Can I use different size apertures for multi aperture photometry?

[Shannon]

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.

ETA: Forgot one last question.  Is there a way to change the shape of an aperture so it isn't a circle?  I'm working with tiny apertures only 2 to 4 pixels across and it would be great to have a custom shape instead (some of my target stars are near the edge of the frame and are distorted triangles).

Thank you for any advice or suggestions, it's much appreciated.

Shannon Morgan

[karenacollins]

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

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[Shannon]

Ah, those are excellent workarounds - thank you for taking the time to provide your detailed response.  AIJ is, hands down, the best software I've used for photometry and I appreciate all of the hard work you put into it.

Thank you,
Shannon Morgan