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CCD Views Vol 2 No. 4
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THE AMERICAN ASSOCIATION OF VARIABLE STAR OBSERVERS
25 Birch Street, Cambridge, MA 02138 USA
Tel. 617-354-0484 Fax 617-354-0665
http://www.aavso.org
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C C D V I E W S
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December, 2001 Vol 2 No 4
Table of Contents
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1. Introduction: Happy Holidays & Winter Solstice
2. More on Observing Faint CV's
2. Tips on Starting an Observing Program
3. Starting Out In CCD Photometry Part 1: Equipment
4. PX And: A Neglected CV With Short, Bright Periods
5. Faintest CCD Observations
6. AIP4WIN & MaximDL Photometry Bugs & Patches
7. CCD Points
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1. INTRODUCTION: HAPPY HOLIDAYS & WINTER SOLTICE
Happy holidays and Winter Solstice to all our observers out there!
Our final issue of CCD Views for this year focuses on some basics
in CCD imaging. We have two excellent contributions about planning an
observing program and choosing the correct equipment for variable
star photometry. Both can be time consuming but important (and fun!)
parts of CCD observing.
JAM's LPV notes will most likely return in the next issue of CCD
Views, expected out in the first of February. Until then please
continue observing the last issue's list and check the AAVSO Bulletin
for LPVs nearing minima in the next two months.
We shall end this introduction on a special note of cheer. The
AAVSO has access to a small number of Schuler Johnson V and Cousins R
filters that we have decided to loan to our observers! If you need
either or both filters please fill out the form below and e-mail it
to aavso@aavso.org. Unfortunately our supply is limited so we will be
choosing recipients based on the quality of past CCD observations,
current activity, and geographic location. Ties will be decided based
on who submitted the request earliest. Please have your requests
submitted by Monday, December 17.
Thank you have an enjoyable holiday season with crystal clear skies
and steady seeing!
Aaron Price Gary Walker
AAVSO Headquarters Chairman, CCD Committee
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
CCD V & R FILTER REQUEST FORM
Name:
Observer Code:
Full Mailing Address:
Telephone Number:
CCD:
What size filter do you need?:
Which filter(s) do you need? (V, R, or both):
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2. MORE ON OBSERVING FAINT CV'S
We have had good response on the article titled "Observing Faint CV's
at Quiescence". Arne Henden, of the US Naval Observatory in Flagstaff
sent a letter with tips and more information on the CV's listed in the
article. The article is reproduced below.
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For many of the CVs that you list as needing quiescent
magnitudes, you can look at the tables on my anon ftp site:
ftp://ftp.nofs.navy.mil/pub/outgoing/aah/cv3/cv3_obs.log
ftp://ftp.nofs.navy.mil/pub/outgoing/aah/cv3/cv4_obs.log
ftp://ftp.nofs.navy.mil/pub/outgoing/aah/cv3/cv5_obs.log
I would suggest listing the expected quiescent magnitudes
for the CVs in your table, whether from my logfiles or other
sources, so that the people who might be going after these
stars have an idea as to how faint the CV is at quiescence.
You might also contact John Thorstensen as he has been
observing many of these stars at quiescence as well.
Also, I've seen many cases now where CVs undergo minioutbursts
(that is, well above quiescent levels but too faint for visual
observers). Your CCD observers should be prepared for such
outbursts and report them.
Finally, the quiescent state is often much redder than at
outburst, depending on which star in the binary is dominant.
Since most observers will be unfiltered to reach these faint
magnitudes, you will have significant transformation differences
between observers, even using the same comp stars. Keep this
in mind.
Arne
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We plan to continue developing this project in the future including
work on charts, observing schedules, and more background information
on the stars. Stay tuned to the next issue of CCD Views for more
information. In the meantime please observe these stars!
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3. PX AND: A NEGLECTED CV WITH SHORT, BRIGHT PERIODS (AND A NEW CHART)
PX And (0024+25) is an interesting cataclysmic variable (CV) that
was observed in CCDV by AAVSO member Lasse Jensen of Denmark (JLT) in
1997 but then promptly forgotten. A new AAVSO f-scale CCD chart with
.01 magnitudes and B-V will hopefully stir up new interest in this
star.
