star magnitude and binoculars

i'm new to this group, and have very little experience with astronomy. was
hoping someone might be able to help with a recommendation...

if i want to view stars with a 12-12.5 magnitude, what is the size binoculars i
need (given that i'm not too greatly concerned about number of stars in my
field of vision)?

unfortunately, the only binoculars i ever use are for bird-watching, and we're
talking two different animals here. *ahem*

thanks,
Ravensong

I'd say you will have to be in the 4" range for that - and that makes pretty
heavy binos then.

Bernhard

"Ravensong" schrieb im Newsbeitrag
...
i'm new to this group, and have very little experience with astronomy.
was
hoping someone might be able to help with a recommendation...

if i want to view stars with a 12-12.5 magnitude, what is the size
binoculars i
need (given that i'm not too greatly concerned about number of stars in my
field of vision)?

unfortunately, the only binoculars i ever use are for bird-watching, and
we're
talking two different animals here. *ahem*

thanks,
Ravensong

(Ravensong) wrote in message ...
i'm new to this group, and have very little experience with astronomy. was
hoping someone might be able to help with a recommendation... [snip]
unfortunately, the only binoculars i ever use are for bird-watching, [snip]

Ravensong -

The following table is the short answer to your question, "What is the
limiting magnitude of my binocular (or small telescope)." I have also
added other advice for the beginner binocular backyard-amateur
astronomers who start during the summer observing season:

Telescope Limiting Magnitude
Theoretical limits - extended down to 35mm

Aperture Aperture Faintest point
Inches mm source observable -
theorectical

1.4 35mm 10.2
2 51 10.3
3 76 11.2
4 102 11.8
6 152 12.7
8 203 13.3
10 254 13.8
12.5 318 14.3
14 356 14.5
16 406 14.8
18 457 15.1
20 508 15.3
24 610 15.7
30 762 16.2

Phillips S. Harrington, Star Ware (2d ed. 1994) (John Wiley & Sons,
Inc.) republished at
http://www.stargazing.net/david/cons...manystars.html

Practical _point source_ (individual stars) magnitude limits are
perhaps 1-2.5 mags less than those listed above. You'll probably get
many different opinions based for every amatuer astronomer you talk
to.

Each binocular or small telescope, when paired with its owner's
individual eyes, is unique. Practice with a star chart and your
personal binoculars will quickly give you a feel at what magnitude of
stars printed on star charts that you can ignore because they are too
dim.

The above table represents the theoretical performance limits by
aperature size. But this theoretical performance is overwhelmed by the
light-pollution differences that vary greatly from night-to-night
depending on how light-polluted the sky is and for a particular
location (e.g. - close to a city or at a rural roadside). For
example, the Moon currently is between its first and full phases and
washes out most of the stars below mag 6 or 7 in binoculars.

The next "dark sky" window, after the new Moon, is around July 28.
Watch your newspaper or find a lunar calendar on the web to identify
these new-Moon "dark sky" windows. That's the best time to plan for a
backyard astronomy night. If possible, drive as far away from a city
as your tolerance for staying up late (or until the next morning) will
allow.

The above table also is valid only for _point light sources_ (i.e. -
individual stars). Many interesting objects in the night sky are
_extended objects_ - like globular clusters (M22 in Sagittarius) or
open clusters (M7 also in Sagittarius) or star patch clouds (M24 the
Small Sagittarius star patch cloud). I do not have my table handy for
the light grasp of binoculars for extended objects, but the following
is a table for all 110 Messier (extended) objects, by size and by
magnitude:

Magnitude (brightness)
Size_arcmin 6 6-7 7-8 8-9 9-10 10-11 11-12 Total
10 3 5 18 20 4 2 52
10-20 6 10 6 7 1 1 31
20-30 5 4 1 1 1 12
30-40 4 1 5
40-50 1 1
50-60 1 1
60-70 1 1
70-80 1 1
80-90 2 2
90-100 2 2
100 2 2
Total 25 18 12 26 22 5 2 110

With my astronomical binoculars (20 x 70mm), I can see (with one or
two exceptions) all the Messier objects less than about mag 9 in
brightness and with a size (in arcminutes or 1/60 of a degree) less
than 10 arcminutes (10) and up to a size greater than 100 arcminutes
(about 1 1/2 degrees). The magnitude and size of the Messier extended
objects are printed on some star charts and are available over the
web.

