Re: Everett and Howell Paper

[Robert Creager <robert@carolinecreager.com>]

Hey Andrew,

Thanks for the info.  I'll chew on this information a
little.

One piece I'm still missing is your comment about the
circularly symetrical illumination, and how it solves
the pixel to pixel variation, but not across the field
of view.  What I'm thinking this means is that the
flatfield differences between pixels 100,100 and
100,101 can accuratly be determined, but because of
the 'polar' nature of the flatfield image, you don't
know the relation between 100,100 and 1200, 1200,
where the illumination is different (not truly flat). 
Is that correct?  If so, how would you treat this
'polar' composit flatfield different from the 'ideal'
flatfield, vs twilight, vs night?

Thanks,
Rob

---- Andrew Bennett <andrew.bennett@ns.sympatico.ca>
wrote:
> On Tue, 6 Nov 2001 17:33:16 -0700 , "Creager, Robert
S"
> <CreagRS@LOUISVILLE.STORTEK.COM> wrote:
> 
> >
> >See questions inserted inline...
> >
> >> -----Original Message-----
> >> From: Andrew Bennett
[mailto:andrew.bennett@ns.sympatico.ca]
> >> Sent: Tuesday, November 06, 2001 5:17 PM
> >> To: tass@listserv.wwa.com
> >> Subject: Re: Everett and Howell Paper
> >> 
> >> Actually, it doesn't need to be anything like
that good. To
> >> use the same set of reference stars, one needs to
do better
> >> than 1/10 image width if one puts up with losing
20% round
> >> the edges. Similarly, with flat field errors of
order 0.1
> >> mags across the field, one should be down at the
0.001 mag level
> >> if one only has to correct inside 1/10 x 1/10
image: basically 
> >> the sort of thing Arne has already done in the MK
III post 
> >> processing.
> >
> >I don't understand your 1/10 references, like
'better than 1/10 image
> >width'.  Is this tracking?
> If the position of the images on the sky are all
contained
> within 1/10 image size in both coordinates, you can
use
> a common reference area 9/10 x 9/10. You lose only
20%
> of the area. The present system can manage this only
> with a lot of luck. The declination drive is not
very
> repeatable. The polar axis alignment is currently
awful.
> I'm not too sure how well Tom can come up with the
same
> right ascension from night to night even though he
has
> got it down to seconds of arc from image to image on
the
> same night ... with luck.
> 
> >  'correct 1/10 x 1/10 image'?  I'm sure this will
> >spring out at me if you speak more slowly ;-)
> 
> With data set "TOM", we were trying to calibrate
> sources essentially randomly positioned within
> the image. This requires a flat field correction
> with small relative errors between pixels separated
> by the entire width of the image. The errors of
> this correction initially run 0.1 magnitudes or
more. 
> One attempts to cook this error by using comparison
> sources by any of a multiplicity of methods - I
> fitted polynomials; others use sub areas. However
> one does it, there are large residual errors. I beat
> the errors down to 0.008 magnitudes rms in one case
> as judged by the noise floor of variation for bright
> sources. This is not very good - entirely useless
for
> planet hunting.
> 
> If the locations of a star on the various images are
> concentrated in a smaller area of the CCD, the
problem
> is very much easier. The above quoted 0.1 magnitudes
> uncertainty is basically a quadratic term which
falls
> to 0.001 magnitudes across 1/10 image and the higher
> order terms are reduced by larger factors. If with a
> good deal of effort one can beat an initial 0.1 
> magnitudes down to 0.008 magnitudes rms, one should
> be easily ably to beat 0.001 magnitudes into
> invisibility. In fact, the residual errors will be
> dominated by the inaccuracies in measuring the local 
> pixel-to-pixel sensitivity variations.
> >
> >> > 
> >> Our flat fielding is entirely inadequate. We need
some
> >> sort of dome flat. The last time this was
discussed, we 
> >> turned up (among others) a design that produced
circularly 
> >> symmetrical illumination. IIRC this was good to
around 
> >> 0.1% (a few digits in a 12-bit system) and could
be built 
> >> to fit the MK IV lens. So flat fielding is a
solved problem 
> >> for pixel to pixel calibration (but not across a
4 degree
> >> image!) We should do it.
> >
> >I understand the circularly symmetrical
illumination, and I get the gist of
> >the pixel to pixel calibration, but how would this
information be used
> >practically?
> 
> A more precise flat field gives more precise stars.
> The present method, even ignoring the presence of
> stars, measures maybe 10,000 electrons/image. With
> say 30 images averaged, the resulting flat has rms
> errors 1/root(300,000) or 0.002 magnitudes. OK - not
> too bad. But in the real world you are not averaging
> but doing a median and the sky is full of stars to
be
> got rid of, not to mention little wispy clouds. This 
> results in a big increase in the noise.
> Worse, because of the star removal, the noise is
correlated
> for nearby pixels so you don't gain root(n) when the
> star covers n pixels - the improvement is less. This
> is not compatible with mmag photometry.
> 
> E & H used 120 images with 40,000 electrons/pixel
giving
> 0.0005 magnitudes uncertainty in the resulting
master
> flat. Per pixel. And the pixels are independent so
the
> resulting star magnitude uncertainties are
0.0005/root(n)
> for an n-pixel star. Overkill for the MK IV but
there is
> no reason not to do it!
> 
> >
> >Thanks,
> >Rob
> >
> 
> Andrew Bennett, Avondale Vineyard
> 
> 
>