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Hubble Space Telescope
Comparison of One-Orbit Dither Strategies

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.