Spanish Monitoring of Comets:

Making sense of amateur photometric data

    Amateur astronomers have always represented an important observing group in cometary astronomy. Much of our knowledge of cometary light curves has come from amateur data, initially in the form of total visual magnitude estimates and now increasing in the form of CCD observations. The increasingly widespread use by amateur astronomers of CCD cameras of excellent sensitivity and good cosmetic quality has revolutionised astrometry, allowing far more intensive astrometric monitoring of comets to be carried out down to magnitude 18 and fainter, with a corresponding enormous increase in the quality of calculated orbits. Although amateur CCD photometry is extensively available in the Internet, its use has been less widespread. The reason is the lack of standardisation in the way that this data is taken that leads to amateur CCD light curves having enormous dispersion.

    All amateur CCD photometry is aperture photometry, but it is seen that neither does it represent well the equivalent of m1 (total visual magnitude), even with a large aperture, nor is it close to the definition of m2 (the nucleus magnitude). The problem is examined using data from the Spanish Comet Observers Group archives to show that by careful standardisation of data acquisition amateur CCD data can produce high quality, well-sampled and physically meaningful light curves. Examples are presented of the results for recent comets including C/2000 WM1 (LINEAR), C/2001 A2 (LINEAR), P/2001 TU80 (LINEAR-NEAT), and 51P/Harrington. A possible precessional signature is identified in the light curve of C/2001 A2 (LINEAR).
 
1. Introduction

    Over the last few years there has been an increasing use of CCD cameras by amateurs astronomers to monitor comets on a regular basis. A CCD camera attached typically a 20 or 25-cm Schmidt/Cassegrain telescope is the preferred configuration (although an increasing number of amateur astronomers use telescopes of 30-cm, 40-cm and even as large as 60-cm). This gives a typical image scale of approximately 2 arcseconds/pixel and a field that may be half a degree or larger. Even from a moderately light-polluted urban site a 20-cm telescope + CCD is capable of detecting comets of magnitude 18 with sufficient s/n to make usable astrometry and photometry.

    Much of the amateur effort has gone into taking astrometry. Over the last 5 years the quantity of astrometry of even relatively faint comets reported to the Minor Planet Center (MPC) has increased many fold. Although a significant part of this increased database is due to professional observatories such as LINEAR, a large part is due to amateur observers. An example of this is seen by comparing the astrometric coverage of a bright pre-CCD comet (27P/Crommelin) or 23P/Brorsen-Metcalf, with recent comets.
 

    Apart from C/2001 TU80 (LINEAR-NEAT) which only became brighter than magnitude 17 when it suffered a major outburst the typical rate of astrometric data acquisition is >100 points per month of visibility (i.e. not including solar conjunction) down to magnitude 15. In most cases the astrometric measurement is associated with a photometric measure. The MPC archive has already been used to produce a catalogue of nucleus magnitudes for Jupiter family comets (Licandro, 2002, Doctoral Thesis, University of La Laguna), but Licandro (2002) notes that a severe handicap with the data is its lack of homogeneity.

    Data in the MPC archive is given as "N" for assumed nucleus magnitude, or "T" for total magnitude. However, the total magnitude rarely is close to the total visual magnitude (m1) which would require a photometric aperture as large as the visible coma and Licandro (2002) has shown that only for low-activity comets at large heliocentric distance is "N" equivalent to "m2", the magnitude of the bare nucleus. Thus although CCD data potentially permits detailed light curves to be prepared for many comets down to V~ 18, the highly variable quality of the data means that cometary photometry taken from the MPC database is often of very limited use. It is not unusual for the magnitudes reported on MPCs and MPECs to show 3 magnitudes or more of dispersion.

2. The Spanish "Cometas_Obs" comet observing list.

    A nucleus of Spanish CCD equipped and visual comet observers started to submit observational data to a new comet observing web page at the Instituto de Astrofísica de Canarias in 1988. One of the advantages of this web page was that it permitted observers to submit their data in Spanish and translated into English by the webmaster thus avoiding linguistic problems that inconvenience many Spanish amateurs for whom English may be their third or even fourth language. The increasing traffic of data to this page led to the suggestion that a formal comet observing mailing list in Spanish should be created to allow observers to share data and information. Since the list’s formation in April 2001 it has increased to more than 70 subscribers, most of whom are active observers, and to mid-January 2002 over 4000 mails had been distributed within the list.

    Although the majority of subscribers to "Cometas_Obs" are Spanish, there are also active observers within the list in Italy, Brazil, Argentina, Uruguay and Australia.

