Re-performing multi-aperture measurement with same apertures

[thisisdogastronomer]

Hello AIJ forum,

I'm currently working on a stack of data on which I want to try several settings for the aperture(s) to reduce errors of both the target and the comparison star(s) flux. To be able to actually make the outcome comparable, I need to use the exact same aperture locations on every run. I know that there's a checkbox which says "use previous x apertures (1-click to set first aperture location)", but what I really want is that ALL the apertures are being placed at their previous positions. As I will always have to set the target aperture on my own, I can't be sure that it's exactly the same location (in pixels e.g.) where I placed it in the previous measurement. Is there a way to do this?

I could try looking at the ImageJ X/Y or FITS X/Y but since it's given till the 4th decimal digit that's probably too much effort if I wanted to perform numerous measurements.

Thanks in advance.

Regards,
Fabian

[karenacollins]

Hi Fabian,

There are a few options depending on exactly what you need to do.

If you want to place the apertures in the first image and then have them stay in the same place for the entire image stack, simply disable "Reposition Aperture to object centroid" in the Multi-aperture setup panel. The "Use last N apertures" option will recall the same locations for the next run. Alternatively, you can save the apertures to a file and reload them each time using menu items in the Image_Display->File menu.

If your images are not aligned and the apertures need to be in different locations for each slice of the stack, you would need to use the "Stack-Aligner" option to align the stack first, then you can place the apertures in one static location as described above.

Finally, there are two centroid options. The setting is in the Aperture Settings panel. If you enable "Use Howell Centroid Method", the centroid operation will find exactly (or nearly exactly) the same centroid position run after run (although I am surprised you are seeing significant differences using either method). The non-Howell centroid method works best for defocused images.

Hopefully, one of the above methods will provide what you need. If I misunderstand what you are needing, please post again with more details.

Karen

[thisisdogastronomer]

Hey,

First of all thanks for the quick answer.

I should've mentioned that I already aligned the stack and disabled the centroid function. All I want to do is perform several multi-aperture measurements with the very aperture locations as in the first run (for target and comparison objects) on the same aligned stack of images but with different radii of the object and outer/inner background annulus.

My problem so far was that I always had to re-select the target as soon as I clicked on the "multi-aperture measurement"-tool to measure again with different radii. (The comparison objects were placed automatically as I had the "Use last N apertures" option enabled.) I just want ALL the apertures (target aperture included) with different radii than before to be placed at the exact same positions as in the previous measurement. I wan't to avoid adding inaccuracy by having to select the target location on my own again in every measurement.

I could try to save and load the aperture settings, but I guess that will either just save the radii settings and maybe the location aswell, but as I want the same location but with different radii that will probably not work out for me. (I'm just assuming this as I can't try it at the moment.)

It would be good if there was a way to just re-run a measurement with the apertures staying at position and the option to change their radii or enable/disable several settings. (Or maybe there is and I just wasn't able to find it, yet.) So far, I always clicked on the "multi-aperture measurement" button in the toolbar to re-perform a measurement, which always clears out the previous apertures.

Regards,
Fabian

[karenacollins]

I understand the problem now. Thanks for the additional info. There is currently no way to have the apertures stay exactly as in a previous run. I can add an option to do that, but for now, I think there is a workaround that may solve your problem.

The work around requires the first image to be plate solved and all of the others to not be plate solved. One you do that, turn on “Use RA/Dec...” to place apertures. Set centroid on or off as desired. Turn off “use previous apertures” for your first run. Also disable “Stop on centroid / WCS error”. Now place your apertures.

Once that run is finished, return to multiaperture again but this time enable “use previous apertures”, leave RA/Dec enabled, and centroid and error message disabled. Now when you click “Place Apertures”, they will be automatically placed in the previous locations without a click. Now run photometry and the exact same apertures should be used again.

Let me know if this works for now.

Karen

[karenacollins]

Note: I think you will need a “daily build” update from within the last couple of months for the above work around to function properly.

Karen

[karenacollins]

One more note: I original said to either enable or disable centroid on the first photometry run, depending on your needs. However, after thinking about it more, you can enable centroid on the first run when you define the apertures, but before starting the run, you will need to turn off centroid for each individual aperture by alt-left-clicking in each aperture (and making sure the "plus" sign in the middle of each aperture is no longer present). If you follow this approach, centroid would fine-tune your aperture locations near where you click, but the alt-left-click in each of the apertures after placing them will keep them from moving throughout the stack during the first photometry run.

If you place the apertures strictly by hand in the first run (i.e. with centroid disabled), then of course you don't need to worry about the above.

Karen