DAWN-FC

                     

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

AD1    DC041 Walkthrough                         DC041_Walkthrough)r2.ppt  
AD2    FC Semi-Annual Checkout Sequence          DA-FC-MPAE-TN-076, 1/a    
AD3    UDP Library 3.04.02 Patch Procedure       DA-FC-MPAE-PR-082, D/     
                                                                                

1.4  Reference Documents
                                                                                
                  
no.    document name                document number, Iss./Rev.  
                                   
RD1    Dawn FC DC038 Report         DA-FC-MPAE-RP-290, 1/       
RD2    Dawn FC DC034 Report         DA-FC-MPAE-RP-285, 1/       
RD3    DC018 Report                 DA-FC-MPAE-RP-286, 1/       
RD4    DC014 Report                 DA-FC-MPAE-RP-287, 1/       
RD5    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 DC041 were planned
to be conducted between July 19th 2010 (DOY 200) and 21st
 (DOY 202). There three major activities:                                 

                                                                                
o     FC1 non-pointed semi-annual checkout and UDP library patch
o     FC2 pointed semi-annual checkout and UDP library patch
o     Geometrical cross-calibration between FC2 and VIR
                                                                                
                                                                                

All the image acquisition FC sequences were merged into the background
sequence, and the UDP patch sequences were uploaded as separate
absolutely-timed sequences, so no telecommand slots were scheduled. However,
for verification of the FC1 patching, in case the sequence had to be aborted
due to a failure in the patch, there was a window from 2010200T21:05 to
2010-201T07:00. Conversely, for FC2 this window was from 2010-201T18:00 to
2010-202T04:45.
                                                                                

                                                                                

                                                                                

2.2  FC1 non-pointed semi-annual checkout and UDP library patch
                                                                                

                                                                                

The background of this activity is the need for the mechanisms on the cameras
to be operated twice a year for maintenance. FC1 was due for this maintenance
because its last operation had been during DC034 in December 2009. The details
of the activity are described in AD2. A total of 65 images were acquired
during this activity.
                                                                                

                                                                                

Additionally, during the Vesta Survey ISB it was identified that the UDP
library loaded during DC034 contained a bug that prevented the macro-command
TCMonitor from acquiring images with the correct exposure times. While this
bug did not endanger the instruments or the mission, it limited the ability of
the camera to return scientifically meaningful images with the already
developed sequences, so the decision was made to patch the UDP library. The
detail information and procedure for this activity is described in AD3. This
activity did not include the acquisition of any image, but some acquisitions
in the semi-annual checkout used TCMonitor to prove that the patch was
successful.
                                                                                

                                                                                

                                                                                

2.3 FC2 pointed semi-annual checkout and UDP library patch
                                                                                

                                                                                

The rationale for this activity is foremost maintenance of the camera
mechanisms. In addition, pointed observations of several targets (stardard
stars, star fields, mission targets) allow a long term monitoring of the
camera performance. The details of the activity are described in AD2. The
selected targets are described in Table 1. This activity produced a total of
186 images.
                                                                                

                                                                                

Table 1: Target selection for FC2 pointed semi-annual checkout
                                                                                
                                                                                
 Type of Target              Selected target  

 Photometric standard star   Vega             
 Solar analog star           HR 2290          
 Mission target              Ceres            
 Star cluster                20 Cephei        
                                                                                
                                                                                

FC2 also underwent the UDP library patch described above, following the
procedure described in AD3.
                                                                                

                                                                                

                                                                                

2.4 Geometrical cross-calibration
                                                                                

                                                                                

This activity is a re-run of DC038 and DC034 (see RD1 and RD2). It was
developed to provide information about the co-alignment between the VIR
instrument and FC2 by acquiring data with both instruments simultaneously. It
includes observations of the star Canopus (Alpha Carinae, V_mag = -0.72)
acquired at several different image stations, and was initiated and outlined
by the VIR team.
                                                                                

                                                                                

