Dátum: 1999. január 31., 15:41
Feladó:
Tárgy: Barnard's Loop
Sziasztok!
Horváth Tibi kérdésére egyértelmüen azt kell mondanunk, hogy
a Barnard-féle iv az Orionban a piros szin árnyalataiban látható
elsősorban.
A kérdező számára küldök külön fotót is erről a tényről.
Itt alul angolul egy kis fotózási tanács a ködre vonatkozóan.
Tuboly Vince
Barnard's Loop (#226)
This is one of those objects which is very difficult to observe visually,
but very much within the range of modest astrophotography. The Loop shines
in the light of H-alpha so a red sensitive film can be helpful. Like other
similar objects, hypered Tech Pan 2415 and an H-alpha filter with long
exposure times work very well for black-and-white. For color photos, you
need a relatively long exposure time. For instance, a 9 minute exposure at
f/2.8 (remember, f-ratio is the most important factor for diffuse objects)
on ASA 800 or 1000 film will faintly, but distinctly show the entire
semi-circle (which extends almost exactly 15 degrees north-south) if you are
using a 105mm or shorter lens.
Note that the feature is much larger than depicted in Uranometria 2000.0. A
3 minute exposure at f/2.8 will reveal the section drawn in that atlas and
this part fits into a 135mm or 200mm frame. With judicious framing, a 135mm
lens will show the main part of the Loop, along with the Orion Nebula, Flame
Nebula, and a hint of the Horsehead Nebula!
H-alpha/H-beta - This is short for hydrogen alpha/beta. These are emission
lines of hydrogen gas. The light that is emitted by each line is
monocromatic ("one color"), and in the red part of the spectrum for H-alpha
and green for H-beta. There are a lot of deep-sky objects that are made up
of ionized hydrogen ("H II regions", as opposed to "H I regions" which are
neutral gas), and these objects emit light at these wavelengths. Most H II
regions emit most strongly in H-alpha. The notable exceptions are the
California Nebula and the bright nebula associated with the dark Horsehead
Nebula, which emit a lot in H-beta.