From sragland@coolstars.cfa.harvard.edu Wed Jun 18 08:59:33 2003 -0400
Date: Wed, 18 Jun 2003 08:59:32 -0400 (EDT)
From: Sam Ragland <sragland@coolstars.cfa.harvard.edu>
To: John Monnier <monnier@umich.edu>
cc: pschuller@cfa.harvard.edu,
Jean Philippe Berger <Jean-Philippe.Berger@obs.ujf-grenoble.fr>,
carleton@cfa.harvard.edu, epedretti@cfa.harvard.edu,
mlacasse@cfa.harvard.edu, rafael@ipac.caltech.edu,
schloerb@fcrao1.astro.umass.edu, Wes Traub <wtraub@cfa.harvard.edu>
Subject: Re: Origin of readout dependent closure phase offset
Hi John,
I noticed this 'readout dependent instrumental closure phase'
about a couple of months back and had a discussion with Wes
earlier. We were speculating various possibilities including
'change of delay between pixels'. I was arguing against this
possibility.
Let us assume (for simplicity) that the ADC sampling
interval is 10 microseconds, and consider only three
PICNIC pixels (0,2,4) out of six outputs.
Case 1: 1 loop & 2 reads
-------
-------------------------------------------------------
Pixel 0 pixel 2 pixel 4
-------------------------------------------------------
Sampling 0,10 40,50 80,90
time (microsec) (2 reads)
-------------------------------------------------------
Mean samp. 5 45 85
time (microsec)
--------------------------------------------------------
Delay between pixels: 45 - 5 = 40 microseconds
Case 2: 2 loop & 2 reads
-------
------------------------------------------------------------
Pixel 0 pixel 2 pixel 4
------------------------------------------------------------
Sampling 0,10 40,50 80,90 (loop 1)
time (microsec) 120,130 160,170 200,210 (loop 2)
------------------------------------------------------------
Mean samp. 65 105 145
time (microsec)
------------------------------------------------------------
Delay between pixels: 105 - 65 = 40 microseconds
I get the same delay irrespective of # of loops & reads.
Am I missing something?
Sam
----------------------------------------------------------
Sam Ragland
Harvard-Smithsonian Center for Astrophysics (MS20)
60 Garden Street
Cambridge MA 02138
Tel: (617) 495-7158 (W) (617) 389-7461 (R)
Fax: (617) 496-0121/ 617 495-7467
E-mail: sragland@cfa.harvard.edu; sam_ragland@yahoo.com
-----------------------------------------------------------
On Tue, 17 Jun 2003, John Monnier wrote:
>
> This may be obvious to some, but I thougth I would send this to the wider
> audience. (Sam: can yo put this on the webpage).
> ---
>
> Statement of problem:
>
> Measure clsoure phase depends on readout pattern.
> Example:
>
> readout Cphase (degs)
> 1r7l 141
> 2r7l 160
> 3r7l 167
> 4r7l 169
>
> Explanation:
> With only 1 loop, there is ~ t_int/6 (sample/6) delay between pixels (or
> sample/3 between beam combiners). For the 'fast' fringe which only has 4
> samples per fringe, then this amounts to 1/12 fringe, or ~30degrees. For
> 2 loops x reads, then this effect is half has bad.. so the difference
> between 1l xreads and 2l x reads is ~ 15 degress, in agreement (ballpark)
> with observations. This also means that the difference gets progressively
> less.. this is waht is observed by me:
>
> W/o taking into account the other fringes (8pixels/fringe), we would
> predict differences similar to the seen above (within a few degrees). This
> also says the TRUE instrumental closure phase is the one that corresponds
> to a readout with many loops (where time differences becomes negligible)
> (~170 degrees in the above example -- I note that there is a possible 180
> degree ambiguity in the above at the current level of analysis). This may
> be important when deriving a color calibration.
>
> Note: This effect would not happen if the piezos were really STEPPING
> between pixels, which is how the ADC is programmed. This readout dependent
> closure phase suggests that the actual motion of the piezo ramps are
> smooth during the whole scan, the steps are so small and sharp that the PI
> servo can not hope to correct this (there is low frequency filter, both
> electronically and probably mechanically that makes the ramp smooth and
> linear. Not surprising I guess.
>
> Comments are welcome.
