VSD @Sheffield 14-Jul-2008

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

Toshihiko calculated the following optical thicknesses would be
required to replace the SDSS u'g'r'i' filters:

Clear filter thickness to replace u' filter: 5.223mm
Clear filter thickness to replace g' filter: 5.498mm
Clear filter thickness to replace r' filter: 5.483mm
Clear filter thickness to replace i' filter: 5.390mm

To save (substantial amounts of) money, it was decided to make all the
clear filters the same thickness of 5.4mm rather than make a separate
clear filter for the blue, green and red arms. The filters are made of
fused silica and both surfaces have been coated with an
anti-reflective MgF2 coating. This coating is identical on all three
filters (to save costs) and has been optimised for 450nm with an overall
average reflectivity of ~2.5%.