One of the traits of PX And that makes it intriguing to observers
is its variability. During JLT's observing run changes in brightness
of as much as 1.5 magnitudes in around 1 hour were detected!
PX And is a SW Sex star, a classification created by John
Thorstensen for novalike objects that have peculiar characteristics.
Some of these strange characteristics include the complete absense of
UV emission from the white dwarf while in the low state (because it
is completely obscured by the accretion disc), zig-zag type features
in the wings of the spectra (possible caused by the stream of
material overflowing or punching a hole in the disc), and many SW Sex
stars show infrahumps (negative superhumps!). Also, some SW Sex stars
appear to be non-eclipsing (such as LS Peg). The current theories
suggest that we are looking along the edges of the cool outer rim of
a flared disc around the white dwarf.
Long term observations of PX And would be valuable to the research
of SW Sex stars. Accurate, high-speed photometry of PX And is needed,
especially in V and I. Observe every few minutes for several hours or
get good coverage of the minimum by observing once per night over
several nights. The key is to get as much coverage as possible.
The new PX And charts are available at the AAVSO Charts WWW site:
/observing/charts/updates.stm
For background reading on PX And consult the following URLs:
http://cdsads.u-strasbg.fr/cgi-bin/nph-bib_query?1994ApJ...431L.107H&db_key=AST&nosetcookie=1
http://cdsads.u-strasbg.fr/cgi-bin/nph-bib_query?bibcode=1998PASP..110..420H&db_key=AST
http://cdsads.u-strasbg.fr/cgi-bin/nph-bib_query?bibcode=2001A%26A...368..183G&db_key=AST
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4. TIPS ON STARTING AN OBSERVING PROGRAM
By Gary Walker (WGR), CCD Committee Chair
So you have finally got your CCD camera and your telescope and
software and you are ready to go. You may even have set up your
equipment, as you have done many times before. Everything is working
well. You are ready to go. And then it hits you, Brain Fade. What
to observe? Well with deep sky, its easy, just pan over to M42 and
you have a start. While looking at this beauty, you usually will
remember another interesting object by looking up at the sky. Then
someone usually say's "I've got M-xx", you go over and look at it,
and then move your scope there. The next thing you know, the night is
over an you have observed many of your favorite objects.
But what if you intend to do some photometry? If you are an old
hand at visual variables, then you probably have a list of favorites.
But what if you are new to variables and did a lot of deep sky imaging
with your camera, and now have found that you want a new challenge?
You are not in the minority, it seems that may of us purchased our
cameras to do pictures first, and did not have photometry in mind.
Now you find yourself wondering what to observe.
I used to have this problem and never knew what variables to observe
until I did a couple of things. They are easy and pretty obvious, but
my observing program really took off after I did them. I want to pass
them on, because I have not seen anyone write them up, and we all
assume that this is really too basic to mention, but I get this
question a lot. Fortunately, planning an observing session is a
cloudy night or daytime activity. As we all know, after purchasing a
new telescope or CCD camera, you are usually in store for some of
this, unless you live in Arizona.
The first thing that I did, was make up a spreadsheet on my
computer, so that I could edit the objects and list their RA and Dec.
This was done mostly to convert the Epoch 2000 coordinates on the
AAVSO charts to the Epoch 1990 coordinates on my digital setting
circles. Most of you will not have to worry about this, but it caused
me a great deal of grief until I figured it out. While I did this to
get the proper coordinates, it provided an observing check list that I
keep at the scope to select the next object. This spread sheet can be
sorted by RA, which makes it very convenient for observing. My site
it open and reasonably dark for the 45 degrees overhead.
