The following list of "Frequently Asked Questions" has been prepared to answer
most inquiries that we receive. If there are questions that are not answered by
this list of FAQ, or that result from further study, please let us know.
What kind of filters should I buy?
There are two options. If you are just starting out and
wanting to keep things simple and
less expensive, you may want to start with a V filter only. This will
allow you to participate in our faint CV and LPV program. The particulars of
that program are included in the "Faint CV and LPV Program" which is available
from AAVSO headquarters if you do not have it. You want to ask for a V filter for CCD
photometry that gives a Johnson V profile when used with a CCD. If you are
purchasing a BVRI set, the RI were defined by Cousins and Kron. Some
suppliers call their filter sets various combinations of Johnson, Cousins and
Kron. They all have the same V response and the alternate names refer to the
designation of filters other than the V filter in their series. The filter is
different from a Tri-color green filter, in that it is more narrow in bandwidth
and centered at a different frequency. The proper filter response is
sometimes referred to as a "Bessell" response which refers back to a paper by
Bessell, who published curves on filters in the IAU Colloquium 136 on Stellar
Photometry. The filter must be properly red blocked. We have observers who
have bought filters from Optec, Murnaghan, Compuscope, and Balzers. Only the
first two are still in business, but they all work equally well.
Both of these companies advertise regularly in Sky & Telescope Magazine. I expect new
companies to offer filters in the future, and as long as you follow the above
guidelines, you should get proper filters.
The second option can be pursued without a filter wheel and without transformations.
The V filter is the same one that an individual would use in the BVRI set that will be discussed
below.
What is BVRI or Four Color Photometry?
BVRI is a measurement procedure in which the magnitude is measured in four
colors (Blue, Visual, Red, & Infra-Red), and then transformed to a standard
system that observers from different sites using different cameras, ccds,
telescopes, and software can merge results to high accuracy (much better that
0.1 magnitude). To transform data one must determine their transformation
coefficients by taking images of a standard field like M67 to determine a
correction factor to make results agree with the standard system. Results in
the standard system are usually capitalized (BVRI) while results before
transformation, also called instrumental magnitudes, are in lower case (bvri).
Why make measurements in multi-colors?
Four-color, or multi-color observations are valuable to determine the star
color and change in color with brightness and phase. Astronomers can
determine much about the physical changes occuring in the star
from these multi-color observations.
How do I specify BVRI filters?
BVRI filters are specified the same as the v-filter described above.
What Software can I use?
Photometric reduction software is constantly changing, just like the entire
software field. We have members using IRAF--a professional package that
requires UNIX and a workstation. This program has many more features than you
need for AAVSO programs. There are several PC versions such as Mira, CCD-IR, as well
as others. They will all do the job. They all have much more capability than
required. If your camera operating system can measure the intensity of a ROI
(Region of Interest), then you can perform the reduction in a spreadsheet. If you
would like a quick derivation of the measurements, contact AAVSO headquarters.
It is not a canned procedure, but will help determine whether or not you are software
inclined. We recommend that you contact those active suppliers who advertise
regularly in Sky & Telescope Magazine and ask for
software that will determine magnitudes by
either aperture photometry or point spread fitting. The second feature, for
future growth, is to ask about a routine to determine transformations.
The transformations can always be done in a spreadsheet as long as you can
determine the instrumental magnitude.
What camera should I use?
We have members using cameras. The only caveat is that cameras with
antiblooming are not suitable for photometry at this time. Some
cameras have the ability to turn off antiblooming. Ask the camera
manufacturer about how this is done, and if it is suitable for doing
photometry at the 0.1, 0.01, and 0.001 level. Most of the AAVSO programs are
done at the 0.1 to 0.01 level. Almost no one can do .001 photometry at this
time. We have members who use the Photometrics S-200, SBIG ST-6, and the
cookbook camera. These devices do not have anti-blooming and work very well. I
am sure that other cameras will work well, however we are not familiar with their
observations at this time at AAVSO. We will add to the list of cameras as
observations with different devices become available.
Is the Optec PFE 1a either useful or necessary?
Optec can advise you on the advantages of this device. None of our observers
are using one at this time, so it is not necessary for everyone. Optec
makes excellent photometric accessories and I am sure they are very useful
for some observers.
Should I use 1", 1.25" or 2" Filters?
The filter size must be large enough so that it does not vinnette the CCD
chip. If you are using a 2kx2k, then 1" filters will not work. Other important factors
include the light cone shape (f-ratio of telescope) and the distance from the CCD
chip to the filter. Your filter manufacturer may be of some help. I
use a 10mm chip, a f3.7 telescope, 1.5 inches and 1 inch filters. They are "just" big enough, and I frequently use
only the center 4mm of the chip since this gives me a 12x12 arc minute field
size.
I have heard the the AAVSO BVRI Photometry is only good to 0.1 magnitude.
Shouldn't I be able to do better than this?
It is true that when combining several observers all at different sites, none
of which are photometric, using different cameras, different filters, different
software, and different telescopes - the data occasionally has points that lie
0.1 magnitude from the mean. The standard deviation is typically 0.03
magnitudes over a four year period, which is no different from the published
data of other sources. Remember that this is absolute accuracy, not just
repeatability of a few points. Some of these observers have also measured
superhumps of dwarf novae with amplitudes of .0025 magnitudes with periods of
90 minutes over a 3-4 hour period. This is instrumental magnitude and is
not the same as an absolute magnitude measurement.
I thought CCD's were super linear. This does not sound like it has much better
accuracy than a PMT photometer.
The reasons are not completely known. CCD photometry to .01 magnitudes takes
much care. Those who push several milli-magnitudes must include many more
error terms in their data reduction. I believe that what we see are
small variations in transparency which may be color dependent and cause short
term variations in the photometry. I have seen light curves from very
experienced CCD photometrists that show a very steady light curve, and then
for no reason, a point at .05 to .10 magnitudes from the population. I have
also personnally had the experience of taking about 6 images, one right after the
other while each exposure was the same, and taken in succession. Suddenly, when the third image came
up on the screen, it was considerably better than the previous two.
Curiously, the following three images were similar to the first two. The
third image was outstanding, and could not be repeated. I believe that I
experienced a particularly transparent few seconds during that exposure. I
suspect that the same thing happens during phototmetric runs. I am sure that
we even get small clouds that are virtually invisible, except to the CCD
photometer to levels of milli-magnitudes.
