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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: Starhawk]
      #6228700 - 12/02/13 09:58 AM

For the curious, here is a link to my project poster (showing only the conclusions).

https://dl.dropboxusercontent.com/u/63497702/Poster-HET615-OrlandoAndico.pdf

The quick and the dirty - there is no statistical difference in FWHM of 600-second images taken at 600mm (1.86"/pixel) with this setup, and a guided Paramount ME at 2.1"/pixel. Granted these are short focal lengths.

Incidentally, the effect of two read heads can be obtained with two rotary encoders mounted co-axially. It would be very expensive however.

I am 100% sure that the TDM's intellectual property is in their mounting, which reduces or eliminates the long-period error due to eccentricity. Now you can relax the mechanical requirement by using two coaxial encoders, at increased cost...


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Raginar
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Reged: 10/19/10

Loc: Rapid CIty, SD
Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #6230095 - 12/02/13 07:56 PM

Are you planning to put up more than the results or is this it? Pretty neat review.

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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: Raginar]
      #6230104 - 12/02/13 08:01 PM

Schematic - https://dl.dropboxusercontent.com/u/63497702/Schematic.pdf

Arduino source code -
https://dl.dropboxusercontent.com/u/63497702/read_rls_2hAGph.7z


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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: orlyandico]
      #6240241 - 12/08/13 04:29 AM

I have gone back to the Baumer sin/cos encoder.

This time I used an ellipse-fitting algorithm from Kapteyn Laboratory Groningen (source code readily available from CSIRO) to find the parametric equation of the ellipse that best fits the encoder data.

Here's a graph showing the raw (x,y) from the data, the transformed (x', y') after solving the ellipse parameters and then applying the translation/rotation from Lepple's thesis, and a model circle.



It is obvious that the corrected (x', y') is much closer to the model circle than the raw encoder data.

But now I am stumped. The angle computed by atan2(y', x') exhibits greater SDE than the naive angle computed by atan2(y, x).

Why??????!!!!


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Raginar
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Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #6242246 - 12/09/13 09:32 AM

I wish I could help. I hope someone can tell you why!

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seryddwr
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Reged: 02/19/10

Loc: La-la land.
Re: Encoder-based PE Correction on the cheap new [Re: Raginar]
      #6377773 - 02/15/14 01:30 PM

Orly,

When you have two read heads, how are the signals averaged? It can't be an arithmetic average, can it? It seems like it would be easy to have the read heads positioned such that their signals were out of phase with each other, thus corrupting or even zeroing-out the signal, if they were simply added together.


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