So to start my spreadsheet, I started with the 8 BVRI Standard Stars
from the CCD Program. Since these were spread from RA 2 hours to RA
14 hours, they were a good start. I next added the 28 Faint CV and
LPV stars from the AAVSO program. These stars were a little more
prevalent and typically passed right overhead. Even this list was a
little thin in some parts of the sky. About that time, I got a notice
from HQ that some new CCD charts had just been added to the web. I
added these to my spread sheet. Some months later, another notice
announced that additional charts were available. Those too were
added.
So now I have 84 stars on my list. The RA and Dec is listed for
each. In addition, a bright calibration star is listed nearby which
can be used to re-sync the setting circles if necessary. I have
increased the font size and used bold print, so that they can be read
by an old guy like me easily. They cover 3 pages, and I have a
clipboard with a ruler, that can be placed on any line.
I start an observing run by pointing at Vega. I use this star
because I can recognize it easily, I have my 7x50 finder with
illuminated cross hairs lined up so that when a star is centered, it
is always on the chip of my ccd. My field is 15 arc minutes. This is
a real time saver, and I urge you to try it. I found that the center
of my ccd chip is not where the center of my eyepieces are,
particularly when using a diagonal, so you may have to realign your
finder specifically for this purpose.
After I have centered Vega in my finder, I go in the house and take
a quick shot. I use this shot to trim the centering to the exact
center of the chip, and also tweak the focus. Next, I go back outside
and sync my setting circles and display the RA and Dec. Then I go to
my list and choose a variable close by (LL Lyr this time of year)
and re-aim the scope. I go back inside and start the exposure. I use
10 minutes for most of these objects, as they are faint and the signal
to noise helps. When the exposure is done, I do the photometry and
obtain my estimate.
Here is another tip that I use. I have a log book where I write on
one line, the Star name, JD, Mag, Std Dev, Comp Stars and time of
observation. I have been keeping this log for about a year, and find
it very useful. Its just an old lab notebook, but it is easy to refer
back to earlier observations, and it much more accessible that my
computer data. I also find that the discipline of filling out each
line during and after the exposure to be beneficial. The log book
also provides all the info I need to submit my observations over the
WWW, so I use it often.
The cycle runs about 17 minutes for a 10 minute exposure. Then I
walk out and point to the next object. I do not observe every object
on my list each clear nite. I skip over some. As the seasons change,
my first star must change as it gets too low to observe. I choose
objects that are well placed-some nites my scope stays just about at
zenith, as I pick off each one. As fall and winter approached, the
early darkness offset the annual progression across the sky nicely.
In the spring, I won't be so lucky.
So those are the two things that I heartily recommend. Have a list
of objects that you evolve. Add objects and take them off. Keep a
whole year's objects on your list. That way, when you observe very
late in the evening, you will have a new supply of objects to point
to. Keep a log in a small notebook that summarizes your work. This
makes it easy to submit them electronically.
I try to observe about 6 stars a night. That takes about 2 hours
on a good night. Some nights, I only get 1 or 2. One night/morning
I got 15 observations. It is really surprising how fast they add up.
I never set a definite goal, after all this is relaxation, not work.
I observe until I feel like stopping. Some nights that means no
observations, as the clouds win. However, I have found that if one
devotes just an hour or two most nights, that a large number of
nights are partly usable. While I never planned this, I have found
that a reasonable number of times, I have an observation that either
supports the quiescent state or signifies the outburst. Its really
neat to see your observer initials on the Alert Notices.
Clear Skies and Good Observing.
- WGR
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5. STARTING OUT IN CCD PHOTOMETRY PART ONE: EQUIPMENT
The following is the first installment in a series of contributions
written by Keith Graham (GKA), who is in his 20th year as an AAVSO
observer. The series will be continued in the next few issues of CCD
Views.
Please note that all views here are views of the author. No
specific makes or models of equipment are officially endorsed by the
AAVSO. The hardware mentioned in this article is here to illustrate
issues that apply to all CCD equipment.
