Suburban viewing is made difficult, not only because the sky is inherently brighter and the objects to be viewed are dim, but also because the eye itself never really dark adapts to take advantage of the darkening effects of telescopic magnification. This darkening effect, when used with appropriate filters, on appropriate objects can improve the contrast even further. Telescopic darkening, and filter use reduce the brightness of the object while improving contrast. Good dark adaptation is needed to offset the dimming of the objects under these conditons.
Over the past few years, light and traffic have increased here in Rockville, and working visually has been a study in increasing frustration. Even at our quasi-local Little Bennet Regional Park (LBRP), sky-glow is sufficient that dark adaptation doesn't really occur, and the sensitive rods and the visual purple produced by the eye just don't quite get there, at least for me they have not.
I finally decided to try one of the dome tents. Kendrick has one, and a company called Clear Night (The TeleDome) also has one. The Kendrick tent is significantly larger than the Clear Night tent, is open to the whole sky (except at the horizons), and is estimated to require about 10 minutes to set up.
The TeleDome tent is small, is open to the sky in only a small area (more like a slitted observatory dome), and requires only about a minute to set up.
I purchased the Tele-Dome because it provided an opportunity to cut sky glow from the open sky significantly. The tent looks like an umbrella that extends to the ground and zips up light tight. An observing quadrant can be opened to the sky. With the quadrant open, after a few minutes, dark adaptation proceeds to the point where the interior of the tent is illuminated by only the light comming in through that small quadrant. To really achieve dark adaptation, I used a large black trash bag, and cut a hole for the C8 to poke through, and fitted the bag on the slit to block out even that light. After 20 minutes or so, I was as dark adapted as I had ever been. (Form more information regarding object visibility visit Mel Bartel's ODM page.)
Using a filter is just dandy, but if the eye is not dark adapted, the image through the filter looks mottled. The Clear Night tent, for the first time has made it possible for me to dark adapt and see a greater percentage of what the filter is actually passing, and I might add, my OIII filter and even the skylight filter do much better than I thought they ever could given good dark conditions.
One evening, before the clouds came in, the sky was ok, not great. I trained my
C8 in the Tele-Dome on M56, and then proceeded to let my eyes really dark
adapt. M56 was at zenith, and I could clearly see the globular. I made a
crude drawing, and later checked out the stars I was seeing in my
planetarium program. I was looking at 12th magnitude stars. Not bad for
bad old DC, and my dinky little C8. I did this with the skylight filter,
and a panoptic 35mm lens.
The tent sets up, and takes down quickly, and for visual work, it has made a
huge difference for me. A couple nights ago, I trained the scope (in the
tent) to M8, and using an OIII, I could clearly see structure in the nebula.
The lagoon stood out clearly. M8 was on the meridian. For me, that placed it right in the center of a light-dome produced by a neighboring jurisdiction.
The tent travels well, and could be taken to our local quasi-dark site and improve on even those relatively good conditions.
