These are examples of a one orbit observation with two filters receiving equal exposure times. They are presented in order of 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) 900s Good No 0.2" Exposure time 901s, CR-SPLIT=0.555
Note: 900s with asymmetric CR-SPLIT into 400s + 500s. Change filter and repeat.
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2 1 (none) 800s Good No 0.2" Exposure time 800s, default CR-SPLIT
Note: default CR-SPLIT of 800s into two 400s. Change filter and repeat.
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2 2 Half 800s Good Good 0.16" Exposure time 400s, POS TARG 0,0
Note: POS TARGs are used to minimize telescope motions. Exposure time 400s, POS TARG 0.249,0.249
Change filter, second half of orbit goes...
Exposure time 400s, POS TARG 0.249,0.249
Exposure time 400s, POS TARG 0,0
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2 2 Half 700s Good Good 0.16" Pattern WFPC2-DITHER-LINE (default)
Exposure time 350s
Change filter, repeat.
Note: here the default dither pattern is used, but it is less efficient
than POS TARGs; total exposure is reduced to 700s.
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4 4 Box 400s Excellent Excellent 0.12" Pattern WFPC2-DITHER-BOX (default)
Exposure time 100s
Change filter, repeat.
Note: 4-point dither provides best resolution, but the total exposure
is only 400s per filter due to overhead times.