Re: It's The Optics Stupid!

[Chris Lloyd <cl@astro1.bnsc.rl.ac.uk>]

Tom,

I have finally got round to asking one of the optics experts here about
the difference in response to a point (unresolved) source and extended
emission. In quotes, theory, they should be identical, but in practice
scattered light from outside the field may find its way onto the detector
and produce a flat field that doesn't correctly describe the response of a
point source. How bad the deviation is of course depends on the
scattering. For a good quality commercial lens in a simple arrangement
with some internal baffles and a deep dew cap I would have thought the
problem would be small. In any event reducing the size of the aperture can
only improve things.

Cheers,

Chris

On Sun, 20 Oct 2002, Tom Droege wrote:

> It has been a struggle, but I have finally found something that changes
> (improves) the error.
>
> Yesterday I had Dan make aperture masks for the I lenses.  These have a
> center hole of 3 inch diameter, so it cuts out nearly half the light.  I
> did this because the I cameras see many more stars than the V cameras, so
> the sensitivity is wasted when we require V and I detections.  If the
> exposure is increased, then the I stars are lost due to saturation.
>
> This mask pretty well balances the two cameras.  The I camera still sees a
> little more sky.
>
> I then made a run with 200 second exposures instead of the usual 100
> seconds.  This required a little work to get the RA drive timed right, but
> now the largest error is due to TOM1 misalignment.  This is not easy to fix
> with the coo coo clock mount.
>
> A run was made (in spite of all that moonlight) where the telescope is
> returned to home after each exposure.  With the longer exposure, the
> brighter stars appeared on four frames.
>
> In the past, the V camera has always had significantly less spread in
> errors than the I camera.  See for example, Figure 3 and 4 from TN-88.
>
>  From this short run, the I data has less spread than the V data, and is
> about half the value of similar runs taken earlier this month.
>
> Is this a scatter problem???  The flat field should correct for optical
> gain unless light from a point source produces a different flat than one
> from a diffuse source.  Optical experts might comment.  Is this a standard
> problem?  Is there a standard solution?
>
> I do remember that Arne Henden wanted to cut down the aperture.
>
> Tom Droege
>
>