Re: Calculation of error ?

Posted by karenacollins on May 20, 2021; 2:29am
URL: http://astroimagej.170.s1.nabble.com/Calculation-of-error-tp1561p1565.html

Hi Pieter,

In your original post, you note:

These are some numbers for the first photo :
Source-Sky_T1     12.459513
Source_Error_T1      10.131787
Source_AMag_T1     9.374163
Source_AMag_Err_T1   0.737586

The Source_Error_T1 = 10.1 is almost as big as the actual number of Source-Sky_T1 = 12.5 integrated counts in the aperture. Is the signal in the T1 aperture really that small?

I can dig deeper if you can send my your measurements table file (and plotcfg file if you've saved one) to karenacollins atata outlook dodot com.

Karen

On 5/19/2021 5:27 PM, PieterVuylsteke [via AstroImageJ] wrote:

Hi Karen,

Thank you for your answer. 

I do find it bizare that the Amag error is indicated as being 0.7, whilst this is the curve of the star, with a "visible" error of about 0.02mag. That is quite a difference ?

Or do I make a mistake in interpretation ?

Kind Regards,
Pieter.

Op wo 19 mei 2021 om 22:55 schreef karenacollins [via AstroImageJ] <[hidden email]>:

Hi Pieter,

Yes, AMag error is in magnitude. However, Source_Error is in ADU (or counts), rather than magnitude. The error will always be given in the same units as the value.

Appendix D in the AIJ paper here describes most or all of the data values found in the measurements table.

Regarding your question: "Do I still have to enter values for the readout and dark current noise, when treating them as in Image Sequence in AstroImageJ ? I did enter the e/ADU value for the gain of the camera, and left those two other values at zero."
- AIJ will still work correctly if readout and dark current are set to zero. However, the photometric errors/uncertainties may be underestimated. This will be less of an issue for bright targets, and more of an issue for faint targets. In either case, only the calculated uncertainties will be affected, not the nominal measured values.

Karen

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