Re: Some Real Work

["Thomas Droege" <tdroege2@earthlink.net>]

Arne,

I just looked at some data.  It is 1 to 1.5 pixels for both V and Ic for a
good night.  Yes, I too have made measurements where the error was less
then 0.01 mag rms.  These were made tracking a star E-W.  Since the gates
run E-W the star would track with a constant fraction on the gate.  Well,
actually it would very slowly move across the chip N-S depending on how
well the telescope was aligned.  So the results are consistent with a gate
transparency problem.  They are not consistent with anything else that I
can detect, and I have sure tried a lot of things.  I am quite interested
in trying your "local ensemble photometry" to see if it will make an
improvement.  

Independent of all this, I still make the point that the data appears to be
photon statistics limited where it will be most useful.  I think that it
good. Well, more a practical fact than good.  To me this indicates  that
most of the data is nearly as good at it can be.

I am still very interested in seeing the results of the plan you proposed
some time back of doing a short exposure survey of the whole sky from your
location.  I recall that you were first going to do a 10 second survey of
the whole sky (TN-48).  To complete such a survey in a reasonable time you
need to observe each field just once.  This would not give the apparent
advantage that I have seen tracking a star.  Such a survey would see the
same problems that I am seeing.  Then possibly we can identify their source
if it is not related to the CCD gate structure.  After a lot of looking,
the CCD gate structure is the only thing that is at all consistent with
what I see.  I would welcome another theory.  

Tom Droege

 


> [Original Message]
> From: Arne Henden <aah@nofs.navy.mil>
> To: <tdroege2@earthlink.net>
> Cc: tass <tass@listserv.wwa.com>
> Date: 11/5/2003 11:12:35 AM
> 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