Re: It's The Optics Stupid!

[Tom Droege <tdroege2@earthlink.net>]

In general, stopping down improves the optics.  Having two different 
exposure times does not save time since we run the cameras in pairs.   So 
given a choice, you stop down.

Tom Droege

At 02:25 PM 10/20/02 -0600, you wrote:

>This might be an easy question for someone in the know.
>
>Any reason the exposures could not be different for the two camera's, by 
>using the shutters at different times?  It wouldn't be to difficult to 
>have both exposures centered at the same time in the software.  This could 
>blur some variablility, but I wouldn't think too much.
>
>What might be the pro's/con's of each method?
>
>Thanks,
>Rob
>
>Tom Droege <tdroege2@earthlink.net> 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