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CCD Views #315

25 Birch Street, Cambridge, MA 02138 USA
Tel. 617-354-0484 Fax 617-354-0665

C C D V I E W S #315
September 24, 2003

Table of Contents
1. Introduction
2. Get Started With Blazars!
3. New Ways to Estimate and Report Error/Uncertainty
4. Computing Limiting Magnitude
5. Return of the V838 Monster
6. Using USNO-B With Astrometrica

1. Introduction</b>

The biggest news of the summer was probably Berto Monard's (MLF)
discovery of the optical afterglow to GRB030725. To read all about the
first time an amateur has discovered a GRB afterglow, with links to
much of the press coverage it spawned, visit this URL:
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Also, please stop by this URL and take our quick survey for the development
of the National Virtual Observatory (NVO). We need as many responses as we
can get. The NVO is likely to be a useful tool for all CCD variable star
observers with its potentially easy access to data and photometry. Please
forward the URL to any local clubs or lists you belong to.
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This issue of CCD Views has been a real collaborative effort. I'd like
to specifically thank Arne Henden (HQA), Bruce Gary (GBL), Gary Walker
(WGR), Arto Oksanen (OAR), Chuck Pullen (PCH), Steve Robinson (RSE) and
Peter Nelson (NLX) for their advice in putting together this issue.

Many of you have probably heard about the illness of our Director,
Janet Mattei. In lieu of flowers please consider donating blood at your
local hospital or blood donation center. She is proof that your donation
could save a life!

Aaron Price (PAH)
On behalf of Janet Mattei and Gary Walker, Chairperson of the CCD Committee


In the last issue of CCD Views (#314) we introduced the AAVSO
International High Energy Network's collaboration with the GLAST
Telescope Network (GTN). The core goal of the collaboration is to
consistently monitor blazars that are going to be studied with the GLAST
& SWIFT orbiting observatories and other ground based telescopes. Below
is a list of blazars we would like to begin observing.

Object Name - Charts (F-scales have CCD tables)
BL Lac - D, DR, E, ER, F, FR
W Com - F, FR
3C 66A And - F, FR
Mark 421 - E, ER, F, FR
PKS2155-30 - E, ER, F, FR
OI+158 - F, FR (a.k.a PKS 0375)
MARK 501 - F, FR
2344+514 - F, FR (a.k.a 1ES 2344+514)
PKS0716 - F, FR

All these charts are *new or updated* except PKS0716, which was
released in March. Note this is just a portion of the blazar program.
More objects and charts will be coming out in the months ahead.

Please monitor these objects as part of your regular observing
program. At least use a V filter and if possible take data in R and I.
The first goal of this project is to establish a baseline of behavior
for the objects in order to eventually notify observers
of outbursts using GRB-style alerts. We'll also coordinate campaigns
the satellites and other members of GTN. There is a lot of work to go
around for the next few years but it all begins with a foundation.
Please observe these objects and report your observations to the AAVSO!

Most of these blazars are faint, but some of the bright ones (such as
Mark 501) may exhibit some fuzziness as the host galaxy is revealed in
your images. In those cases be sure to keep the sky annulus outside of
the galaxy's light. The measuring aperture should include only the
nucleus of the galaxy. We are currently studying techniques to handle
these situations and more precisely define "nucleus" as used here. A
future CCD Views will go into in-depth discussion on the topic. For now
it is important to do your best and *be consistent* with your annuli as
much as your seeing permits.

AAVSO member Bruce Gary has created a couple of web pages to help get
started with this project:

"AAVSO Blazar List: Useful Information for Observers"
Locations, transit times, and magnitude estimates.

"Blazar 'W Com' Monitoring Feasibility Demonstration"
W Com observations as a way to discuss all aspects of blazar monitoring.

