for a few hundred bucks more.

If there was a $2K encoder-equipped mount with the capacity of an iEQ45 or Atlas/CGEM, then that would fly off the shelves.

Avago has a cheap 20-bit (1 million tick) digital encoder (this one - http://www.avagotech.com/docs/AV02-0865EN), that if combined with something like the SiTech tick management (possible if the RA encoder is integrated with the mount drive electronics) would enable this price point.


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vdb
sage


Reged: 12/08/09

Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #5774706 - 04/03/13 04:44 AM

Problem is that Renishaw encoder is already around 1K so cheapest I can see for such a mount is around 3k ...

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orlyandico
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Re: Encoder-based PE Correction on the cheap new [Re: vdb]
      #5774709 - 04/03/13 04:46 AM

The iEQ45 in pinbout's link is a Renishaw RGH22 (or similar) linear read head. These have built-in interpolation and output a digital signal. They are cheaper than rotary encoders and you don't need as high a resolution because you would put an encoder tape around the circumference of the RA housing, which is a much larger radius, so the grating pitch on the tape can be less. I believe the tape is $12/inch or something.

the Gurley 320K tick encoders used on SiTech are about $600 and have built-in interpolation. The resolution is not high enough by itself, but in conjunction with tic management using the motor encoders, SiTech does a very good job. I cannot see how the iOptron guys could not use a similar approach..

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


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freestar8n
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Re: Encoder-based PE Correction on the cheap new [Re: Alph]
      #5774716 - 04/03/13 05:16 AM

Quote:

What are the bare minimum hardware requirements to get the output from the encoder to the PC? I see a lot of potential in having just that.




Years ago in my P.E. days, I was involved in other ways to measure PE. Why do you think it's so valuable or desirable to measure PE without a star? Are there others that share your interest?

I suppose for people making mechanical adjustments to their mounts it would be helpful to have prompt feedback, or when iterating to find an optimal PEC curve to apply.

Frank


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orlyandico
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Re: Encoder-based PE Correction on the cheap new [Re: freestar8n]
      #5774771 - 04/03/13 07:04 AM

Frank, there was a long discussion of this before.

Basically, having a mechanical method of measuring PE would allow a mount manufacturer to do QA cheaply. They would be able to ensure that no stinker mounts got out the door, and perhaps charge a premium for a "certified" mount. Surely a certification would be worth another $100 to $200?

But if you have to use a star to measure the PE, the labor alone of the guy doing the measuring will wipe out your profit.


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freestar8n
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Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #5774777 - 04/03/13 07:10 AM

That makes sense - but would only apply to a mount manufacturer... And presumably they can afford an expensive and high-res encoder in the first place since they would use it in production.

My own work involved other methods for measuring PE - but I lost interest. If there is a general need or desire for such things then I can take a look.

Frank


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vdb
sage


Reged: 12/08/09

Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #5774782 - 04/03/13 07:17 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)




But you loose the polarscope ... which for a portable setup is nice, especially the iOptron as it's very accurate, I could do 30 min subs without field rotation.


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orlyandico
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Re: Encoder-based PE Correction on the cheap new [Re: freestar8n]
      #5774785 - 04/03/13 07:19 AM

One method I thought that a manufacturer could use to measure PE.. (not useful for astronomers)

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. Then post-process each frame of the movie to quantify the laser dot's movement. A perfect mount would show the dot moving at a fixed rate, any variation in the dot speed is PE.

QED. No need for a fancy encoder.


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