With basic field binoculars (7 x 35mm), with which I have less
personal experience, you should be able to see most of the extended
Messier objects from the 10-20 arcmin row and to the mag 7-8 column.
That's still a respectable total of about 45 objects and is much more
than enough to get familar with the night sky. Do not get
disappointed if you cannot see everything with binoculars. You can
see enough with binos to get an idea of what's up in the night sky.

M22, M7 and M24 are marked on almost all star maps and are good
beginner objects to start getting a feel for "how bright" and "how
big" things are in the night sky. M22, M7 and M24 are visible in 35mm
basic field binoculars and in larger 50mm birding binoculars. Good
beginner binocular objects along the summer galactic spine (the Milky
Way) between Sagittarius through Scutum (from the horizon to up to
about 40 degrees), for a North American summer observer, include:

M7 Ptolemy's open cluster (a very near open cluster). Between
Scoripus and Sagittarius (the "teapot") at the south horizon in
summer.

M22 Globular cluster (a far but bright globular cluster). It is the
fuzzy ball just north-east of the top star in the "lid" of Sagittarius
"teapot" figure at the south horizon.

-- Large Sagittarius star cloud (8-10 degree naked-eye object in
good dark skies, but very interesting to scan with binoculars). The
Large Sagittarius star cloud is really the bottom of the galactic core
sticking out beneath the dark clouds of the next galactic arm in from
our Orion galactic arm. The Large Sagittarius star cloud sits on top
of the "spout" of the Sagittarus "teapot" and extends up to Lagoon
Nebulae, M8 discussed below, to the north and west. To get an idea of
what to look for, go to and look for the bright patch sitting on top
of the Sagittarius "teapot spout" at:
http://canopus.physik.uni-potsdam.de/~axm/mwpan_vr.html

-- Dark nebulae in the Large Sagittarius cloud area (two tubular
pitch black clouds lead off the Large Sagittarius star cloud, north
west into Ophiuchus.) In good dark skies, you can follow these with
your binoculars. They are pitch black dark clouds surrounding and
obscuring the bright galactic core from our view. They are
distinguishable from the relatively lighter but still "dark"
background by their rich inky-pitch-blackness. To get an idea of what
to look for, go to:
http://www.allthesky.com/constellati.../constell.html (Press
the button "Mark deep sky objects" and look for the dark tubular cloud
labeled the "Pipe Nebulae".)

M8 Lagoon Nebula (it will look tiny in 35mm binoculars, but is
visible. There is a small open cluster right next to it, in your
line-of-sight, but in 3-dimensions, located some distance in front of
the Lagoon.)

M21 Open cluster

M24 Small Sagittarius star patch cloud (a hole in a galactic
arm's cloud bank revealing the bright galactic core that is densely
packed with stars).

M28 Globular cluster

-- Scutum star cloud (another hole in the galactic arm's cloud bank
revealing the bright arm inside of the next galactic arm closer to the
Milky Way's core). A photograph, to give you an idea what to look for,
is at:
http://www.allthesky.com/constellations/scutum/ (Press the button
"Mark the Deep Sky Objects". The Scutum star cloud will be marked.)

-- Dark nebulae around the Scutum star cloud (seen at the
southern end of the Scutum star patch cloud.) Look for extraordinarily
inky-black clouds against the bright background of the Scutum star
patch.

To see greatly enlarged pictures (as compared to what you will see in
your binoculars) of the Messier "M" objects discussed above (so you
know what to look for), go to:
http://www.seds.org/messier/data2.html

The following link is to a photographic panorama of the galaxy that
may be of help in getting oriented to the Sagittarius to Scutum to
Aquila to Cygnus night sky:
http://canopus.physik.uni-potsdam.de/~axm/mwpan_vr.html

A few other good objects for 35mm's binoculars, assuming a summer
North American location, include:

Antares alpha Scorpius - a bright red colored star - name means
"rival to Ares" or "rival to Mars", a red colored planet visible in
the southeast after 1 am (A.D. 2003).

