Creative Commons License Copyright © Michael Richmond. This work is licensed under a Creative Commons License.

Some tips for photometry of comets

Michael Richmond
Apr 5, 2013

In late 2013, the comet ISON (comet C/2012 S1) will pass through the inner solar system, zipping past the Sun in late November and then flying out again past the Earth in December.

Click on the image to start animation

It will be closest to the Earth around Dec 25, 2013, at a distance of roughly 0.43 AU. It will appear brightest about one month earlier, in late November and early December.

But how bright will it be? Will it turn out to look like Comet Hale-Bopp,

Image courtesy of Drew Sorenson and the Ames Astronomers

or more like Comet C/2011 L4 PanSTARRS?

Image taken Mar 14, 2013, around 8 PM, 10 sec at f/16

Well, it's a risky business to predict the behavior of comets, especially new ones which are making their first pass through the inner solar system. Some people just can't resist, though, and they will need all the observations they can get in order to improve their predictions. So, how can we help?


Basic photometry

Photometry is a fancy word for "measurement of light". The goal here is to determine the apparent magnitude of a comet by comparing its brightness to that of a star (or stars) of known brightness. Let's begin with a simple example, involving just stars; we'll leave comets for later.

The first step is to acquire an image of some stars of interest. Below is a CCD image taken through a V filter with the RIT Observatory's 12-inch telescope.

Do you recognize this field?

      Hint: there's a spiral galaxy in the background 
      Hint: one of the stars is a supernova 

Yes, that's right: it's the spiral galaxy M101, and the supernova is SN 2011fe!

Suppose we want to measure the magnitude of the supernova: we'll call it our "target". We must do three things:

  1. Make some quantitative measure of the brightness of the target object and several nearby comparison stars.
  2. Look up the catalog magnitudes of the comparison stars.
  3. Use some math to convert the relative brightness of the target object into a magnitude.

The trouble with comets

The basic techniques of photometry work just fine when one is comparing stars to other stars, because the light from all stars has the same shape and size.

If a comet is so far from Earth that it appears nearly stellar, then we might still be able to use simple aperture photometry to measure the light from the comet. What do you think of this image of Comet ISON taken on Feb 23, 2013, with the WIYN 0.9-meter telescope?

When a comet approaches the Earth, its light may appear spread out over a large area. How can we now compare the light from the comet to that from a comparison star?

Image of Comet Hyakutake copyright Rick Scott and Joe Orman

Well, there's no easy answer. Observers who measure magnitudes visually (with their eyes) often try to defocus their binoculars or telescopes until starlight is spread out over an area similar to that of the comet; they can then fairly compare the brightness of the comet to that of the stars.

Can you do this with CCDs, too? Give it a try and find out!

The wierd "absolute magnitudes" of comets

There's one final point concerning the measurement of comets (and asteroids, and other bodies in the solar system) which may confuse you -- it certainly confuses me.

When we talk about the magnitudes of stars, we normally distinguish two varieties:

What's the difference? Well, take a look at the constellation of Canis Major:

You all know that Sirius is the brightest star in the sky, right? That is, it appears brighter to human eyes than any other star. The star called Mirzam, at the end of one of the doggy's forepaws, is not nearly as bright. The apparent magnitudes of these stars are:

But this is not really a fair comparison of the stars. Why not? Because Sirus is one of the closest stars to the Sun ...

... but Marzam is much farther away.

In order to compare these stars' luminosities fairly, we need to observe them at the same distance from us. Suppose that, with the help of some friendly aliens, we could move both stars to the standard distance of 10 parsecs from the Sun.

We define the absolute magnitude of a star to be the magnitude it would have if observed from this standard distance. In this particular case, Sirius would be moved farther away, and so fade, while Marzam would grow much brighter.

The notion of absolute magnitude is to level the playing field: by putting all the stars at the same distance, their relative apparent brightness really does reflect their relative luminosity: powerful stars will now look brighter than feeble ones.

Okay, that was the story for stars. It turns out that one can do something similar for objects within our Solar System: to remove the effects that different distances can have on the apparent brightness of objects. In the solar system, though, it's a bit more complicated because comets and asteroids shine by reflected light, not by producing their own energy.

Consider the effect of distance from the observer alone. Asteroids A and B are identical: same size, same shape, same surface properties.

  Q:  Which asteroid will appear brighter to the
      observer on Earth?

Okay, we can fix that difference by placing both objects at the same distance from the observer, right?

But suppose that we imagine two astronomers: one on Earth, looking at an asteroid exactly 1 AU from him, and an observer on Saturn, who is looking at an asteroid exactly 1 AU away from HIM.

  Q:  Will asteroid A, seen from Earth, have the
      apparent brightness as asteroid B, seen from Saturn?

No, of course not: the intensity of sunlight at the distance of Saturn is much lower than the intensity at the distance of the Earth. That means that the light striking asteroid B will be weaker than that striking asteroid A; and, therefore, asteroid B will appear fainter to the astronomer on Saturn than asteroid A appears to the astronomer on Earth.

In order to make a fair comparison between all asteroids and comets and moons in the Solar System, astronomers have come up with a standard procedure:

Step 1: place the object at a distance of 1 AU from the Sun.

Step 2: place the observer at a distance of 1 AU from the object.


If one could somehow arrange for the observer to survive, the measurements he made would be called absolute magnitudes, denoted with the letter H.

There are additional complications, dealing with the angle between the Sun, the object, and the observer. See the references below for more information on the subject.

For more information

Creative Commons License Copyright © Michael Richmond. This work is licensed under a Creative Commons License.