> -------------------------------------------------
> John D. Monnier, Assistant Professor of Astronomy
> University of Michigan 941 Dennison Building
> 500 Church Street Ann Arbor, MI 48109-1090
> monnier@umich.edu 734-763-5822 (FAX 734-763-6317)
>
From sragland@coolstars.cfa.harvard.edu Wed Jun 18 08:59:33 2003 -0400
Date: Wed, 18 Jun 2003 08:59:32 -0400 (EDT)
From: Sam Ragland <sragland@coolstars.cfa.harvard.edu>
To: John Monnier <monnier@umich.edu>
cc: pschuller@cfa.harvard.edu,
Jean Philippe Berger <Jean-Philippe.Berger@obs.ujf-grenoble.fr>,
carleton@cfa.harvard.edu, epedretti@cfa.harvard.edu,
mlacasse@cfa.harvard.edu, rafael@ipac.caltech.edu,
schloerb@fcrao1.astro.umass.edu, Wes Traub <wtraub@cfa.harvard.edu>
Subject: Re: Origin of readout dependent closure phase offset
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References: <Pine.SOL.4.55.0306171830150.11940@imager.astro.lsa.umich.edu>
<Pine.SOL.4.53.0306171937400.1471@coolstars.cfa.harvard.edu>
<Pine.SOL.4.55.0306172001340.11991@imager.astro.lsa.umich.edu>
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Hi John,
I noticed this 'readout dependent instrumental closure phase'
about a couple of months back and had a discussion with Wes
earlier. We were speculating various possibilities including
'change of delay between pixels'. I was arguing against this
possibility.
Let us assume (for simplicity) that the ADC sampling
interval is 10 microseconds, and consider only three
PICNIC pixels (0,2,4) out of six outputs.
Case 1: 1 loop & 2 reads
-------
-------------------------------------------------------
Pixel 0 pixel 2 pixel 4
-------------------------------------------------------
Sampling 0,10 40,50 80,90
time (microsec) (2 reads)
-------------------------------------------------------
Mean samp. 5 45 85
time (microsec)
--------------------------------------------------------
Delay between pixels: 45 - 5 = 40 microseconds
Case 2: 2 loop & 2 reads
-------
------------------------------------------------------------
Pixel 0 pixel 2 pixel 4
------------------------------------------------------------
Sampling 0,10 40,50 80,90 (loop 1)
time (microsec) 120,130 160,170 200,210 (loop 2)
------------------------------------------------------------
Mean samp. 65 105 145
time (microsec)
------------------------------------------------------------
Delay between pixels: 105 - 65 = 40 microseconds
I get the same delay irrespective of # of loops & reads.
Am I missing something?
Sam
----------------------------------------------------------
Sam Ragland
Harvard-Smithsonian Center for Astrophysics (MS20)
60 Garden Street
Cambridge MA 02138
Tel: (617) 495-7158 (W) (617) 389-7461 (R)
Fax: (617) 496-0121/ 617 495-7467
E-mail: sragland@cfa.harvard.edu; sam_ragland@yahoo.com
-----------------------------------------------------------
On Tue, 17 Jun 2003, John Monnier wrote:
>
> This may be obvious to some, but I thougth I would send this to the wider
> audience. (Sam: can yo put this on the webpage).
> ---
>
> Statement of problem:
>
> Measure clsoure phase depends on readout pattern.
> Example:
>
> readout Cphase (degs)
> 1r7l 141
> 2r7l 160
> 3r7l 167
> 4r7l 169
>
> Explanation:
> With only 1 loop, there is ~ t_int/6 (sample/6) delay between pixels (or
> sample/3 between beam combiners). For the 'fast' fringe which only has 4
> samples per fringe, then this amounts to 1/12 fringe, or ~30degrees. For
> 2 loops x reads, then this effect is half has bad.. so the difference
> between 1l xreads and 2l x reads is ~ 15 degress, in agreement (ballpark)
> with observations. This also means that the difference gets progressively
> less.. this is waht is observed by me:
>
> W/o taking into account the other fringes (8pixels/fringe), we would
> predict differences similar to the seen above (within a few degrees). This
> also says the TRUE instrumental closure phase is the one that corresponds
> to a readout with many loops (where time differences becomes negligible)
> (~170 degrees in the above example -- I note that there is a possible 180
> degree ambiguity in the above at the current level of analysis). This may
> be important when deriving a color calibration.
>
> Note: This effect would not happen if the piezos were really STEPPING
> between pixels, which is how the ADC is programmed. This readout dependent
> closure phase suggests that the actual motion of the piezo ramps are
> smooth during the whole scan, the steps are so small and sharp that the PI
> servo can not hope to correct this (there is low frequency filter, both
> electronically and probably mechanically that makes the ramp smooth and
> linear. Not surprising I guess.
>
> Comments are welcome.
> -------------------------------------------------
> John D. Monnier, Assistant Professor of Astronomy
> University of Michigan 941 Dennison Building
> 500 Church Street Ann Arbor, MI 48109-1090
> monnier@umich.edu 734-763-5822 (FAX 734-763-6317)
>