Re: Calculation of error ?

Posted by PieterVuylsteke on May 20, 2021; 9:25am
URL: http://astroimagej.170.s1.nabble.com/Calculation-of-error-tp1561p1566.html

Hi Karen,

I tried to send the information to your outlook email address, but gmail says you do not exist... 
Could there be some slight imperfection in this mail adress :  arenacollins atata outlook dodot com.

I found several other mail addresses of you, but did not want to spoil those mailboxes. 

this is the content of the mail I wanted to send :
Hi Karen,

I ran the calculations again, now with the apertures suggested in the seeing plot. They are a bit smaller than in my first run, but give basically the same values.

In the word document I tried to document as much as possible the settings.

I added in the beginning of the word document some statistics that I took with PixInsight of the background, and of the star. 
The maximum ADU of the star is 25000, maybe "a bit low", but not "terribly low" (?), as the max for this 16bit sensor is 65383.


The only bizare thing I found after comparing the values in the excel with the explanation in the pdf that explains the formules is that 
Mean_T1 = Source-sky_T1 /  N_src_Pixels

Although in the document it is stated : 
Mean Xnn: the mean pixel value in aperture nn (not background subtracted)

But, if I compare Mean_T1 with Sky/Pixel_T1 it reads :
0.03308 compared to 0.003069

So, it can not really have a big impact.




In any case, thank you very much for willing to investigate this !

Kind Regards,
Pieter Vuylsteke.  

Op do 20 mei 2021 om 04:29 schreef karenacollins [via AstroImageJ] <[hidden email]>:

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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