Visual observation of comets
Visual comet observing
Most comets are not very spectacular, just faint smudges of light, similar in appearance to many deep sky objects. This similarity prompted Messier to produce his famous catalogue, which was designed to help him discover new comets. The average comet appears as a small patch of light, a few arc minutes across (a distance similar to the separation of the Galilean satellites from Jupiter), possibly brightening a little towards the centre. This patch of light is known as the coma and is produced by outgassing of material from the icy rubble pile a few kilometres across that is the true comet nucleus. This nucleus is far too small to be seen in even the largest telescope, but you may see a false nucleus or disc like nuclear condensation at the heart of the coma. The head of the comet consists of the coma and nucleus. A coma 5' in size when a comet is 1 au from the Earth is actually over 200 000 km across. A bright comet may develop a tail or even tails which generally point away from the sun and the longest may be over 150 000 000 km long.
Unfortunately, prospects for visual observation, particularly of the fainter comets are not encouraging because of the wide-spread increase in light pollution in the UK. A dark site helps you to make good observations: if you can't see a 7m comet in 7x50B then you should try and find a darker site. The higher the site the better as this puts you above more of the atmosphere so that there is less scattering and absorption of light.
You should keep a record of all your observations. It is best to have a rough notebook to use at the telescope, as it will inevitably get damp and dirty, and a finished version in which to complete notes and drawings. Your book can record extra details not reported on the standard forms, eg personal narrative. You should always record your observations as you go, rather than committing them to memory and writing them down later. It is worth recording regularly used details in the front of the book, eg the latitude and longitude of your observing site, the magnification and field diameter for various eyepieces, separation of standard double stars etc.
You will need to get properly dark adapted before starting to observe and during this time you can write down some background information in your observation book. Record the name and official designation of the comet that you are going to observe (eg 2011 L4 (PanSTARRS)), and where you are observing from. Next write down the year, month and day (record the double date to avoid confusion, eg if it is the evening of March 22, then write down March 22/23). As with all astronomical observations it is important to record the time of the observation; for visual cometary work an accuracy of around 15 minutes (0.01 day) is sufficient.
The appearance of a comet very much depends on the type, aperture and focal ratio of the instrument; on the type and magnification of the eyepiece being used and on the observing conditions, so these facts need to be recorded. A comet generally appears fainter with higher magnification, larger aperture and poorer conditions. To find the comet, plot its position on a suitable star atlas and then star hop or use setting circles to find the field. For a faint, diffuse comet accurate positions are essential, as the object may be barely observable above the sky background. Here it may help to print a computer generated chart showing the comet and faint stars prior to the observing session. If you have to plot the position by hand, you may need to interpolate the positions given in the ephemerides to get the position at the time you are going to observe; this is particularly important if the comet is faint.
Observers are encouraged to go on and make more detailed observations of a comet, however these observations are not easy. For some comets simply reporting that you have seen the comet is worthwhile. Some periodic comets undergo outbursts and a report that a comet is within range may encourage other observers to go out and make more detailed observations. It is always worthwhile attempting a drawing, as this helps train the eye and you will find that you begin to see more detail.
Estimating a comet's magnitude
It can be very difficult to make good magnitude estimates of comets because they are often large and diffuse; practice helps. The observed visual magnitude is dependent on a number of factors including how much defocusing you use and the telescope aperture and magnification. You should use the smallest aperture and lowest magnification that clearly shows the comet. For a comet brighter than 3rd magnitude this will normally be the naked eye.
To make the estimate you use a similar technique to that used for making a variable star estimate; here you choose one comparison star (from the AAVSO atlas or a variable star chart) that is a little brighter than the variable (A) and another that is a little fainter (B). If the variable is closer in brightness to the brighter star then your estimate might be A 1 V 2 B (here the range is divided into three parts), if it is about midway then A 1 V 1 B (the range is divided into two parts), and if closer in brightness to the fainter star then A 2 V 1 B (the range is divided into three parts again). If the variable is much closer to the brighter star your estimate might be A 1 V 3 B (the range is divided into four parts), but it is rarely necessary to divide the range into more than four parts. The brighter star is always given first. If the variable appears identical in brightness to A, then record it as =A. If you can't see the variable at all, just record the faintest comparison that you can see, for instance if you can see B, but not the variable then record <B or [B.
To estimate a comet the same procedure is followed, but you have to put the comparison stars out of focus. Select some likely comparison stars, from the AAVSO atlas, from a nearby BAA or AAVSO variable star field or from the Tycho catalogue. For fainter comets it may be preferable to use the North Polar Sequence. Ideally the comparison stars should be in the same field, but this is rarely possible; if they are not you will have to memorise the brightness of the comet and may have to swap between the fields several times.
First centre the comet in the field and try to memorise its appearance. Next choose one comparison star that is a little brighter and another a little fainter than the in-focus comet and defocus (racking the eyepiece in) until they appear the same size as the in focus comet. Are they still brighter and fainter?, if not choose again! When you are happy, make the estimate in exactly the same way as for a variable star; this technique is known as the Sidgwick (S) method.
As an example, suppose you had used the variable star T UMa (on the BAA VSS programme), and made the estimate: P (1) comet (2) Q (ie the comet is closer in brightness to P). P is mag 9.85, Q is mag 10.29, the difference between them is 0.44; 1/(1+2) of this is 0.15, so the comet is 9.85 + 0.15 = mag 10.00, however we only record it to 1 decimal place, ie 10.0.
If you have time, repeat this for another pair of stars because there is often some discrepancy in the magnitudes of the comparison stars from different fields, and several independent estimates will improve the accuracy of the magnitude determination. If the observation is uncertain for any reason (for example discrepant comparison star magnitudes, poor sky conditions etc), always note the magnitude as approximate by putting a colon (:) after it.
Where possible use comparison stars from the AAVSO variable star atlas, a BAA variable star chart, the Tycho catalogue or the North Polar sequence. Always give the source of the comparison star magnitudes, but there is no need to record the actual estimate on the report form, unless you do not have a suitable catalogue or atlas. In this case show the stars you have used by means of a field sketch on a separate sheet.