Post-launch observations required to characterize flatfields

Michael Richmond
Dec 22, 2005
revised Dec 22, 2005

This document describes briefly the type and number of observations required to determine the SNAP detector "flatfield" effects before we can begin normal operations.

The various effects which we expect to see, and the methods by which one can measure them accurately, are described in

• for the spectrograph: section 2 of "Flatfielding the SNAP Focal Plane".
• for the imaging detectors: section 4 of "Flatfielding the SNAP Focal Plane".

Let us list the required observations and add up the time required for each.

• spectrograph: not done yet

• imaging detectors: we adopt a standard procedure of sets of three exposures of duration 100 seconds each to yield high signal-to-noise values for stars in the range 17 < V < 20. Unless otherwise indicated, the SNAP North field will suffice to provide the stars.

  1. variations in sensitivity on length scales of a single detector chip. This is especially important for the IR detectors. Since each side of a chip subtends roughly 6 arcminutes, the slews for this operation will be roughly 1 arcminute in length.

              5x5 grid of exposures of SNAP field, 3x100 seconds each
         

  2. variations in sensitivity on length scales of a single optical filter (one quarter of a chip). Since one side of a single filter subtends about 3 arcminutes, these slews will be smaller than those for category a; they will be about 30 arcseconds each.

              5x5 grid of exposures of SNAP field, 3x100 seconds each
         

  3. variations in sensitivity or bandpass between chip/filter combinations of the same type. One must move the same set of stars onto each IR chip or optical filter of each type. There are 36 optical filters in each quadrant, so 36x4 = 144 total individual optical filters. There are 9 IR detectors in each quadrant, so 9x4 = 36 total individual IR detectors. If we must put the same set of stars onto each of these detectors, we need 144 + 36 = 180 pointings. It is possible that by choosing the right field, we may find two sets of stars with the required properties, separated by roughly the offset between quadrants; in that case, we could decrease the total number of pointings for this purpose significantly. The number below is a worst-case scenario.

              180 pointings of field with range of stellar types, 3x100 seconds each
         

  4. errors caused by non-linearities in the detectors and/or due to motions of the shutters; need a range of exposure times on the same set of bright stars. For example, a series of exposures with durations 1,1,1 then 3,3,3 then 1,1,1 then 3,3,3 then 10,10,10 then 30,30,30 then 10,10,10 then 100,100,100 then 30,30,30

              1 pointing of a low-latitude field with many bright stars,
                27 exposures with durations varying as shown
         

To estimate the readout time, I took the IR detector pixel count, 2048x2048 = 4.2 million, and divided by a "slow" readout rate of 100 kHz = 100,000 pixels per second. This yields about 41 seconds to read one IR chip. I assume that the readout time for the optical detectors is the same.

The grand summary of required observations listed above is as follows:

• spectrograph: not done yet

• imaging detectors:

           69,534      seconds of exposure time
  
           29,397      seconds of readout time
                           (717 readouts @ 41 seconds each)
  
              201      slews of roughly a chip's length or smaller
                4      slews from one quadrant to another
                2      slews (there and back) from the default SNAP North field
                              to a special dense starfield
  
       ---------------------------
  
          100,000      seconds of exposure + readout time
  
              ???      slew time
          
       ---------------------------
   
          1.5 days     total elapsed time, very roughly