Visual observations of comets


Updated 2017 January 14






Although most serious scientific observations are conducted using imaging devices comets also lend themselves to visual observations. Such observations can be merely for the excitement of seeing one of these objects which may be visiting the inner Solar System for the first time or to record the aspects of the comet which will be described here. The BAA Comet Section Observing Guide contains much useful information and is well worth a read. It is hoped that this tutorial will be of help to those who prefer visual observation and/or do not have access to imagers.


This tutorial is a follow on to the tutorial Known comets – observation planning tools and methods ?



- How to observe them

- Telescope set-up

- Field of view – what am I seeing ?

- Limiting magnitude

- Finding the comet

- Recording and reporting what you see

            - Recording your observations

            - Inputting to the COBS database

            - Reporting to the BAA Comet Section


First some very useful resources which considerably expand on what is described her and are well worth referring to;

- BAA Comet Section Visual observations page

- ‘Observing Comets’ by Nick James and Gerald North, published by Springer

- ‘Observing Guide to Comets’, BAA Comet Section handbook


How to observe them


Most important - be safe and warm AS BELOW




Telescope set-up


What sort of telescope should you use ? A good question !!! Perceived wisdom is a 4”/100mm refractor or 6”/150mm reflector is a good starting point. Local astronomical societies may have a fixed site with one or more telescopes and/or members with various types of kit so, if you don’t yet own a telescope, then examine what others are using and find what best suits your needs. For the brightest comets requiring a field of view of several degrees then binoculars would be more suitable, preferably mounted for stability. You don’t necessarily need a pair of Fujinon 25x150 binoculars as shown below being used by members of a beginners course run by the Hampshire Astronomical Group  - 10x50 are more likely to be available to you at a reasonable price (and are more portable !!!).




Having chosen a comet to observe the next step is to point your telescope at the required spot in the sky.


If you have a goto telescope then you need to level it, polar align and calibrate it. If your telescope is devoid of such gizmos then leveling and polar alignment is all that is required. My first telescope had manual drives, no leveling facility and polar alignment was done by elevating the polar axis to one’s latitude marked on a dial. This might all seem quite inaccurate but it worked so don’t worry unduly if your telescope is similar.


Polar alignment is, as the name suggests, pointing the RA axis at the Pole Star. To be pedantic the Celestial Axis is a little off the Pole Star but your telescope handbook should explain how to cope with this. To be honest for small telescopes operating at lowish powers a reasonably accurate pointing is all that is required – arc second accuracy is not !!! The above of course applies to telescopes on an equatorial mount – with which I am familiar. Those on alt-az mounts still need leveling and calibrating though.


Some websites which should help with polar alignment;


Calibration is aligning the telescope with one or more reference stars so that its electronics knows where it is. Having done that you can be reasonably sure that the telescope will point where you tell it to. If you can, try and use a reference star near the comet’s position as there is then less chance of any misalignment taking you far from the target when you actually move from the reference star to the comet. If your telescope has GPS it may well make the calibration process almost fully automatic.


Fields of view - what am I seeing?


One of the trickier aspects of visual observing is understanding what you are seeing through the finderscope or eyepiece i.e.: the orientation and size of your field of view (Once using a different telescope I hadn’t realised that the finder field of view (FOV) was not the same orientation as my own and spent most of the evening ‘lost’). This is best done by using an area of the sky which has some very obvious features e.g. a star cluster such as the Pleiades in Taurus.


If you are using a computer and planetarium software such as Megastar you can overlay circles relating to your finder and various eyepieces as this really does make finding your way around the sky much easier. If you are using printed charts then make up some cardboard overlays equivalent to the sizes of the various FOVs you are likely to encounter or use a flowchart template. Planetarium software will also allow you to orientate the star chart to match your FOV (north or south up, mirror for example). A further advantage of computerized star charts is that you can adjust the magnitude of the stars displayed to match what you see through your finder or eyepiece. The diagrams below show views through a typical finderscope (5º FOV and showing stars to mag 8) and  low power eyepiece (~1º FOV and stars to mag 12).




