Friday, August 10, 2012

Guiding - a major milestone

One of the things that makes ODI unique is its detectors, orthogonal transfer arrays (OTAs), which are more complicated than the typical CCDs found in most astronomical imagers.  The OTAs in ODI are each divided into an 8 X 8 array of "cells", each of which contain 480 X 494 pixels.  The reason for the separation into cells is so that the guide function can be accomplished by reading out a part of a single cell frequently to measure the position of a guide star without affecting the other 63 cells on the detector.  Ultimately, this will allow us to assign 4 cells on each OTA to the guide function and independently guide each quadrant to remove the local motion from atmospheric turbulence and telescope shake.  At this point (the first week on the telescope), the challenge has been to read out a cell at video rate, extract the guide star information, and send it to the telescope every second or so. 

We accomplished this last night for the first time.  The first picture shown is the OTA Listener, which displays the entire pODI focal plane on the left and one of the OTAs on the right.  For this exposure, we had pointed at M57 the ring nebula in order to take some long exposures and measure the crosstalk between cells.  The OTA shown is OTA 33, the center one in the 3 X 3 science field.  If you look closely at the lower left corner cell, you can see that it is truly black.  This is because during the exposure (about a minute long), we read out a piece of that cell and used it to guide the telescope.  This is the first step towards OT operation, and it is prerequisite to taking exposures longer than a minute or two.  The other thing you will notice are the amplifier glows. In order to read out an OTA, we have to turn the amplifiers on (throughout that entire OTA), and they glow in this batch of detectors.  The amps are off until readout on the OTAs not used for guiding - one of these, OTA 44, is shown in the lower picture.  Our planned mode of operation for pODI is to use one of the outer OTAs for the guide function, leaving the 3 X 3 "science field" free of amplifier glow.  For the full-up ODI, we'll manufacture new detectors that do not have the amplifier glow.  The images shown are raw - no flat fielding applied yet, so small variations in gain among cells are apparent.

Todd and Daniel

A 60 second long exposure of M57, with guiding, shown in the OTA Listener
OTA 44 from the above exposure - note that the amplifiers are not glowing

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