1 General aspects

1.1 Scope

This document contains the results of the analysis of the data acquired by the Framing Camera 2 during the DC038 operational slot. The scope of the activities was exclusively engineering, so no associated science report will be released.

1.2 Introduction

This report is structured in several parts.

Section 2 describes the planned operations, including the different activities and a brief description of each.

Given that this was a re-run of a former activity, all sequences were merged into the background sequence, so no activity log is provided.

Section 3 reports on the general health status of the cameras.

Section 4 explains the evolution of the detector since launch and analyses its change in performance.

The conclusions are covered in section 5.

1.3 Applicable Documents

no.     document name                  document number, Iss./Rev.                                                                                         	
AD1      DC038 Walkthrough                DC038_Walkthrough_r2.ppt, 2/-         
                                                                                

1.4 Reference Documents

                
  no.    document name                   document number, Iss./Rev.  

  RD1    Dawn FC DC034 Report            DA-FC-MPAE-RP-285, 1/-      
  RD2    DC018 Report                    DA-FC-MPAE-RP-286, 1/-      
  RD3    DC014 Report                    DA-FC-MPAE-RP-287, 1/-      
  RD4    Framing Camera ICO Report       DA-FC-MPAE-RP-268, D/c      
                                                                                

2 Description of the activities

2.1 Overview

The operations of the Framing Cameras within the frame of DC038 were planned to be conducted between May 2^nd 2010 (DOY 122) and May 3^rd (DOY 123). There was only one major activity:

o    Geometrical cross-calibration between FC2 and VIR 
                                                                                

All the FC sequences were merged into the background sequence, so no tele-command slots were scheduled.

2.2 Geometrical cross-calibration

This activity is a re-run of DC034. It was developed to provide the co-alignment between the VIR instrument and the Framing Camera (FC2) by acquiring data with both instruments simultaneously. It consists of five concurrent observations of the star Canopus (Alpha Carinae, V_mag = -0.72) and was initiated and outlined by the VIR team.

The first 10-minute observation shows Canopus in the center of the FOV of both instruments with the FC acquiring a clear filter image at the beginning, in the middle, and at the end, followed by two OpNav images. The second observation shows Canopus in the upper left corner of the FC FOV. One clear filter image was acquired at the beginning, one after 5 minutes and one after 10 minutes. During the third observation the FC acquired 3 clear filter images with Canopus in the lower right corner of the FOV. In the fourth observation Canopus shows up again in the center of three images. During the final observation the spacecraft performed a slow slew that made Canopus appear to move through the FOV from bottom to top on twelve consecutive images.

The total number of FC images acquired with the door open was 26. Among these were 24 cross-calibration images (125 ms exposure time) and 2 windowed OpNav images (500 ms exposure time). All of them were acquired in the clear filter F1.

3 Health status assessment

During the operational slot the camera performed nominal from the engineering point of view.

All the images were acquired as scheduled and received without any missing packet.

4 Image analysis

The images acquired during the operational slot were analyzed in three aspects. First, we assessed whether the exposure times were adequate. Second, the dark current was determined to monitor its long term evolution. Finally, the position of the reference star Canopus throughout the observation run was calculated for comparison with the acquisitions made by VIR.

4.1 Exposure times

The updated exposure values worked better than the DC034 values; no image of Canopus was overexposed, except for the two OpNav images.

4.2 Dark current

The dark current generation rate was analyzed and compared with previous measurements, including some on ground. The bulk dark current, shown on Figure 1, is consistent with that determined during ICO/MGA (dotted line).

Figure 1: Comparison of FC2 dark current with previous measurements.

4.3 Canopus positions

The position of the star was calculated by fitting a 2D Gaussian to the stellar brightness profile for each of the 26 images acquired during the activity. The positions are numbered 0 to 25 indicating the sequence in which they were acquired. 0 to 4 are centered in Canopus, 5 to 7 on the upper left quadrant; 8 to 10 in the lower right, 11 to 13 close to the center and 14 to 25 were acquired during the slow slew. Figure 2 depicts the position of Canopus for each image in DC038.

Figure 2: Canopus position in the CCD frame during DC038

Correspondingly, Figure 3 displays the position of the star during DC034. It is remarkable that, while the pattern is obviously the same for both runs, DC034 shows a poorer alignment with the s/c Y axis. This is due to the fact that the pointing commands were not updated for DC034 from the originally planned DC033.

Figure 3: Canopus position in in the CCD frame during DC034

Table 1 shows the value of the calculated positions of Canopus both for DC034 and DC038.

Table 1: Canopus position in the CCD frame during DC034 and DC038 (in pixels)

Img #  DC034 X    DC034 Y  DC038 X  DC038 Y 
	
 0     510.451   509.143  508.936  510.767  
 1     509.231   509.777  511.565  510.801  
 2     509.934   509.878  510.935  510.836  
 3     510.037   510.142  509.188  510.707  
 4     510.389   510.055  510.929  510.911  
 5     234.973   771.372  240.134  778.978  
 6     236.017   770.362  240.516  778.197  
 7     234.594   770.631  241.163  778.308  
 8     781.964   246.225  778.212  241.795  
 9     782.938   246.197  778.121  243.105  
10     782.423   246.302  778.152  242.956  
11     545.692   498.817  545.454  498.707  
12     545.420   497.978  545.210  498.081  
13     544.821   498.164  545.387  497.998  
14     549.760   155.548  546.673  156.030  
15     548.061   216.444  545.203  220.138  
16     547.123   280.189  545.112  284.178  
17     546.739   342.886  545.084  348.794  
18     545.286   404.469  545.230  413.049  
19     544.014   467.282  545.013  477.326  
20     542.886   530.045  546.101  541.485  
21     541.821   592.255  544.800  605.823  
22     540.931   654.616  544.037  669.866  
23     539.097   717.196  543.792  734.005  
24     537.240   779.850  543.653  798.783  
25     534.042   841.975  544.153  851.045  
                                                                                

5 Conclusions

Concerning the hardware, the operational slot demonstrated that the camera health status is nominal, including the mechanisms.

With respect to the operational procedures, this slot demonstrated again an excellent performance of the instrument, spacecraft and mission teams.

At mission level, we can confirm that no streak [RD4] was found in the images acquired during this operational slot. This does not mean that there are none, because the exposure times were so short that a streak, if present, would also be very short and easy to mistake for a high-energy particle hit. The FC team will continue to monitor the images acquired in flight in search for occurrences of streaks.

Finally, the observation of Canopus under different pointing conditions was successful and avoiding saturation enabled better determination of the position of the star compared to the earlier D034 observations. The quality of the data set is sufficient to determine the geometric co-alignment between FC and VIR.