[AZ-Observing] Re: Tonight
- From: "Bob Christ" <bchrist@xxxxxxxxxxxxxx>
- To: <az-observing@xxxxxxxxxxxxx>
- Date: Fri, 23 Feb 2007 14:33:56 -0700
Impressive Jeff, please - what is next week's winning lottery number. Do
tell.
Bob
Tom,
Since you are unfamiliar with photometry I'll try to explain.
While it is try that with CCD photometry you can produce a FWHM of
pixels for star images. The problem is it is much more a function of
equipment than the sky. If you used the same equipment, same exposure
time on the same star during different nights you might be able to
get some indication of the sky condition by examining the FWHM of
given star images. During some experiments with using a focal reducer
with the CCD camera I found the FWHM considerably smaller with the
focal reducer on the same stars the same evening than without the
focal reducer.
Most of my photometry is done with single channel UBV photon counting
and JH (infrared bands) analog detection. With the photon counting I
count each detected photon. A photon creates a pulse of current that
is amplified and then counted with a frequency counter. The brighter
the star the more photons per second get detected. Different filters
block photons outside a narrow energy/frequency range. The infrared
analog unit produces a current proportional to the flux of photons
striking the detector. The current is converted to a proportional
frequency and that frequency is then used as an indication of the
star's brightness.
Last night's observing produced sets of counts very close in number,
i.e., the counts for three readings were in some cases exactly the
same to 4 places. I take 3-10 second readings of a star in each
filter, correct for dead time (photon counting), gate time,
amplification (analog) and subtract the sky readings. The resulting
values are then converted to raw magnitudes which are then corrected
for color and extinction. A program star is bracketed by a comparison
star three times and the result normalized based on the standard
value of the comparison star. This results in three magnitude values
for each band. The three magnitudes are then averaged and a standard
deviation found. The standard deviation is an indication on how
precise the data is. Besides the equipment, which is constant, the
main factor determining the data spread or standard deviation is the
sky. A sky that produces a standard deviation of 0.01 magnitudes is
considered very good. Last night's V filter standard deviation was
0.0022 magnitudes, B was 0.0011 magnitudes and U was 0.0023
magnitudes.
Also I use a 12 mm eyepiece to center the stars for measurements and
last night the stars were very steady (exceptionally steady) in the
eyepiece. There was no noticeable wind at the observatories' levels.
Perhaps my location is unique where things that adversely affect
others seeing are cancelled out. I have recent records that go back
to 2003 that show a large number of good to extremely good nights for
this location.
Jeff
At 07:32 -0700 02/23/2007, Tom Polakis wrote:
>Jeff,
>
>Does your software measure the FWHM seeing, and if so, do you have a
>value from last night? I, too, was out last night attempting in
>vain (again) to split Sirius with my 4". By the look of things, the
>seeing wasn't as bad as it gets, but I would guess that it was its
>usual 2 or 3 arcseconds, when it was at its best.
>
>There is quite often very little correlation between scintillation
>viewed with the naked eye and seeing that plagues us when we're
>looking at an object through a telescope. I have seen a lot of
>nights like last night when the stars were hardly twinkling at all,
>but the telescope revealed "fast" seeing currents that makes a mess
>out of star images.
>
>Tom
>--
>See message header for info on list archives or unsubscribing, and please
>send personal replies to the author, not the list.
--
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.
(623)849-5889
www.hposoft.com
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