Re: The Prize #7

["Richard Miles" <rmiles.btee@btinternet.com>]

>
> There are now 15,000 or so images up on matt.  Sample darks and flats
> could be made available there, but the images on matt are dark subtracted
> and flat fielded.  The uncorrected images are simply not available.  What
> has been saved is the dark and flat corrected images.
>

Yes - A login and pw for matt or mike would be useful as I am likely to have 
to search out what I need to test out the ultimate precision achievable 
using archived TASS images.

BTW: My proposal for a new data reduction methodology will also include 
proposals for some minor changes to the observing methodology in future 
(e.g. certain specific calibration routines) and a very minor tweek that 
will give you some extra precision/accuracy with little to no additional 
effort.

I should also mention that I have now been operational for about 7 weeks 
using my own twin V- and I-photometric telescopes of 60mm aperture so I am 
learning a great deal.  I am achieving high precision and accuracy but of 
course my fields are significantly smaller than TASS ones, which eases the 
requirements.
To give you an idea, I have had 12 observing runs and 9 good clear nights 
with V-band extinction coefficients, kv < 0.250.  For these 9 clearer 
nights, the exo-atmosphere zeropoint magnitudes in V- and V-I yield a 
standard deviation of 0.013 mag and 0.015 mag respectively relative to the 
mean of the zeropoints for all 9 nights.
Remember that it is quite a challenge to measure exo-atmosphere zeropoints 
accurately since you effectively derive 'local' zeropoints for stars over a 
range of airmasses and then extrapolate to zero airmass using a measure of 
the extinction coefficient.  Uncertainties in both these measures compound 
the error in the final exo-atmosphere zeropoint.  The local zeropoints are 
affected by catalog errors for the stars used as well as any inaccuracies 
arising from photon statistics and the way in which raw mags are extracted 
from the images.  The extinction coefficient depends on the change in the 
local zeropoint as a function of airmass, so errors in this slope lead to 
further increases in the scatter in the exo-atmosphere zeropoints.

I have been amazed by how well my set-up is performing, so I do hope this 
bodes well for TASS as I am sure some of the methodology I am using can be 
exploited by the project itself.

Richard Miles