[sac-forum] Re: Couple oddball questions...
- From: Tom Polakis <tpolakis@xxxxxxx>
- To: sac-forum@xxxxxxxxxxxxx
- Date: Mon, 15 Sep 2008 14:55:26 -0400
David,
Actually, they are both very smart questions. And there is such a thing as a
dumb question.
For limiting magnitude, you should pull out all the stops, and use averted
vision. You mentioned "at the zenith" which is actually only a point exactly
straight overhead. That's a good approximation where to look for your faintest
star, but you might have to compromise using the charts you have. Try to find
a field whose altitude is at least 45 degrees, or better yet 60 degrees. It's
best not to do the limiting magnitude test around Polaris, as you are certainly
losing some starlight and contrast that low in the sky.
Binoviewers can help with the aesthetic aspect of telescopic views, but they
can only harm seeing faint detail. Even if there was a hypothetical *zero*
light loss in splitting the beam, each eyepiece is seeing half of your scope's
light-gathering area. In other words, a binoviewer makes a 12-inch telescope
behave like two 8.5-inch telescopes. So each eyepiece loses 0.75 magnitudes
relative to cyclops observing.
The best expert on binoviewers I know of, Ed Zarenski, says that the commonly
accepted magnitude *gain* using two eyes is about 0.3 magnitudes. So even if
you "re-gain" all this light thanks to your physiology, you still lose nearly
half a magnitude when you look through a binoviewer.
But be careful who you tell this to. I have learned the hard way that it can
be difficult to convince those who have dropped a grand on a binoviewer that
it reduces their limiting magnitude.
Tom
Other related posts:
