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.
- The first needs short exposures to freeze the motion of the object and is usually made unfiltered to get the highest possible signal to noise ratio (S/N) to detect faint object.
- The second needs filtered images and longer exposures to get a suitable S/N.
Our approach concerns photometry performed in R and I bands, and also with narrow-band filters on bright comets. It is a different approach respect, for instance, to V band photometry made with the goal to complement visual magnitude estimates. V band data cover a spectral range dominated by strong gas emissions (mainly due to C2) and by dust reflected sunlight.
R and I pass-bands instead cover spectral ranges usually interested by relatively weak emissions and dominated by reflected sunlight. Gas emission contamination can be not negligible in very active comets (up to about 30-40% or so contamination) but usually on a large number of targets emissions are negligible or within the measuring error.
Instead of the classical magnitude we derive the Afrho quantity, that is nearly not (or a little) sensitive to instrumental effects and is related to the dust production rate.
This quantity, obtained according to a standard procedure, allows to get data comparable to the ones usually performed by professional astronomers.