Albireo beta Cygnus - a double star that appears bright reddish
yellow.

del eps Ophi delta and epsilon Ophiuchus - two orange stars that make
up the right-hand of Ophiuchus that grasps the body of the Serpent's
head (Serpen Caput).

Arcturus alpha Bootes - a bright golden yellow star - name means
"bear-keeper" referring to Bootes, the herdsperson, herding the bears,
Ursa Major and Ursa Minor.

Vega alpha Lyra - a bright blue-white star - name means "swooping
vulture" in Arabic and refers to an animal killed by Hercules in one
of the "Tweleve Labours of Hercules" Greek myths.

Altair alpha Aquila - a bright white star - name means "eagle" in
Arabic, same as the constellation name, Aquila, in Greek. Another
animal dispatched by the Hercules in the "Tweleve Labours of Hercules"
Greek myth.

16-17 Draco An easy binary to split with large binoculars. Moderately
difficult to "split" with 35mm's binos. The constellation "Draco"
means "dragon." It refers to Ladon, a dragon, and another unfortunate
animal also dispatched by the ever-busy Hercules during one of his
"Tweleve Labours" myths, while stealing apples from earth-goddess
Gaea's apple-tree.

zet-80 UMa zeta Ursa Major and 80 Ursa Major. Zeta Ursa Major is a
named star - "Mizar" which means "horse" in Arabic. These are the
second to last stars in the handle of the Big Dipper. Mizar's
companion, 80 UMa, also is a named star - "Alcor" which means "rider"
in Arabic. These two binary stars are separated by about 11.75
arcmins. (An arcminute is 1/60 of a degree.) It is a good test
object to get a feel for how big things are when seen through your
binoculars.

-- Perseus-Cassiopeia double open-clusters. (Far in the north in
the early morning after 1am.) Good example of two clusters that are
very far away. Compare their size to M7 - also known as Ptolemy's
open cluster, a much closer open cluster.

M13 Great Hercules globular cluster. (Hercules is high in the
northwest.) Compare to the M22 globular cluster in Scorpius.

-- The Milky Way galactic core (south horizon) with the Orion
galactic arm of the Milky Way sweeping over your head to the
north-north-east horizon. Best northern view in the galaxy!

M31 Andromeda Galaxy (rises over the north east horizon after 1:00
am.) A fuzzy ball in 35mm's binos, but still light from a fuzzy ball
that left our closet neighbor galaxy, 4 1/2 million years ago, when
some lemurs in Africa got the idea to evolve into homo sapiens.

Various parts of the Milky Way galaxy are visible during each of the
four seasons. Currently, the Milky Way between Sagittarius (south
horizon) to Cygnus (overhead) to Cassiopeia (north-northeast horizon)
is visible.

Binocular numbers (e.g. 7 x 35mm) consist of two numbers. The first
number is the magnification power and the second is the size of the
objective aperature (the big end of the binos). E.g. 7x35mm,
10x50mm, 20x80mm. The back of the housing of the binoculars will
usually be stamped with the true field-of-view size in degrees. E.g.
3 degrees, 5 degrees, 6 degrees. etc. (You may have to use a
magnifying glass to find that number.) This is how much of the sky,
in degrees, that you see when you look through your binoculars. It is
useful number to know. You can relate that field-of-view size to the
angular distances between stars on your star chart.

Small field binoculars (7 x 35mm) can be held steady in the hands
without a tripod. To look at objects directly overhead (like Alberio
in Cygnus), plan to bring a blanket and pillow to lay on. Larger
bincoluars (10x50mm and 20x80mm) need to be mounted on a tripod in
order to be held steady.