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Part One: Equipment
Let's begin with the equipment required. By now, you probably have
the telescope and maybe even a CCD camera. I knew when I was looking
for a camera that I wanted to use it primarily for photometry. My
knowledge of CCD cameras was limited to say the least. So I
researched web sites and user groups. Upon reading the many posts on
user groups (such as MAPUG and SBIG) I soon learned who on those
lists knew and had experience with CCD cameras. I would write to
them, and found them to be extremely helpful. In fact, I don't know I
would have gotten this far without their help. Their comments helped
me to decide on the right camera based on my intended use
(photometry) and my telescope system (LX200 12" f/10). I narrowed my
choices down to the SBIG ST6 and SBIG ST7 cameras.
I should mention here that the ST6 is no longer manufactured, and
has since been replaced by the SBIG ST9. The ST9 has everything the
ST6 has and more, including a larger FOV and an onboard guiding chip
(more on this later). There are other brands of cameras out there
with similar characteristics as the ST6 and ST9 and very suitable for
photometry. But never having used them, I will leave their
attributes to those who know their capabilities. I am comparing the
ST6 with the ST7 to show my reasoning in selecting an appropriate
camera for photometry. New cameras can cost $3,000 - $4000 and up. If
you want to save some money, you might be lucky enough to find a used
ST6 and have a camera that is very well suited for photometry.
I ultimately decided on the ST6 camera for a number of reasons.
First, the ST6 has 23x27-micron pixels compared with the 9 micron
pixels of the ST7. These larger pixels of the ST6 had a larger well
capacity, meaning they can absorb many more photons than can the
smaller pixels of the ST7 before they become filled to capacity (or
saturated). By comparison, think of a juice glass versus a larger
drinking glass. Just as the larger drinking glass can hold more
water than can the juice glass before overflowing, so, too, the
larger pixels can hold more photons before they "overflow" than can
the smaller pixels. When a pixel is full to the brim, it is
saturated. The problem here is that if you continue to expose beyond
saturation, you will not be able to determine the true magnitude of
the star because you will not know at what point it became saturated.
If you expose your image so that your stars are always well below
saturation (1/2 well capacity is about right), your software can
accurately compare the number of photons collected by the comp and
variable in the same time period. Another plus for the larger pixels
is that they are more sensitive than the 9 micron pixels. This is
like comparing a 12" scope to a 4-inch scope. Due to its larger
aperture, the 12-inch scope absorbs many more photons than the 4 inch
in the same period of time. This means with the ST6, I would need far
less time to take images than with the ST7 resulting in more images
taken during an observing session.
Another advantage of the ST6 over the ST7 was the higher QE (that's
quantum efficiency) of the ST6. You want your CCD camera to have a QE
conducive to photometry. QE is simply an expression of just how
sensitive your CCD chip is to the various wavelengths of the
spectrum. I found that the QE of the ST7 was almost 0 at the blue end
of the spectrum and was lower at most other wavelengths than the ST6.
This meant that the ST7 was almost blind to the shorter wavelengths
and did not see the other wavelengths as well as the ST6. Ideally,
you would want a chip that has a high and even QE for all wavelengths
of the visible spectrum, but don't look for that to happen.
Another factor I considered was the field of view (FOV) of the CCD
chip. The ST6 had a slightly larger FOV than the ST7. This might not
seem too important at first glance, but when you are trying to fit a
small piece of the sky onto a small CCD chip, it is much easier to
put it on a larger chip. The larger FOV is also very helpful when
comparison stars around the variable are not always adjacent to the
variable. When doing the photometry on the image, you definitely want
your comp and check stars on the same image as the variable (more on
check stars later).
Finally, you will want your camera to have a chip with a NON anti
blooming gate. Although the ST6 chip is not totally NAB, it can be
set to low, medium or high AB. I keep it at the low setting, and this
has produced good results. The problem with ABG chips is that part of
each pixel is used for this purpose.of bleeding off excess electrons
to prevent saturation. This means the pixel size is reduced,
resulting in less sensitivity and a reduction in well capacity. The
ABG does work very well for those who use their cameras to take
"pretty pictures", but it reduces the chip's ability to get good
photometric data.
