Introduction

Perhaps unknown to many amateurs there is a new kind of “discovery” astronomy: finding supernovae. Until recently, amateur discoverers were limited to the occasional comet and many great observers have applied themselves to this diligently. Perhaps less known are those amateurs who search for new CVs (cataclysmic variables) or look for new outbursts from known CVs. Finally, especially with the advent of the CCD camera, amateurs are essential to the enormously growing database of minor planets.

While there is tremendous value in these studies, the discovery of supernovae has vast implications as new distance indicators to the very tool that will help astronomers determine the fate of the universe. Never before have amateurs been able to so readily add data to the really big questions begged by theoretical and observational cosmology. Supernovae may help determine answers to those really big questions: Where is the missing mass in the universe? Is the inflation hypothesis for the beginning of the universe correct? Is there a repulsive component to gravity? Why and how precisely is our part of the universe moving towards the Great Attractor? If you wish to be part of the solution to these great questions then read on!

What is a Supernova (SN)?

On many levels this is still debated among observational and theoretical astronomers. What is clear is that supernovae are enormous explosions that come from a single star or binary star system. They are so very bright that they can collectively outshine all the stars in the galaxy where the SN occurs.

Generally speaking, there are two major types of SN. Type I SN are thought to come from binary star systems. A dwarf star revolves around a larger, much less dense companion in an orbit such that it “steals” some matter from its more extended companion with every close approach in its orbit. The net result is that the dwarf takes on more matter than its supporting core can handle. An immense implosion occurs with a rebounding of a huge amount of energy into space at near light speed. More precisely, these are known as type Ia SN.

A Type II SN is similar except it gains this excess of matter by being born a heavyweight contender. Unlike most other stars, the cores of these monsters are used up on a relatively quick time scale, millions of years. When the core is gone the star suddenly falls upon itself, again resulting in an enormous explosion.

The two types of implosions differ on a physical level, though. Type II supernovae release more energy than type Ia, but most of this is in the form of those most ghostlike creatures, the neutrino. In type Ia SN more of the energy released is in the form of visible light therefore they are about one or so magnitudes brighter than type II. This fact becomes become more important when we discuss how to find SN.

SN History at Light Speed

Changes in the skies have always fascinated astronomers, perhaps because the night sky offers so little apparent change compared to the world around us. The novae and SN were in a class of their own, so different from the apparition of comets and the relatively common rain of meteors. The most famous early SN was the one in 1054 that resulted in the Crab Nebula, M1. Chinese astrologers wrote extensively about it. Given certain evidence from petroglyphs, it may have been noted by American Indians. Dark Age Europeans were apparently keeping their gaze to the ground.

The SN of 1572 was documented by Tycho Brahe and considerably added to his renown. Similarly, the SN of 1604 was studied by Johannes Kepler. The Swiss-American astronomer Fritz Zwicky was the first modern astronomer to extensively research SN. He coined the term “supernova” and to this day is the individual who still holds the record of the most SN discovered. He was also known as one of the most offensive people to have ever worked at Palomar and was not beneath dropping down and doing one armed push ups in order to intimidate associates.

Reverend Robert Evans, The Pioneering Amateur

The original Palomar Sky Survey and Zwicky’s work were the major sources of SN discoveries. There was the occasional serendipitous discovery by professional astronomers when studying particular galaxies.

The possibility of amateurs discovering SN seemed to go unconsidered until Dr. Robert Evans turned his relatively small 16” reflector to the task in 1981. To date his discoveries number 37 and have yet to be matched by any amateur. Dr. Evans showed that the discovery of SN required two primary ingredients: An unparalleled level of stick-to-itiveness and the ability to memorize star fields around galaxies. He also searched intelligently. He knew that there was no
sense in looking for something in a place where he couldn't find it. He searched nearby galaxies where he knew that supernovae could be seen visually with his instruments. Intelligent choice of target galaxies is a most important aspect in SN discovery and must be considered by amateurs and professionals alike.

Dr. Evans worked almost exclusively from the Australia. Perhaps it was the low frequency of his finds (less than 2 per year) and the lack of Internet communications, he had no counterpart in the northern hemisphere. Surely a concerted effort by a handful of amateurs in either hemisphere would have resulted in a much greater number of discoveries and would have substantially added to the currently data base of discovered SN.

