Unable to understand how to proceed

[Amlan mihir]

while working on a cluster NGC2419 to plot light curves as a part of my graduation project, i came across this asteroid , which can be seen clearly in the video attachment. (the video is a flashing of multiple observation images captured in a single night).

my guide says to align the frames , find ra/dec of the centre from calibration , intensity of the asteroid and use Aladin etc to estimate its brightness and find and correct it's orbit.

although i understand it very superficially , I don't  understand how to proceed in a quantitative manner and produce results.

Please any guidance would be highly appreciated.

[Dustin Lang]

Hi,

My advice would be to ignore measuring its brightness for now, and first get the orbit.

1. get the x,y coordinate of the asteroid in each frame.  Maybe you want to use photutils for this - https://photutils.readthedocs.io/en/stable/detection.html

2. use the WCS to convert from x,y to RA,Dec for the asteroid in each frame.  Your video looks like difference images, so either your telescope pointing is very good, or you're already aligning the frames using WCS.

cheers,

dustin

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[Dustin Lang]

Replying on-list...

It looks like the Astrometry.net web service handles your individual frames without trouble, for example:

    https://nova.astrometry.net/user_images/7965350#annotated

so you can get the WCS (for each frame) that way.

I don't use IRAF so I can't tell you how to do star detection for your asteroid, but the IRAF routine is probably writing its outputs into some file, right?  ("output = default"?)  You may be able to figure out which "star" in that file is actually your asteroid?

cheers,

dustin

[Swagat Bordoloi]

Wow dustin! Thanks a lot even though this is not my thread. I needed this discussion for a very long time, and I'm glad someone asked. Thank you for providing with the steps

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[Wayne Green]

Hey,

If you take the raw images and get a good astrometry.net WCS, then you can run

sextractor with a few of the, like 400, parameters chosen. This will give you the

flux for asteroid even if it is not 'round'. Sextractor will find lots of oddball things.

ALPHAWIN_SKY             Windowed right ascension  (native)                        [deg]
DELTAWIN_SKY             Windowed declination (native)                             [deg]
X_IMAGE                  Object position along x                                   [pixel]
Y_IMAGE                  Object position along y                                   [pixel]
FWHM_IMAGE               FWHM assuming a gaussian core                             [pixel]
FWHM_WORLD               FWHM assuming a gaussian core                             [deg]
ELONGATION               A_IMAGE/B_IMAGE                                          
ELLIPTICITY              1 - B_IMAGE/A_IMAGE                                      
FLUX_ISO                 Isophotal flux                                            [count]
FLUXERR_ISO              RMS error for isophotal flux                              [count]
ISOAREA_IMAGE            Isophotal area above Analysis threshold                   [pixel**2]
BACKGROUND               Background at centroid position                           [count]
THRESHOLD                Detection threshold above background                      [count]

These are the ones I use in the default param file. Check for elongation, ellipticity FWHM

etc to reject all but stars and your asteroid. Be sure to fill out the default.sex file for

your scope, camera, and seeing conditions.

I am not wild about aperture photometry for non round PFSs. I do use IRAF, and imexam

will pull flux with a little hand holding. I found sextractor better for pipelines.

--W

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-- W