Re: [TASS] Photometry filter question

["Arne A. Henden" <aah@NOFS.NAVY.MIL>]

Tom Krajci asked,
>I'm getting closer to finishing my cookbook CB 245 CCD camera.  When I get
>around to getting proper photometry filters, will I need any additional
>filters, such as an IR cutoff filter?  (I know that this is needed for the
>RGB or CMY "pretty picture" processes, but I don't know about the UBVRI
>filters.)
  You should not need any additional filters.  VRI filters that conform
to the Johnson-Cousins standards don't need any blocking.  U&B filters
normally do, but as long as you order them from a reputable vendor (or,
at least, they specify that they use the 'Bessell prescription'), they
will have the blocking glass as part of the filter.
  You should make sure the filters are parfocal; that is, you won't
have to change your focus when moving from one filter to another.  The
usual method to achieve this is just to have all of the filters the
same thickness of glass.  The Bessell prescription does this.  There
are some differences in the U&B filters for front-illuminated CCDs
versus back-illuminated CCDs; tell your vendor what kind of CCD you
have and they should be able to match your system.  Also, note that
the TC-245 does not have great blue response.  You might consider just
getting VRI or even just VI for now, and get the other filters later.
Arne