Strategy for determining the instrumental polarization: winter 2003

• The raw polarization measurements made with SPARO (e.g. those currently posted to the web page under the Data Analysis menu item) include contributions from the source but also, unfortunately, from the instrument. All polarimeters have this problem. In order to correct this, we need to determine the instrumental polarization of each pixel in the array. (Except pixel 3, not used for polarimetry.) Then we can subtract this spurious polarization from our raw measurements. We can determine the instrumental polarization by observing sources of known polarization. For SPARO, this means Sgr B2 and the Moon. We really only need to observe one of these two sources, but it is better to observe both of them. We will start with Sgr B2 as it is operationally simpler.



• The instrumental polarization was about 0.5% for SPARO in 2000, but it varied from pixel to pixel (see Dave Chuss' thesis). We expect that this year it will also be less than 1%. Therefore the polarization we have measured in rcw57(West) is probably mostly real.



• But none of our results are publishable until we measure the instrumental polarization, and correct our measurements for this. For this reason, we should move to the lower actuator postion and observe Sgr B2 reasonably soon.

First step: we need to determine the pixel pointing offsets (This step is DONE)

• The peak of Sgr B2 has a polarization that is well known from observations at other telescopes. But we will need to point each pixel at the peak of Sgr B2. So this will require us to calculate eight different sky positions for polarimetry. Each of these sky positions will correspond to one of the active pixels in SPARO. We don't use pixel 3.



• But we had a problem, which is that we didn't accurately know the relative pointings of the pixels in SPARO.



• So the first step was to go to a peaky source like rcw57(West) and make a map that has the following characteristics:
  
  
  • phot mode
  
  
  
  • xstep = ystep = 0.03
  
  
  
  • nx = ny = 11



• Using one such map, we used Greg's software to determine the pixel pointing offsets for each pixel in SPARO. Then we used these to determine the 8 polarimetry sky positions that we will use for the instrumental polarization calibration observations of Sagittarius B2.

Second step: instrumental polarization calibration observations of Sagittarius B2. (This step is now also DONE.)

• These instrumental polarization calibration observations of Sgr B2 involve doing polarimetry at each of 8 different sky positions



• First, the Northwestern team must provide these 8 different polarimetry sky positions, each one corresponding to a different pixel of the array (1,2,4,5,6,7,8,9) pointed at Sgr B2. (This has now been done)



• The next step involves going to the lower position of the course actuator. Updated instructions on changing the course actuator position are found here.



• When you get to Sgr B2, the first step is to do photometry of Sgr B2 using the sky position corresponding to pixel 5 [called SgrB2(MAX)], and use this to iteratively find the CDCC that will put the peak of Sgr B2 precisely centered in pixel 5.



• A good guess for what CDCC to start with when you first go to look for SgrB2(MAX) can be found by clicking here.



• After you have iterated enough to have found a good CDCC for SgrB2(MAX), that keeps the source centered in pixel 5, the next step is to do polarimetry at each of the 8 different polarimetry sky positions that have been provided. Use the same CDCC for all for all of these 8 sky positions.
  
  
  • We will need at least 10 polarimetry files for each of the 8 sky positions, maybe more, depending on the weather.
  
  
  
  • It will be best to cycle through these 8 sky positions, in other words...
    
    
    • do this: 1,2,4,5,6,7,8,9,1,2,4,5,6,7,8,9...
    • not this: 1,1,1,1,1,1,1,1,1,1,2,2,2,2,2,2,2,2,2,2,4,4,4,4,4,4,4,4,4,4...

Third step: instrumental polarization calibration observations of the Moon.

• The idea is the same as for Sgr B2, but the Moon moves at a different rate from other objects we look at. So Greg has some specialized scripts. It will take some preparation before we are ready for these Moon observations.

Other steps: dependence of instrumental polarization on flux distribution within pixel

• This is a subtle effect. We never did very careful tests for this in 2000. It would be nice if we get some time for this in 2003. Further explanation to be posted at a later time.