G2V Calculation Value

[Jkulin]

Hi All, I wonder if someone can advise, I want to experiment with the difference using G2V to calibrate my RGB durations.

I've captured my images at 1, 2 and 3 secs on a G2V star (Androm) and just wanted to do a simple ratio calculation by identifying the ADU of the star under each filter, is there a simple way of doing that within Pixinsight which I already have or is there a free bit of software that will do this?

I have asked on the PI forum but no one has provided a simple answer.

Thanks for your help?

[wornish]

Not sure exactly what you want but does this help?

 

https://pixinsight.com/doc/legacy/LE/07_image_windows/reading_pixel_values/reading_pixel_values.html

[vlaiv]

Regular ImageJ or AstroImageJ can help you there.

AstroImageJ is just distribution of ImageJ (or Fiji, again distribution of ImageJ with lots of different plugins) - that has plugins suitable for astronomy use (did not check it out, just heard of it).

Easiest way to do this in ImageJ is like this:

Take your sub, use circle/ellipse selection tool, and make selection around star including any halo. Next perform Analyze/Measure and record mean pixel value, and area (when you multiply those two you get total ADU sum under selection).

Next thing that you want to do is to make slightly larger selection around the star - about 4-5px larger in diameter than previous, and again do Analyze/Measure and note average and area.

Star ADU count is following:

smaller_selection_average * smaller_selection_area -  (larger_selection_average x larger_selection_area - smaller_selection_average x smaller_selection_area) / (larger_selection_area - smaller_selection_area) * smaller_selection_area

(someone, please check above expression , point is to remove background mean ADU from total star ADU - classical aperture photometry).

BTW I believe AstroImageJ has above implemented as macro or plugin

 

[Jkulin]

17 minutes ago, wornish said:

Not sure exactly what you want but does this help?

 

https://pixinsight.com/doc/legacy/LE/07_image_windows/reading_pixel_values/reading_pixel_values.html

Thanks Dave appreciate your reply, I know about that but they aren't ADU values.

[Jkulin]

13 minutes ago, vlaiv said:

Regular ImageJ or AstroImageJ can help you there.

AstroImageJ is just distribution of ImageJ (or Fiji, again distribution of ImageJ with lots of different plugins) - that has plugins suitable for astronomy use (did not check it out, just heard of it).

Easiest way to do this in ImageJ is like this:

Take your sub, use circle/ellipse selection tool, and make selection around star including any halo. Next perform Analyze/Measure and record mean pixel value, and area (when you multiply those two you get total ADU sum under selection).

Next thing that you want to do is to make slightly larger selection around the star - about 4-5px larger in diameter than previous, and again do Analyze/Measure and note average and area.

Star ADU count is following:

smaller_selection_average * smaller_selection_area -  (larger_selection_average x larger_selection_area - smaller_selection_average x smaller_selection_area) / (larger_selection_area - smaller_selection_area) * smaller_selection_area

(someone, please check above expression , point is to remove background mean ADU from total star ADU - classical aperture photometry).

BTW I believe AstroImageJ has above implemented as macro or plugin

 

Thanks Vlaiv,

I do appreciate your help, but as this is going to be a routine thing dependant on the environment, I want something easy and I have seen in Maxim where you can just click the star and it will give the intensity, which is fine and then I coudl divide the intensity into the green values and get a ratio, which would then tell me if there what timings to use.

I just rather hoped there was a simple straight forward way that I could just click the star and get the ADU value.

I'm OK with Maths, but not in your league?

[vlaiv]

2 minutes ago, Jkulin said:

Thanks Vlaiv,

I do appreciate your help, but as this is going to be a routine thing dependant on the environment, I want something easy and I have seen in Maxim where you can just click the star and it will give the intensity, which is fine and then I coudl divide the intensity into the green values and get a ratio, which would then tell me if there what timings to use.

I just rather hoped there was a simple straight forward way that I could just click the star and get the ADU value.

I'm OK with Maths, but not in your league?

Try astro imageJ then, here is website:

https://www.astro.louisville.edu/software/astroimagej/

It is free / open source, written in Java so it should run "everywhere", and it has bunch of advanced features (by the looks of it), so I'm pretty sure there is simple aperture photometry mode (even star detection with photometry, so fully automated).

[vlaiv]

Oh, it works like a charm, just download AstroImageJ zip, extract to desktop (or preferred folder like My Programs in user home), start, click on star and read off values:

[Jkulin]

Thanks Vlaiv,

I really appreciate your help.

So which figure do I take for the ADU?

This was a 3sec of Androm, I have 1 & 2 secs if needed, as I'm advised not to use a saturated image: -

[vlaiv]

I think it is like this:

You open up fits image with file/open. Next you select single aperture photometry tool, like this:

you click on wanted stars (below I selected 2 stars and one "empty" spot)

Source-Sky should be total ADU for star (total source minus total sky), Source_Error is estimated error of measurement (probably deviation of pixels from gaussian, or something), Source_SNR is obviously those two divided.

Peak is peak ADU value for star, Mean is total divided with aperture area (so it is mean pixel value in star profile). Sky/pixel should be sky background per pixel. Width is, I'm not sure, probably either FWHM or Sigma of Gaussian fit, or Half flux.

Next two are previous value decomposed in X and Y direction, Angle is angle of elongation (2d Gaussian profile) and Roundness should be measure of how much star is round (ratio of short and long axis of FWHM/Sigma Gaussian ellipsis).

These are my assumptions, and there should be (probably) some sort of help file somewhere to explain exactly these values - just to make sure I got it right.

Btw, you can change all sorts of aperture photometry settings in Edit/Aperture ... dialog of image.

Main 3 that you wish to set for your images are:

Here aperture photometry works a bit different then I described above as there are 3 radii that you can set. First is radius of actual star (you should include any halo star has when you stretch it extremely), second and third define "ring" or "zone" where background sky value is measured so it can be subtracted from from star (in my example first and second radius where the same, but here you can put them at a distance - to avoid halo, depending if you want to include it - it is area of very low SNR and can skew results sometimes - when comparing relative brightness for example).

[Jkulin]

Thanks Vlaiv, you are gentlemen, much appreciated

[Jkulin]

Where are you getting the measurements panel up from Vlaiv?

[vlaiv]

7 minutes ago, Jkulin said:

Where are you getting the measurements panel up from Vlaiv?

When I select single aperture photometry tool and click on the star it shows up with all the measurements (add later measurements to the list).

It is the same measurement window that appears in regular ImageJ when you select analyze/measure, but I think it is triggered here with single aperture photometry tool.

[Jkulin]

Thanks Vlaiv, Yeah its a bit clunky but works, thanks for the help ?