SAO Guest Contribution


AMATEUR SUPERNOVA HUNTING

by The Rev. Robert Evans OAM

Robert Evans holds the all-time record for visual discoveries of supernovae - 32. His first discovery was in 1981. Ten of these were found using a 25cm telescope, one using a 31cm telescope, eighteen using a 41cm telescope (all three were "backyard" variety newtonians), and three using the forty-inch telescope at Siding Spring Observatory (Australian National University). In addition he has discovered four supernovae (plus a comet) on U.K. Schmidt films especially exposed by Anglo-Australian Observatory staff in a Pro-Am project which Evans shared with Dr. Brian Schmidt of Mount Stromlo Observatory, and with Robert McNaught.

The first discoveries of supernovae by amateur astronomers were made either visually or by using photography through the telescope. Nearly all discoveries made by professional astronomers up until the late 1980's were made by photographic searches; however, amateurs have not used photography much, probably because few could afford the equipment needed to make photographs of galaxies good enough for this purpose. It was much simpler for amateurs to operate visually, and supernova searching of this kind soon began, often using the most elementary backyard telescopes.

By the end of the 1980's, most of the supernovae brighter than 15@h magnitude were being found visually by amateurs! In the early 1990's when professional astronomers were beginning to use supernova studies to address major problems in cosmology, they had to rely very much upon amateur searches to provide them with the brightest and best supernovae in nearby galaxies. These nearby supernovae provided much of the benchmark information needed for studying supernovae at remote distances, in order to arrive at independent estimates for the expansion, age, and fate of the universe.

CCD Supernova Hunting

In the last few years, the cost of charge-coupled devices (CCDS) has fallen to the point where some amateurs can afford to use them on computer controlled telescopes. Some of these amateurs are hunting for supernovae with resounding successes. Telescopes with CCDs and computer control have a number of advantages:

(1.) It is possible to observe in locations with some light pollution, or in the presence of fairly strong moonlight.

(2.) With a computer to direct the telescope to individual galaxies, it is no longer necessary to know the sky well.

(3.) With appropriate computer control of the telescope, the observer can sit in a warm room facing the computer screen.

(4.) If your equipment is good enough, it will find supernovae for you, without you being present.

(S.) Stars as faint as 18th or 19th magnitude become accessible, which includes the brighter and fainter supernovae in nearby galaxies, plus the brighter supernovae in a great many of the more distant galaxies, out to about 300 million light years. Indeed, you would have so many thousands of galaxies within range of your equipment that you would never have enough time to observe them all! However, you will need reference material for all the galaxies on your observing list, even if you make the reference images yourself, so that you can tell when a new object appears in or near a galaxy.

Visual Supernova Hunting

Visual supernova hunting has special requirements, but it has a number of advantages over using a CCD. The requirements are:

  1. A reasonably dark observing site is needed.
  2. Your telescope needs to be easily manageable so that you can locate objects quickly, and the aperture needs to be big enough so that you can see down to about 15th magnitude. You can then observe all the nearby galaxies (out to, say, 100 million light years). You will then be able to see the brighter supernovae and most of the fainter ones out to about 25 million light years, and the brighter supernovae out to 100 million light years or more. But, naturally, your chances of success decline with the distance of the galaxy. Fainter supernovae in any given galaxy may be more numerous than the brighter ones, although the latter are the more interesting scientifically.
  3. As in CCD searches, charts or suitable photographs of all your target galaxies are needed, so that you can tell when a new object appears.
The advantages of visual searching are:
  1. You can be successful with much less expensive equipment than is needed for CCD work.
  2. An experienced visual observer usually knows the location and the normal appearance of many target galaxies, and thus can work through observations of galaxies at up to ten times the speed of anyone using a CCD on an amateur telescope. Professional-standard CCDs are quicker, but are still much more expensive than ones used by amateurs.
  3. The observer becomes very familiar with the night sky. Personally, this is a great benefit of visual searching.
  4. Amateurs who rely on computers to find galaxies are deceived by technology into being ignorant of the sky. Thus, when the technology fails (as it does from time to time), the search halts because the observer does not know where to locate target galaxies. A visual observer who knows the sky is immune to this problem.
Verification and Reporting

Verification of any suspected new discovery is vitally important. The first step is to check any suspect against all available photographs, CCD images or charts of that galaxy. Measure carefully the offset of the new star from the nucleus of the galaxy. Watch the object for any possible movement against nearby stars. Note the time of your discovery in universal time (i.e. the standard time in Greenwich). It is necessary to have a team of other observers who can make independent observations of the new object for you, and who will do so immediately, if asked. These other observers must also have enough galaxy resources so that they can eliminate anything which is not a supernova, and they should lived in a number of locations in case bad weather or other commitments put the main back-up observer out of action.

The Central Bureau for Astronomical Telegrams has issued instructions describing how much verification they want about any possible new supernova, and these should have been consulted beforehand. CCD observers need at least five observations covering 24 hours. A visual observer should have independent observations by people who know what they are doing. Even after the Central Bureau has been notified, spectra will probably need to be obtained before the Bureau will finally announce a discovery.

When the Central Bureau is notified, full details of all observations of the new object should be provided: the name and location of the person making the report; the discoverer's name and location; details of the reference materials consulted; details concerning the equipment used; universal time of all observations; name and position of the galaxy; offset and brightness of the supernova; and similar details about each verifying observation. Observers who are not already known at the Bureau should take special thoroughness and care in detailing and supporting their report. For all discoveries, the email address of the Central Bureau is cbat@cfa.harvard.edu.

Much helpful advice is available in the AAVSO Supernova Search Handbook, which is available from the AAVSO, 25 Birch Street, Cambridge, MA 02138, USA, for the cost of postage only. It can also be downloaded through the internet (aavso@aavso.org).
 


This text is to appear in "Observers' Handbook 2000." edited by Roy L. Bishop.

Copyright by The Royal Astronomical Society of Canada. 136 Dupont Street, Toronto. Ontario. MSR 1V2. Canada. (Reproduced with permission.)


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Monday, 19-Nov-2007 11:17:06 AEDT

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