Below is a sampling of recently received observations of these objects:

BL LAC SEP 01.1160 14.61 WDZ CCDV ERR: 0.06
BL LAC SEP 10.1769 14.95 RSE CCDV ERR: 0.024
BL LAC SEP 24.1764 14.6 SXN Visual
3C 66A AND SEP 11.0153 14.85 OAR CCDV ERR: 0.02
3C 66A AND SEP 14.9034 14.67 MMG CCDV
3C 66A AND SEP 17.2069 14.40 RSE CCDV ERR: 0.024
3C 66A AND SEP 21.2597 14.5 SXN Visual
OI+158 SEP 21.4208 15.7 PVA Visual
MARK 501 SEP 17.0576 13.57 RSE CCDV ERR: 0.008
MARK 501 SEP 24.0403 13.7 SXN Visual
2344+514 SEP 14.2951 15.63 GBL CCDV
2344+514 SEP 24.1739 15.6 RSE CCDV ERR: 0.038

Special thanks to these observers for the above observations:


Download Charts:
AAVSO International High Energy Network:


In CCD Views #314 we published a way to derive uncertainty
measurements (aka error) for your photometry. Here is an additional
way to measure uncertainty with even less work:

* If your CCD software supplies SNR, simply supply 1/SNR as your
measurement uncertainty.

* If it does not give SNR, then use the formula we provided before,
except that we have simplified it to: 1/sqrt(net_counts*gain)

That's it!

The measurement uncertainty just described is a way to report the
"precision" of your measurement. At some future date we may suggest a
method for estimating "systematic error", which is needed for
combining with precision to obtain measurement "accuracy."

For those of you who apply transformation coefficients (derived
within the last 12 months) then put "Transform: Yes" in the "Comments
Explained" field. This will tell the person analyzing our data that your
error estimates are probably more accurate.

Remember to report measurement uncertainty in the "Comments
Explained" portion of your observation report using the term "Err:"
followed by the uncertainty value. If you are using WebObs new fields
have been added for this data. A new version of PCObs with support for
this is in development and will be published soon.

A more detailed discussion about estimating uncertainty, the theory
behind it, and several other ways to do it is available in our CCD
Observing Manual at this URL:


For some observing projects, such as GRB afterglow hunting, reporting
the limiting magnitude of your observation is important. But where do
you draw the line between a star and background fuzz? We've decided to
standardize on the magnitude of an object with a SNR of 3. This is the
usual definition of limiting magnitude for photometry. However, for
some projects a value of SNR that is as low as 3 doesn't make sense,
so feel free to use whatever SNR you feel appropriate. However, if you
use an SNR of *other* than three, please let us know in the "Comments
Explained" portion of the observation report.

Here is a simple way to calculate limiting magnitude with any
aperture photometry program that displays SNR. Use this formula after
selecting any faint star (without nearby stars within the reference

M_limiting = M + 2.5 * log10 (SNR/3).

where M = Magnitude of star and SNR is the SNR of the star.

This gives you a 3-sigma limiting magnitude.


V838 Mon (the "light echo" object that appeared a year ago) is still a
very interesting object. It recently has brightened by 0.5 mag (V) and has
been measured by Arne Henden at V = 15.50, B-V = +1.00, V-R_c = +2.70,
R_c-I_c = +3.25 (formal errors of 0.01 mag). The spectra has changed as
the cool dusty atmosphere of the outbursting star begins to dominate
shorter wavelengths and is now encroaching on the V band. This could be a
very exciting time to monitor V838 Mon. It is now becoming viewable in the
morning sky. It can be a challenging object to observe in this state. Make
sure your sky annulus is larger than the light echo so you are only
measuring the sky. Use a very small measuring aperture for V838 Mon itself
since there are a couple of close neighbors.

More information on this development is in IAUC 8202:
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V838 Mon Variable Star of the Month:
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AAVSO Published V838 Mon observations (IBVS 5315):
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The program "Astrometrica" has recently been upgraded to
support the USNO-B catalog. The following article was
contributed to CCD Views as a tutorial for Astronometrica users
but it also includes useful information for all CCD users (such
as how to get V from USNOB data).