There are a number of websites which can be used to calculate fields of view for various telescope/eyepiece combinations – one such being ‘Telescope Field Of View and Power Calculator at


Most, if not all, star charts, computerized or in paper form, show brighter stars as larger and larger dots. This can be misleading as it tends to make the stars look closer together than they do through the eyepiece. If your planetarium program has a facility to plot all stars the same size try it out to see if identification of the star field is made easier.


It is recommended that you try out all the telescope/eyepiece combinations available to you to ascertain which gives you the best view of any particular comet.


Limiting magnitude


By this I mean what is the faintest magnitude comet that can be seen with any given telescope. Some idea of the theoretical limit can be obtained from the table below taken from the book ‘Observing Comets’ mentioned above. Limiting magnitude towards the horizon will be a magnitude or two fainter than observing at higher elevations. In practice you may find that the faintest comets you can see are two magnitudes less than listed here because comets are diffuse objects meaning that the total magnitude is spread over a larger area than that for a point source star (deep sky observers will be familiar with that concept as some seemingly bright galaxies can be difficult to observe if they are quite large which means their surface brightness is low). Don’t forget that with magnitudes more is less i.e. higher numbers are less bright so if you are using a 10in/254mm telescope in less than pristine skies or a suburban location you might get down to stellar mag 13, comet mag 11 rather than the predicted 15.


Telescope aperture (in/mm)

Limiting (stellar) magnitude (Observing Comets)

Limiting (comet) magnitude

m = 8.0 + 2.53 log (A2)



11.7 (11.0)



12.3 (12.0)






13.2 (13.5)






13.8 (14.5)











The magnitude formula was obtained here and the numbers in brackets are those quoted in the Section handbook. So much depends on your viewing location and sky transparency so you will just have to see what you can see in your particular location. The numbers quoted above do tend to explain why we couldn’t see a mag 14.5 comet using the Hampshire Astronomical Group’s 24” reflector – it was right on the border of theoretical visibility. Being at a low altitude would have cost us a couple of magnitudes or thereabouts.


Finding the comet


Once you have decided that a particular comet may be observable form your location using one of the methods described in Known comets – observation planning tools and methods the next task is to pint your telescope in the right direction. There are two ways of doing this;

- star hopping from a known bright star

- entering coordinates into a goto telescope (some have the facility to enter the comet’s orbital elements and calculate the comet’s position).


If at all possible I suggest you use a laptop computer or other electronic device to plot the comet’s position as this saves paper (give yourself an environmental pat on the back) and allows you to adjust your star map to what you can actually see at the eyepiece or through the finderscope. Goto systems can operate using the telescope’s handset or linked to a computer. Typically the position of the comet is plotted and the telescope pointed directly from your computer or the coordinates of the comet’s position are fed into the telescope’s handset. Experience suggests that you verify the accuracy of the comet’s position as given by your star charting software with that given by the Minor Planet Center ephemeris. If there is a difference then use the latter.


I will cover star-hopping here. The screen shot below shows 5º finderscope Fields of View starting on Gamma Andromeda (Almach) and moving Westwards towards comet 260P/McNaught and finally centering on Upsilon (υ) Andromedae. It is advisable to move in either RA or Declination and not both at the same time. In that way, if you get lost, you can backtrack until a recognisable starfield comes into view.



Once the telescope has been located on Upsilon (υ) Andromedae one can then move to the eyepiece and track to the comet as shown on the chart below. Note that a low power eyepiece giving a 1º field of view is illustrated here. Having found the comet try experimenting with different focal length eyepieces to see which one gives the best view.



