avshift -- Combines (averages) the shifts measured on the 4 WFPC chips.
avshift table angle
- table [file name]
- Output file generated by task shiftfind. This table contains the list of images and associated shifts.
- (angle = 0.0) [real]
- The rotation angle.
- (weight = "0. 1. 1. 1.") [string]
- Weights for each chip.
- (wfpc2_chips) [pset]
- Pset containing the WFPC2 chip offset parameters.
This task produces a weighted average of the shifts measured for each of the 4 WFPC chips, and then outputs the average shift in the coordinates systems of the four chips. Avshift uses the known relative differences in orientation between the four chips in computing the common shift, and can also account for a small change in orientation between the shifted and reference images. Avshift is designed to work on the output file generated by the shiftfind task. In order for this task to properly associate the measured shifts from the four groups of a single image the first entry in the input table must use the naming convention employed by the offsets task.
The use of a non-zero rotation angle is only recommended in cases where the rotation is substantially less than one degree, as often results from a guide star misacquisition. For larger rotations, the user must presently use rotfind to solve for each chip independently.
The user can weight the four WFPC chips. This is can be useful in cases where the placement of objects on the image provides much more information in one chip than another. This feature was particularily useful before the adoption of precor, because the number of cosmic rays in a PC image is comparable to that in a WF image, yet the power in the objects is typically less. However, once cosmic rays are removed by precor, the information in the PC is generally comparable to that in a WF chip (as the smaller pixel size compensates for the smaller field of view).
Below is an example of applying avshift. to two dither positions of the HDF. In acquiring the second of these dither positions, the fine guidance sensors locked up an a secondary null, and as a result the pointing of the telescope was off by about an arc second, and was rotated with respect to the desired orientation by a few arc minutes. In the first use of avshift the user puts in a suggested rotation of 0.00 degrees, and a large error is found in the shift determination. However, in the second attempt the user (correctly) guesses the correct orientation and the error becomes a small fraction of a pixel. In practice, it only takes a few tries with avshift to home in on the rotation angle.
di> avshift pos2x11.out angle=0.00 Assumed angle of rotation = 0. degrees Weights: 1. 1. 1. 1. # # Image xsh_in ysh_in best_xsh best_ysh tot_sh_in delta_xsh delta_ysh pos2x11_g1 45.068 31.945 45.709 31.410 55.461 -0.641 0.536 pos2x11_g2 14.095 -20.226 14.559 -20.780 25.372 -0.464 0.553 pos2x11_g3 -21.278 -14.065 -20.858 -14.446 25.372 -0.420 0.381 pos2x11_g4 -14.839 21.432 -14.245 20.996 25.372 -0.594 0.436 # RMS error (all chips on WF scale)): 0.6187 # RMS error (WF only): 0.6795 # weighted RMS error (all chips on WF scale)): 0.6187 # weighted RMS error (WF only): 0.6795 di> avshift pos2x11.out angle=0.08 Assumed angle of rotation = 0.08 degrees Weights: 1. 1. 1. 1. # # Image xsh_in ysh_in best_xsh best_ysh tot_sh_in delta_xsh delta_ysh pos2x11_g1 45.068 31.945 45.068 32.066 55.311 0.000 -0.121 pos2x11_g2 14.095 -20.226 14.105 -20.231 24.663 -0.011 0.005 pos2x11_g3 -21.278 -14.065 -21.289 -14.018 25.490 0.012 -0.047 pos2x11_g4 -14.839 21.432 -14.821 21.416 26.044 -0.018 0.016 # RMS error (all chips on WF scale)): 0.0392 # RMS error (WF only): 0.0322 # weighted RMS error (all chips on WF scale)): 0.0392 # weighted RMS error (WF only): 0.0322 di>
Avshift does not properly handle rotations larger than a few tenths of a degree.
This task was written by I.Busko based on a program by R. Hook and A. Fruchter