(Editor's Note: Those of you who already own ABG cameras can still do
good photometric work, you just have to follow special rules and your
camera's sensitivity is somewhat compromised. We will have an
article on that issue in a future CCD Views.)
When choosing a camera, you must also be certain that your camera
is a match for your scope. There have been MANY, MANY discussions
about pixels, focal lengths, etc. on user lists. I will not go into
this here except to say that if you are going to err, then err on the
side of oversampling rather than undersampling. If your images have
square stars, then you are undersampled. My particular set up
employs an LX200 12". At F/10, I get nice round stars with my ST6. At
F/6.3(using a focal reducer) I am just teetering on the brink of
undersampling. Since I have been getting good photometric results
when imaging at F/6.3 I continue to use that focal ratio, because my
imaging time is reduced and I get a larger FOV.
Once you have your scope and camera, the only other required piece
of equipment is a V filter. The filter will screw into the nose of
your camera. This filter is used to record stellar magnitudes in the
480-650 nm (peaking at about 540 nm) range of the visible spectrum.
(As you may have determined, a camera that has a high QE in this
range would be optimal.) This filter best replicates what the human
eye sees, so it keeps CCD observations on a close plane with visual
observations. If you were to perform BVRI photometry, you would need
the entire set of 4 filters so you could record magnitudes for each
of these ranges. When doing V photometry, you need to take only 1
image of the variable and use your software to determine the V
magnitude. If you were to perform BVRI, you would need to take 4
images - 1 through each filter- and then use your software to
determine the magnitude as portrayed in each image. (There is
actually much more to it than that, but that is another story). There
are circumstances in which you can do unfiltered photometry, but it
is highly recommended that for AAVSO variable star observations, you
use a V filter.
So you have your telescope, CCD camera, and V filter. The fun now
begins. I might interject a side note here. Like me, you probably
spent some time observing variables visually before making the
decision to enter the realm of CCD photometry. I began observing
variables about 20 years ago. I started out with my trusty C8, and
spent many hours using setting circles and craning my neck just to
get the variable into the field of view. I now have my GOTO LX200
that does the entire finding for me. A CCD chip is quite small, and
it is much more difficult to place the variable's FOV onto the chip
than it is into 40 mm eyepiece. The LX200 places the variable onto
the chip every time. Whereas some might say that this takes the fun
out of it, I say that I would rather spend my fun time imaging
variables and doing photometry. I can accomplish more science in one
observing session if the effort of finding the variable is done by
the scope. If you can afford it, I would highly recommend a GOTO
mount for photometry.
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6. FAINTEST AAVSO CCD OBSERVATIONS
For you trivia buffs, here are the faintest observations found in
the AAVSO International Database.
Unfiltered: HT CAS at 19.3 on Jan 14, 1997 by DRG
B: VX UMA at 18.78 on April 5, 2001 by ZRE
V: HT CAS at 21.500 on Sep 22, 1996 by ZRE
R: FO PER at 16.45 on Nov 7, 2000 by SFK
I: BHS 1804+67 at 15.6 on Sep 18, 1996 by RR
Currently Ron Zissell (ZRE) holds the record for the faintest
overall observation in the AAVSO database for his 21.5 observation.
Here are some notes from him about the observation, made with a 24"
telescope and Photometrics CH250 CCD:
"My CCDV observation of HT Cas at 21.500 should have an error
bar of +/- .1 mag. The HT CAS observation occured during
mid-eclipse of the system. The eclipse lasts for only about
six minutes. The exposures for that eclipse were 180 seconds.
Unfiltered faintest mag is a function of the color of the star.
A Mira variable at Mv=21.0 may have an Unfiltered (V+R+I) mag of 16.5
which is an easy observation. If one quoted the Mpg for that star
one would get an even fainter magnitude than 21.0"
- Ron Zissell (ZRE).
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7. AIP4WIN & MAXIM DL PHOTOMETRY BUGS & PATCHES
If you are using AIP4Win or MaximDL for your photometry please be
aware that some versions of these packages have bugs regarding the
calculation of julian dates. Fixes for both of these software
programs have been posted online at the URLs below. Please update
your software where appropriate.
