December 17, 2021
COS NEWSLETTERS

About This Article

In this STAN we announce the delivery of an updated SPOTTAB file to correct for a recently discovered transient hotspot in the lower LP4 background region of the FUVB detector segment.

Correcting for a Recently Discovered Transient Hotspot

A transient hotspot was recently discovered that affects a small fraction of COS FUV data. The hotspot is on segment FUVB, centered near (XCORR, YCORR) = (11205,423) with an extent of ±10 and ±8 pixels in the dispersion and cross-dispersion directions, respectively. It first appeared on February 20, 2018, and continues to sporadically reappear for intervals ranging from several minutes to several days. It falls inside the lower region used by CalCOS for background subtraction of data obtained on FUVB at Lifetime Position 4 (LP4). The affected wavelengths for settings observable on FUVB at LP4 are listed in the table. (G140L/1280 is affected below 900 Å.) For faint and/or background-limited LP4 FUVB COS observations, the hotspot could cause an over-subtraction of the background in the science spectrum. Therefore, we updated the Hotspot Table (SPOTTAB) reference file so that CalCOS will properly flag and ignore the hotspot pixels during processing. The figure shows an example of the hotspot's effect on a faint spectrum and its correction with the new SPOTTAB (5b91820fl_spot.fits). Although most users should not expect their data to be significantly affected by the transient hotspot, those with background-limited FUVB data taken at LP4 on or after February 20, 2018, who are interested in an affected wavelength range are encouraged to re-retrieve their data from MAST.

 

 

Hotspot Wavelength Ranges (Å)

Cenwave

FP-POS 1 FP-POS 2 FP-POS 3

FP-POS 4

G130M/1222

1174.1-1174.9 1171.6-1172.4 1169.2-1170.0

1166.7-1167.5

G130M/1291

N/A on FUVB N/A on FUVB 1236.0-1236.8

1233.5-1234.3

G160M/1533

1472.6-1473.6 1469.6-1470.6 1466.5-1467.5

1463.5-1464.5

G160M/1577

1517.3-1518.3 1514.3-1515.2 1511.2-1512.2

1508.1-1509.1

G160M/1589

1528.9-1529.9 1525.8-1526.8 1522.8-1523.8

1519.7-1520.7

G160M/1600

1540.4-1541.3 1537.3-1538.3 1534.2-1535.2

1531.2-1532.2

G160M/1611

1552.2-1553.2 1549.1-1550.1 1546.1-1547.1

1543.0-1544.0

G160M/1623

1564.3-1565.3 1561.2-1562.2 1558.2-1559.1

1555.1-1556.1

 

 

 

 Example spectrum of new and old SPOTTAB

Figure 1: An example spectrum was taken during a period when the hotspot was contributing significant counts to the background region. The updated SPOTTAB file corrects the background over-subtraction and restores the true flux values in the affected region. Note that this is an extreme example, and most users will not detect any significant differences with the updated Hotspot Table.

 

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