The first image station was occupied for 10-minutes. Canopus was in the center
of the FC2 FOV. The FC acquired a clear filter image at the beginning, in the
middle, and at the end of this period, followed by two OpNav images. The
second image station had Canopus in the upper left corner of the FC2 FOV.
Clear filter images were acquired at the beginning, after 5 minutes, and after
10 minutes. At the third image station the FC acquired 3 clear filter images
with Canopus in the lower right corner of the FOV. At the fourth image station
Canopus was again in the center of the FC2 FOV for the duration of 3 image
acquisitions. For acquisition of the final observations the spacecraft
performed a slow slew that made Canopus appear to move through the FC2 FOV
from bottom to top in twelve consecutive images.
                                                                                

                                                                                

The total number of images acquired with the door open was 26. Among these
were 24 science images (250 ms exposure time) and 2 windowed OpNav images
(500 ms exposure time). All of these were acquired in the clear filter F1.
                                                                                

                                                                                

                                                                                

3 Operations summary
                                                                                
                                                                                

All the sequences were uploaded to the s/c without incidence and were started
according to schedule.
                                                                                

                                                                                

During the operations of FC1, the spacecraft was transmitting the H/K and
event stream in real time to ground, allowing a quick confirmation of the
success of the UDP library patch. After all images were played back and FC1
was already off, we were able to confirm that the images acquired through
TCMonitor showed the correct exposure times.
                                                                                

                                                                                

The verification approach for the FC2 patch was different in that it relied on
the playback of the engineering virtual recorder (VR3) to confirm that the
patching of the UDP had been successful. However, a glitch in the H/K and
event stream prevented this confirmation and forced the FC team to make the
assessment based only on the exposure time shown by the images. Fortunately,
the images acquired with TCMonitor were transmitted early to ground and the
confirmation of success arrived well in time. After the activity was over, the
event stream was reconstructed and the camera notifications of success were
retrieved.
                                                                                

                                                                                

                                                                                

4 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, both the ones associated to the mini-cal and VIR geometric
cross-calibration.
                                                                                

                                                                                

                                                                                

5 Image analysis                                                                               
                                                                                
                                                                                

The images acquired during the operational slot were analyzed in four aspects.
First, the correctness of the exposure times was assessed, given that this was
the first time that these sequences were run following the design in AD2.
Second, the dark current was analyzed to evaluate the evolution of bulk dark
current and warm pixel generation rate. Third, the status of the extra charge
was assessed. Finally, the position of the reference star Canopus throughout
the geometrical cross-calibration was calculated for comparison with the
acquisitions made by VIR.
                                                                                

                                                                                

5.1 Exposure times
                                                                                

                                                                                

The exposure times used for most of the activity were inherited from DC038 and
provided images of good quality.
                                                                                

                                                                                

Four callamp-illuminated images in the extra charge investigation sequences
erroneously had their exposure times updated to 10 seconds, resulting in
saturation. Our recommendation for future instances of the checkout is to
reduce the exposure time for these images to 0.8 seconds.
                                                                                

                                                                                

5.2 Dark current
                                                                                

                                                                                

The dark current generation rate was analyzed and compared with previous on
ground and in flight measurements. The bulk dark current on FC1, shown on
Figure 1, is consistent with that determined during ICO (dotted line). For
FC2 the situation is very similar, as depicted in Figure 2.
                                                                                

                                                                                

                                                                                
                                                                                

Figure 1:                                                                        
FC1 bulk dark current for DC041
                                                                                



                                                                                

                                                                                
                                                                                
                                                                                

Figure 2:                                                                        
FC2 bulk dark current for DC041
                                                                                


                                                                                

To assess the generation rate of number of warm pixels, we plot histograms for
DC041 in Figure 3 and for ICO in Figure 4. The number of warm pixels has
increased slightly since launch.
                                                                                

                                                                                

                                                                                
                                                                                


                                                                                

Figure 3:                                                                        
FC1 warm pixel histogram in DC041
                                                                                


                                                                                

                                                                                
                                                                                


                                                                                

Figure 4:                                                                        
FC1 warm pixel histogram in ICO
                                                                                


                                                                                

Figure 5 and Figure 6 show the same plots for FC2. For both cameras, the
hottest pixel has a dark current generation rate below 50 DN/s.
                                                                                


                                                                                

                                                                                
                                                                                


                                                                                

Figure 5:                                                                        
FC2 hot pixel histogram in DC041
                                                                                