    Chief among the aims of "Cometas_obs" have been

• To exchange information and data between observers and to co-ordinate observing activities.

• To standardise observing methods and data analysis techniques.

• To aid observers to obtain an office MPC site code and to contribute to the MPC database.

• To observe objects as intensely as possible given the limitations of equipment, observing opportunities and site conditions, searching in particular for unusual or unexpected activity.

3. Standardising amateur cometary photometry

    In many cases cometary light curves are built up from a wide variety of data: total visual magnitude estimates and CCD aperture photometry taken with a wide range of apertures from a few arcseconds to several arcminutes. These data are not comparable except in the case of a totally inactive object.

    The following example is of C/2000 WM1 (LINEAR) and taken from Seichii Yoshida’s Japanese comet web pages that represent one of the finest databases in the world of amateur data. Note the wide dispersion that, at some epochs, may reach five magnitudes.

    The light curve of C/2000 WM1 (LINEAR) taken from Seichii Yoshida’s Japanese comet observing page. Note the huge dispersion in the data (5 magnitudes or more) at certain epochs due to mixing data taken in different modes. Even the post-discovery CCD data has a 2-magnitude dispersion.

    To standardise amateur data a series of criteria have been adopted within the "Cometas_obs" observing group:

1. CCD photometry taken with astrometry and reported to the MPC should be obtained with a 10-arcsecond aperture centred on the comet’s centre of light. This aperture is sufficiently large to be several times the typical seeing, but small enough to reflect nucleus activity. At D =1AU, this aperture corresponds to a radius of 7250km around the nucleus and thus a typical crossing time of a few hours.

2. The photometry should be reduced in a standard way using a standard catalogue. Where possible an R filter should be used. Where white light is used the photometry should be treated as being R except where a blue-sensitive CCD is used that gives a passband closer to V. Given the realities of conditions usually being non-photometric, standard stars are required within the field of view of each comet. Although not ideal as it is not primarily a photometric catalogue, the USNO A2.0 catalogue has been used as standard given that it has very wide sky coverage, high spatial density, and a wide range of magnitudes that have been linked as accurately as circumstances permit to the Tycho standards.

    An example of the resultant data is shown below.

    The light curve of C/2000 WM1 (LINEAR) taken from the "Cometas_obs" database.

    We see that the dispersion of the data taken with the standard aperture is greatly reduced. And is, in most cases, is less than 0.5 magnitudes despite mixing data that is effectively V-band with R-band data and despite the wide range of instrumental responses between observers.

    When we look at the data from October to December 2001 on an expanded scale we appreciate the improvement in definition of the light curve evolution.

    The light curve of C/2000 WM1 (LINEAR) taken from the "Cometas_obs" database.

    We see that a 2-magnitude outburst took place between mid-November and early January. This outburst is seen in the total visual magnitude data, but is only poorly defined. The small quantity of data available after December 10th 2001, most of which was taken in difficult conditions with the comet at high airmass, does not allow a conclusive conclusion to be reached about the behaviour after outburst. However, the comet may have continued its normal increase in brightness with decreasing heliocentric distance after the termination of the outburst.
 
 
4.Comparison of MPC archive data with standardised "Cometas_obs" light curves

    An example of the potential for amateur CCD monitoring is shown in the light curve of C/2001 A2 (LINEAR). Some evidence of structure is seen in the light curve during the decline with occasional outbursts that may be of up to 1 magnitude in amplitude.

    If we fit the data with a simple least squares regression and calculate the residuals we find some evidence that, as the coma activity dies down, there is a saw-tooth structure in the data with an amplitude of approximately 0.5 magnitudes and period 25± 4 days. This may be due to an artefact in the data, but it is possible that this may be the precessional signature of the comet’s rotation in the light curve.

    Given the large torques induced by the fragmentation of the nucleus between March and July 2001 it may be expected that the rotation is in an excited state. The saw-tooth pattern would, in this case, be caused by variable gas production as the insolation of a small number of active areas on the nucleus varies with the precessional cycle.
 
 
5. Conclusions

    Amateur CCD monitoring of comets offers the chance, if properly treated to carry out detailed studies of the activity and gas production rates of many comets brighter than magnitude 18. Despite the many difficulties involved, if the data is taken in a standard manner the absolute errors on the photometry can be reduced to approximately 20%, compared to a typical dispersion of several magnitudes in raw data from the MPC archives.

    We have shown that the large and increasing group of CCD equipped amateur astronomers can provide a valuable service to the study of comets that goes far beyond simple astrometry.

Regresar a la página principal