[Aavso-photometry] Low-pressure sodium lights and photometry

[Michael Newberry]

Hi Bob,

There is another broadband system that was designed to reduce the effect of 
the Na-I 5890 doublet on broadband photometry: the Gunn-Thuan "uvgriz" 
system (as in Jim Gunn, or James E. Gunn, now at Princeton). The uvgri, and 
z refer to the bandpasses, of which g is blue shifted relative to Johnson V 
(the Gunn v refers to vio;et, not visual) and the tail of the r filter 
begins near the edge of the sodium emission---hence the system is almost 
blind to D-line emission from low pressure sodium lights. Although broadband 
V photometry is devastated by low pressure sodium emission, the Gunn-Thuan 
system permits sampling the spectrum across the visible range by defining 
the g and r bands so as to avoid the D line. The Gunn-Thuan system was 
developed at Palomar to allow photometry mainly of faint, high redshift 
galaxies in spite of the growing brightness of sodium vapor lights from the 
ever-sprawling Los Angeles basin. There is no reason this system cannot give 
good results for stellar photometry. One wrinkle is that there are well 
defined standards but not large numbers of them, so that means more 
calibration time spent moving the telescope. Another, wrinkle---the real 
show-stopper---is that you must find a filter set and I don't believe there 
are any commercial, off-the-shelf alternatives. However, if there was 
"enough" interest, I am sure a competent filter vendor would see the 
business potential in producing one.

Michael Newberry



----- Original Message ----- 
From: "Robert Harmon" <roharmon at owu.edu>
To: <aavso-photometry at mira.aavso.org>
Sent: Saturday, February 25, 2006 10:51 PM
Subject: [Aavso-photometry] Low-pressure sodium lights and photometry


> Hello, all,
>
> I'm an astronomer at Ohio Wesleyan University, which is about 20 miles
> north of Columbus. OWU owns and operates Perkins Observatory
> (www.perkins-observatory.org), which houses a 32-inch reflector. Because
> it's f/17, it's not very useful for CCD photometry as the field of view
> is about 3x2 arcmin with our SBIG ST-8E. Thus, the telescope is used
> mostly for visual observing by students and the many visitors to our
> twice-weekly public programs. However, I do use 8-inch and 12-inch
> telescope on site for BVRI photometry.
>
> To make a long story short, there is a strong possibility that a
> Wal-Mart Supercenter and a Toyota dealership will be built within a mile
> of the observatory to the south. The good news is that both would be on
> land annexed by the city (pop. ~22,000) in which OWU is located (the
> observatory is on the outskirts of town) and thus subject to lighting
> code restrictions which would require them to use low-pressure sodium
> lamps on 28-foot poles, as opposed to typical broad-spectrum parking lot
> lights on 40-foot poles. (Wait until the Toyota dealer sees what his
> cars look like under LPS light!) The bad news, of course, is that the
> lovely yellow glow will still most likely annihilate the naked-eye views
> from the observatory grounds and degrade telescopic views of objects
> like galaxies and clusters.
>
> I have two questions for the list. Any insight would be greatly 
> appreciated!
>
> 1. It seems to me that LPS lights should have a minimal effect on BRI
> photometry, but that V might be a different story. Am I correct in my
> thinking? Has anybody succeeded in doing good V-filter photometry in the
> presence of strong LPS skyglow? Any tips?
>
> 2. Does anybody know of references for estimating the level of skyglow
> from a given number of LPS lights at given wattages from a given
> distance? It would certainly help me make my case more effectively in
> community meetings if I could give some quantitative estimates of how
> bright the skyglow will be. Alternatively, does anybody know of any
> photos showing skyglow from LPS lights at a single large parking lot
> from a mile or a few miles away? There aren't any comparable facilities
> in my area.
>
> Regards,
> Bob Harmon
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