These are examples of a one-orbit observation with a single filter. They are presented in order of increasing dither points, and decreasing total exposure time. All will just fit into a 52.5 minute orbit. Rough indications of the quality of cosmic ray rejection, warm pixel rejection, and effective resolution are given. While the WFC has 0.1" pixels, the effective resolution is lower since a star will typically split across several pixels.
Number Number Pixel Total Cosmic Warm Approx. WFC Implemetation
of Dither Offsets Exp. Ray Pixel Effective
Exp. Points Time Reject. Reject. Resolution
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2 1 (none) 2400s Good No 0.2" Exposure time 2400s
Note: this will default to a CR-SPLIT with a pair of 1200s exposures.
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3 2 Half 2100s Very Good Good 0.18" Exposure Time 700s, CR-SPLIT=NO
Then a second exposure with....
Pattern WFPC2-DITHER-LINE (default settings)
Note: this is a compromise between simple CR-SPLIT and dithering. Expsoure Time 700s, CR-SPLIT=NO
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3 2 Half 2100s Very Good Good 0.18" Exposure Time 1400s
Expsoure Time 700s, CR-SPLIT=NO,
Note: this just another way to write the above observation. and POS TARG=0.249,0.249
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4 2 Half 1900s Excellent Good 0.16" Exposure Time 1000s
Then a second exposure with....
Exposure Time 901s, CR-SPLIT=0.556, and
Note: 2 exposures at each of 2 positions. Compromise between POS-TARG 0.249,0.249
CR-SPLIT and dithering. Asymmetric CR-SPLIT on the second exposure.
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4 2 Half 1800s Excellent Good 0.16" Pattern WFPC2-DITHER-LINE (default settings)
Exposure Time 901s, CR-SPLIT=0.556
Note: 2 exposures at each of 2 positions, but written with pattern
instead of POS TARG. Asymmetric CR-SPLITs (500s + 400s) at each position.
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3 3 Integer 1900s Very Good Very Good 0.2" Exposure Time 700s, CR-SPLIT=NO
Exposure Time 600s, POS TARG=0.498,0.498
Exposure Time 600s, POS TARG=0.996,0.996
Note: this is a 3-position integer pixel dither to improve cosmic ray and
warm pixel rejection.
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4 4 Half 1600s Excellent Excellent 0.12" Pattern WFPC2-DITHER-BOX (default settings)
Exposure Time 400s
Note: this is default 400s dither box. Gives best rejection of artifacts
and best resoltuion, but overhead times reduce total exposure to 1600s.