Roger Clark's "Visual Astronomy of the Deep Sky" has an excellent
chapter on "The faintest star visible in a telescope".
He lists ideal limiting magnitudes as:
Apertue (inches) Faintest stellar magnitude
1 10.7
2 12.2
3 13.1
4 13.7
5 14.2
6 14.6
7 14.9
8 15.2
10 15.7
12 16.1
14 16.5
16 16.7
18 17.0
20 17.2
22 17.4
24 17.6
30 18.1
36 18.5
These estimates are based on
Limit mag in scope = eye limiting mag +2.5 log(Dscope*(T)/Deye)
Where Dscope and Deye are telescope and pupil diameters in mms and T
is a telescope transmission factor.
Clark takes the ideal eye limiting magnitude as 8.5 at zenith under
ideal conditions, and a typical telescope transmission factor around
0.7 , so his equation above reduces to
Limit mag = 3.7 + 2.5 log(scope diameter in mm squared).
Note that the eye limiting magnitude factor depends on the
background surface brightness of the sky.
This can be improved either by
a) going to a very dark location
b) using very high magnification in the telescope, which reduces the
background surface brightness of the sky in the eyepiece, without
reducing the apparent brightness of point like objects like stars
seen in the same eyepiece field.
Or stated another way:
If you are only going to use low power in a telescope, your limiting
magnitude will go up dramatically as you go to darker sites, in
direct relation to the change in your naked eye visual limit.
However, if you are willing to use high magnifications in the
scope, you can reach near the theoretical limiting magnitude for
stellar objects even from a non ideal site. Clark goes through an
example of an 8 inch scope which could reach its ideal limiting
magnitude of 15.2 at low power from a very dark site (like Chile).
In a suburban sky (naked eye limit 5.5), the same scope would reach
mag 12.5 when observing at 27x magnification, but would improve to
14.6 if you increase the magnification to 200x, and would reach the
near ideal limiting magnitude of 15.2 if you crank it up to 570x (at
which point the background surface brightness in the scope would be
reduced to about the same point as observing from a much darker site).
Note that the magnification trick only works for chasing point like
objects, not diffuse objects like large galaxies or nebulae.
Diffuse objects have their surface brightness reduced along with the
sky background when you crank up the magnification, unlike stars that
remain as virtual point objects even at high magnifications. For
the best viewing of extended objects, you should go to the darkest
skies possible. However, for reaching faint stellar magnitude
limits, try using higher magnfications, even from less than pristine
sites. The magnification trick has been very useful for me when
hunting Pluto in small scopes, trying to see faint companions of
double stars, chasing down near stellar extragalactic globulars in
M31, etc. And Clark's table above is a pretty good match to my own
experience with a 7 inch Dob and a 14.5 inch Dob from sites in
California. I reached mag 14.9 with the 7 inch Oak Classic when
hunting Pluto at Dino point a couple years ago. And I have gotten to
a stellar mag of 16.7 with averted vision using the 14.5 inch
Starmaster and a magnitude of 400 x from Bumpass Hell at Mount
Lassen.
Combining ideal skies plus high magnification is an experiment I'm
looking forward to trying in Australia in less than two weeks!
--David Kingsley
Received on Wed Mar 23 02:11:53 2005