Re: Can I use different size apertures for multi aperture photometry?
Posted by
karenacollins on
Sep 28, 2020; 5:43am
URL: http://astroimagej.170.s1.nabble.com/Can-I-use-different-size-apertures-for-multi-aperture-photometry-tp1448p1449.html
Hi Shannon,
Yeah, the current AIJ implementation supports only a single aperture
size for all apertures within an image. As you have found the
variable aperture changes the size of all apertures from image to
image. We have had the per aperture size setting on the to-do list
for a while, but do not currently have the resources to implement it
in the near term. We will keep this high on the priority list, but
that doesn't help with your current scenario. My only suggestion is
to offset the aperture from the center of the star (with centroid
off - which is a per aperture setting) that you want smaller, if you
are trying to avoid blending from a nearby star. If the field is
super crowded, then this may not work. Also, possibly try a smaller
aperture for all stars.
You can disable sky-background subtraction by setting the background
outer radius to the same size as the inner radius (you can also turn
off the display of the background annulus using the icon above the
image display, if you want). Then place your extra aperture to use
for background subtraction (probably with centroid disabled), and
then use the Multi-plot "Function" setting to subtract the
background Source-Sky (in this case Sky=0) aperture from the
Source-Sky(s) of interest. Use the Y-operand column to set the
Source-Sky for the aperture you are using for background.
Also, note that AIJ using an iterative median routine to remove
pixels with stars from each background annulus, so if you make the
background annulus large enough, there's probably no reason to need
to use a separate aperture to measure sky. Maybe there is a
diabolical case where even this routine would not work
satisfactorily, so you still need the separate aperture to measure
background.
Karen
On 9/27/2020 4:07 PM, Shannon Morgan
[via AstroImageJ] wrote:
I noticed after doing a multi aperture run on a stack of images, I
could click and change the size of the apertures while they were
still on the screen. i tried saving these resized apertures, but
when I reloaded them they were back to the same uniform size
again. I've played around with the feature which lets AIJ change
the aperture sizes on its own but unfortunately that won't work
for my situation.
What would be helpful is if I could create custom apertures for
each one giving them different radii for the annulus, background,
etc.
Also, is it possible to have an aperture that is a single radius
circle? It would be wonderful if I could place an aperture with
only one radius and mark it as the sky value. I suspect there's a
workaround for this that lets me "trick" AIJ into measuring the
background in the center radius but I haven't learned enough about
the inner workings of AIJ to know what that would do to the math.
I could use an aperture and then find the raw counts for the
center of my "fake background aperture" on the spreadsheet,
though... would that be a reasonable workaround to get a pure
background measurement from a crowded field? I could do the
remaining calculations and graphs in Excel if needed.
Thank you for any advice or suggestions, it's much appreciated.
Shannon Morgan