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Hubble Space Telescope
Comparison of Half-Orbit Dither Strategies

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.