In observing celestial targets most amateur astronomers routinely do not use filters; this helps in getting a better signal to noise ratio, a quite important task when very faint objects are observed and when time exposure is a crucial parameter for instance with fast moving objects or for observing a large number of targets each night.

This is true also with comets as unfiltered images provide generally better imaging and are specially needed for recording the maximum possible extent of the tail.

So why to use optical filters with the need of much longer time exposures ?

The answer is that, to better understand the chemical and physical processes in comets, we need to collect more selective data gathering detailed information about the gas and the dust components.

From our experience unfiltered data can be also sometimes useful but it is quite enough for a scientific approach.

For this reason the use of standard filters is encouraged in order to provide at least images giving information about a well defined and not too large, spectral range. This increases the scientific meaning of our results and allows a better general calibration of the data. Furthermore our observations will be more easily compared with the ones performed by professional astronomers that use the same standard filter sets (or other standards close to ours).

So, if you do not have a photometric filter, do not matter: you can start to observe and to produce some useful data, but consider for the future to plan to use at least of one filter.  An R or I  (Cousins) filter is enough for making a really good work.

The purchasing of a single filter is not so expensive and nowadays there are some affordable solutions. It is a matter of fact that (nowadays) a photometric filter is less expensive than an average quality eyepiece! 

The choice depends mainly on the spectral sensitivity of your CCD; of course we suggest to buy a filter close to the peak response of the chip. Most observers use an R band filter (Cousins or  Bessel standards), but pay attention to purchase a photometric filter and not an RGB series filter, or other kind of “red” filters made for other purposes. 
Also, take into account that a photometric filter can be useful also for other kind of observations, as variable stars and minor planets. It is a quite useful tool if you have in mind to make a little of science with your telescope !

If you are interested in deep sky imaging or in monitoring a large number of objects than our project will not be good for you, but if you are attracted from obtaining scientific information from a limited number of good quality observations, than you can find interesting these pages.

(courtesy P. Valisa, L. Buzzi, "Schiaparelli" Observatory, Varese, Italy)

Wide & narrow-band filters

The spectrum of a comet usually shows two distinct components: a continuum part due to the sunlight scattered and reflected by dust grains and a superimposed emission spectrum due to the coma and tail gas components, see for instance the nice spectra collected at Osservatorio G.V.Schiaparelli.
Specific filters are needed to obtain quantitative data on the different components.

Best results are obtained with narrowband filters centred at specific wavelengths and for this purpose specific standard sets have been defined (IHW set, Hale Bopp set).

Unluckily these filter are expensive and not easily available. An alternative was found among cheaper commercial filters where some have characteristics very close to the professional standards. The first tests on these filters have been done at the Crni Vrh Observatory, especially for tail imaging; dust and H2O+ components.

The dust continuum one, proved good also for coma photometry and Afρ quantity measurements on bright comets.

The photometric calibration of the 647 nm filter is made by approximating its passband to the one of the S Vilnius photometric band, centred on the hydrogen line and very close to our filter.

The uncertainty introduced by this approximation does not exceed 1-2 percent.

Wide band filters were born for stellar studies and were not planned for observing comets. But some of them can  be used for a general work, especially on distant or faint comet, as well with objects that display a very strong dust component.

The advantage is to work in a well defined spectral range and that a number of reliable reference stars are commonly available. Among the many photometric systems adopted by professional astronomers one widely used is the Johnson-Cousins system (originally used with photoelectric photometers and photographic films and plates). Filters sets useful for CCDs have been reviewed in detail by Bessel (PASP, 1990, vol. 102, p.1181).

Take into account that with wide band filters the effective photometric band is defined by the filter, the CCD spectral sensitivity and the telescope characteristics (spectral reflectance or transmission).
It is a matter of fact that the (our) system will be close to the standard photometric band but will not match it exactly.

At our level of accuracy and using stars with nearly the same colour of the Sun,  this do not introduces relevant errors. On the other hand the narrow-band filters define by themselves the photometric band.

The R and I pass-bands cover spectral ranges where usually comets display weak emissions and the continuum dominates. For this reason these filters are suggested for comets imaging and photometry  related to the Afρ quantity.  Anyway some active comets can show relevant emissions in the R and I bands and in this case we cannot get reliable Afρ quantity values.

An example is 153P/Ikeya-Zhang; hereafter some data of this comet on 11 March 2002 the:

• 647 nm filter (10 nm FWHM) Afρ = 5330 ± 180
• I (Cousins) filter Afρ = 8600 ± 250
• V (Johnson) filter Afρ = 12880 ± 400

We can see that the I band value is approximately 60% higher than the red continuum 647 nm filter one. The discrepancy with the R band could be even greater if we take into account the reddening of the light.
As a reference the V band Afρ value have also been reported. As discussed above he V band can be strongly contaminated by the C2 gas emission bands, and the very high value found in this comet clearly confirms this fact.

It is evident that for active comets the assumption that R and I bands contain negligible gas contaminations is not always true and we must be very careful in adopting default assumptions while analysing the data taking also into account that the characteristics of our comet can change with time and if unusual events are occurring. Where possible a cross check against the 647 nm filter is  always recommended.

Suggested narrowband filters for non professional astronomers

Commercial narrowband filters useful for cometary observing selected among the ones available from worldwide manufacturers (e.g. Edmund Optics,  Andover Corporation).

The FWHM (Full Width Half Maximum) of the filters is 10 nm and the peak transmission indicatively close to 50% or so (it can slightly vary depending on the filter type, on its age and also on the spectral range).

• C₃ (coma) - 405 nm
• Blue (continuum) - 440 nm
• C₂ (coma) - 515 nm
• Red (continuum) - 647 nm
• H₂O+ (tail) - 620 nm
• Na (coma and tail) - 589 nm

Except the 647 nm filter, that as mentioned above can be conveniently calibrated on the S Vilnius band, the other filters indicatively need spectrophotometric reference standard stars for a proper data reduction.