Re: (IAAC) Obj: M 42, Trapezium - Inst: 25" Obsession
Interestingly, I noted color in M42 from a very dark site with as small an
instrument as 10x50 binocs (see IAAC archive). It seems generally that color is
either visible in M42 from a given site, or it isn't: I've even missed the
faint green hues with the 20" from only moderately dark sites!
As for reds, I remember reading (probably in Roger Clark's book) that much of
what we often note as "pale green" is actually other colors (such as red)
stimulating the rods JUST over the threshold for color perception. This effect
is very common in meteor observing, where meteor "heads" or persistent trains
are most frequently seen as pale green, if color is seen at all.
I've seen red tinges with the 20" at very dark sites, but suspect they probably
would have been visible even in smaller scopes. Todd Gross (an active poster to
IAAC) has also logged "red/pinks" with a 16" dob: