[Aavso-photometry] Which Photometry Filters to Begin With ?

[Wolfgang Renz]

Hi Steward

Beside what Arne already said, the usage of the different filters
depends on many conditions and your actual imaging aims.

The ppi filters are usually 1-3 and the (Bessell prescription) pho-
tometric filters 4 or 5 mm thick. As the optical depth of a plane
filter in the convergent beam is ~ 1/3 of its physical thickness,
one gets different foci with differnt thick filters and without filter.
So if one merges filters from these two types or goes unfiltered,
one will need to focus when changing filters. Then you will need
a remote controlable focuser to account for this.
As all scopes except of reflector-only systems show some more
or less severe kind of chromatic aberration (especially in the
NUV and far-red where visually optimized scopes are not well
corrected), one will usually have to focus in every filter to get the
sharpest possible star images.
As the focus of most system also depend on the temperature,
one will have to focus regularly if the night has a larger tempe-
rature gradient. Some systems also change the focus slightly
when the pointing changes (flexture, mirror flow, ...). So if one
wants to make ppi with the sharpest possible star images,
one probably will focus after every filter and target change
anyway.

The ppi and more expensive photometric filters usually have
an AR coating to optimize their transmission and to minimize
issues due to reflections. Inexpensive filters usually don't have
an AR coating and will therefore show an at least ~ 4-8 %
worse transmission (~ 4 % reflection per uncoated surface
instead of usually 0.x-1.x % per coated surface) and increa-
sed reflections due to this.

No filters are used to go as faint as possible in the shortest
possible time. But the star images are with most systems
not as sharp as when using a filter that rejects at least the
NUV and far-red. In photometry they are used e.g. for out-
burst monitoring and "higest-possible-speed" time series
of CVs when its more important to go as faint as possible
or with the shortest possible time interval with an as high
as possible SNR when the color of the target and a mag
value in a photometric band is not important or relevant.
Most people who image to derive times of minimum/ma-
ximum of short periodic stars (e.g. EB and RR Lyr stars)
also don't use a filter. This is usually sufficient for vars that
don't change color (e.g. W UMa type EBs) and if one does
not do any transforms to a photometric standard passband
anyway. In ppi clear filters are usually just used for fainter
non-emission-line targets (galaxies, star clusters, stars)
that have a significant continuum that would be blocked
with a luminance filter.

Clear filters are basically "just" a filter glass of the same
thickness of the other filters of the filter set to not to have to
(raw) focus when wanting to go unfiltered. The clear filters
are used for the same targets as no filters, but they are ac-
tually just used by most, if one wants to take filtered and un-
filtered images in the same session without having to (raw)
refocus.

Luminance filters are basically "NUV+NIR-cut" filters that
block the NUV plus far-red light (~ 400-700 nm transmis-
sion band) and usually match the transmission bands of
the ppi RGB filters well. In comparison to clear filters, they
get rid of the blurring halos of the star images due to not
well focused NUV and far-rastroed light when one focuses on
the visual range. These halos get severer with scopes that
show more chromatic aberrations in the NUV+blue and far-
red. For inexpensive achromatic refractors and camera
lenses one might even need a more restricting luminance
filter that blocks all light below 430-450 nm to get the sharp-
er star images. In ppi they are used especially for emission-
line targets (that don't have emissions in the NUV and far-
red) and to get sharper star images than with clear filters.
With astrometry they are used to get more accurate posi-
tions that are less affected by chromatic differentiyl refrac-
tion. With photometry they are usually just used to get the
mag values closer to a photomtric standard band (e.g. for
blocking the far-red to get better matching V mags; to get
better matching Rc mags a long-pass filter can be used)
or if the NUV or far-red blurring is an issue with close com-
panions.

The foci of the ppi RGB and luminance filters and the pho-
tometric RcVB filters are with most scopes usually close
together (but in most cases still not 100% identical). Ne-
vertheless it might not be necessary to change focus for
photometry when changing between these filters (photo-
metry is not as demanding as ppi in this respect if one
doesn't want to get as faint as possible and there are no
close companions to the comp and vars stars of interest).
But the foci of the U band filter and often of the Ic band
filter too are usually pretty far off and will require a refo-
cusing if the stars of interest are not very bright and one
defocuses anyway to get a longer exposure time to beat
the scintillation noise with less subframes.

