Using ASTAP for comet measurements

Forums Comets Using ASTAP for comet measurements

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    Mark Phillips

    Ni Nick
    I’ve tried to roughly write down the details of how I did the measurements on your files. Attached a PDF.
    It was a bit of a fix but see what you think. I’m not an experienced comet observer so happy to be proved wrong with this technique. I just think there’s a lot of potential with ASTAP, for comets and many other objects. Han is very responsive to adding new features too.
    PS Currently it is only the development version of ASTAP that contains some of this:

    • This reply was modified 1 year, 2 months ago by Mark Phillips. Reason: added link to development version of ASTAP
    Grant Privett

    I just had a quick look at the ASTAP manual.

    If I am reading it right, then ASTAP needs two FITS keywords* populated with the approximate field centre coordinates in RA/Dec and also guidance from the user (field diameter/search size) to run. So, its not doing blind solving, but the much simpler user assisted solving. Which probably explains the speed.

    I’m not sure though how most of us would get the RA/Dec values into the header. Is there a command line tool or something available? I imagine you could supply a target name and the software then use Simbad or JPL Horizons online to supply the current position from your lat/long.

    Does anyone know of such a tool? That would be extremely useful – and not too hard to write in Python.

    Certainly, I control my telescope via TheSkyX, but I don’t like its imaging interface and so use my own CCD control software (or sometimes AstroArt) – which just takes images and does not talk to the mount. I imagine others may also be in the position of generating FITS from their camera without RA/Dec coordinates in.

    *Or possibly 4, as I have seen two different definitions of keywords for RA/Dec.

    Mark Phillips

    I use NINA which automatically writes all the FITS keywords to the file header. Definitely recommend NINA as the best I’ve used by far and it’s become very popular – for obvious reasons.

    Nick James


    Thanks for writing that up. It is good to see different approaches.

    You are using an elliptical aperture which I don’t think is correct and probably explains the slightly brighter magnitude that you are getting since you will have included some of the dust tail. Comphot always uses a circular aperture which is reasonable since the comet’s coma is likely to be spherical and the rules are that you should not include the tail in the estimate. Also, how do you exclude stars that appear in the aperture? The sigma-clipped comet stack does significantly attenuate stars but they are not completely removed.

    You make the comment that you would not normally use this approach with ASTAP. How would you normally deal with the fact that the comet is moving relative to the background since stacking over long periods is often necessary to get sufficient SNR in the outer coma?


    PS for Grant – Surely your home-brew camera software can query the mount’s commanded pointing position from SkyX and then insert the required FITS keywords. Blind solving is great but it is relatively slow so it is always best to have this data in the header. In particular Astrometrica is a bit of a pain to use if this is not there.

    Grant Privett

    Yes, I could make the code look for the TheSkyX COM process running and try to interrogate it for what the current pointing is, catching an error if need be. I have never bothered as I usually take many dozens of images of the same target and use a platesolving script that feeds the RA/Dec coordinates of the previous frame solved into an astrometry.NET instance. Upshot is that solving the first frame is slow, but the later frames solve in a second or two (depending on how many cores I use).

    Worth thinking about – after I make the code rattle through the filters….

    Mark Phillips

    Nick – I’m beginning to see why Comphot is the best way of doing comet photometry and my lack of understanding. Finding the extent of the coma is too subjective in my technique but useful for a quick analysis.
    The way I normally work with ASTAP is to use a small number of images in the stack so as not to trail too much for a magnitude measurement. Coma size is measured using just one frame. Not ideal for a good SNR though. Also basically using Han’s technique that he shows in the video, removing any stars from the measurement area.

    Any further forward on porting this to Windows and a GUI?

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