HDF Plans for Reprocessing:

The preliminary superbias frame was constructed from individual bias frames taken from January to November, 1995. The final superbias will include bias frames taken during and after the HDF observations. This will probably have a negligible effect on the noise in the combined images, but is something that might as well be done as long as the data are being reprocessed.

New Superdark Frame:

The preliminary superdark frame was constructed from individual bias frames taken from January to November, 1995. The final superdark will include frames taken during and after the HDF, and will eliminate frames taken more than 6 months before the HDF. This should reduce the number of hot pixels.

Time-dependent Hot Pixel Masks:

In the initial processing we masked all pixels that became hot (or stopped being hot) at any time during the observations (hot being defined as more than 5-sigma deviant from the mean dark current). With more time, we can do a better job of finding all the hot pixels, perhaps to a lower sigma cut, and can mask them in a time-dependent fashion (probably using a different mask for each 24 hour period).

Moving Target Masks:

In the first pass, moving targets were only eliminated as well as the crrej task can do it. As many of the moving objects are resolved, this actually does not work very well. In the reprocessed data, these objects will be masked before the images are combined.

Scattered-Light Correction:

We are currently experimenting with different ways to remove the scattered-light X pattern. There are very few frames where the scattered light background significantly affects the rms pixel-to-pixel noise. However, the varying backgrounds increases the pixel-to-pixel noise in the final subsampled (drizzled) images, and is best removed before the drizzling stage. Our tentative plan is to drip the images that have no scattered light, geometrically transform them to the positions of the frames that are affected by scattered light, scale by the exposure time, and subtract. This will remove the sources. The resulting image can be median filtered to produce a fairly smooth background model that can be subtracted to remove the X pattern.

True Inverse-Variance Weighting:

In the combining the data for the preliminary products, the individual data frames were weighted by exposure time. This is close to optimal for the F606W and F814W images, but not for F450W and F300W, where readout noise makes a significant contribution. Version 2 of the data will have a more optimal weighting, which takes into account the readout noise and dark current, but still uses a constant weight for each frame. For version 3 we plan to let the weight vary pixel by pixel depending on the brightness of the source and the number of cosmic rays rejected.

Copyright © 1997 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved.