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2000-2001 Procedures 2004-2005 Procedures

Below are the procedures for the 2000 apparition of Tempel 1. Please be sure to look at the procedures and current objectives for the 2005 apparition.



Table of Contents

Observing Objectives

There are 2 observing objectives for this science program:

  1. Record brightness changes: We want to have nearly continuous coverage of the comet's brightness changes as a function of distance from the sun. This will tell us how much dust is being produced and will allow us to see if the comet undergoes any outburst activity. Photometry of Tempel 1 from photometric R images taken by STSP observers will supplement the rotation curve being produced from professional, large telescope data.

  2. Record coma features and dust activity: We want to model the dust coma to get information about dust ejection velocities, dust particle size distributions and when the activity turns on and off in the comet. This is done by looking at many images of the dust coma and tail spread over a long time period (months), but at high time resolution. The high time resolution images give us information about the smallest dust coming from the nucleus and the long time bases give us information about the slower moving, large dust grains. Professional astronomers usually have to compromise on the models because it is impossible to get both long-term coverage and frequent images (this would mean observing every night)! For this modeling, we also need to see as much of the dust coma as possible. To do this, we need a wide field of view. Usually the large professional telescopes do not have a very large field of view. Small telescope observers can make a huge contribution in this area! However, care must be taken to collect the data properly and in a uniform manner so that it will be possible to put together observations from different observers into a model.

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Outline of Observing Procedures

For each observing session, you will need to:

  1. 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.
  2. Take dark frames for all flat fields, calibration frames, and comet frames.
  3. Take flat fields using each filter.
  4. Take calibration frames using each filter: Image standard UBVRI stars, needed for accurate photometry.
  5. Take exposures of the comet using one of the following filter systems:
    • Bessel (Kron-Cousins) photometric V and R filters. If your system has as infrared-cutoff filter, it must be removed for comet observations!
    • No filter.
  6. Record all of your exposures, including bias frames, dark frames and flat fields in an observing log.
  7. Format all of your images as FITS files.
  8. Send a copy of your observing log.
  9. Transfer all of your raw, unprocessed, FITS files to Stef McLaughlin at the University of Maryland. Please contact stefmcl[at]astro.umd.edu for anonymous file transfer (ftp) instructions.

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Observing Logs

Observing logs are required or we cannot use your data! Your other option is to put observing-log information into the FITS header for each of your images! 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:

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Procedures for Dark/Bias Frames

Bias Frames for CCD cameras that are not thermoelectrically cooled:

Take 5-10 bias frames (zero time exposures without opening the shutter) each time you observe. Record each bias frame in your observing log. Send each bias frame to us in FITS format. We need your bias frames for consistent data reduction.

Dark Frames for CCD cameras that are not thermoelectrically cooled:

Take 3-5 dark frames using an exposure time of 1-2 minutes. Record each of these dark frames in your observing log and send your dark frames to us in FITS format. We need your raw dark frames for consistent data reduction.

Bias Frames for thermoelectrically cooled CCD cameras:

Thermoelectrically cooled CCDs perform automatic biasing for each exposure. Therefore, the CCD software for most of these CCDs does not allow bias frames (a simple CCD readout using a zero exposure time and a closed shutter) to be taken. However, it is scientifically useful to make 3-5 dark frames using the shortest time exposure allowed by your CCD software. We will use these frames to estimate a bias frame.

Dark Frames for thermoelectrically cooled CCD cameras:

For thermoelectrically cooled CCDs, the amount noise is very dependent on the time of day of the exposure (i.e., the ambient temperature) and on the length of the exposure. Therefore, manufacturers of these CCDs recommend taking a dark frame immediately before or after each light frame with the same exposure time as the light frame.

  • If your CCD software automatically takes and subtracts the dark frame from the light frame, you may use this feature to avoid sending us dark frames. Please note in your observing log if the dark frame was automatically subtracted.
  • If your CCD software does not automatically take and subtract the dark frame, make 3-5 dark frames using the same exposure times that you used for light frames. Record each of these dark frames in your observing log and send your dark frames to us in FITS format. We need your raw dark frames for consistent data reduction.

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Filters

We need the observations made through standard photometric broadband V and R filters (Kron-Cousins, Johnson, or Bessell). If your filter system has an infrared-cutoff filter, it MUST be removed when observing with photometric V and R filters! Depending on your site, the V or R filter may take longer to expose. CCDs usually have a greater efficiency in the V band, but the comet is usually brighter in the R band since the dust scatters the sunlight preferentially in the red. Also, the phase of the moon will effect your filter exposure times.

We will accept observations taken without filters for total visual magnitude studies.

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Flat Fields (white screen flats vs. sky flats)

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). Send each unprocessed flat field to us in FITS format. You may send an "averaged" flat field but we need your raw, unprocessed flat fields for consistent data reduction.

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.

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Comet Exposures

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:

  1. 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.
  2. 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:
    1. You can take multiple, short exposures (under 1 minute) without manual guiding, or
    2. 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:

  1. 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.
  2. 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.

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Standard Stars and Calibrations

We need each observer to make at least 1 set of standard star observations using the same filters from comet images. Since these sets will be used for calibrating ALL of your comet images, you must make these images on a "photometric night": a night when seeing is good and there are no high, thin clouds! You may take standard star sets during one of your comet observing runs or during a run dedicated to standard star work.

You must choose standard stars from a list compiled for comet Tempel 1 observing. Go to the list of standard stars, which includes background and instruction for making calibration images.

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Data-Formatting Requirements

FITS!

We need all the data to be sent to us in FITS format. When you send the data, 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)
  • END

What data to send?

For each observing session, we will require the following from you:

  • Bias frames, if applicable.
  • Dark frames.
  • Flat fields.
  • Comet images.
  • A copy of your observing log or observing information loaded into the FITS header.

How to send your data to us?

FTP INSTRUCTIONS:

  1. Launch an FTP session.
  2. FTP to ftp.astro.umd.edu.
  3. Enter "anonymous" for the userid.
  4. Enter your email address for the password.
  5. Change the remote directory to "incoming/stefmcl".
  6. Make sure your FTP session is in binary mode.

  7. Put/copy your images into "incoming/stefmcl". To avoid writing over other observers' images, use the following format for FITS file names:

    mmddooxfz.fit

    where
    mm = 2 digit month of the observation (UT)
    dd = 2 digit day of the observation (UT)
    oo = observer's initials
    x = type of exposure:
       i=comet image
       b=bias
       d=dark
       f=flat
       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).

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Observing Tools on the WWW

Ephemeris Generators

Time Links

Sky Surveys

Astronomical Catalog Search Engines

  • Simbad from Centre de Données astronomiques de Strasbourg.

Photometry References

Comet Links

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STSP Coordinator: Stef McLaughlin
Webmaster: Elizabeth Warner
Last Updated: Wednesday August 29, 2007

 Deep Impact Small Telescope Science Program