[Aavso-photometry] GMRT Campaign Update

[Brad Walter]

Oh, one more thing that I learned from the experience was to make sure
the hole was located between the secondary vanes. The first time I
mounted the mask, my Monk like quest for symmetry led me to put the hole
on a straight line between the center of the secondary and the mount. Of
course, that happens to be where one of the three secondary vanes is
also located.  The result was very pronounced diffraction spikes.
Rotating 60 degrees Solved the problem. Another testimonial that the
stupidity quotient increases after dark. 

-----Original Message-----
From: aavso-photometry-bounces at mira.aavso.org
[mailto:aavso-photometry-bounces at mira.aavso.org] On Behalf Of Brad
Walter
Sent: Friday, March 18, 2005 3:32 PM
To: aavso-photometry at aavso.org
Subject: RE: [Aavso-photometry] GMRT Campaign Update


Captured about 4.5 hours of V data last night for 70 Vir using a
restricted aperture on a 10" cassegrain variant fitted with an ST7E
NABG. That got me to 7 second exposures for around 20,000 ADUs. I used a
76mm off-center hole as the restriction. The restricted aperture caused
some "bloating" of the bright object star similar to the effect of a
poor seeing night with an unrestricted telescope.  The flats for this
configuration also showed significant off-center vignetting that results
from the restriction. The flats should correct for it but since the
intensity variation over the surface of the chip is 5 to 6 times as
great as with an unrestricted aperture, I expect it to cause some
reduction in accuracy.

Has anyone experimented with an annular ring restriction rather than an
off center hole? The vanes to hold the center of the inner obstruction
of the annulus in place could be made to line up with the secondary
holder vanes to mitigate any additional spiking.