Multi-aperture Data - What do values describe?

[FRogers]

After performing Multi-aperture photometry using three concentric apertures, the data for each star collected (C-2 through C-188 in our images) the Multi-aperture Reference Star Settings window opens. From here we can save a table that contains the data contained in the Measurements window. Some of the values are: rel_flux, rel_flux error, flux SNR, Source SNR, Source_Sky, Source_Error, etc.

I've found that source_sky relates to Signal and source_error relates to Noise, as dividing these two give the Source_SNR. However, which value from the Measurements window gives the actual counts from the source (star)? I've looked through the manual, but can't find any description of what the values are or what units are associated with the values. Can someone please outline the characteristics and units for the values in this table?

Thanks!

[karenacollins]

The data naming convention for the measurements table columns is listed at Multi-Plot_Main->Help->"Data naming convention...".

The closest data set to raw integrated counts is Source-Sky_xx, which is the sum of all the ADU within the aperture, after subtracting the sky background from each pixel. In other words, this value is Source minus Sky, or net integrated counts.

If you really need the total counts in the aperture without sky background subtraction, set the inner and outer radii of the sky background annulus to the same value (say r1=40 and r2=40) and rerun photometry. Then the sky value subtracted will be zero and the result will be the total of all ADU inside the aperture.

Does this give the measurement you are seeking?

Karen


P.S. Here is a copy of the data at the help panel mentioned above:

Data Naming Convention

NOTE: A suffix _xx below denotes the object identification, where xx = T1, C1, T2, C2, T3, C3, …

Saturated – zero indicates that the peak value of the target and all comparison stars is less than the value set in the “Set_Apertures” module. A number other than zero indicates that highest peak value that exceeded the set limit.

J.D.-2400000 – Julian Day of the observation (UTC based). Availability requires UTC time-of-day information in the FITS header of the observed data.

JD_UTC – Julian Day of the mid-point of the exposure for this observation (UTC-based). This value is inserted into the FITS header by the Data Processor module when the “FITS Header Updates” option is enabled and target coordinates and observatory geographic location are provided. In addition, UTC time-of-day and exposure time information are required to exist in the FITS header of the observed data.

HJD_UTC – Heliocentric Julian Day of the mid-point of the exposure for this observation (UTC-based). This value is inserted into the FITS header by the Data Processor module when the “FITS Header Updates” option is enabled and the target coordinates and observatory geographic location are provided. In addition, UTC time-of-day and exposure time information are required to exist in the FITS header of the observed data.

BJD_TDB – Barycentric Julian Day of the mid-point of the exposure for this observation (BJD_TDB-based). This value is inserted into the FITS header by the Data Processor module when the “FITS Header Updates” option is enabled and the target coordinates and observatory geographic location are provided. In addition, UTC time-of-day and exposure time information are required to exist in the FITS header of the observed data.

ALT_OBJ – altitude above the horizon of the target object at mid-exposure. This value is inserted into the FITS header by the Data Processor module when the “FITS Header Updates” option is enabled and the target coordinates and observatory geographic location are provided. In addition, UTC time-of-day and exposure time information are required to exist in the FITS header of the observed data.

AIRMASS – airmass of the target object at mid-exposure. This value is inserted into the FITS header by the Data Processor module when the “FITS Header Updates” option is enabled and the target coordinates and observatory geographic location are provided. In addition, UTC time-of-day and exposure time information are required to exist in the FITS header of the observed data.

Source_Radius– radius of the aperture used to calculate total integrated counts. In fixed aperture mode, this value is the aperture radius set by the user. When the variable aperture mode is used, this value represents the actual aperture radius used considering the object’s full width half maximum (FWHM). In variable aperture mode, Source_Radius = (Width_x1) * (FWHM_Mult).

FWHM_mult– the FWHM multiplier value set by the user in variable aperture mode. In variable aperture mode, Source_Radius = (Width_x1) * (FWHM_Mult). In fixed aperture mode, this column is not included in the table.

Source_Rad(base) – in variable aperture mode, this value represents the fixed aperture radius set by the user to measure the FWHM. This value should be set to a number greater than 1.5 times the maximum FWHM expected. In fixed aperture mode, this column is not included in the table.

Sky_Rad(min) – radius of the inner edge of the annulus used to calculate the sky background

Sky_Rad(max) – radius of the outer edge of the annulus used to calculate the sky background

X(IJ)_xx – x-location in ImageJ coordinates of the center of the aperture located at object xx

Y(IJ)_xx – y-location in ImageJ coordinates of the center of the aperture located at object xx

X(FITS)_xx – x-location in FITS coordinates of the center of the aperture located at object xx

Y(FITS)_xx – y-location in FITS coordinates of the center of the aperture located at object xx

rel_flux_Tx – the ratio of target Tx’s net integrated counts to the total of all comparison star net integrated counts. Mathematically the value is Source-Sky_Tx / SumOfAll(Source-Sky_Cx )

rel_flux_Cx – the ratio of the comparison star Cx’s net integrated counts to the total of all OTHER comparison star net integrated counts. This value requires at least two comparison stars be selected. Mathematically the value is Source-Sky_Cn / (SumOfAll(Source-Sky_Cx)), where x not equal n. For clarification, target star net integrated counts are not used for these calculations.

rel_flux_err_xx – the error in the relative flux for object xx. The error is calculated by propagating all Source_Error_xx values through the equations defined for rel_flux_xx above.

rel_flux_SNR_xx – signal-to-noise ratio for rel_flux_xx. This value is simply rel_flux_xx / rel_flux_err_xx.

Source-Sky_xx – net integrated counts within the aperture located at object xx. Net integrated counts is defined as the sum of all pixels whose center falls within the aperture source radius, less the sky background per pixel. Sky background is either the average value of the pixels in the annulus defined by Sky_Radius(min) and Sky_Radius(max), or the value from a plane fitted to the values in the sky annulus. The user can select which method to use for calculation of sky background in the “Set_Aperture” module.

Source_Error_xx – error in Source-Sky_xx. The calculation includes contributions from read-out-noise, dark current, and source and sky poisson noise as defined by Merline, W. & Howell, S.B., 1995, EXP. ASTRON., 6, 163. The equation is:
SourceError = SquareRoot ((NetSourceCounts * gain +SourcePixelCount * (1 + SourcePixelCount / SkyPixelCount) * (TotalSkyCounts * CCDgain + DarkNoisePerPixel + ReadOutNoisePerPixel * ReadOutNoisePerPixel + CCDgain * CCDgain * 0.289)) / CCDgain)

Source_SNR_xx – signal-to-noise ratio for Source-Sky_xx. This value is simply Source-Sky_xx / Source_Error_xx.

tot_C_cnts – the sum of all comparison star net integrated counts. tot_C_cnts = Source-Sky_C2 + Source-Sky_C3 + …

tot_C_err – the error in tot_C_cnts. The error is calculated by combining Source_Error_C2 + Source_Error_C3 + … in quadrature.

Peak_xx– the highest pixel value within the aperture for object xx (NOT background subtracted)

Mean_xx– the mean pixel value within the aperture for object xx (NOT background subtracted)

Sky/Pixel_xx – the mean pixel value within the sky-background annulus for object xx

X-Width_xx – the X-direction FWHM of the PSF of object xx. The aperture radius must extend beyond the object’s PSF for this number to be accurate.

Y-Width_xx – the Y-direction FWHM of the PSF of object xx. The aperture radius must extend beyond the object’s PSF for this number to be accurate.

Width_xx – the mean of the X- and Y-direction FWHM of object xx. The aperture radius must extend beyond the object’s PSF for this number to be accurate.