Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.stsci.edu/hst/stis/documents/isrs/199904.pdf Дата изменения: Tue Dec 7 21:15:56 2004 Дата индексирования: Sat Dec 22 23:35:50 2007 Кодировка: Поисковые слова: dust

STIS Instrument Science Report 99-04

DUST MOTES AND BLEMISHES IN THE CCD SPECTRAL MODES
Ralph Bohlin July 1999

ABSTRACT A set of 84 dust mote features and blemishes have been identified in STIS CCD flat fields. These 84 regions are flagged in the bad pixel table and will be propagated to extracted spectra. The current defaults place the spectra at the Y center of the CCD, where two motes can cause spurious spectral features. Since flat field correction of the dust motes is inadequate for spectra of point sources, these artifacts can be up to 3% deep with a width of several pixels after flat field correction. Several Figures are included as guides to placing spectra onto regions of the CCD where blemishes have minimal impact.

1. Introduction
Particulate contamination on the window of the CCD produces out-of-focus regions of lower sensitivity on the CCD. In addition to these dust motes with sizes in the 8-30 pixel range, a few other blemishes of similar sizes appear on the CCD itself. Figures 1-2 are flat fields for the low and medium resolution modes, respectively, that illustrate the blemishes. Because of differences in the optical magnification, the low dispersion motes are smaller and deeper than the medium resolution motes. Figure 3 is an example of the effect of a mote in an extracted spectrum of a star.

2. Results
The mote positions are flagged in circular regions of radius 12 or 16 pixels for the low and medium resolution modes, respectively. The two motes that influence spectra that are centered on the detector have smaller radii of 8 and 12 pixels, respectively. Since the first order spectra lie nearly along the X direction of the CCD, an inventory of the effects of the motes as function of Y (row number) is required. Figures 4-5 count the number of pixels in motes encountered in each row, while Figures 6-7 are the corresponding total depth below the unit average flat field level for all of those flagged pixels.

1


STIS Instrument Science Report 99-04 Examples of regions that avoid blemishes are near Y=457 or 773. The number of rows in a region needed to totally avoid blemishes is the seven-pixel width of the spectral extraction region, plus the tilt of the spectrum across the detector (typically < 7px), plus a couple of pixels for mechanism repeatability. For spectra of the highest purity, Figures 4-7 can be used to select dither positions or place a target spectrum at a Y location that avoids the CCD blemishes. For example to center a target at a particular Y pixel value, use the spectral Y locations in Table 1 and the CCD cross-dispersion plate scale of 0.05071 arcsec/pixel to calculate the proper POSTARG value. The STIS mode select mechanism usually positions the spectrum at the same place on the CCD to 1-2 pixels. All Y pixel locations quoted in this ISR are based on counting the first pixel as one, rather than the IDL convention of zero. Table 1. Y Location of Spectra for the CCD Primary Modes
Mode G230LB G430L G750L G230MB G230MB G230MB G230MB G230MB G230MB G230MB G230MB G230MB G230MB G230MB G430M G430M G430M G430M G430M Cenwave (A) 2375 4300 7751 1713 1854 1995 2135 2276 2416 2557 2697 2836 2976 3115 3165 3423 3680 3936 4194 Rootname O45A13010 O45A13020 O45A13030 O4A506020 O4A506030 O4A506040 O4A506050 O4A506060 O4A506070 O4A506080 O4A506090 O4A5060A0 O4A5060B0 O4A5060C0 O4A5050L0 O4A5050M0 O4A5050N0 O4A5050O0 O4A5050P0 Y (px) 509 509 510 512 511 513 515 510 510 512 513 508 513 508 517 515 517 516 516

2


STIS Instrument Science Report 99-04

Mode G430M G430M G430M G430M G430M G750M G750M G750M G750M G750M G750M G750M G750M G750M G750M

Cenwave (A) 4451 4706 4961 5216 5471 5734 6252 6768 7283 7795 8311 8825 9336 9851 10363

Rootname O4A5050Q0 O4A5050R0 O4A5060D0 O4A5060E0 O4A5060F0 O4A505030 O4A505040 O4A505050 O4A505060 O4A505080 O4A5050A0 O4A5050C0 O4A5050E0 O4A5050G0 O4A5050I0

Y (px) 516 512 516 532 511 516 515 516 517 510 513 514 509 512 511

3


STIS Instrument Science Report 99-04

0.98

1.02

STIS CCD P Flat PGCCDL

BOHLIN: prtimg 21-Jun-1999 16:05

Figure 1: Flat field for low dispersion STIS CCD modes scaled from 0.98 to 1.02

4


STIS Instrument Science Report 99-04

0.98

1.02

STIS CCD P Flat PGCCDM

BOHLIN: prtimg 21-Jun-1999 16:07

Figure 2: As in Figure 1 for the medium dispersion spectral modes. The dust motes are larger and shallower than in low dispersion.

5


STIS Instrument Science Report 99-04

Figure 3: Effect of the CCD dust mote at (220,505) on the sum of two G430L spectra of G191B2B. Plotted as a function of wavelength is the ratio of the STIS data to the noiseless model spectrum for that standard star. There are no features in the standard spectrum, while the dip in the 3515A region of the lower panels is the dust mote in the data. The lowest panel (G191B2B.G430L-NOFLT) used no flat field; G191B2B.G430L-ORIG is with the original current pipeline flat from prelaunch data; G191B2B.G430L-PGCCDL is the new flight flat; and G191B2B.G430L-MOTFLG is the same flight flat with the mote region flagged, so that the region of the mote can be automatically ignored. The two numbers at the lower left of each panel are the mean level and rms scatter in percent of the data points in the 3200-3467A region delimited by the left hand axis and the short vertical dotted line. The new flat produces the best signal-to-noise ratio with a sigma=0.38% that approaches the quantum statistical limit of 0.3%.

6


STIS Instrument Science Report 99-04

disk$data16:[bohlin.stisccd.dat]pgccdL.fits
150

Number of Pixels in Motes

100

50

0 0

200

400

600 Y (Row Number)
BOHLIN: motemeas 21-Jun-1999 16:09

800

1000

Figure 4: Number of pixels that lie in mote regions as a function of row number in the STIS CCD low dispersion modes. The vertical dotted line is the nominal location of the spectra at row 512 in the 1024x1024 CCD image.

7


STIS Instrument Science Report 99-04

disk$data16:[bohlin.stisccd.dat]pgccdM.fits
200

150

Number of Pixels in Motes

100

50

0 0

200

400

600 Y (Row Number)
BOHLIN: motemeas 21-Jun-1999 16:10

800

1000

Figure 5: As in Figure 4 for the medium dispersion modes.

8


STIS Instrument Science Report 99-04

disk$data16:[bohlin.stisccd.dat]pgccdL.fits
4

3

Total Mote Depth

2

1

0 0

200

400

600 Y (Row Number)
BOHLIN: motemeas 21-Jun-1999 16:12

800

1000

Figure 6: Cumulative depth below unity in motes for each row in low dispersion.

9


STIS Instrument Science Report 99-04

disk$data16:[bohlin.stisccd.dat]pgccdM.fits
4

3

Total Mote Depth

2

1

0 0

200

400

600 Y (Row Number)

800

1000

Figure 7: As in Figure 6 for the medium dispersion modes.

10