COR1 Ahead

At the time of launch COR1 Ahead had a higher level of contamination on its lens and thus had a much higher stray light background than COR1 Behind. Since that time, the level of contamination of the COR1 Behind objective has increased, and the two are now more comparable. The Behind spacecraft has more pointing jitter, which can been seen in COR1 time-difference movies (e.g., April 20, 2007).

Comment from Bill Thompson: I think that the comment about which spacecraft has more jitter is somewhat anecdotal. We know that both spacecraft encounter periods when the jitter is enhanced, presumably related to reaction wheel speeds, making it difficult to make blanket statements about which spacecraft has more jitter. On the other hand, COR1-Behind is probably more sensitive to jitter, because of the steeper gradient of the scattering function, and because of the many distinct "fingernail" features near the edge of the occulter.

This image (link to image appears to be lost—WTT) shows the difference between pB determination using monthly minimum ('standard') backgrounds vs. the backgrounds determined from the minima of the stepped roll maneuvers. Notice the distortion in streamer belt due to the monthly minimum subtraction. However, the monthly minimum seems to do a better job on the 'fingernail' artifacts. The observation date is Feb. 9, 2007. The monthly minimum backgrounds for the first few months of the mission, including February 2007, are of significantly lower quality than those for March 2007 and onward. However, effects like those shown below can occur for all dates, though not usually so dramatically.

More information about background subtraction can be found on this page.

bkg_compare-ahead_roll022007.gif

A discussion about the effect of the pointing calibration on A vs. B data comparisons has been removed. You can see a more up-to-date discussion of the comparison between COR1-A and B here. The figure that used to be on this page was mislabled, reversing Ahead and Behind. The software has been updated to correct the COR1 pointing for the first few months of the mission, so that direct comparisons can now be made.

A few comments from Rich Frazin:

A major pointing adjustment, which was quite large on the order of an arcminute, was made to the Behind spacecraft on Feb. 21 (smaller and probably insignificant pointing adjustments were made on Aug 1, 2007 for Ahead and July 26 for behind). (These were not pointing adjustments.) As Bill's analysis above indicates the pointing makes a dramatic difference in the background estimation. These roll backgrounds are only good for data before Feb. 21. It is well understood that the 1 arcmin pointing adjustment has a big effect on the COR1 stray light level (however, this is not the case for COR2). I think we need to arrange for new stepped cal rolls to be made ASAP.

Clarification from Bill Thompson:

The last pointing adjustment was made to Ahead on February 3, 2007, so the stepped calibration roll maneuver made on February 20 is applicable to the current Ahead pointing. There was a stepped calibration roll on Behind on April 17, 2007 which is also applicable to the current pointing. Rich's comment on arranging for new calibration rolls has been taken to heart, and a new calibration rolls was performed on January 3, 2008 on Ahead, and is scheduled for February 19, 2008 on Behind. Roll maneuvers will be performed at roughly 3 month intervals on each spacecraft, at orbital perihelion, aphelion, and the midpoints. The planned and historical schedule of roll maneuvers can be found on the SSC planning page.

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