WFPC2 Filter Wavelengths in the HST Archive

April 16, 2003: WFPC2 Filter Wavelengths in the HST Archive

-Anton Koekemoer and Gabe Brammer

The HST Archive/CDBS catalog contains a number of different quantities that characterize the properties of each filter in the WFPC2 filter set. These quantities include the "Central Wavelength", which appears as a default output column in archive searches, as well as the "Pivot Wavelength" and other related quantities that are used to calculate photometric keywords such as PHOTFLAM, which can be used to define the zeropoints for WFPC2 photometry.

The CDBS keyword referring to "Central Wavelength" was populated with a value for each filter before the launch of WFPC2, based on ground-based measurements of the filters. This quantity is not used in determining any other keywords, but appears as a default output column when archive searches are performed. There are a number of issues related to this quantity, which include the following:

• The "Central Wavelength" is not a well-defined quantity, to the extent that other quantities such as "Pivot Wavelength" have precise mathematical definitions based on the filter throughput curves (as described in the WFPC2 Instrument Handbook).
• The "Central Wavelength" values have apparently not been updated since the launch of WFPC2, whereas the other photometric quantities are regularly re-populated whenever there is a major update of the system throughput curves used by Synphot.
• There are a few cases of very large discrepancies between the "Central Wavelength" and other filter quantities (including Pivot Wavelength, Mean Wavelength and Average Wavelength, which all agree with one another).

Therefore, the WFPC2 group, in conjunction with personnel in charge of CDBS, have chosen to replace the use of "Central Wavelength" with the use of "Pivot Wavelength" instead. The only effect that this will have is that, when archive searches are performed, the column that used to list "Central Wavelength" will now contain values populated from the "Pivot Wavelength" instead. No other quantities are affected by this change; in particular, photometric keywords within the WFPC2 dataset headers will remain unchanged, since they are already calculated based on the Pivot Wavelength.

For reference, we are providing here a table that lists the old values, representing "Central Wavelength", together with the new values that are based on "Pivot Wavelength".