Date: Tue, 17 Jun 2003 20:17:18 -0400 (EDT) From: John Monnier X-X-Sender: monnier@imager.astro.lsa.umich.edu To: Sam Ragland cc: pschuller@cfa.harvard.edu, Jean Philippe Berger , carleton@cfa.harvard.edu, epedretti@cfa.harvard.edu, mlacasse@cfa.harvard.edu, rafael@ipac.caltech.edu, Sam Ragland , schloerb@fcrao1.astro.umass.edu, John Monnier , Wes Traub Subject: Origin of readout dependent closure phase offset 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)