Mimir Flat Fielding Check List

Flat Fielding for Mimir in the JHK bands is normally performed in the dome against the flat field screen. The following procedure may be followed and the numbers listed used as verification that Mimir is operating normally. The integration times have been set to achieve less than 50% of saturation, while being short enough to avoid excessive dark+thermal current effects and long enough (seconds) so that the first read of the CDS does not have a significant slope across it (from a bright illumination and the 0.5 s readout time). The following list assumes the user has already brought Mimir up normally.

This page is organized into five sections:

1. Basic Set-Up
2. Bias Images
3. F/5 Imaging Mode Flats
4. F/5 Polarimetric Mode Flats

Note that taking data for the linearity correction is located on another page (click this link).

For advanced users, the following table summarizes key settings:

Basic Set-Up

1. Turn off the sun
   1. that is, dome flats should be done at night since sunlight scattered into the dome will change brightness with time. While this may not harm imaging flats, it render polarimetric flats useless.
2. Close the dome slit
   1. same reason
3. Point the telescope at the flat field screen
   1. this is done by issuing the FS command in MOVE
   2. after motion has stopped, use the hand paddle to fine position the telescope to accurately point at the flat screen.
4. Open the mirror cover
5. Open the instrument cover
6. Turn on the flat field illuminator light
   1. Select the "Narrow Band" option of the switch marked "Broadband / Narrow Band"
   2. Turn to "On" the other switch marked "On / Off / Test"
   3. Set the black rotatable knob (located just above the flat field illuminator switch plate) as indicated in one of the sections below

Bias Images

It is vital that users obtain bias frames (zero exposure darks) before their flat field images and after the flat field images have been acquired (for all modes).

1. Take a set of Bias Images
   1. Put a value of "32" in the argument of the "go_bias" button and push that button

F/5 Imaging Mode Flat Fields (for F/17, substitute values given in table above)

1. For the J filter (J+PK50)
   1. Select the J filter, by clicking the "set_J" button
   2. Set the integration time to 5.0 seconds
   3. Put the black knob setting to "18"
   4. Take a test image
      1. the mean count level should be about 4,000 counts
   5. Populate the FITS header
      1. "J-Band Dome Flats; lights On" , "J+PK50"
   6. Select N_exp = 32
   7. Press "Go" to collect 32 images
   8. After this "lights on" sequence has finished, turn the illumination lights off using the "On / Off / Test" switch
   9. Change the FITS header to indicate the lights are off
   10. Press "Go" to collect 32 images
       1. the mean count levels at the image center for the lights off condition should be about 115 counts

        

2. For H-Band (H+PK50)
   1. Select the H filter, by clicking the "set_H" button
   2. Put the black knob setting to "18"
   3. Set the integration time to 0.7 sec
   4. Repeat the steps 2-4 through 2-10 above
      1. lights on should give about 4,000 counts/pixel
      2. lights off should give about 35 counts/pixel

 

3. For K-Band (K+PK50)
   1. Select the K filter, by clicking the "set_K" button
   2. Put the black knob setting to "18"
   3. Set the integration time to 0.7 sec
   4. Repeat the steps 2-4 through 2-10 above
      1. lights on should give about 4,000 counts/pixel
      2. lights off should give about 450 counts/pixel

       

4. For H2ON (H2ON+PK50)
   1. Select the H2-ON filter, by clicking the "set_H2On" button
   2. Put the black knob setting to "25"
   3. Set the integration time to 6 sec
   4. Repeat the steps 2-4 through 2-10 above
      1. lights on should give about 4,000 counts/pixel
      2. lights off should give about 240 counts/pixel

       

5. For H2OFF (H2OFF+PK50)
   1. Select the H2-OFF filter, by clicking the "set_H2Off" button
   2. Put the black knob setting to "25"
   3. Set the integration time to 5 sec
   4. Repeat the steps 2-4 through 2-10 above
      1. lights on should give about 4,000 counts/pixel
      2. lights off should give about 240 counts/pixel

F/5 Polarimetry Mode Flat Fields

The following will provide a set of polarimetric flats sufficient to achieve 0.8% photometric uncertainty in each pixel for each HWP, or under 0.01% polarization uncertainty for stars spanning 16 pixels (about 1.6 arcsec FWHM). For cruder uncertainties, reduce the number of exposures or number of cycles (step 9).

As a benchmark, the full set of observations takes about 2 hours of clock time.

1. Set up Polarimetry Mode
   1. In LOIS window, "cd ../tcl.mimir" then "source gpips.tcl"
   2. Click "Show User Buttons"
   3. Select H-band filter, PK50 blocker, H-band Half-wave Plate, and Wire Grid Analyzer
      1. Click the "set_POL" button
1. Test the HWP rotation
   1. Click the "pol_reset" button
2. Put the black knob setting to "18" with the illuminator lights turned on to the narrow band setting
3. Set the integration time to 1.6 sec
   1. Take a test image - the mean counts should be around 4,000 ADUs. If greatly different, adjust either the light setting or the integration time to achieve a mean of about 4,000 ADUs.
4. Run the pol_FF_cal polarimetry data collection script
1. Do a "go_bias 32" first
2. Edit the object title to say "Pol Flat Lights On"
3. Find the "pol_FF_cal" user button and set the exposure time ("expt") to 1.6 and the number of exposures ("nexp") to 4
4. Click this button to collect 32 biases, then the polarimetry data and to move the HWP to different positions taking 4 exposures at each position
5. After the script ends, click the pol_FF_cal button, again with the lights still on
6. Turn the illuminator lights off
7. Run pol_FF_cal twice with the lights off (edit the object title to "Pol Flat Lights Off")
8. Repeat steps 5-8 three times
   1. totaling 32 lights on and 32 lights off images at each of 16 HWP angles.

    

DPC 20070927