Aperture photometry - number of stars

[miska]

Hi !
I am trying to use the aperture photometry script to estimate the magnitude I reach after a given exposure time.
The script runs fine. However, when I look at the results, I see that many stars detectable by eye are not detected by the script (the star that are detected have a green square on them).
So how do I get more faint stars into the routine ?

I have increased the magnitude of the catalog stars to get ~700 stars in my field. But still, I only get about ~80 PSF stars analyzed in the PSF extraction step.
I am a bit fuzzy as to what variables I need to tweak...

Thought ?
Thanks !

[Andres.Pozo]

Hi Miska,

could you upload one of the images that you are trying to measure? It would be also useful the result image with the detected stars and a copy of the log text of the process.

[miska]

The output I get is at the end of this message. All values at default, except the magnitude in the catalog ("stars" sub-menu).
I have asked for some storage space on the Endor server so I can show you the image file.
I did manage to go through the plate solve step. The image comes from a BPP processed DSLR data (and I extracted the luminance channel to work on the photometry). I did not touch any of the parameters in the "star flux" sub-menu. I have no idea what values I should change. Probably this could be the problem ?


run --execute-mode=auto "/Applications/PixInsight/src/scripts/AdP/AperturePhotometry.js"

Processing script file: /Applications/PixInsight/src/scripts/AdP/AperturePhotometry.js
Reference image: M51_RC10_L_Backgroundextracted
Catalog PPMX already loaded
Catalog PPMX size: 123 objects inside the image

Catalog stars: 728

DynamicPSF: Global context
1 view(s)
100 star(s)
88 PSF fittings
706.770 ms
Found 1 stars with invalid position
Reference stars: 88
Validating reference stars
Stars with problems: 1
Calculating image noise: sigma=13.462 count=61.52% layers=4

MultiscaleMedianTransform: Processing view: M51_RC10_L_Backgroundextracted_MMTbkg4
Multiscale median transform: done
Multiscale reconstruction: done
Normalizing sample values: done
24.413 s
Calculating Photometry: Processed 88 stars in 0.014000 seconds
Astrometric error: RMSx=0.1642" RMSy=0.1288" RMS=0.2087"
Writing output file: /Volumes/PICSBACKUP/Astrophotos/M51_RC10/M51_RC10_L_Backgroundextracted.csv
Invalid out format
Writing table: /Volumes/PICSBACKUP/Astrophotos/M51_RC10/Table_Flux_Ap8.csv

*******************************
Photometry process finished:
   1 of 1 images processed successfully.
   0 images with errors.

Thanks !

[Andres.Pozo]

You are using the catalog PPMX. There are only 100 stars of this catalog in your image. The script has got 88 good PSF measures. All of this seems correct.

If you want to measure more stars you should use a catalog with more stars and set a good magnitude filter.

I did not touch any of the parameters in the "star flux" sub-menu. I have no idea what values I should change. Probably this could be the problem ?

The documention of the script contains descriptions of all the parameters of the script.

[miska]

Ha !
I now set the catalog to USNO, and indeed, it finds many more stars. Progress ! Thanks !
I will now proceed to analyze the data that I get.

What I am after is a plot of measured magnitude (on my image) vs. known magnitude (from the catalog) for an individual frame and from the BPP processed sum of my frames.
Of course, on the screen, I see many more stars in the field in the BPP processed data, but I'd like to know how many magnitudes I gained by staying ~ 1hr on the source.

Thanks again for your help !

[Andres.Pozo]

The script doesn't calculate absolute magnitudes and it won't do it. The script only measures the flux of the stars and for performing absolute photometry it would need to do a lot of things more.

In any case, I think that you are using a DSLR without photometric filters, so you can not compare your images with stars from a catalog that uses one of the photometric standards. However you can analyze the fluxes in order to find what you are looking for.

[miska]

Yes, you are right, extracting absolute photometry is a can of worms I may not want to get into.  Although, perhaps extracting the R channel of my DSLR is not completely wrong if I take the R magnitudes of the stars, and use an average of many stars to "calibrate" the ADU to magnitudes conversion factor.

However, what I did was to compare the measured SNR (provided by the photometry package) as a function of the catalog star magnitude. That shows quite clearly the difference between a 2min exposure and a 1h one. No surprise there.

My idea behind all this, is that I want to explore how long I can expose before hitting the systematics (quality of the darks for example).
What I plan to do now is to get a really long exposure, and see if after 1,2,3,4 hours, I still improve in detectivity.