To have a clearer idea about what is needed for please take a look at the following basic information and suggestions.

The basic instruments needed are:

1. Telescope : any telescope is potentially useful for photometry. In the first year of trials the GOC observers contributed with telescopes ranging from 7.5 cm to 40 cm of aperture. Keep in mind that refracting telescopes can give problems of chromatism when coupled with CCDs due to the wide spectral sensitivity.
2. CCD : the main characteristic of a CCD is to have a linear response. Usually CCDs without anti-blooming are linear within 1-3%, CCDs with antiblooming need a test to check how the response is and in what range they are linear. A calibration of the response of the sensor on the other hand is always recommended.
3. Resolution : CCDs with larger pixels are usually better as they allow a larger full well capacity and a wider dynamic range. The scale on an average is between 8 and 2 arcsec/pixel.
4. Observing site : ideally we should observe from an high mountain site but on the other hand most amateur astronomers live close to cities and in light polluted areas. We found that, using better nights, and with the proper technique, also from sites far than ideal good photometric data can be obtained.
5. Filters : For a general monitoring of comets an R (or I) photometric filter is recommended. In specific cases and for bright comets narrowband filters (647 or 650 nm,10 nm FWHM) provide a much better result. Unfiltered data produced with first generation red sensitive CCDs (Kodak, Texas chips for instance) can in some case be accepted (after a check against R filtered data). Take in mind that with these old generation CCDs usually the I band can be well matched simply with an RG9 Schott or RG712 glasses; it is cheap, allows to operate in a spectral range less affected by light pollution and haze noise and the increase in the time exposure is relatively small (indicatively 2-3 times).
6. Software : the observer must have software allowing to perform simple basic pre-processing procedures (dark frame subtraction, flat field correction, alignment, average and sum of images). The photometric measurements and Af[rho] quantity determination are made by means of a dedicated software (Winafrho) realized for the CARA project. Winafrho is freely provided to all active observers.

Suggestions, tips & tricks

• Images must always be pre-processed, i.e. dark frame and flat field corrected.
• Dark frame must be obtained at the same temperature and exposure time than comet images, and it is recommended to average several dark frames (ideally at least 9 or so) as this reduces noise and improve accuracy.
• Flat field must be dark frame subtracted and also in this case as above it is recommended to average several flat field images.
• Flat field exposure should be enough to cover about 70% of the total dynamic range, and each filter needs his own flat field.
• The measured images must be “original” and, except for the dark frame and flat field corrections mentioned above, no image processing must be applied (image processing alter or destroy the information contained in the original frames!) .
• Exposures time has to be set in order to never reach the CCD camera saturation level for both the comet and the comparing stars. If the comet is a dim one it could be easy to average many images; centred in the nucleus in order to improve the signal to noise ratio and precision of the measures. In presence of bright comets it can be also useful to averages some images.
• Bright stars superimposed to the coma will lead to unreliable Afrho results. In this case, if the star is in the outer part of the coma, you can limit the measuring window to the inner unaffected region. Generally speaking, if possible, it is better to wait the comet moves far enough from the star to get some images useful for photometry.
• The filters recommended for general studies are the R and I (Bessel or Cousins standards). Narrowband filters are suggested for bright comets only because of the strong light attenuation of the filter (the light is indicatively 100 times dimmer than is in unfiltered images). A V filter (Johnson) usually is not useful for Afrho measurements but can be used to provide total magnitude values (comparable to visual total magnitude estimates) and for a qualitative monitoring of the C2 component of the coma (that strongly affects this band).
• Reliable reference stars are needed, as for instance the Landolt sequences or the Northern Bright Standard List by Brian Skiff but unluckily these stars rarely close to a comet path in the sky. A good alternative is to use the Tycho and Hipparcos catalogues, selecting the stars with highest photometric accuracy. For our purpose, where not available, the R and I magnitudes are extrapolated (this is done directly by the Winafrho software).
• The choice of comparing stars should be fall in those with colour close to the Sun (B-V = 0.67) and in general with a colour index in the range 0.4 < B-V < 0.8. Of course variable stars must be avoided.
• Stars in the same comets field should be used. If it is not possible, take a snapshot of a nearby star possibly no more than 1 degree far away (to minimize the sky brightness and extinction differences).
• Exposure time for the comparison stars should be possibly greater that 5-10 seconds. In case of very short time exposures (bright reference stars) the atmospheric scintillation becomes an important factor, so it is better to average at least several images. In this case verify also the CCD shutter timing on both operations, specially if it is a mechanical one that could introduce systematic errors.
• Relatively bright reference stars should be preferred when possible as they provide higher signal to noise ratio (higher accuracy on measurements) and allow better final results. Furthermore the magnitude and colour indexes from the catalogues are usually more accurate for bright stars than for fainter ones. Usually stars between 8-13 magnitude are fine for small telescopes