To: MSI/NIS Science Team

From: Mike Malin (malin@msss.com)

Subject: NEAR Calibration Update #3

Date: 9 January 1995

Status as of 9 January 1995

This page continues reporting analyses I am performing on the "dirty" CCD calibration data. I am doing these analyses in order to gain some insight into performance issues, as well as to practice the techniques for application to the "clean" detector and to the other detectors as data become available; all of these measurements, and the interpretations and conclusions drawn from them, are subject to complete revision when data from the "clean" CCD are available.

NOTE: If any of you find mistakes or errors or omission or co-mission, please let me know so I can correct these in future reports.

"High" Temperature Measurements (-20 deg C) (Continued)

More re: Dark Current

It was pointed out to me that I did not include an appropriate description of the accumulation rate of dark current. The explanation for the shutter speed dependence I noted in Report 2 is overly complicated; the explanation is much simpler. Dark signal accumulates whenever the detector is not clocking (i.e., when the signal is accumulating. The longer the exposure, the more thermally-generated electrons can accumulate. This dark current rate is the number usually given in the literature. As can be seen in Figure 1, there was some scatter in the observed DN values as a function of shutter speed independent. The vertical offset between the two bounding curves may be explained by the ~1 deg C of change in focal plane temperature, but as can be seen in Figure 3 of Report 2, the higher dark current values actually occur at lower temperatures (which is not likely). In any case, the dark current accumulation rate at -20 deg C is between 2.5 and 4.2 DN/sec.

Figure 1: Dark Current Accumulation Rate at -20 deg C

Raw Spectral Response (i.e., No Lamp Correction)

The final set of data acquired at -20 deg addressed spectral response. I don't feel confident correcting for the spectral form of the lamp (see Report #2 for the probably spectral shape of the lamp's light), and I don't actually know the bandpasses of the filters used [Note to Scott: any progress on getting these digitized? I have a program I can use here if you can FAX me the curves]. I also got somewhat variable results with respect to the density of the neutral density filter when I compared images acquired at the same shutter and with the same spectral filter but with and without the neutral density filter. I also ran into some uncertainty as to which filter was used (I think one wavelength was not run with ND1), so much of this stuff should be taken with a large grain of salt. With that said, I adjusted the values for those images taken through the ND filter to fit on the same scale as those taken without the ND filter, and these are all plotted in Figure 2. You guys know more about spectral stuff than I--what do you think?

Figure 2: Raw Spectral Response at -20 deg C

That's all for today...I should be moving on to -30 deg tomorrow.