Astrophotography with small telescopes

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Observing comets

Hello to all comet observers of the world.

I started regular comet watching in 1990 and want to share my experiences with all observers and especially with the beginners in that field. As I started I had many problems with finding, estimating the brightness and a lot more. So I would be satisfied if I can help you to step in this exciting field of amateur astronomy. So lets start.

First I want to tell you something about comets but not too much. Most things can be read in common astronomy books. Comets are small, dirty snowballs containing dust, dirt, frozen water and several frozen gases like methan. Comet Halley, which returned in 1986 to the inner solar system is 5 km wide and 15 km long. So it is not that great as you might think. What we see from any comet is the atmosphere of the comet called "coma", which exists only near the sun and appears when the sun's light warms the icy snowball. Then the dust ejects and the frozen gases sublimate. The coma appears and veil the core. So always when you see a comet you see the coma around the core. The famous tail and for this are comets known is what we want to see. Unfortunately most comets develop only a very faint tail that only can be photographed but approx. only every 10 years a really bright comet appears at the sky. The last really bright comet was comet West in 1976 so we are waiting for a big one ! Yes, we had a great comet display with Hale-Bopp which was discovered in July 1995. But I think it is enough to know at the beginning. Much more important is how to get notice of a comet, how to find a comet and other things around comet observing. I hope I can give you as much experiences as possible to get most out of your observing sessions.

Get notice of a new comet

Comets move to the inner solar system, brighten up unnoticed and become discovered by professional or amateur astronomers. If a comet was discovered and announced by the International Astronomical Union, astronomers measure as many positions as possible to get orbital elements calculated by CBAT. If the new comet is announced you need to get information about it. Use the Internet an go to NASA  or German Cometgroup or the Comet Mailing List at Yahoo. There are many sources to get up to date information about new comets. Don't wait until you get your subscribed magazine. The magazines are almost always out of date when published. Another possibility is to join a local or national comet observer group. These groups often provide up to date information about new discovered comets.

How to get ephemerides ?

Ephemerides are tables of positions for a comet or asteroid. With these positions you can check your star atlas where the comet is at a specific day and point your telescope to these coordinates. Much easier is to use a computer program that calculates positions or plots star charts with the comets position. Many different programs are available on the market that provide star charts including ephemeride generators. I would recommend "Guide " that plots stars with magnitudes down to 14.5 mag. I use version 3.0, because the newer version 4.0 uses photographic magnitudes and these differ from visual magnitudes. The latest may uses other magnitudes. Check this out before you use it for estimations.

What telescope, observing site...?

I'm sure that many amateurs with small telescopes think that their telescope is to small to observe comets. First I want to tell you that you can observe comets. Observing comets depends much more on your observing site your sky and your practise than on the aperture of your telescope. When I started on watching comets, first I used a 5" newtonian, later a 6" f/5 to observe comets and I observed comets with 12.0 mag !!. Other things are more important. One thing is the darkness of your observing site. It is necessary to have a reasonable dark sky to observe comets. Altough a 8.0 mag comet can be a difficult object under a perfectly dark sky. But every comet looks different and you must watch at it to see if you can see it. At my hometown I have normally 5.0 mag visual limit and I can only observe comets up to 9.0 mag. when the comet is condensed. If it is more fuzzy I can't observe it.

What is important ?

1. Dark sky
2. Don't magnify to much, use low (20 - 40 x) magnification
3. Use a Lumicon Swan-Band Comet filter, also at very good sites
4. Star-hop to the position and watch, use inverted sight, move the telescope slightly

With that information you can start watching. But don't stop too fast when you can't find a comet. I missed several comets because of its different look. You must learn how a comet looks like and your brain must learn to separate a small fuzzy blob of light against the sky.

Doing science

When observing comets you can observe it only while you are interested in comets but also measure the brightness of a comet. Another possibility is to make accurate position measurements of comets to improve the orbit. But the following information is for visual comet observations. First it is important to know what you can estimate at a comet. You can make a brightness estimation, a measurement of diameter of the coma and about the length of the tail. Another thing is to say how the coma looks like e.g. does it look fuzzy or more concentrated like a star.


1. Brightness
2. Diameter of coma
3. Look of the comet
4. Length of the tail

Methods to estimate a comet apparation

The brightness measurement: There are some different methods to measure the magnitude of a comet. Methods:

Sidgwick Method
Bobrovnikov Method
Morris Method

Sidgwick Method:

Watch the comet and keep the image of the comet in your mind. Then move your telescope to the field of comparison stars ( if necessary). Defocus the comparison stars until the stars are as big as the comet is focused. Compare the defocused stars with the image of the focused comet.

Bobrovnikov Method:

Watch the comet and identify the stars within the field of the comet. Defocus the image until the comet and the stars have the same size. Compare the brightness of the defocused comet with the brightness of the defocused stars.

Note that the Bobrovnikov Method is only possible with bright comets because it is impossible to defocus dim comets. They disappear in the sky background.

Morris Method:

Find the comet. Defocus the comet until its brightness appears equal about the complete size of the comet. Keep the brightness and diameter of the defocused comet in your mind. Seek comparison stars and defocus the stars until the stars look equal as you saw the defocused comet image. Compare the brightness.

Star charts for the estimation

To make good comet estimations it is necessary to have good reference stars. Several catalogues are available e.g.

AAVSO star charts
SAO catalogue
Guide Star Catalogue

The AAVSO star charts are very accurate but the comparison stars are often not near the comets position. The SAO catalogue provides also good reference stars but only to about 10 mag. The GSC catalogue provides stars and the brightness up to magnitude 14.5. But the brightness differ up to 0.4 mag from the real brightness.

Measure the diameter of the coma

The next thing is to measure the diameter of the coma. The easiest method is to use the earth rotation to measure how long the comet needs to move across a cross-hair. So stop the motor drive and measure the time how long it takes until the comet moves across it. If you don't have a cross-hair ocular then stop the time when the comet starts to disappear from your view until is has disappeared completely.

Then use the following formula to calculate its diameter:
D = diameter(in arc minutes)
S = measured time in seconds
X = declination of the comet

D = 0.25 * S* Cos(X)

Appearance of the comet

The next is to say something how the comet looks like. It is called the Degree of Condensation (DC). It goes from 0 - 9 with
0 = comet is very diffus, no concentration of brightness can be seen
3 = comet gets brighter to the middle of the coma
5 = brightness increases to the middle of the coma, very good to see
but no central condensation is to see
6 = central condensation is to see
7 = central condensation is very bright
8 = very bright central condensation, coma is much fainter than the central condensation
9 = comet is one central condensation, it looks pinpoint or like a star

Tail length

If you can see a tail then it is very interesting to know the length of the tail. Measure the length by estimating the position ( right ascension and declination) of the head of the comet and the end of the tail using the star chart.
Then calculate the length with the formula:

L = length of the tail
R1 = right ascension of the end of the tail
D1 = declination of the end of the tail
R2 = for the head of the comet
D2 = for the head of the comet

L = Arccos(sin(D1)*sin(D2)+cos(D1)*cos(D2)*cos(R1-R2))

Use the right ascension in degree, R1 = R1 * 15

The estimate the position angle of the tail. North is 0 degree, East = 90, South = 180, West = 270.

What to do with your observations ?

When you made all estimations you can send it to the International Comet Quarterly .
Send it to Dan Green, SA0, 60 Garden Street, Cambridge, MA 02138, USA.

But it must be sent in a special format.