FreeLists / az-observing / [AZ-Observing] Re: Photometric Night Definition

["Keith Schlottman" <keith@xxxxxxxxxxxxxxxxx>]

Hi All,
Thanks for the discussion, helps a lot to hear a few different perspectives.
Best wishes for many photometric nights to come!
Keith Schlottman

-----Original Message-----
From: az-observing-bounce@xxxxxxxxxxxxx
[mailto:az-observing-bounce@xxxxxxxxxxxxx] On Behalf Of Jeff Hopkins
Sent: Monday, February 12, 2007 10:05 PM
To: az-observing@xxxxxxxxxxxxx
Subject: [AZ-Observing] Re: Photometric Night Definition

Hello Bill in Flag,

Those who have actually done photometry will understand that it is 
not the brightness of the sky and it is not the extinction. What is 
important is that the sky remains stable while the photometry is 
being done.
Extinction can be determined and it is whatever it is. A low 
extinction coefficient only means the sky is fairly transparent. On 
smoggy night in Phoenix when the transparency/extinction is horrible, 
good photometry can still be done as  long as the sky is stable. Is 
it a photometric night? In my book it is.

It can be raining hard early in the evening and then clear and 
provide an excellent sky for photometry. Then a few hours later it 
could be raining hard again. Was it a photometric night? Indeed, the 
only time that mattered was when photometry was being done. Was it a 
photometric rainy night, in my book, for my photometry it was. Some 
of those nights required measurements between lightning flashes, but 
it worked and produced excellent photometry.

The most important place to do photometry is at the meridian and the 
closer to the zenith the better. That is why one plans photometry of 
a star so it is close to the meridian. Sometimes a time series set of 
observation all night makes it necessary to do as much photometry as 
possible and indeed the further from the meridian the worse the data.

Surprisingly the Phoenix area is actually excellent for photometry of 
stars brighter than 12th magnitude.  There seems to be a bowl over 
Phoenix. Many nights are clear with clouds all around the border of 
Phoenix. Finding stars fainter than 6th magnitude can be a challenge, 
however, but with CCD photometry and GOTO scopes, it's relatively 
easy.

How do I know all this? I have been doing backyard photometry in 
Phoenix since the very early 1980's.

Jeff

At 19:35 -0700 02/12/2007, BillFerris@xxxxxxx wrote:
>From Brian's montly and annual weather summaries, he defines a photometric
>night as being "cloud-free from dusk to dawn." A night that has "at least
>three-consecutive hours of cloud-free skies" is partial. Thin cirrus 
>causing  "one
>magnitude or less extinction" puts a night in his spectroscopic category.
>Anything else is cloudy.
>
>Hopefully, Brian will chime in with the extinction he would associate with
a
>photometric night. My guess is, he'd consider any night with extinction  of
>0.2 to 1.0 magnitude per airmass to be spectroscopicf. Extinction of  about
>0.15 magnitude per airmass represents a typical cloud-free night in
northern
>Arizona. And extinctions of about 0.11 mag/airmass are seasonally  enjoyed
>during the winter when the atmosphere is virtually free of  aerosols.
>
>Bill in Flag

-- 
Jeff Hopkins
HPO SOFT
Counting Photons
http://www.hposoft.com/Astro/astro.html
Hopkins Phoenix Observatory
7812 West Clayton Drive
Phoenix, Arizona 85033-2439 U.S.A.
www.hposoft.com
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