The angular distances in the night sky can be estimated using your
fingers and hands:

Table of angular distance estimates:

width of index nail at arm's length: 1 degree
1st-2nd knuckles of clenched fist at arm's length: 3 degrees
2nd-4th knuckles of clenched fist at arm's length: 5 degrees
1st-4th knuckles of clenched fist at arm's length: 8 degrees
span of open hand at arm's length: 18 degrees
two clenched fists, end-to-end, at arm's length: 30 degrees

Adapted from Sedwigk's Amateur Astronomer's Handbook at p. 522.

Again, few of above objects will be easily visible (if at all) until
the new Moon later at the end of July. In the meantime, concentrate
on locating the above objects on your start charts, or through an
online star chart like:

http://www.fourmilab.to/yoursky/

which will allow you to print a horizon star chart for your city.

The "Your Sky" site also includes point-and-click features to print
star charts to find all of the Messier objects and the named-stars
mentioned above:

http://www.fourmilab.to/yoursky/cata...atalogues.html

Enjoy - Kurt

P.S. - Corrections from experienced newsgroup members are welcome.

PrisNo6 wrote:
(Ravensong) wrote in message
...
i'm new to this group, and have very little experience with
astronomy. was
hoping someone might be able to help with a recommendation... [snip]
unfortunately, the only binoculars i ever use are for bird-watching,
[snip]

Ravensong -

The following table is the short answer to your question, "What is the
limiting magnitude of my binocular (or small telescope)." I have also
added other advice for the beginner binocular backyard-amateur
astronomers who start during the summer observing season:

Enjoy - Kurt

P.S. - Corrections from experienced newsgroup members are welcome.

Aren't Binos classed as Two Telescopes so that the light collected by the
two apertures has to be taken into consideration when working out the
limiting magnitude?

Just asking. Mr M.

Paul Schlyter wrote:
In article ,
/\\/\\R /\\/\\ wrote:

Aren't Binos classed as Two Telescopes so that the light collected
by the two apertures has to be taken into consideration when working
out the limiting magnitude?

It's a bit more complex than that.

The limiting magnitude will be fainter that if you used just one of
those scopes on one eye. But it won't be quite as faint as if you
used a larger scope, with the same entrance pupil area as those
two scopes combined, on one eye. It will be somewhere in between.

Are you saying that my binos with two 60mm objectives will give me a
limiting magnitude similar to a single telescope with an objective of
approx. 85mm (given the same eyepieces)?
I'm sorry I'm being thick about this.
Mr M.

"/\/\R /\/\" wrote in message
...
Paul Schlyter wrote:
In article ,
/\\/\\R /\\/\\ wrote:

Aren't Binos classed as Two Telescopes so that the light collected
by the two apertures has to be taken into consideration when working
out the limiting magnitude?

It's a bit more complex than that.

The limiting magnitude will be fainter that if you used just one of
those scopes on one eye. But it won't be quite as faint as if you
used a larger scope, with the same entrance pupil area as those
two scopes combined, on one eye. It will be somewhere in between.

Are you saying that my binos with two 60mm objectives will give me a
limiting magnitude similar to a single telescope with an objective of
approx. 85mm (given the same eyepieces)?
I'm sorry I'm being thick about this.
Mr M.
No, it'll be slightly less.
Take the limiting magnitude of a 60mm 'scope' = 'X'.
Work out the equivalent scope size for the two tubes = just under 85mm, and
this then has a limiting magnitude 'Y'. The limiting magnitude for the pair
of binoculars, will be _between_ these two figures. Not quite as good as the
85mm scope, but better than a single 60mm scope (so X M Y).
The exact 'figure', will depend on the observer, but if you say that the
60mm scope has a limiting magnitude of perhaps 11.9, and the 85mm scope a
limiting magnitude of perhaps 13.2, the binoculars will give a figure around
perhaps 12.5. However the observer, and the conditions, make so much
difference, that accurate comparison is very difficult...

Best Wishes

"bwhiting" wrote in message
...
I personally think you can add anywhere from 0.5 to 1 whole
magnitude to that table below....IF you are in very dark skies...
we have typically split the gravitationally lensed quasar up
in Ursa Major at CSSP with the 30 inch...mags 17.1 and 17.4,
having to go averted for the dimmer one. (and obviously those
are point sources).
IMHO,
Tom W.



