It's The Optics Stupid!

[Tom Droege <tdroege2@earthlink.net>]

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