                                                                                

                                                                                
                                                                                


                                                                                

Figure 6:                                                                        
FC2 hot pixel histogram in ICO
                                                                                


                                                                                

                                                                                

5.3 Extra charge
                                                                                
                                                                                

An important part of the semi-annual checkout is monitoring the evolution of
the extra charge for both Framing Cameras. FC1 is known to show extra charge,
and it is found again in the DC041 images, as shown in Figure 7. The level of
extra charge has not increased compared to DC034 (Figure 8). No extra charge
has been detected in the past for FC2 at the illumination levels provided by
the calibration lamp, and this is still the case in DC041. As the spacecraft
approaches Vesta, there are a number of activities planned to conduct the same
investigation with the higher photon flux that the asteroid can provide.
                                                                                

                                                                                

                                                                                
                                                                                


                                                                                

Figure 7:                                                                        
FC1 extra charge levels in DC041
                                                                                


                                                                                

                                                                                
                                                                                
                                                                                

Figure 8:                                                                        
FC1 extra charge levels in DC034
                                                                                


                                                                                

                                                                                

5.4 Canopus positions
                                                                                

                                                                                

The position of Canopus in the FC2 image frame was calculated by fitting a 2D
Gaussian to the stellar brightness profile for each of the 20 images acquired
during the activity.  The positions are numbered 0 to 19 indicating the order
in which they were acquired (0 to 2 are centered on Canopus; 3 to 5 are in the
upper right quadrant; 6 to 8 in the lower left; 9 to 11 had Canopus at
VIR’s boresight; 12 to 14 are slightly off the boresight and 14 to 19
were acquired during the slow slew). Figure 9 depicts the position of Canopus
for each image in DC041.
                                                                                


                                                                                

                                                                                
                                                                                
                                                                                

Figure 9:                                                                        
Canopus position in the FC2 image frame in DC041
                                                                                


                                                                                

Correspondingly, Figure 10 displays the position of the star during DC038. The
observation pattern was flipped with respect to the previous slots, but
otherwise they are very similar.
                                                                                


                                                                                

                                                                                
                                                                                
                                                                                

Figure 10: Canopus position in the FC2 image frame in DC038
                                                                                


                                                                                

Table 2: Canopus position in DC041 in pixel coordinates
                                                                                

                                                                                

Img #  DC041 X  DC041 Y  

  0    511.146  510.789  
  1    510.711  510.736  
  2    210.564  510.748  
  3    662.789  662.078  
  4    662.848  662.213  
  5    663.838  662.227  
  6    358.116  359.082  
  7    359.055  359.055  
  8    358.608  359.223  
  9    561.919  485.530  
 10    561.502  484.530  
 11    561.205  484.873  
 12    529.893  511.460  
 13    529.964  511.173  
 14    529.375  511.428  
 15    561.095  193.720  
 16    561.902  321.842  
 17    562.073  449.870  
 18    561.968  577.771  
 19    562.116  705.663  
                                                                                
                                                                                

                                                                                

6 Conclusions
                                                                                
                                                                                

Concerning the hardware, the operational slot demonstrated that the camera is
in good operational condition, including the mechanisms.
                                                                                

                                                                                

With respect to the operational procedures, this slot demonstrated again an
excellent performance of the instrument, spacecraft and mission teams.
Additionally, it demonstrated that the procedure for patching the UDP library
is working fine. The command sequences for the pointed and non-pointed
semi-annual checkout for both FC1 and FC2 need to be updated to address the
issue of the overexposure of the illuminated images in the extra charge
monitoring block.
                                                                                
                                                                                

At mission level, we can confirm that no streak [RD5] was found in the images
acquired during this operational slot. This does not mean that there are none,
because 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 observations of Canopus under different pointing conditions were
as successful as for DC038, and allowed an accurate determination of the
position of the star. The FC team believes the data set is of sufficient
quality to determine the geometric co-alignment between FC and VIR.
Additionally, FC also observed for the first time one of its targets, Ceres,
shown in Figure 11
                                                                                


                                                                                

                                                                                
                                                                                
                                                                                

Figure 11: F1 image of Ceres as seen by FC2 during DC041