FreeLists / az-observing / [AZ-Observing] Re: A Sunset Excursion

["Jimmy Ray" <jimmy_ray@xxxxxxx>]

Loved it Dean...Great sequence! 

Jimmy Ray

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Sent: Sunday, February 11, 2007 8:42 PM
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Subject: [AZ-Observing] A Sunset Excursion


Hi All-

A couple friends suggested I post this, so since the clouds make any sort
of observing difficult, I hope you enjoy...

Back nearly 20 years ago, I happened to be going up the Mount Lemmon
Highway nearly every evening to use the then-16" Schmidt camera to image
Comet Bradfield (it is now the 30" Schmidt of the Catalina Project...). 
Anyway, this was near winter solstice, and as a former Kitt Peak employee,
I thought it would be cool to catch the sun setting behind the
observatory.  A friend of mine, Ed Strittmatter, (or was it his brother
Don?) imaged the sunset behind the then-smaller observatory near Ryan
Field back in the 60s...  Anyway, over the course of a few trips up Mt
Lemmon, I found where to set up near winter solstice (near milepost 9.5 at
a wide pullout!).  I got a couple frames back in '88 taken with my 12"
Newt that I had, and in fact, it was made into a postcard once sold at
KPNO.  Since the profile changes with time (the #3-16 is gone, the #1-36
is replaced by WIYN, the 20" public scope and the 72" Spacewatch is new
also) an update was overdue.

So I went out on the 23rd of December to reshoot a sequence to make into a
GIF file.  I used the 11cm F/7 William Optics APO and eyepiece projection
to get the correct image size on the Canon 20Da.  Aligning this shot is
always tough - usually I set up about an hour before sunset, but you have
to compose and focus looking very nearly right into the sun.  The images
are taken through a Thousand Oaks filter.  There was a very nice inversion
layer that I was above, and the seeing was quite good.  Taking a frame
every 5 seconds, I got the following sequence:

http://alice.as.arizona.edu/~ketelsen/KPNOSunset061223.gif

Unfortunately, I wasn't in the perfectly correct spot - the sun never
fully clears the outline of the 4-meter on the right, so I went up again
the next day to do it again.  But where to set up?  Looking at the image
carefully, I figured I needed to shift the sun's image about 10% - about 3
arcminutes to the north.  From a computer program, I saw that the sun
moved about an arcminute north by itself, since it was post-solstice.  So
I needed to shift the image of the mountaintop an additional 2 arcminutes
south.  In round numbers, the mountain subtends about 3,000 feet in
profile, and nearly matches the sun's diameter of 30 arcminutes.  Since
the sun is off at infinity (for this purpose), each 3,000/30=100 feet
shift of my setup point will "shift" the mountain an arcminute against the
sun.  So on the 24th I set up an additional 100 feet north to get a
hopefully perfectly aligned sequence.

Of course, fate always conspires...  The inversion layer had raised, the
seeing was terrible, and when taking this sequence, while adjusting focus
between frames (every 4 seconds...) I managed to shift the telescope and
only recently got a photoshop expert to help me realign the last dozen
frames with the rest.  But the observatory is perfectly aligned to the
sun's image.  When looking at the individual images, because of the
northward march of the sun, there really is only 1 image that you could
consider perfect - otherwise the observatory is closer to one side or the
other:

http://alice.as.arizona.edu/~ketelsen/KPNOSunset061224.gif

Questionable seeing, but what can you do?  I know, do it again next year
and cross your fingers!

-Dean
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