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Re: Database error estimates
So then, for a given star, the ratio of starlight to sky
background light captured by the aperture depends on
where on the frome the image of the star falls. If you
point the camera randomly then the ratio varies randomly
with each observation.
Ok, finally it is clear to me why the scatter is so much less
when the system was tracking a frame across the sky.
Seems then that there would be much to be gained from precision
pointing. If every time a field were revisited the camera
could point exactly as it did the previous time the scater
in the data could be reduced, and we know by how much.
I know the Mk IV lacks encoders on the mount but now with the
possibility of real-time data reduction you can know _exactly_
where the camera was pointing when it took the previous frame.
--- Stupendous Man <richmond@stupendous.cis.rit.edu> wrote:
>
> >> The values stored in the database are calculated for
> >> each instrumental magnitude extracted from each image.
> >> They are based on simple photon statistics: an aperture
> >> of radius N pixels contains a total signal of E counts
>
> Chris asked:
>
> > Is the aperture fixed at N pixels?
>
> Yes.
>
> > Wouldn't it be better
> > if it were fixed at a constant angular diameter?
>
> No, I don't think so. A constant angular diameter
> would mean a radius which gets _smaller_ (in pixels) towards
> the corners. The FWHM tends to get larger (in pixels)
> towards the corners. So an aperture of fixed
> angular size would give the same number of
> electrons in the background for all stars (good)
> but provide a smaller fraction of the total stellar
> signal for stars near the edge (bad).
>
> The constant-pixel-size aperture gives a larger
> background contribution for stars near the edge,
> but also contains a less-variable-fraction of the
> stellar signal across the frame. I think, on the
> whole, that's better.
>
> Tech Notes 51, 52, 59 and 63 show in various ways
> how the PSF shape and size change across a Mark IV
> frame, and some effects on the photometry through
> a fixed aperture.
>
> > If someone would send me the terms of an X,Y to RA,.DEC
> > Nth order transform I'll work out the size of the error
> > I'm sure this transform must be a byproduct of the pipeline
> > and the numbers are available.
>
> I do not have these transformation values handy;
> all my old Mark IV analysis data was lost when some
> disk drives died a while back.
>
>
> Michael
>
>
> =====
> Chris Albertson
> Home: 310-376-1029 chrisalbertson90278@yahoo.com
> Cell: 310-990-7550
> Office: 310-336-5189 Christopher.J.Albertson@aero.org
> KG6OMK
>
>
>
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=====
Chris Albertson
Home: 310-376-1029 chrisalbertson90278@yahoo.com
Cell: 310-990-7550
Office: 310-336-5189 Christopher.J.Albertson@aero.org
KG6OMK
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