Re: Calculation of error ?
Posted by PieterVuylsteke on May 21, 2021; 12:44pm
URL: http://astroimagej.170.s1.nabble.com/Calculation-of-error-tp1561p1569.html
As you can see, the values of the calbrated files (with the very low ADU (source) values) are not soooo much off, but they are.
The values for the Source_Amag_Err are also much more logical (0.02 to 0.002, depending).
By the way : Is there a way to get these calculated magnitudes directly inside the measeruments csv file ? I think they are interesting to evaluate the stability of the measurements ?
Now I will try proceed with a calibration in AstroImagJ itself. and that should even get better results.
Conclusion :
1. Don't ue PixInsight to calibrate the images
2. Yes there was a problem, Yes the spreadsheet of AstroImageJ indicated there was a problem, No the problem was NOT in in AstromImage, YES it was due to a bizare manipulation of myself.
Happily there are no stupid questions, they say !
Kind Regards,
Pieter.
Op do 20 mei 2021 om 19:36 schreef karenacollins [via AstroImageJ] <[hidden email]>:
URL: http://astroimagej.170.s1.nabble.com/Calculation-of-error-tp1561p1569.html
Hi Karen,
Your remark " Is the signal in the T1 aperture really that small?" made me think. A lot.
In fact, the calibration and extraction of the green channel in PixInsight did some bizare things with the ADU counts in the fits files. The numbers remain relatively (to each other) about the same, but are indeed much smaller. I did this calibration in PixInsight because I know the program, and because I had problems with other photometry programs that were not able to handle my files. But, AstroImageJ can !
I made a measurements of another amge sequence. Once of the not calibrated files, and once of the "treated" files.
Then, I calculated the magnitude of my 8 comparison stars through the fluxes, and the given formula in the paper.
From the not calibrated files (upper row is Source_Amag_Txxx, lower row is the calculated value as if it were a target star):
Your remark " Is the signal in the T1 aperture really that small?" made me think. A lot.
In fact, the calibration and extraction of the green channel in PixInsight did some bizare things with the ADU counts in the fits files. The numbers remain relatively (to each other) about the same, but are indeed much smaller. I did this calibration in PixInsight because I know the program, and because I had problems with other photometry programs that were not able to handle my files. But, AstroImageJ can !
I made a measurements of another amge sequence. Once of the not calibrated files, and once of the "treated" files.
Then, I calculated the magnitude of my 8 comparison stars through the fluxes, and the given formula in the paper.
From the not calibrated files (upper row is Source_Amag_Txxx, lower row is the calculated value as if it were a target star):
| 13 | 13.1 | 13.2 | 10.1 | 12.9 | 11.8 | 12.7 | 14.4 |
| 12.99394767 | 13.10828659 | 13.18441 | 10.09822 | 12.87875 | 11.79391 | 12.75057 | 14.40099 |
| From the calibrated files : | |||||||
| 13 | 13.1 | 13.2 | 10.1 | 12.9 | 11.8 | 12.7 | 14.4 |
| 12.98078259 | 13.09688139 | 13.16789 | 10.11251 | 12.8695 | 11.79084 | 12.74073 | 14.36663 |
As you can see, the values of the calbrated files (with the very low ADU (source) values) are not soooo much off, but they are.
The values for the Source_Amag_Err are also much more logical (0.02 to 0.002, depending).
By the way : Is there a way to get these calculated magnitudes directly inside the measeruments csv file ? I think they are interesting to evaluate the stability of the measurements ?
Now I will try proceed with a calibration in AstroImagJ itself. and that should even get better results.
Conclusion :
1. Don't ue PixInsight to calibrate the images
2. Yes there was a problem, Yes the spreadsheet of AstroImageJ indicated there was a problem, No the problem was NOT in in AstromImage, YES it was due to a bizare manipulation of myself.
Happily there are no stupid questions, they say !
Kind Regards,
Pieter.
Op do 20 mei 2021 om 19:36 schreef karenacollins [via AstroImageJ] <[hidden email]>:
Hi Pieter,
I sent you my email address in a private message. The address above is an attempt to encode to try to spoof automated searches for email addresses. You are correct that Mean Xnn should more precisely say the mean background subtracted pixel value in the aperture.
I think I'd need a measurements table to check to see if there is an amag uncertainty calculation error, or if the values reported are correct.
Karen
On 5/20/2021 5:26 AM, PieterVuylsteke [via AstroImageJ] wrote:
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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