Recording and reporting what you see


Vis obs process flow


Recording your observations


There can be much to observe and report particularly if the comet is bright and sports a tail or tails. The BAA Comet Section Visual Observation Report Form shown, in part, below can be obtained from that section’s website at  This can be used by observers for recording data at the telescope and for their own records.


Magnitude estimation


The position of a comparison star wrt to the comet in terms of both PA and where they are wrt the centre of the field of view (FOV) can affect the magnitude estimate. The correct procedure, to overcome any such induced errors, is to centre the comet, remember its brightness, then centre the comparison star and defocus so it appears the same size as the comet. This applies whether or not the comet and comparison star are in the same FOV.





An example of a comet observation taken from 2013 January issue of  The Astronomer magazine


Observer                               Shanklin

Comet                                   168P/Hergenrother

Year                                      2012

Location                                Cambridge. England

Month                                   December

Day.dd UT                            03.75

MM (Magnitude Method)    S(idgwick)

Total Magnitude                   11.3

Ref (Magnitude Reference)  TJ (Tycho catalogue, Johnson V magnitudes)

Tel(escope) ap(erture)          20 (cms)

Tel(escope typ(e)                  R(efractor)

f no.                                       14

Tel(escope) mag(nication)    90          

Coma Diam(eter)                  1.1 (arc mins)

D(egree of) C(ondensation)  4

Tail len(gth)                          Not recorded

Tail P(osition) A(ngle)         Not recorded

Sky                                        Not recorded (limiting magnitude at the time of observation should be entered here)

Rel                                        Not recorded

Comments                             None


In addition a drawing, using the Comet Section Observing blank form can be made. This can be obtained from that section’s website at A completed example is included in the Comet Section Observing Guide as is a description of the various codes used.


The drawing below is of comet C/1995 O1 (Hale-Bopp) by Robert Bullen (compass points added) and is from the CD-ROM included with the book Observing Comets by Nick James and Gerald North. Also shown is a Megastar chart of the same area indicating that stars down to mag 10 were visible. The tail Position Angle is estimated to be 338º (measured from N through E and S) and its length to be 1.8º (using Megastar chart). The Position angle is measured at the point the tail exits the coma and not along a line joining the beginning and end of the tail. In this case the tail is straight but that previous point would be relevant if the tail were curved as many are.


Comet drawing


Jeremy Perez’s website The Belt of Venus is an excellent place to start for those wishing to draw comets or any other celestial object for that matter.



The Degree of Condensation is an indicator of how much the surface brightness of the coma increases towards it s centre – see diagram below. A DC of 0 indicates a totally diffuse coma and a DC of 9 indicates a star like object. As the DC increases the size of the coma usually decreases as it and the central condensation become more sharply defined. Further details on describing the coma can be found in the BAA Comet Section’s Observing Guide to Comets.


Inputting to the COBS database


The Comet Section recommends that observations are logged in the COBS database


The first step is to obtain a login username and password.


To enter observations select the Observations tab and Visual observations below the tab and complete the form as directed. While this might seem a little tedious, once the various ICQ codes have been negotiated and a set of observations for your telescope or binoculars has been entered, adding further observations is much simpler. When I have made a visual observation I will enter an example here. An example of a CCD observation can be seen in this tutorial


Your visual observations, in ICQ format, can be downloaded from COBS and submitted to the BAA Comet Section Director as mentioned below.


Reporting to the BAA Comet Section


The primary method of reporting observations to the Section Director (see Section website or handbook for email address) is by email using the ICQ format. The various ICQ codes can be found here If not using COBS data, and you are urged to do so, a data entry program to produce ICQ formatted files is available – see BAA Comet Section webpage contributing observations This generates three files, BAA, TA and ICQ formats, but please use only the ICQ format for submitting observations. The ICQ file can then be attached to an email and sent to the Comet Section Director.


The above example in ICQ format, generated using the above mentioned program is shown below.


168        2012 12 03.75  S 11.3 TJ 20  R14  90   1.1  4            ICQ nn SHAXX