Re: Database error estimates

[droege@snapmail.us]

Andrew,

If you are looking at early data, then we may have been running in track
mode where we followed one field for a long time.  This indeed results in
smaller sigmas.

Most of the "Engineering" data was taken where we looked at a random spot
in the sky.  This results in stars being imaged at many RAs but mostly the
same declination.  This tracking data does have a smaller spread.  I
think, as I have said many times, that tracking just hides the real
errors.  Comments anyone?

Tom Droege

> I have downloaded and plotted error estimates
> for some selected areas.
>
> I don't understand the results!
>
> For example, selecting MV in 11.9 to 12.1 I find a
> range of standard deviations in the Database from
> 0.012 to 0.091 mags with clusters around 0.020,
> 0.040 and up, implying a wide range of
> exposure times. Not true, I think?
>
> The case that I can check, because I have the raw
> data is Day 2452605: files HV/IRA2604797-863.
> Specifically for a star of MV 12.0 near the centre of
> HVRA2604797 I estimate a standard deviation of
> 0.061 mags for my least squares PSF fitting method
> and around 0.064 to 0.067 mags for aperture methods,
> depending on the size of the aperture.
>
> The standard deviations in the Database are 0.019
> to 0.022 mags for MV 11.9 to 12.1 in this image.
>
> My estimate agrees with the Database values for data
> on Day 2452301 (i.e. before the start of "Engineering"
> data) but not for any of the more recent data. Did
> we have it right once and then "improve" the system?
>
> I used:
> Background s.d. 28.56 digits excluding sky background
> Sky background 1886.3 digits
> MV 12.0 equals 3321 digits total, 175.8 digits peak pixel
>
> Andrew Bennett, Avondale Vineyard, NS, Canada
>
>
>