Bright Star Removal

Bright stars and features associated with bright stars must be removed from the coadds before the JHK backgrounds can be determined and star-galaxy discrimination can be conducted. This process is discussed at length in the memos: GALWORKS Bright Star Cleansing and in Bright Stars in the MSX Field . To reiterate the basic removal steps:

The horizontal stripe blanking is performed only when the star is very bright, except for the "middle" strip which seems to appear for any bright star. The number of persistence ghosts that are blanked (including along adjacent coadds) is controlled by the input flux of the star.

With the Coma data, we can attempt a more refined "tuning" of the masking windows, to minimize the loss of area due to bright star removal. The following images show a number of bright stars in the Coma scans, with and without their removal from the coadd images.

Bright Stars from Scan 003, 970418

J = 7.3, H = 6.1, K = 6.9

The diffraction spikes are clearly visible in this moderately bright stars. Horizontal stripes are just barely visible in the J and H images. Adjusting the masking values by increasing the bright star thresholds 0.5 mag results in slighter less area masked/blanked from the coadd images. This technique is described in more detail in Bright Stars in the MSX Field .

J = 6.7, H = 5.3, K = 5.6

This star is brighter still, with stripes and persistence ghosts easily seen in the J and H images. The modofied masking values again look better than the nominal values (less area blanked, with minimal contamination).

H = 7.1, K = 6.5; K = 6.9

The Coma cluster center contains one bright star near the edge of the coadd field. The star has a K mag of 6.9 (probably wrong), but no J mag is recorded (because it is too close to the edge). However, the star must be very bright in J since horizontal stripes are readily apparent in the coadd images (suggesting that the star is at least J = 5 or 6 mag). A large fraction of the coadd area is blanked using the nominal masking values. With modification of the masking values, the blanked area is minimized (thus sparing small satellite galaxies in close proximity to the giant ellipticals that rule Coma).

J = 6.6, H = 6.4, K = 6.9

JCAT: J = 5.3, H = 4.9, K = 4.7

Here is a case in which the bright star is located just off the coadd edge. There are no read-1 or read-2 detections for this source. Fortunately the bright star is catalogued in the JCAT, with location 64 arsec to the east of the coadd, and with JHK values of 5.3, 4.9, and 4.7 respectively. The masking effectively removes the horizontal strip(s) associated with this bright star.

Conclusion

All bright stars (K < 9) located within (including near the edge of) the Coma scan 003 are effectively removed from the coadd images by GALWORKS. The masking windows appear to be best determined from slight modification of the nominal values; the modification is 0.5 mag in the masking thresholds. Bright stars should not be a problem with Coma and similarly other low stellar density regions.