Q.: For which filters are photometric transformations available to ground-based (e.g. Johnson) filters?

A.: currently none. There isn't a straightforward, painless way to transform photometry in one filter of some arbitrary source into another filter with different passband. The transformation is highly dependent on the nature of the source. None of the NICMOS filters correspond with any `standard' ground-based filter, although the F160W is fairly similar to many ground-based H-band filters, the F110W may represent an approximation to the J filter ( although it is a bit broader), and the F222M may be close to the K filter ( although it is quite narrower that the "typical" filter).

Q.: Can I get hold of the filter transmission curves for the NICMOS filter set in electronic form?

A.: Yes, you can. You should ftp to the site ftp.stsci.edu, and once connected cd to /cdbs/comp/nicmos, where you will find all the filter curves (and a variety of other information). The filter curves are in files named with the filter name, exactly as listed in the Handbook, and are in the STSDAS Tables format. You can convert them into ascii files if you wish by using the tdump utility in the IRAF STSDAS package.