VSD @Sheffield 14-Jul-2008

Here are some details on the construction of the SDSS filters for
ULTRACAM. (I no longer have the emails from Toshihiko on-line, so I've
typed these details in from my notes):

- All multilayer coatings were done by IAD (Ion Assisted Deposition),
which is excellent in terms of environmental resistance and spectral 
stability.
- AR coatings were done by Thermal (Electron Beam) Deposition.
- BG38, UG11, GG400, OG550, RG695, RG830 and BK7 were all manufactured
by Schott Glaswerke.

Here are the details of the construction of each filter (according to 
an email from Toshihiko on 21/10/2002):

u'  UG11  (1mm) + BG38 (1mm)  + Fused Silica (2.9mm)
g'  BG38  (3mm) + GG400 (2mm) 
r'  OG550 (4mm) + BK7 (1mm)
i'  RG695 (4mm) + BK7 (1mm)
z'  RG830 (4mm) + BK7 (1mm)

Each filter has a multi-layer coating on both surfaces.

In October 2002, Mark Stevenson measured the filter thicknesses in
each corner of the filter and also in the centre, using a
micrometer. The measurements are accurate to 5 microns. Here are the
results (in mm):

     centre    bottom-left    bottom-right    top-left    top-right

 u'  5.046     5.035          5.032           5.043       5.041
 g'  5.058     5.045          5.057           5.055       5.063
 r'  5.121     5.092          5.097           5.090       5.106
 i'  5.094     5.068          5.081           5.088       5.080

(z'  5.088     5.103          5.085           5.105       5.081)

I've not got a record of the measured central wavelengths and FWHM,
but these should be measureable from the data file available on the
ULTRACAM filters web page. The figures quoted in the table on this web
page are from Fukugita et al.  (1996).