Reply To: Using ASTAP for comet measurements

Forums Comets Using ASTAP for comet measurements Reply To: Using ASTAP for comet measurements

Nick James

Hi Mark,

Astrometrica may be old and it would be a lot better if Herbert Raab open sourced it but it is the current standard for amateur astrometry and many comet and asteroid observers use it. It is worth the effort to learn and it can use a wide range of different catalogues including UCAC4 and Gaia DR2. Comphot was designed to work with Astrometrica since most observers will have already done astrometry on their frames using it and it provides a fast way of getting a measurement from stacks of frames that have been used for astrometry.

The main problem with photometry of comets is that the magnitude is fairly strongly dependent on the estimated coma diameter. Comphot uses a rather arbitrary, but at least objective, way of estimating this. For brighter/larger comets you have the additional problem of needing to remove the contribution of stars in the photometric aperture. Comphot does this by assuming that the coma is circularly symmetric. Remember that the magnitude is the magnitude of the coma, not including the tail and so a circular aperture is best since the coma is generally pretty spherical. You generally don’t need an elliptical aperture since any elongation is probably the tail.

There are a number of tools out there to do accurate comet photometry. Thomas Lehman’s AIRTOOLS is a good example. It is great to see that ASTAP is being used for this too and I’d be happy for people to use any tool that they want as long as they get consistent results.

It would be good if we could all have a go at measuring a reference image with different tools and see what results we get. I’ll dig out one of my C/2022 E3 images that we could use as an example.