Re: It's The Optics Stupid!

[Tom Droege <tdroege2@earthlink.net>]

Chris,

One problem with the lenses, there are no internal baffles.  I do have a 
light shield on TOM1 that is about a focal length long.  In case you have 
not been around, these lenses were designed from scratch for us by Elliot 
Burke.  He ran out of time, and I had someone else design the lens 
tubes.  These were designed without any internal baffles.  Sigh!  To remind 
you all, the lenses are designed to be narrow bandwidth.  Thus with fewer 
elements, we get a much flatter field than a common (or not so common) 
camera lens.  Our lenses are down 5% at the edges, and at most 10% in the 
corners.  This compares to Pojmanski's camera lenses which are down 50% in 
the corners.

I guess I need to take a lens apart and do an internal walnut shell paint 
job and see if there is any improvement.

Tom Droege

At 03:12 PM 10/25/02 +0100, you wrote:
>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
> >
> >