Optimum usage of AstroImageJ for diffuse objects such as comets

[GordonMcKay]

I am using AstroImageJ in a multi-aperture photometry exercise to derive the relative flux for several comets. The potential issue with circular apertures is that it is almost certain that often some of the tail of the comet will be in the background anulus. The background algorithm in AIJ is good and in all probability there ought not to be a significant bias due to material in the comet tail. Does anyone have any experience with the use of AIJ for comets or diffuse objects?

[karenacollins]

Hi AIJ Users,

I have little experience observing comets and diffuse objects. If you have experience reducing such observations with AIJ, please let Gordon know here. Also, if you have specific suggestions for how AIJ could better handle these type data reductions, please post those suggestions here too.

Karen

[GordonMcKay]

After much work and testing of newly written post-processing software I believe AstroImageJ can be used in a highly effective way for the photometric analysis of diffuse objects such as comets. The methodology is relatively simple although a thorough knowledge of how AIJ computes target and reference flux in both the object field itself and the background annuli is required so that once the standard AIJ measurements table is exported it can be post-processed to provide a significantly enhanced flux/magnitude estimation of the diffuse object (in my case a comet). I am using a multi aperture scenario (MAP) choosing a range of ever widening apertures so that a profile of flux values is obtained. 2 sigma cleaning of background in AIJ is extremely effective for point sources but does not readily apply to diffuse objects. A quick comparison of the excellent seeing profiles in AIJ shows up the "noise" issue quickly. Once the standard MAP exercise is completed the measurements table is exported to an external tool custom written for the task. Only the availability of detailed parameter values and how they are derived in AIJ makes this possible. The background flux around the target object is re-computed from the reference fields. This effectively nulls out attendant noise. Using the best reference star G magnitudes available from GAIA EDR3 data the cometary coma profiling results are encouraging. It seems possible that a software enhancement in AIJ could accommodate this methodology but it is appreciated that no software enhancement is ever simple!! If anyone wishes to discuss this methodolgy please message me here.

[karenacollins]

If the 2-sigma cleaning is causing problems for your case, did you try disabling it in AIJ? If not, go to Image Display > Preferences > "Ignore pixels > 2 sigma from mean in photometer background region". If this option does not solve your problem, please let us know the algorithm you used to compute the background that is giving lower noise than AIJs current two background computation options.

Karen

[GordonMcKay]

Hi Karen,

The email address from I have received emails from you before doesn't accept responses. I have comparative  data, results and plots which I would like to share with you. Can you update your email inbox to accept emails from me temporarily?

Thanks
Gordon