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. This happens for instance with fast moving objects or for observing a large number of targets each night.
Cometary photometry is not simple and some rules must are important as it has characteristics quite different than classical star photometry. Comets move in the sky and finding each night proper reference stars for photometry is not obvious. As routine we ask to check in advance the field to where the comet will be that night to look for reference stars in the field of view. Sometimes you need extra images of nearby fields to get reference stars.
The contribution to the CARA project is free, but some basic requirements are needed.
A telescope coupled with a CCD camera (no anti-blooming is highly preferred) with an image scale around 1-3 arcsec/pixel and at least one photometric filter (R or I Cousins / Bessel filters are suggested). A sophisticated equipment is not specifically needed, but must be of enough quality and solidity to grant standard results and match the requirements to perform a basic scientific research (reproducible results).
Comets are a typical target for amateur astronomers. Scientific observations span from basic visual techniques (drawing and total magnitude estimates) to imaging and CCD techniques, the latter devoted in particular to astrometry and photometry.
Astrometry and photometry require two different approaches.