RE: [APML] The Glory of Rho

["Thomas W. Earle" <twade@bmi.net>]

Chris,

Yet, another excellent image from your trip to NM.  

I hope to capture Rho with my Pentax 300mm EDIF F/4.0 lens this
upcoming New Moon.  I'll be using E200 too since it surprisingly
recorded the blue spectrum quite nicely under your very dark sky.
It's definitely a challenge capturing these southern objects from
45N.  We only have 3-3.5 hours of total darkness this time of year.
I feel for those that are even farther north.  Despite having some
of the darkest skies in the continental US, Mother Nature doesn't
allow many "cloud free" opportunities this far north, especially
during New Moon.  Just this past New Moon, I sure was wishing I
still lived in West Texas.  The Water Vapor was red the entire New
Moon window which means excellent transparency.  I had to battle
high clouds the whole time.  I did try capturing Rho with Provia
100F pushed 1 stop.  I exposed it for 75 minutes.  I initially was
going to go for 90 minutes but I knew I didn't have good enough
polar alignment judging from the corrections the STV was making.
Albeit, I'm not certain if it was poor polar alignment or the gusty
wind causing the corrections.  It was a bad night anyway with 15-25
mph winds and patchy cirrus.  I'll get them developed this upcoming
week.  If I have any images worth publishing, they'll be ready for
viewing late July.

It still floors me that you are able to expose for 40 minutes at
F/2.8.  Using Michael Covington's Astrophotography Calculator, it
shows the maximum exposure (i.e. sky fog limit) for your setup is 45
minutes.  Since I'll be shooting at F/4 and pushing E200 a full stop
I figure my limit is probably around 75 minutes.  As a result, I'll
take two shots of Rho Ophiuchi: one 60 minute and one 75 minute.

Again, thanks for sharing your excellent images over the past week.

Wade


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