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Table of Absolute Plate Scale Measurements for NICMOS

Absolute Plate Scale Measurements

We have taken the relative plate scale changes at each of the 3 epochs where there are absolute measures from the 7039 proposal (these made by dithering a star around the detector by known ammounts and measuring the target star's position relative the the guide stars used). We then back out the value of the absolute plate scale at the reference epoch to which all the relative plate scale measurements are referenced to (in this case, the fine-alignment activity 7134, day 63). We get X and Y numbers for each of the three 7039 epochs, extrapolated to day 63, which are presented here:

     Extrapolated NICMOS Plate Scales for March 4, 1997

	      X                     Y     (from measuement on)
  	  0.0431151 	        0.0429543    3/24/97
NIC1   	  0.0431015    	        0.0429366    4/17/97
    	  0.0431544    	        0.0429582    5/28/97
   mean:  0.0431237 (0.000027)  0.0429497 (0.000012)    

	      X                     Y     (from measuement on)
          0.0758757             0.0752028    3/24/97
NIC2      0.0758160             0.0751554    4/17/97
          0.0759084             0.0751973    5/28/97
   mean:  0.0758667 (0.000047)  0.0751852 (0.000026)

	      X                     Y     (from measuement on)
          0.201957              0.200844     3/24/97
NIC3      0.201478              0.200990     4/17/97
          0.201737              0.201123     5/28/97
   mean:  0.201724 (0.00024)    0.200985 (0.00014)

Then just use the mean zero-point value to scale the absolute value of the relative plate-scale curves in each camera. The shape of each curve is not changed, only its absolute value. The plots show the agreement of the absolute measurements to the scaled relative measurements.

We do not have measures of the relative plate-scale changes in NIC1 because its long focal length makes it rather insensitive. So to get the long-term behavior of NIC1 we have scaled the relative plate scale measurements of NIC2 using the inverse ratio of the NIC1 and NIC2 focal lengths and then used the absolute numbers from the 7039 as for the other cameras.