MaximDL 3.03 Update: http://www.cyanogen.com/upgrades/3x/
AIP4Win 1.3.14 Update: http://www.willbell.com/aip4win/AIP.htm
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8. CCD POINTS
As we stated in the last issue, we are beginning a new system of
assigning points to each CCD observation. These points are based on
the quality of observation and the difficulty of the target. Lots of
"variables" go into the formula (pun intended), which is still secret
but we _may_ be pursuaded to reveal some hints in the future! Stay
tuned.
Remember, these points are only for fun and are not official in any
way. So without further ado..
3093 RSE ROBINSON, STEPHEN E. MD, ROCKVILLE
1345 DRG DIETHELM, ROGER SWITZERLAND, RODERSDORF
828 OFA O'FEARGHAIL, AENGUS IRELAND, DUNSHAUGHLIN
684 GKA GRAHAM, KEITH A. IL, MANHATTEN
651 ZRE ZISSELL, RONALD E. MA, SOUTH HADLEY
630 WJD WEST, JERRY DOUG KS, MULVANE
563 SFK SCHEDER, FRANK L. MD, LEONARDTOWN
555 WGR WALKER, GARY MA, SHERBORN
512 OAR OKSANEN, ARTO FINLAND, MUURAME
428 MDW MACDONALD II, WALTER J. CANADA, OSHAWA ONT.
304 OCN O'CONNOR, STEPHEN D. CANADA, NORTH MONTREAL, QUE
176 COO COOK, LEWIS M. CA, CONCORD
172 HDU HURDIS, DAVE RI, NARRAGANSETT
152 JEA JENKINS, ALLAN C. SC, TAYLORS
147 MTK MICHALIK, TOM VA, LYNCHBURG
130 WRX WILLIAMS, ROGER MI, KALAMAZOO
106 NMI NICHOLAS, MIKE AZ, GLENDALE
95 SDY SCHARNHORST, DANNY GERMANY, ERFURT
61 RIX RICHARDS, THOMAS J. AUSTRALIA, ELTHAM, VI
50 CLF COHEN, LOUIS MA, CAMBRIDGE
48 SDB STARKEY, DONN RAY IN, AUBURN
22 SYZ SANCHEZ, CRISTINA SPAIN, GIJON-ASTURIAS
16 GBL GARY, BRUCE L. CA, SANTA BARBARA
14 RGY RUBRIGHT, GARY PA, LANCASTER
14 CJI COLOMA, JOSEP MARIA SPAIN, BARCELONA
13 MDA MORTON, ART WA, EDMONDS
12 MAU MACIOLEK, ARDIS MI, PLYMOUTH
10 MZG MAINTZ, GISELA GERMANY, BONN
10 RZD RODRIGUEZ, DIEGO SPAIN, VILLALBA, MADR
10 MHE MAIER, HARALD GERMANY, MUNICH
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CCD Views is published bimonthly and when circumstances warrant
via e-mail. An archive is available at http://www.aavso.org/ccdviews/ .
Please send comments and suggestions to aaronp@aavso.org.
To receive CCD Views via e-mail send a message to
majordomo@aavso.org with "subscribe ccdviews" in the body of the
e-mail. To unsubscribe, place "unsubscribe ccdviews" in the e-mail.
The AAVSO has many free online publications including "Eyepiece
Views", a similar newsletter intended for visual observers. To learn
more and subscribe visit: http://www.aavso.org/mailinglists.stm
Good observing!
Aaron Price, AAVSO Technical Assistant (PAH)
Gary Walker, Chairman of the AAVSO CCD Committee (WGR)
Copyright 2001, American Association of Variable Star Observers
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THE AMERICAN ASSOCIATION OF VARIABLE STAR OBSERVERS
25 Birch Street, Cambridge, MA 02138 USA
Tel. 617-354-0484 Fax 617-354-0665
http://www.aavso.org
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