Dr. Evans’ success really depended on dedication and patience. Literally thousands of observations were required to find one supernova. Not many people are willing to do this. Finding SN requires an appreciation of the beauty of the old “nebular” wisps and the stars that frame and crown them. Obviously, Dr. Evans has these qualities and more.

Current notables include Michael Schwartz and Tim Puckett of the USA, Mark Armstrong and Tom Boles of the UK, and Aoki of Japan.

Limiting Magnitudes

As has been stated and restated, the CCD made the casual back yard telescope into a research grade instrument. Among other remarkable qualities it added two qualities that greatly extended the abilities for finding SN. The first is sensitivity. While the limiting visual magnitude of Dr. Evans telescope was around 16, the typical CCD on a 10 inch telescope reaches 17 in a relatively short exposure. The second is coverage. With such short integration times it is possible to cover many more and fainter galaxies in a single night, therefore increasing the chances of finding that SN. These values are, in general, the keys to success. How many galaxies can you observe and how faint can you see?

A histogram of magnitudes of the discoveries of Dr. Evans shows that the vast majority are between magnitudes 13.5 and 14.5. Dr. Evans was always careful to try to catch SN as they first brightened, as this rising portion of the “light curve” is quite valuable to astronomers. Therefore, it is safe to assume in general that the magnitudes of Dr. Evans’ discoveries are as faint as he could detect visually, most between 13.5 and 14.5. At the other end of the spectrum is the discovery magnitudes of SN found by Zwicky using the first Palomar Observatory Sky Survey (POSS) and earlier photographic surveys. The photographic surveys are still the deepest, most SN being magnitude 17 to 20.

Amateurs with CCDs use telescopes ranging from 8” to 24” in aperture. Given the sizes of these telescopes and the need for covering as many galaxies as possible, most discoveries are between magnitudes 16 and 17, considerably deeper than a visual search yet falling short of the deep photographic surveys of old.

The premier professional supernova hunting machine is KAIT (Katzman Automated Imaging Telescope) run by Drs. W. Li and A. Fillipenko of the U. of California, Berkeley. It utilizes a 30” telescope Mount Hamilton, the home of Lick Observatories. Therefore the KAIT program is often referred to as LOSS (Lick Observatory Supernovae Search). Most LOSS discoveries are between 17.5 and 18.5, appreciably dimmer than the amateur group.

So, these are the choices. Deep photographic surveys are expensive. So is a 30” robotic telescope. That leaves visual and CCD methodology. Obviously, a CCD is most helpful, especially considering the increasing amount of competition, but visual discovery is still not out of the question. Dr. Evans recently discovered SN2000cj. So, these are the choices. Obviously the best and in reach of many amateurs is the relatively small SCT with a sensitive CCD and a mount that points and tracks well enough. There are lots of considerations here and are beyond the scope of this article. I literally waited for the first really accurate permanent mounts, convinced that it was the only mount that would meet required specs for applying myself to an extensive effort.

Choosing the Galaxies

Galaxies need to be chosen according to their distance. Beginning SN hunters make a common mistake. They choose their galaxies only according to brightness and assume these are the closest. NGC galaxies are their targets. While this is generally correct, there are many UGC and other catalogued galaxies are just as close, but less luminous. The author has discovered SN in NGC, IC, UGC, MCG, CGCG and even anonymous galaxies.

Galaxies need to be chosen according to their type. Remember that type Ia SN require a dwarf with a companion star. Dwarf stars are generally very old; therefore type Ia SN can happen in older star populations in the central bulge of a spiral galaxy. Similarly they can happen in the old populations of elliptical galaxies. But, type II are the result of quickly evolving and massive young stars therefore only happen in galaxies where star formation is still occurring. The net effect is that spirals will produce both type Ia and type II. Ellipticals can only produce type Ia. If you wish to increase the chances of finding SN it is best to ignore elliptical galaxies. Keep in mind, though, that ignoring these galaxies is only for a research design optimized for finding the most SN. Given that type Ia SN may be the only cosmological standard candle we have, and the lack of obscuring dust in ellipticals, this are often the most valuable SN that can be found.