"Astrometry and Photometry From CCD images with ASTROMETRICA
v4.1.2.310 and USNO-B Catalog Reference Stars"

By David A. Cornell and Herbert Raab

Recent release of the USNO-B catalog enables improved precision
of astrometry and photometry from CCD images. The large new
catalog is not generally available for distribution, but users
can request specific reference star set for download via the
Internet. The purpose of this note is to tell how to download
these star sets and use them to process CCD images with

The first step is to obtain CCD images of the object and star
field. If filtered magnitudes are desired, be sure to insert
the proper selection (e.g., "V" filter) in the optic path of the
telescope. If long exposures are required, owing to faintness
of the object, it may be necessary to correct for flat field
effects. Save the images in a 16-bit format, in order to
preserve precision in data reduction. For this purpose the
*.FIT format is suitable, and is standard for some CCD camera
control software like CCDSoft ( SBIG
file formats may also be suitable, but employ some compression.

The second step is to obtain USNO-B catalog data on stellar
positions and magnitudes for your star field. To do so, go to
th e USNO Catalog Archive on the Web at Specify parameters for
the image star field and request data extraction from the USNO
B1.0 catalog in the form of a catalog list but not image data.
Check all rows in the catalog list box, except plate file index.
To limit the number of stars in your request, and to use
brighter stars whose positions and magnitudes are known to
highest precision, limit the range of magnitudes. (Limiting
magnitudes to the range V=10 to 15, one finds about a dozen
reference stars in a square star field measuring 9x9 arc
minutes. Change the range to suit your need.) Once you have
the reference star list, edit/copy and edit/paste the list into
a text editor like MS-Notepad. You may append as many star
field lists as you wish. You need not delete headings in the
lists. Once you have the reference star lists assembled in a
text file, save them as USNO-B.DAT in a directory that you
expect to refer to in ASTROMETRICA. When it searches for
reference stars, ASTROMETRICA knows to ignore headings.

Once you have the CCD images and the USNO-B.DAT catalog data
file, the final step is to execute ASTROMETRICA (available from The following presumes that you have the
program configured for your telescope in the Files/Settings
menu. To prepare for analysis, set the Files/Settings/Program
menu to configure the program to expect the USNO-B 1.0 star
catalog. Also set the Files/Settings/Environment menu to point
to the directory in which you saved the catalog file USNO-B.DAT.
Now load an image file (checking the time and date stamp for
correctness) and select the Astrometry/Data Reduction option.
ASTROMETRICA loads objects in the designated star field,
including the reference set in your data file. After comparing
your image and the reference data set, ASTROMETRICA displays
circles around stars that correspond to those in the reference
set. Next single click on the object whose astrometry and
photometry you wish to calculate. The display shows calculated
RA/DEC and magnitude, together with a display of the digital
data. Enter the name of the object in the appropriate box and
click on accept if you are pleased with the results. This
completes analysis of one image. The file MPCreport.txt saves a
concise line for each object. Another file Astrometrica.log has
many related details.

Readers may wish to be aware that the USNO-B catalog does not
list V magnitudes at all, but only B, R, and I. How then does
ASTROMETRICA calculate V magnitude for objects in the catalog?
Herbert Raab, ASTROMETRICA author explains that the program reads
the B and R values from USNO-B data. Then the value of V is
calculated from the formula, V = B - (0.571*(B-R)+0.03). This
formula works well for main sequence stars, but may not for
others. If the residual in magnitude for an object is higher
than that set in the program settings (default limit 1.0
magnitude), that object is not included in the reference set.


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Good observing!

Aaron Price, AAVSO Technical Assistant (PAH)
Gary Walker, Chairman of the AAVSO CCD Committee (WGR)

Copyright 2003, American Association of Variable Star Observers
25 Birch Street, Cambridge, MA 02138 USA
Tel. 617-354-0484 Fax 617-354-0665

AAVSO 49 Bay State Rd. Cambridge, MA 02138 617-354-0484