[Aavso-photometry] Transformation coefficients question

[arne]

gianlucaros at gmail.com wrote:
> Arne,
> what if the filters have different thickness? I've heard that the
> Shuler's are 4mm in thickness while the Custom Scientific's 5mm. Does
> it affect precision in the coefficients or not?

Different thicknesses is an interesting question.  Sometimes, a vendor
goes a little thicker to make the colored glass a bit thicker - in some
circumstances, this provides a slightly better fit to the Johnson/Cousins
system.  Sometimes they go thicker because they want to include a clear
cover glass on both sides of the filter to prevent moisture contaminating
the soft colored glass.  In general, both 4mm and 5mm filters will work.

However, what *does* happen is that 4mm filters have a different focus
than 5mm filters.  So as you switch between filters, you will have to
refocus.  Most sets from a single vendor are built so that they are
parfocal (how close they get also depends on the price).  With modern
robotic focusers, this is less of a consideration than when you had
to manually focus the telescope, but keep it in mind.
Arne