Putting It All Together

So you want to discover a SN? Then pay careful attention to the following factors:

(1) Don't look where you can’t find something. It is most important that you know the magnitude limitations of your telescope. It makes no sense to search in galaxies where SN at their brightest will barely be visible. You should take exposures of the same galaxies to determine this limitation under the many conditions you will work. Also, as  mentioned above, the chance of finding a SN is much greater if you look at spiral and irregular galaxies. A large number of electronic compilations of galactic data provide the radial velocities, therefore the distance (using whatever Hubble constant you wish) to galaxies. The best is RC3 by de Vaucoulers. This is a compilation of galaxies from all large catalogs and the radial velocities and galaxy types easily let you select your target galaxies. See the LINK section of this web site for to a link to ADC to obtain this catalog and its format. For my 14” telescope/CCD combination I chose a cutoff point of 13,000 km/sec. If a radial velocity was not known for the galaxy (quite often) I added the galaxy to my list if its major axis was 1 arc minute or larger, my guess that chances are the galaxy is within range.

(2) Look at as many galaxies as possible. Your success depends on numbers. Statistically speaking, the number of SN you discover is directly proportional to the number of galaxies that you can observe. Obviously, the automated GOTO telescope has a distinct advantage for SN hunters. Some software systems, such as the suite of programs from Software Bisque, allow you to program a GOTO telescope to begin at sunset and end at sunrise and even sleep during the process. This is tremendous asset to those who must get up and go to work!

(3) Your own images are your best references. It can be tempting to compare your images with outside resources such as RealSky or paper photographic atlases of galaxies. While this can be helpful, CCD images are very different than these photographic sources. Collections of glowing gases in galaxies, such a HII regions, can look much more star-like on a CCD image. In any case, on your own images are true reference images for a possible SN. None-the-less, the DSS (Digital Sky Survey) is a valuable asset for a first level of confidence when a reference image does not exist.

(4) Go to Internet sites or other sources to look at SN discoveries with your telescope. How can you really know what a SN looks like without imaging those discovered by others? This is an essential exercise and fun as well. You can also submit brightness measurements to VSNET or IAU circulars. This will also give you the opportunity to use practice with astrometric programs to measure the offsets of supernovae from their host galaxies as well as make sure that your brightness measurements are correct. It is very easy to make astrometric mistakes when you have the deadly combination of tired and excited.

(5) Make yourself aware of the procedure for reporting SN. There is no space in this article to begin to address procedures for reporting a SN to the IAU but it is essential that it is presented in the exact format required and will no more or less than the required information. You can always get help from an established SN discoverer through the International Supernova Network; see links at this website.

(6) Have a standard method for checking your images. Remember, your best reference images are your own. Establish a method to easily compare a new image to your reference image. Methods used vary from individual to individual, from simple direct visual comparison to blinking. This can be an arduous task, but you get to see the glory and beauty of galaxies. I have imaged and admired over 20,000 galaxies, each a jewel in its own right. Remember to look inside the nuclei of galaxies! The distribution of stars in galaxies favors that SN will generally be closer to the center than farther out. This means that each image should be enhanced to see the faint outer regions and processed again to see to the core. This does not require fancy image processing programs. Both the core and outer regions of a galaxy can be examined easily using brightness and contrast adjustments.

(7) Do not get discouraged! While the methods overviewed in this article will maximize your chances, these are still chances. Discovery has a heavy component of luck. It can be particularly disturbing to discover a SN and find that it has already be discovered. This should not trouble you! It means that your system for maximizing your chances of find a SN is working! No one can take away the fact that you discovered one, only that you were not the first! My search comes up with a supernova every 1400 images or so. Again, this is a statistic. I have discovered 3 supernovae in 3 weeks. I have also gone 4 months without a discovery. Just follow the guidelines in this article, get and plan and stick to it. You will find a supernova if your equipment satisfies basic requirements.

There are other issues surrounding supernova searching that go much deeper than this short introduction. Among them is plate scale (arc seconds per CCD pixel), always seems to be a focus of fights even out of the supernovae realm. Other issues surround how to trade-off limiting magnitude against bad tracking, i.e. determining the best exposure time for your telescope drive. I invite your questions and look forward to seeing more and more amateurs join the ranks of successful supernova hunter.

I fully realize that this article stops short in answering many questions and providing complete information about how to find supernovae. I encourage you to contact me at mbs@tenagraobservatories.com.

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