Observation Strategies

7.6 Dither Strategies


There
is no single observing strategy that is entirely satisfactory in all circumstances for WFPC2. One must consider cosmic rays, hot pixels (i.e. pixels with high, time variable dark count), spatial undersampling of the image, and large-scale irregularities such as the few arcsecond wide region where the CCDs adjoin. One strategy that can be used to minimize the effects of undersampling and to reduce the effects of hot pixels and imperfect flat fields is to dither, that is, to offset the telescope by either integer or sub-pixel steps. The best choice for the number and size of the dithers depends on the amount of time available and the goals of the project. In the following we will address a few issues related to dithering:

  1. Undersampling: Individual images taken with sub-pixel offsets can be combined to form an image with higher spatial resolution than that of the original images. A single dither from the original pixel position -- call it (0,0) -- to one offset by half a pixel in both x and y, (0.5,0.5) will produce a substantial gain in spatial information. On the other hand very little extra information is gained from obtaining more than four positions, if the standard four point dither is used, and if the telescope has successfully executed the dither. Therefore the recommended number of sub-pixel dither positions is between 2 and 4.

  2. Hot Pixels: There are three ways to deal with hot pixels: correct using "dark frames" that bracket the observation, dither by an integer amount of pixels, or use a task such as "WARMPIX" within STSDAS to filter out the known hot pixels. Note that the integer dither strategy would ideally use six images, i.e. two CR-SPLIT images at each of three different dither positions. This is because in addition to hot pixels, low or "cold" pixels*1 can be present and simple strategies selecting the minimum of two pixel values can fail. However, even four images (two each at two dither positions) will greatly aid in eliminating hot pixel artifacts.

  3. Cosmic Rays: Although dithering naturally provides many images of the same field it is better to take several images at each single pointing in order to remove cosmic rays. In principle, it should be possible to remove cosmic rays using only sub-pixel dithered data. At present, however, no publicly released software is available for this task. Hence we recommend obtaining two or more images (i.e. CR-SPLITing) at each position in the dithered sequence. For very long integrations it is convenient to split the exposure into more than two separate images. As an example, for two 2000s exposures, about 1000 pixels per chip will be hit in both images and will therefore be unrecoverable. Moreover, since CR events typically affect 7 pixels per event, these pixels will not be independently placed, but rather will frequently be adjacent to other unrecoverable pixels.

  4. Accuracy of dithering: We do not yet have good statistics on the accuracy of HST dither offsets. During the Hubble Deep Field, nearly all dithers were placed to within 10 mas (during +/- 1.3" offsets and returns separated by multiple days), although in a few cases the dither was off by more than 25 mas, and on one occasion (out of 107 reacquisitions) the telescope locked on a secondary FGS peak causing the pointing to be off by approximately 1" as well as a field rotation of about 4 arcminutes. The software which was developed for the Hubble Deep Field is able to reconstruct images even for these non-optimal dithers, still gaining in resolution over non-dithered data. This software will be available in STSDAS in mid 1996 and is based on the variable pixel linear reconstruction technique developed by Fruchter and Hook (this procedure is also known as "dripping and drizzling"). The Richardson-Lucy-Hook non-linear deconvolution technique (STSDAS task ACOADD) can also use non-optimal dithers.

The simplest way to schedule dithers is to use the options DITHER-TYPE=LINE (e.g. for two-point diagonal dithers) or DITHER-TYPE=BOX (for four-point dithers). Alternative approaches are to specify a spatial scan or to use POS TARGs. Note that when the WF3 is specified as an aperture, the POS TARG axes run exactly along the WF3 rows and columns. For the other chips, they only run approximately along the rows and columns due to the small amount of rotation between CCDs. For small dithers these rotations are not important.

Some specific offsets allow one to shift by convenient amounts both the PC and the WFC chips. For instance an offset of 0.5" is equivalent to 5 WFC pixels and 11 PC pixels. The default DITHER-LINE spacing of 0.3535" along the diagonal is equivalent to shifts of (2.5,2.5) pixels for the WFC and (5.5,5.5) pixels for the PC.

Note that large dithers will incur errors due to the camera geometric distortion which increases toward the CCD corners and alters the image scale by about 2% at the corners. Hence a 1.993" offset will be 20.3 WF pixels at the field center, but suffer a 0.4 pixel error at the CCD corners. Large dithers may also occasionally require a different set of guide stars for each pointing, thus greatly reducing the expected pointing accuracy (accuracy only ~1" due to guide star catalogue). One set of software presently available to handle the reduction of large dithers (which show the effects of geometric distortion) is the Fruchter-Hook code developed for reduction of the Hubble Deep Field images.

For related articles on dither strategies, see the January, 1995 issue of the WFPC2 Space Telescope Analysis Newsletter and the February, 1995 issue of the ST-ECF Newsletter. For more information on dithering, please see "Questions about Dithering WFPC2 Observations" on the "Frequently Asked Questions" page of the WFPC2 WWW area. For detailed information on POS TARGs, please see the report "Dithering: Relationship Between POS TARGs and CCD Rows/Columns" obtainable from the WFPC2 WWW pages or help@stsci.edu.