Observer's Forum

edited by John Isles (ILS)

This is the place where you can share observing experiences, make suggestions, ask for advice, and sound off on any topic likely to interest other observers. Material for inclusion in the Observers' Forum can be sent to the Observers' Forum editor:
John Isles, 11105 Tremont Lane, Plymouth, MI 48170   e-mail: jisles@voyager.net

In this Observers' Forum we are pleased to have interesting contributions from Mike Simonsen and Marv Baldwin. I've also included a few extracts from a discussion from the AAVSO's Internet group on observing from the city. You can read the whole thread in the archives at http://mailman.McMaster.CA/pipermail/aavso-discussion/.

Observing from the City

Bobby Williams wrote: I live in downtown Atlanta, surrounded by the worst light pollution you could imagine. I usually drive to sites well outside the city to do my observing. However, last night I took out my binoculars (15x70s) to take a look at Orion and Auriga from my balcony. I was amazed to discover that I was locating 8.0-magnitude stars with little difficulty. I quickly realized that I could do a number of variables from my apartment.

My main concern is this: Are there certain types of variables that shouldn't be observed from extremely light-polluted skies? I know that the Moon can bias certain types of observation, so I assume that man-made light would raise similar issues.

Claudio Veliz replied: For almost 14 years I've been observing from downtown Manhattan. No variables — I travel to Vermont for that. But if confined to the city, I have discovered that Sunday evenings are best for more intense observing runs. Why? Well, here in New York, the maintenance crews in the buildings are off. During the week, many of the business towers are illuminated at night, less because of late workers than because of the small armies of cleaning crews. Also, Sunday evenings are (by NY standards) relatively quiet regarding regional traffic, so the cumulative effect of a few million fewer headlights is palpable.

About 6 years ago I engaged in a relatively loose (read: unscientific) PEP survey of different nights over about an 8-month period. Using an SSP-3, I aimed straight up, no scope, no lenses, just the SSP-3. The only requirement was that the sky be clear; no clouds or haze. Anyway, over that period, Sunday evenings displayed an average of 1.5 magnitudes improvements over other evenings, all of which were within 0.5 mags of each other, fairly randomly. Worst night was Friday, followed by Saturday, not too surprisingly.

Bob Young commented: Those results are roughly what we found with a null visual photometer around the Harrisburg, Pennsylvania, area about 10 years ago. I believe the effect of vehicle lights is greatly underestimated. We found Friday nights to be the brightest, owing to many outdoor activities and traffic. It's good to have a numerical value for the Sunday-night difference.

Also, we found that from evening astronomical twilight, the sky darkened linearly until about 11:30 PM, then leveled off at the darkest level about 12:15 AM, where it remained until morning Astronomical Twilight. We nicknamed this 12:15 AM moment, "local artificial twilight."

Claudio Veliz replied: For a couple of months I noticed a consistent 0.25-mag improvement in seeing (based on the SSP-3 readings) at EXACTLY midnight. This remained a mystery on the charts for some time. The gods were having fun with me. It was a bewitching hour. It was hocus-pocus. Well, not quite....

One evening many years ago when doing this whole thing, I was at a late party and standing with some friends, martini in hand, on a balcony. I was looking at the glorious old Empire State Building about 7 blocks away. Midnight arrived and the building's display lights (the multi-colored things that shine vertically, up, every night) shut off. As did those of about two dozen other buildings in midtown over the next 3 to 5 minutes. End of mystery.

The point is, local conditions can affect these findings considerably. One simply needs to get to know the 'habits' of the light pollution sources in one's region.

Richard Huziak (HUZ) reported: I've been observing variable stars for 24 years, often from the dead center of Saskatoon (Canada), a city of 220,000. About half of my observing is done from the city, half at the dark site. Comparing my estimates with everyone else's by using the really cool light-curve plotter on the AAVSO website, I find that it makes no difference at all from where I do my estimates. Whether in town or out, the estimates usually fall into the center of the distribution. I suppose the rule is "if you can see the variable and comp stars, you can make an estimate!"

By the way, the in-town sky here allows for estimates often down to 14.5 mag with my 10" Newtonian, while dark sky goes to about 15.6. So if you can find a DARK backyard with few neighboring yard lights and no streetlights, you may not lose that much in town. I've also talked about variable astronomy to all of my backyard neighbors—they understand, and turn off their porch lights most clear nights!

I often encourage the members of our local astro club to observe variable stars FROM THE CITY, since many of them whine and complain that they can never 'get out (of town) to observe' the deep-sky stuff they're hooked on . Easy solution—observe variables from IN town! It's a really great way to keep the observing fix going!

Patrick McDonald added: I've found that having the C14 Schmidt-Cassegrain on the third-floor walkout enables me, looking east over the suburbs, to get down to magnitude 13+ on a clear night. Looking west over the center of the city, below 40 degrees or so, I'm more in the 12's. Like my Saskatoon colleague, I've found that a light hood (In my case, an old dressing gown) helps a lot. One has to be careful on cold nights, though, of condensation from breath fogging up the lens in this small space. I also have pieces of cardboard or plywood veneer arranged around the "fence" of the walkout space as light blockers. These arrangements have been most helpful to me—certainly better than the old days with the scope in an alley trying to dodge my neighbor's sensor lights" and the street lamps. When the school put horizontally-directed mercury vapor lamps in, I gave up and shifted.

[Editor's comment: I've made thousands of visual observations from light-polluted central London, England. While I believe they were useful, I'm sure they were not as accurate as those I've made from dark-sky sites. When I traveled to Edinburgh, Scotland, where low-pressure sodium lights bathed the sky in an orange glow, I found that all my red variable stars looked fainter in binoculars against the whiter comparison stars. Circumstances vary!]