The U band filter is scientifically very interesting as it does
pass some light below the Balmer jump. But most scopes
(except of reflector-only systems) do absorbe most to all
light below 370 nm due to glas in the optical beam or re-
flect it due to visually optimized AR coatings. CCDs that
are not especially blue and NUV sensitive (e.g. the old
Kodak non-E chips) also don't have a sufficient QE in the
NUV. If  the CCD chip has a cover glas or the window of
the camera is not optimized for the NUV too, the NUV
transmission is reduced too. So the actual system re-
sponse of most amateur setups will not match the requi-
rements of the U band mag standard well.
Nevertheless there are some types of vars that are worth
to be observe in U band even if the actual system response
with the U band filter does not let through much light below
370 nm. This are e.g. the flares of UV Cet stars that can
have flare amplitudes that are several mags larger in the
U band than in the B and V band. Or e.g. zeta Aur type
EBs (binarys whose early, hot dwarf gets occulted by a
late, much colder giant) whose amplitudes are much lar-
ger in the U band than in the B band and already pretty
small in the V band.

The Bessell prescription Ic band filters don't match the re-
quired transmission of the Cousins Ic band standard well
when used with CCDs as originally a PMT was used that
has a steep cut-off at ~ 875 nm while the CCDs have a
significant QE up to 1000-1200 nm. This red tail makes
it difficult to transform Ic band measurements especially
for red vars that have strong absorbtion bands there. But
as there are not many really red constant standard stars
anyway, the requirements to reach a high accuracy or to
make a transform at all are usually relaxed with these vars.
If one needs to reach a high accuracy with red stars, it
might be recommendable to use the Rc band filter instead.

As your KAF-6303E has its max QE in the 550-700 nm
range you will get the highest signal and SNR with most
stars with the Rc band filter (for very blue stars it might
be with the V band filter). Next are with the V and Ic band
filters. With the B band filter it will be already consider-
ably longer and with the U band filter very much longer
if one can get a sufficient signal at all (e.g. for red stars).

With astrometry its basically not very important which
filter one uses. With imaging at small airmasses it is
usually sufficient to use a "NUV+NIR-cut" filter to get
a bit sharper stars and therefore a higher accuracy of
the positions. But when imaging at higher airmasses,
the atmospheric chromatic refraction shifts the star
images up to the few arcsec range and might cause
significant differences due to differential chromatic re-
fraction of different colored stars in the FOV. Then its
recommendable to use a passband filter. As higher the
airmass as narrower the passband should be to get
accurate positions.
With photometry and imaging over a wide range of air-
masses and especially at higher airmasses, atmos-
pheric extinction gets an issue. If the FOV is small and
the comps therefore close to the var and the comp stars
are of the same color as the var, extinction can be usually
be neglected as long as its effect is smaller than the pre-
cision of the measurements themselves to derive good
mag values in the full FOV. If the color of the comps and
the var differ significantly, every kind of passband filter
will help to reduce the influence of the differing colors. If
the FOV is larger, effects due to differential extinction in
the FOV might not be neglected anymore. At the zenith
its usually no issue even with large FOVs, but at higher
airmasses one should correct for extinction to be able
to derive good mag values in the FOV. The its recom-
mendable to use photometric filters to be able to derive
an extinction coefficent by observing standard stars in a
wider range of airmasses.

For many vars its sufficent to image them for photometry
with no or a clear filter (see above).
Using a single filter gives often an important benefit. But
its often still not very important what kind of filter it actually
is. But if one wants to compare or even merge the own
measurements with the ones of others (that don't have
exactly the same scope and camera setup and are loca-
ted close together), its always recommendable to use the
same (photometric) filter for imaging. As the visual obs
are close to the V band standard, one should usually cho-
ose the V band filter as first filter to reach this goal.
If one wants to transform the mag values to the standard,
one must image with at least two photometric filters as
one need a measured color index for the transformation.
As most of use don't have many real photometric nights
that are good enough for all-sky photometry, it depends
on what kind of mag values for the comp stars are avail-
able in the FOV. For real bright vars it will be usually B-V
from old ground based photometry. For brighter vars it
can be also V-Ic (e.g. from the HIP catalog). For lightcur-
ves of real faint astroids it might be also recommendable
to image in B and Rc if just USNO B and R mags are
available.
To observe a var in more than two photometric bands is
just needed if one wants to study a var in more details.
But its done by amateurs usually just if one has a full set
of photometric filters, the additional time for multiband
flats etc. are worth the effort, and the var does not vary
much during the time it takes to complete a full set of
measurements.

For vars that do change color (e.g. Cepheids, RR Lyr
stars, ...) significantly, the derived ToMs usually also sy-
stematically depend a bit on the actual system response
(transmission of the scope, filters and other optical ele-
ments in the beam times the QE of the CCD). For these
its better to use a photometric standard filter to be able
to compare and merge the resulting measurements and
the derived ToMs with the ones of others.

Clear skies
 Wolfgang

-- 
Wolfgang Renz, Karlsruhe, Germany
Rz.BAV = WRe.vsnet = RWG.AAVSO