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(IAAC) Color in the deep-sky?



Malcolm Currie, professional astronomer and amateur meteor observer,
asked me for my thoughts about visually observable colors in objects
of the deep-sky. Here is my response: I welcome input (including the
occasional polite brickbat) from other deep-skyers on this subject,
even considering that much has already been said on both these fora
about this topic in previous years...
Malcolm notes regarding colors:
>Stars without question. I can see colour in M42 and some planetary
>nebulae. I'm not a great visual deep-sky observer, and from Britain
>most of the best objects are low in the haze, so there cold be more.
>I must try looking through a big Celestron at Hale Pohaku [Hawai'i].
Not too much time at work today for astronomy (funny how that happens
at times! :>), but here are my quick "thinking points" on this, based
purely on my own experiences with visual deep-sky color:
1) Visibility of color in "nebular" objects *is* dependent on both ap-
erture and magnification - though clearly in non-linear, non-monotonic
ways. Again, some put forward theoretical reasons why this shouldn't be
so... Nonetheless, my comments derive purely from personal experience,
rather than any theoretical considerations - however interesting.
2) Visibility (and optimal aperture and power for seeing it) is also
highly dependent on sky contrast and transparency - in ways which are
too complex for my limited experiences to characterize. (And on this
topic, some theoretical input would be very welcome indeed!)
3) Relatively MANY amateur-observable PNe show color, including many
which are nearly stellar, and so infrequently observed. In fact, so
many PNe show either a green, blue-green, or blue, that I'd exclude
them from this discussion! Same goes for stars - from the blue-white
of an O-class star, to the deep crimson of an M, RC or RN star. Again,
*not everyone* will see these colors - and some will call green what
others call unequivocally blue-green or even blue. But the EXISTENCE
of visual color in such object classes is not in question, I think.
4) Individual perception plays an overriding role... Some people do in
fact see color in M42 naked-eye! Others cannot see this at any aperture
or magnification. More interestingly, some are most likely to perceive
pale green in M42 before any other color; while others may see nothing
but the "default" pearly gray, until suddenly (at the right power, sky
contrast, and aperture) they will perceive reddish tinges first.
(Malcolm, this "wild" scatter in color perception is certainly borne
out by the available data on fireball colors... One might assume that
there was little room for perception variation in descriptions of an
object of mag -3 or brighter - yet this is decidedly NOT the case!)
5) Objects (excluding stars & PNe) in which I definitely logged color
are listed below, with minimal data about the WORST (or in some cases,
ONLY) instrument and conditions under which color was logged so far:
M42 (6" scope in Bortle 3 sky); M8 (12.5" scope in Bortle 2 sky); M20
(20" scope in Bortle 2 sky - both red in the emission area and lovely
contrasting pale blue in the reflection region!); M45 nebulosity (I
have seen color only once, in a 6" under Bortle 3(!) skies - a very
pale blue in the Merope nebulosity); pale yellow in core of M22 (30"
scope in Bortle 1 skies of NE New Mexico); very pale red in NGC 281
(again, 30" in Bortle 1 sky).
I noted that extraordinary yellow in the core of M22 while doing a
very unusual sort of observing: exploring the globular at ABSURDLY
high power, seeking a small PN superimposed on the inner regions...
(I finally logged the presence of the PN by "blinking" the field at
about 1000x with a UHC filter. Obviously the yellowish tinge in the
core was only seen when the UHC was NOT interposed on the field! :>)
Clear skies,
Lew Gramer
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