Muniwin aperture selection

Hi Paul,

I use Muniwin for high precision exoplanet transit photometry.  A great strength of Muniwin is the ability to see the impact of aperture choice on scatter.  Muniwin does not choose an aperture.  You choose a set of apertures in the Project Settings.  As part of the process of plotting a light curve Muniwin displays a chart of scatter vs aperture choice.  I generally see that ~FWHM x 2 gives me minimum scatter.

As for dropping data points for some aperture choices, I don't see that behaviour at all.  It does not remove outliers.  There are plots you can choose from which you remove outliers manually.

Regards,

TG Tan

http://pestobservatory.com/

Thanks TG,

I do appreciate the abiltiy to see changes to the scatter by altering the Aperture, but I'm still confused about the aperture choices it offers after computing a light curve - with the bottom of the curve (ie, best choices) usually represented by smaller apertures than one would expect. Should I be performing some preliminary inspection of my data using the Quick Photometry tool and defining / fine tuning the aperture ranges in the Project Settings (ie, setting the minium Aperture to better represent my FWHM characteristics)?

Re: dropped data points, I'll post a screen shot. It definitely occurs in my data.

Regards,

Paul

Two screen shots showing the effect of increasing the aperture. A few data points disappear, and this is reflected also in the table and output report.

Muniwin will drop points if any pixel within the aperture is higher than the linearity limit that you set.  It is possible that there is a hot pixel near either V or C that gets included with an increased aperture.  I would check that particular image for this.

TG

My FWHM is usually 3-4 pixels.  I define the range of apertures to be 1,2,3..to 12.  This way I have a choice that is much wider than needed to pick the optimum.  I have never seen a situation where the optimum is 3-4 x FWHM.  My optimum is usually about 6.  I have seen a prescription for 1.5 x FWHM, but I think this may be for consistently sharp images.  My photometry is usually done with slight de-focus, and stars are also sometime a little bit oval due to imperfect guiding.

I don't see what manual inspection with Quick Photometry does for you.  Surely what you are after is minimum scatter, at that is what the muniwin plot of scatter vs aperture gives you - regardless of the theory!

TG

Thanks TG - very useful information, particularly the likelihood that there is a hot pixel or two at play in my dropped points. The move from the very 'visual' mode of aperture adjustment in MaxIM DL is taking a bit of getting used to, but I am starting to appreciate how the plots in Muniwin provide confirmation of the best settings.

Paul

Are the aperture sizes Paul and TG describe radii or diameters?

Steve Howell describes the "curve of growth" phenomena for selecting aperture sizes in a paper and in his book.  VPHOT also gives a visual display of signal/noise vs. aperture size.  For any given frame, the best aperture may be different than for the next field or even the next frame.  Most books give you general guidelines as to which aperture to use, much like the rule of thumb of 2-3 pixels per fwhm, seeing being 2 arcsec, etc.

It is usually a good idea to drop a magnitude estimate if the measuring aperture includes a bad pixel.  Otherwise, you have to "fix" the bad pixel by interpolation using its neighbors, an action that can have serious consequences in the result.  That said, it is easier to identify the bad pixels if you are oversampled, so there are always tradeoffs in photometry.

Muniwin is an excellent program.  You might also look at AstroImageJ as another freeware program, or talk to the AAVSO about gaining student access to VPHOT.  There are another half-dozen programs out there, like AIP4WIN, MaximDL, Mira, etc. where the vendor might also be willing to support students if asked.

Arne

Ahh.  I'm guilty of sloppy talk.  The aperture sizes I was talking of were radii.  FWHM of course is 'somewhat like' a diameter.  So for FWHM=4 pix, I use usually aperture radius=6, or diameter=12.  So indeed 3 x FWHM!