The Skyhound's Guide to Finding Comets

Hunting known telescopic comets can be both challenging and rewarding. I recommend starting with the brightest comet visible from your location and gradually working your way fainter. Some comets are very tiny and look like stars. Others are large and diffuse--these are usually the most difficult to spot because their light is so spread out. Many comets are somewhere in between in appearance, showing a compact bright core surrounded by a larger diffuse region. Most telescopic comets are round in appearance, but some will show a faint stubby tail, particularly in larger telescopes (>10").

SkyTools users can see the special guide to finding comets for SkyTools users.

Note: the comet magnitudes computed via the orbital elements circulated by CBAT are often grossly in error. An example of this is 88P/Howell which at the time of this writing was reported to be near 10th magnitude, yet the CBAT elements claim 14th magnitude. For this reason it is often important to check the current observed magnitude rather than that predicted. See the table on the previous page for the latest magnitudes and coma diameters.

Is The Comet Visible In My Scope?
In general, the limiting magnitude for comets is much less than the limiting magnitude for stars. For instance, a 6" scope will show stars down to about 13.8 magnitude under ideal conditions, but in general comets fainter than 12.0 mag are invisible. You have a better chance of seeing a point-like comet near this limit than one that is very diffuse. The following table gives a general idea of the practical comet magnitude limit for various size scopes. Dark sky conditions are assumed.  The formula used is m = 8.0 + 2.53log(A2) where A is the aperture in inches.  This is an empirical relation that fits the limits reported by experienced comet observers.
Aperture (inches)
Mag Limit
Aperture (inches)
Mag Limit
2 (50mm binoculars)
4 (100mm binoculars)

How Big Is It In The Eyepiece?
The table of comets on the Comet Chasing page gives the diameters of the current crop of comets as reported by observers.  If you know the field of view of your eyepiece when used with your telescope then you can use the following procedure. It is usually best to use your widest field eyepiece.

Draw a circle on a piece of paper with a diameter identical to your field of view. You can use the field of view in either minutes or seconds of arc and in inches, centimeters or millimeters on your ruler--whatever makes things work out the best. For instance, if your field of view is 1.2 degrees (72 arc minutes) then draw a circle with a diameter of 72 mm. Now you know that 72 arc minutes is equivalent to 72 mm on the paper. Now suppose that the comet has a diameter of 1.2 arc minutes. This means that it will be represented by a circle that is 1.2 mm in diameter on the paper. Draw this smaller circle inside of the larger one. This will give you a good idea of how large the comet will appear in the eyepiece. Remember, you are looking for something similar to a round, fuzzy galaxy.

What Magnification Should I Use?
You want to use as much magnification as you can get away with. Higher magnifications (eyepieces with smaller focal lengths, such as a 9mm vs. a 35 mm), will generally give you more contrast. In other words the sky background will often appear darker, making it easier to discern faint fuzzies. On the other hand, you don't want to use so much magnification such that the comet looms too large. This will also make it hard to detect. I like to start with a low power eyepiece (~50X). This is often best if for no other reason than to more easily identify the star field. I find that ~135X works best for most comets, but in a few cases, particularly for the smaller comets, as much as 200X can make the comet more easily visible.

What Should I Look For?
You are looking for something similar to a round, fuzzy galaxy. If the comet is near your limiting magnitude or your sky is not completely dark, you may only see a bright center. If there is no bright central concentration, you may not see anything at all under these conditions.

Begin by positively identifying the field in the eyepiece. Look for a round fuzzy object. Make sure you are dark adapted, in a relaxed position, and try to keep both eyes open. If close to your limiting magnitude you may have to use averted vision. Look just to one side of the comet's predicted location. Sometimes gently moving the scope can bring it out.

Try using higher magnifications as this usually makes the comet appear more easily visible because of the increase in contrast. Be carefult not to use too much, however. If you spread the light over more than 1/4 of the field of view it may be more difficult to detect. Note that in some cases comets can move significantly in only a few minutes, so look in the vicinity of the predicted location if you don't see it there.

How Do I Find It?
This is the most critical problem. For most faint comets those finder charts you see in the magazines are just about worthless. The problem is that comets are faint and they move. A finder chart showing the path of a typical 10th magnitude comet, with stars to 10th magnitude, would either be only four days long or so dense with stars as to be a mass of dots.

This is clearly a case for good software that can accurately plot the position of a comet at any time in a star field similar to that which you would see in your telescope. Without software, your best bet is to interpolate the comet's position from an ephemeris for a precise moment in time and go about finding the comet in the same way that you find deep sky objects.

Here's how to use interpolation to determine the position of a comet at the time you plan to observe it: the ephemeris is a list of positions for the comet at particular times, usually 0h UT on different dates. Find two times that bracket the time you wish to observe and write down both the times and the positions. You will have T1, P1 and T2, P2. Where T1 is the first time, P1 is the position that goes with it, and T2, P2 is the second time and position. You will also have the time you wish to observe, which we will simply call T. The position of the comet at that time will be P. Don't forget that T needs to he in UT.

It is best to work in hours, so the first step is to add 24 hours to T2 for every day it is ahead of T1. Do the same for T.

Now apply the following formula twice, once using the RA values for P1 and P2, and again using the Dec values for P1 and P2. The first result (P) will be the RA at the time of your observation (T), and the second will be the Dec.

P = (P2 - P1)/(T2-T1)*(T-T1) + P1

Note: * is the multiplication symbol.

You can find ephemerides here.

T1 = 0h UT on Oct 10. At this time RA = 10h23m, Dec = 11d15m
T2 = 0h UT on Oct 12. At this time RA = 10h12m, Dec = 10d23m

The time you wish to observe is 9 UT on Oct 11.

T2 is two days ahead of T1, so adding 2*24, we get T2=48h.
T is one day ahead of T1, so adding 1*24, we get T = 33h.

We need to convert our positions into decimal hours and decimal degrees:
P1(RA) = 10 + 23/60 = 10.38333h
P1(Dec) = 11 + 15/60 = 11.25d

P2(RA) = 10 + 12/60 = 10.2h
P2(Dec) = 10 + 23/60 = 10.38333d

Now to compute the RA at time T, using our modified values:

P(RA) = (10.2 - 10.38333)/(48 - 0)*(33 - 0) + 10.38333 = 10.2573h
P(Dec) = (10.38333 - 11.25)/(48 - 0)*(33 - 0) + 11.25 = 10.6542d

Converting our positions back from decimals:

RA = 10 + 0.2573*60 = 10h15m
Dec = 10 + 0.6542*60 = 10d39m

That's a lot of trouble, isn't it?  This is why software is the way to go; all the work is done for you.

Skyhound's Guide to Comets

The Comet Observation Home Page
Astronomical Headlines (IAU)