Flux levels of the present Star Tracker CCD

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Sam Ragland dated on 20th Nov. 2003

The flux levels of the present Star Tracker CCD are characterized using data from March 2003 (Mira run). Sixty-three independent measurements taken on seven calibrators, all early K type stars, are used here. Measurements were made with an integration time of either 10ms or 4ms. Data taken with 4ms integrations are scaled (by a factor of 2.5) and used with that of ‘10ms integration’.

We find that I magnitude is a better magnitude than V & R magnitudes to characterize this Star Tracker CCD. Fig. 1 & 2 show plots of observed Maximum CCD counts as a function of I magnitudes. While taking one telescope data, shutters corresponding to the other two telescope beams were closed. Also, background data has been taken and taken into account.

Assuming that the CCD has linear response over its dynamical range, the expected fluxes for different integration times and for a range of I magnitudes are listed in Table 1.

I mag

Integration time (ms)

 

4

10

20

30

40

50

3.0

3105

 

 

 

 

 

3.5

1959

 

 

 

 

 

4.0

1236

3090

 

 

 

 

4.5

780

1950

3901

 

 

 

5.0

492

1230

2462

3691

 

 

5.5

310

776

1553

2329

3105

3880

6.0

196

490

980

1470

1959

2448

6.5

124

309

618

927

1236

1545

7.0

78

195

390

585

780

975

7.5

49

123

246

369

492

615

8.0

31

78

155

233

310

388

8.5

20

49

98

147

196

245

9.0

12

31

62

93

124

154

9.5

8

19

39

59

78

97

10.0

5

12

25

37

49

61

10.5

3

8

16

23

31

39

11.0

2

5

10

15

20

24

Table 1: Max. CCD counts as function of I magnitudes for different integration times

 

Fig.1: Maximum CCD counts are plotted against I magnitudes for all three telescopes. A least square fit to the data are shown and the best fit parameters are given.

Fig2: Same as figure 1, but all three telescope values are used together. A least square fit  to the data and the best fit parameters are  given.

The slope of the best fit (Fig. 2) is -0.394, which is close to the expected value of -0.4, suggesting that this calibration is probably independent of the spectral type of the target. This slope for calibrations based on V magnitude and R magnitudes (Fig. 3) are respectively -0.299 and -0.353. Strictly speaking, these (V & R magnitude based) calibrations are good for only early K type stars. And, targets of later spectral types would have more counts and earlier spectral types would have less counts.

Fig.3: Maximum CCD counts are plotted against V magnitudes (left) and R magnitudes (right) for all three telescopes. Top three rows correspond to Telescope A, B & C respectively and the bottom row shows all three telescope data together.