Table of Contents
Outline of Observing Procedures
For each observing session, you will need to:
- If your CCD camera is not thermoelectrically cooled, take bias frames. If your CCD camera is thermoelectrically cooled, you do not need to take bias frames.
- Take dark frames for all flat fields, calibration frames, and comet frames.
- Take flat fields.
- Take calibration frames using each filter: Image standard UBVRI stars, needed for accurate photometry.
- Take exposures of the comet.
- Record all of your images, including bias frames, dark frames and flat fields in an observing log.
- Dark/bias subtract and flatten your comet images and save your reduced data in FITS format.
- If your data is for photometry of the dust coma, perform photometry. Or you may send your reduced data to us and we will perform photometry.
- Zip your reduced data files and any photometry results for uploading.
- Login as an observer and submit your observation data via the form. If you have never logged in before, you can sign-up to be an observer by filling out an observer profile form.
- Please keep a copy of your raw data! If we find discrepancies, we may need to go back to your raw data.
[Back to Top]
Observing logs are required or we cannot use your data! Actual observations of the comet and well as your flat field and dark frame images must be recorded in your observing log. To perform scientifically meaningful studies of your images, we need the following information about your observations:
- Location of observations (latitude and longitude).
- Telescope type, size, and f-ratio.
- CCD type, pixel scale (arcsec), read noise, and gain.
- Filter characteristics (e.g. central wavelength, FWHM, Was the infrared-cutoff filter used OR removed?).
- For each observation: Filter, exposure time, start time (UT), and CCD temperature.
- Conditions during observation: seeing, clouds, and moon phase.
A downloadable observing log is available:
[Back to Top]
For photometry, we need the observations made through standard photometric broadband RI filters (Kron-Cousins, Johnson, or Bessell). If your filter system has an infrared-cutoff filter, it MUST be removed when observing with photometric RI filters!
We will accept observations taken without filters for total visual magnitude studies.
[Back to Top]
Take 3-5 flat fields for each filter you plan to observe with. Each flat field should have a signal level approximately 2/3 the level of the linear range of your detector. Therefore, you will need to vary the exposure times between filters since CCD chips usually have greater efficiency in the R band.
Record each of your flat fields in your observing log. Please indicate in your log what you used as your flat field (an illuminated white screen, the twilight sky, the dark sky).
Some flat field methods:
- White screen: Many professional astronomers take their flat fields using a white screen illuminated by lamps on the inside of a dome. This will work well to flatten the image to about 1%. This has the convenience of being able to take calibrations in the afternoon, or when conditions are poor, and it will give high signal to noise flats because the light levels are high. However, the screens often do not accurately match the illumination pattern of the night sky, nor the colors. But, for small telescopes which are often fast systems with a wide field of view, this method tends to provide better results than twilight flats which often have too many stars.
- The sky + white plexiglass: This combination produces results that are as flat as dome flats and eliminates the need to remove stars. At twilight, point your telescope at the zenith and place a 1/8-inch to 1/4-inch thick piece of white plexiglass over your telescope. You may need to integrate longer than a dome flat since the signal to noise may be lower.
- The sky: Accurate flat fields are very important when trying to detect the faintest portions of the coma and tail of comets. To very accurately flatten the images, you need to use the sky as a flat field. You can take flats during the twilight, beginning just after sunset in your bluest (V) filters, and progressing toward redder (R) filters as the sky darkens. Be sure to have the telescope tracking on and to move the telescope significantly between each exposure so that the stars can be removed later. Take a minimum of 3 flats for each of the filters you plan to observe with. Ideally, at least 5 good flats per filter are needed, each with a signal level approximately 2/3 the level of the linear range of your detector. Using these flats will more accurately represent the illumination pattern of the sky. However, the colors will be different than the dark sky. A small correction can be made for this by making flats out of dark sky images. Any time you take an exposure during the dark part of the night if there is no comet or extended object or very bright star in the frame, these frames can be combined to remove stars and be used as a correction to the twilight flat. Be sure to move the telescope (dither) by an amount a few times the size of the stars in between each picture taken at the same place in the sky.
[Back to Top]
A general note about comet exposures:
If the exposures of the comet are less then 1 minute, then the images can later be added together in two ways:
- Assume the comet is well-guided then add the images to create a deep image of the comet for coma and tail studies. This deep image preserves the comet's components and shows star trails.
- Assume the stars are well-guided then add the images, creating a comet trail. This addition method allows us to accurately model the tail's brightness and enables us to remove the stars which interfere with the tail's brightness.
Details about comet exposures and tracking:
- If your tracking software allows you to enter offset rates to sidereal, you should use offset rates based on the right ascension and declination rates of change for the comet at the time of the observation. Right ascension and declination rates, in arcsec/sec, are available from the JPL Ephemeris Generator. Make sure you convert from arcsec/sec to the units used by your tracking software, usually degrees/hour. Offset rates will allow you to take longer, multiple exposures of the comet. Exposure times of several minutes should be possible if your equipment tracks accurately.
