RE: Some Real Work

["Gutzwiller, Michael" <mgutzwiller@lanvision.com>]

From my experience I don't think it's a gate structure problem either.  I
have successfully gotten the noise floor to 0.005 mags when using multiple
images but only if the set of images were from a single set of exposures
where the Mark IV system was tracking.  Because my alignment was not perfect
the stars were not at exactly the same position from frame to frame but
would drift up to 20 pixels over a two hour period.  When I combine images
from more than one night however there were systematic changes.

Foe example, one night I would have a set of twenty images of star A that
would have an average V mag of 7.000 with a sigma of 0.005 and the next
night an average of 7.050 with a sigma of 0.005.  The two sets of images
would not be pointing at the same part of the sky but may be off by a degree
which put the star in a completely different area on the CCD, not just 20
pixels.

To me this points to either flat field errors or zero point errors.

Thanks,

Mike Gutzwiller



-----Original Message-----
From: Arne Henden [mailto:aah@nofs.navy.mil] 
Sent: Wednesday, November 05, 2003 12:21 PM
To: tdroege2@earthlink.net
Cc: tass
Subject: Re: Some Real Work


Thomas Droege wrote:
> We see plainly the "noise" floor at 0.05 mag.  As we collect more and
> more photons, the measurement does not get better.  Sigh!  I think the 
> reason for this is the construction of the CCD.  The front side gate 
> causes differing sensitivity depending on where the image falls on the 
> gate structure That is what you get for trying to cover the whole sky in 
> a lifetime.  The tiny star image is small compared to the pixel and so 
> the measurement cares where the image is sitting.  This limits the noise 
> to the 0.05 mag range.  This is *not* the accuracy measure for a large 
> number of points.  That is something else.  It could be much better for 
> a large enough set of measurements, it could be much worse.  It is the 
> subject of another set of calibrations. 
>  

I'm pretty sure we discussed this back many years ago since front
illumination is a known limitation in photometry.  However, I don't
think this is the case for the Mark IV.  Tom, can you remind me of
the current fwhm in pixels of the images you are taking?  Usually
the gate structure problem is a blue thing since that is where the
gates become opaque.  At V, there is little effect; at Ic, there
is no effect.  My photometry of BP Vul and CY Aqr from 2000 had
much less than 0.05mag rms error (I can post those plots again
if anyone is interested), and I recall Michael R. showing
some results from TOM1 with a floor much less than 0.05 per image.
I can believe night-to-night offsets that cause such a noise floor,
but that is not CCD related but rather calibration related.
Arne



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