taskGNU Astronomy Utilities - Tasks: task #14124, Include correlated noise and...


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task #14124: Include correlated noise and aperture errors

Submitter:  Mohammad Akhlaghi <makhlaghi>
Submitted:  Thu 18 Aug 2016 09:30:09 AM UTC
Should Start On:  Wed 17 Aug 2016 03:00:00 PM UTC Should be Finished on:  Wed 17 Aug 2016 03:00:00 PM UTC
Category:  MakeCatalog Priority:  5 - Normal
Item Group:  Enhancement Status:  Postponed
Privacy:  Public Percent Complete:  30%
Assigned to:  makhlaghi Open/Closed:  Open
Effort:  0.00

Thu 18 Aug 2016 11:18:48 AM UTC, comment #1: 

The first part of this task (to add a magnitude error column) has been pushed to the repo.

As explained in the commit message, this does not complete the issue, because we have to find good ways to include correlated noise and aperture estimation errors in the magnitude error (and the S/N) value too. So the task is still open.

But since I can't work on it right now, the status is marked as postponed, and the 30% progress is just an arbitrary value signifying that the column is made, but work still remains. If you would like to work on it while I am occupied, please leave a comment here and assign the task to your self ;-).

Mohammad Akhlaghi <makhlaghi>
Project Administrator
Thu 18 Aug 2016 09:30:09 AM UTC, original submission:  

Currently MakeCatalog can output the magnitude of an object, but it doesn't calculate the magnitude error. MakeCatalog does calculate the signal-to-noise ratio (S/N) however.

Converting from the S/N to the error in estimating the magnitude is an easy operation (detailed below) which can be done with something like AWK, but it is common enough to justify its inclusion as a column in MakeCatalog.

To derive the error in measuring the magnitude from the S/N, let's take `F' as the flux, `Z' is the zeropoint, `M' is the magnitude, `S' is the S/N, and `D' to stand for capital delta (or error in a value) then from

    `M = -2.5*log10(F) + Z'

we get the following equation after calculating the derivative with respect to F.

    `dM/df = -2.5*( 1/(F*ln(10)) )'

From the Tailor series, `DM' can be written as:

    `DM = dM/dF * DF'


    `DM = |-2.5/ln(10)| * DF/F'

But `DF/F' is just the inverse of the Signal to noise ratio, or `1/S'. So

    `DM = 2.5 / ( S * ln(10) )'

Note that this is just the raw flux measurement S/N, it does not include correlated noise, or aperture errors (which are also present in a realistic situation). This will be noted in the manual.

Maybe later, we can find ways of including these extra error sources directly in MakeCatalog (with the help of NoiseChisel in the form of FITS header values, ofcourse).

Mohammad Akhlaghi <makhlaghi>
Project Administrator


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    Follow 3 latest changes.

    Date Changed by Updated Field Previous Value => Replaced by
    2016-08-18 makhlaghi StatusIn Progress Postponed
        Percent Complete0% 30%
        SummaryMagnitude error in MakeCatalog Include correlated noise and aperture errors

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