- If you cannot use offset rates:
- You can take multiple, short exposures (under 1 minute) without manual guiding, or
- You can take fewer but longer exposures (over 1 minute) with manual guiding. However, we recommend short exposures to minimize tracking errors.
- Finally, if your CCD and tracking software allows you to automatically track and accumulate images, you may use this feature. However, you must send us a copy the offset files that the software used and indicate how dark frames were taken!
How to calculate an ideal exposure time:
Generally, the exposure time should be short enough that neither the stars nor the comet trail. This will depend on the rate of motion of the comet, the size of your pixels, and the seeing or atmospheric conditions. At large telescopes, plate scales are small - around 0.2 arcsec/pixels, and the seeing is often quite good, between 0.5-1 arcsec. Normally, one might limit the exposures so that the comet (or stars, if guiding on the comet) won't trail by more than half the seeing. The exposure time then is calculated by dividing the seeing size by the rate of the comet motion in arcsec/sec which you can get from the JPL Ephemeris Generator.
How to determine the total exposure time:
The total exposure time depends on which program objective you are trying to contribute to:
- If your objective is to provide a brightness estimate of the nucleus as a function of distance from the sun, we need to know these numbers to 5-10% accuracy. Several short exposures (one minute or less) for a total of 3-5 minutes would be adequate.
- If your objective is to image the coma and tail for features and dust modeling, we need to see the faintest extend of the coma and tail. This requires combining many short exposure images. For example, for this purpose on a 2-m class telescope, one might try to get a total exposure time of 1 hour or more. However, any coma and tail observations are useful! The fainter they go and the more structure that is seen, the more useful they are. Again, several short exposures (one minute or less) for a total of 5-10 minutes would be useful.
[Back to Top]
Standard Stars and Calibrations
We suggest imaging at least one Landolt Equatorial Standard star during each night of observing. Images before, during, and near the end of your run should suffice. At this time, we suggest the following Landolt standard:
||13 35 24
||-00 23 47
||13 37 17
||-00 38 05
If you wish to select your own stars, you may select from either a subset of stars with RAs near the comet or search the entire Landolt Equatorial Standards (although we suggest picking a standard near the RA of the comet):
Landolt Equatorial Standard Stars
[Back to Top]
Please save your reduced data in FITS format. When you send the data to us, please include information about the program you used to generate the FITS images.
The following FITS records should be contained in the FITS header:
- SIMPLE = T
- BITPIX (for example, -32 for single precision floating point or 16 for 8 unsigned integer)
- NAXIS = 2
- NAXIS1 (# pix in slowest changing axis of your array)
- NAXIS2 (# pix in fastest changing axis of your array)
- DATE-OBS (YYYY-MM-DD observation start date, in UT)
- TIME-OBS (HH:MM:SS observation start time, in UT)
- EXPTIME (exposure time in sec)
- SET-TEMP (CCD temp setpoint in Celsius)
- CCD-TEMP (CCD temp at start of exposure in Celsius)
We suggest the following format for FITS file names:
mm = 2 digit month of the observation (UT)
dd = 2 digit day of the observation (UT)
oo = observer's initials
x = type of exposure:
s=standard star image
t=standard star bias
u=standard star dark
f = filter used:
N = no filter
C = clear
V = photometric V
R = photometric R
I = photometric I
1 = photographic R (as in RGB)
2 = photographic G (as in RGB)
3 = photographic B (ad in RGB)
z = alphabetical image number:
a = first image,
b = second, etc.
The file name is 9 characters long! if this presents a problem for you, shorten the month to a single digit ( 05 -> 5).
What data to send?
For each observing session, we will require the following from you:
- Reduced comet images in FITS format (a single image or zipped collection of images) and/or photometry measurements.
- size of photometric aperture at the comet
- circular or square aperture?
- estimated magnitude and error
- UT of photometric measurement
- A copy of your observing log.
- Optional: Submit a displayable image for the Gallery. This image should be a jpeg or gif image, no spaces in the file name, and if text is embedded that the spelling is correct.
How to send your data to us?
We have now setup the STSP site so that the zipped data may be submitted through a web form. You must first register. Then, when you login the next time, you will be taken automatically to the data submission page. If you have not yet registered, please do so, because the system will only accept data from approved profiles (and it takes about a day to approve).
[Back to Top]
Observing Tools on the WWW
Astronomical Catalog Search Engines
- Simbad from Centre de Données astronomiques de Strasbourg.
[Back to Top]
STSP Coordinator: Stef McLaughlin
Webmaster: Elizabeth Warner
Last Updated: Wednesday August 29, 2007