Telescope Limiting Magnitude
Theoretical limits - extended down to 35mm

Aperture Aperture Faintest point
Inches mm source observable -
theorectical

1.4 35mm 10.2
2 51 10.3
3 76 11.2
4 102 11.8
6 152 12.7
8 203 13.3
10 254 13.8
12.5 318 14.3
14 356 14.5
16 406 14.8
18 457 15.1
20 508 15.3
24 610 15.7
30 762 16.2
Yes. Your comments agree, with a lot of other people's opinions on this.
There is a standard 'formula', which runs:
Mag=6.5 - (5 log d) + (5 log D)
where 'd' is the eyes exit pupil, and 'D' is the scopes apperture. However
it is allmost invariably accompanied by a 'coda', reading something like
'observed magnitude limits often exceed these figures'. The figures above,
are a little low, even from this formula (corresponding to an exit pupil of
about 8mm, rather than the normal 7mm max). Really dark/stable skies, and a
good observer, can often add at least half a magnitude to the theoretical
figures.
Like most such things, these figures are 'guides'. You know full well, that
there is no point trying to observe a mag 15 object, with a 150mm scope,
but, it may well be worth trying for a mag 13 object with the same scope.
All these formulae, start with assumptions (the primary one on the above, is
the visual limiting magnitude without any 'aid', at around mag 6.5). On a
lot of nights, seeing this 'deep', may well be impossible, but just
occasionally, when everything conspires to help, it can be possible to far
exceed this expectation, and this 'moves the goalposts'.
I remember on one amazing night, when the sky was both stable, and clear,
managing to count thirteen seperate stars in the Pleides, naked eye. The
very next night, I was straining to see six, which is much more 'normal'.

Best Wishes

In article ,
/\\/\\R /\\/\\ wrote:

Paul Schlyter wrote:
In article ,
/\\/\\R /\\/\\ wrote:

Aren't Binos classed as Two Telescopes so that the light collected
by the two apertures has to be taken into consideration when working
out the limiting magnitude?

It's a bit more complex than that.

The limiting magnitude will be fainter that if you used just one of
those scopes on one eye. But it won't be quite as faint as if you
used a larger scope, with the same entrance pupil area as those
two scopes combined, on one eye. It will be somewhere in between.

Are you saying that my binos with two 60mm objectives will give me a
limiting magnitude similar to a single telescope with an objective of
approx. 85mm (given the same eyepieces)?
I'm sorry I'm being thick about this.

No, I'm saying that the limiting magnitude of 60mm binoculars will be
somewhere between the limiting magnitudes for 60mm monoculars and 85mm
monoculars (where "monoculars" = normal single-tube scopes).


--
----------------------------------------------------------------
Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stockholm dot bostream dot se
WWW: http://www.stjarnhimlen.se/
http://home.tiscali.se/pausch/

Kurt wrote:
Telescope Limiting Magnitude
Theoretical limits - extended down to 35mm

It should be emphasized that only the *differences* between the limits
are in any sense theoretical. There is also an empirical element to
the formula.

Specifically, these "theoretical" values are derived from some formula
that is similar to

M = 1.8 + 5 log A

where M is the limiting magnitude, A is the aperture in mm, and log is
to the base 10. The fact that this logarithm is multiplied by 5 does
have a basis in theory: To increase the log of the aperture is to
multiply it by 10, which multiplies the area by 100, and 100 times as
bright is 5 magnitudes.

However, the 1.8 is entirely empirical. It undoubtedlly varies quite
considerably from observer to observer, from site to site, and from
night to night. One should not refrain from attempting a target simply
because it falls outside these limits.

Personally, I think 1.8 is terribly conservative. It corresponds to a
limiting magnitude of about 6.0 for the unaided eye with a pupil of
7 mm, and only 5.3 with a pupil of 5 mm. This is routinely exceeded in
many sites by many observers.

Practical _point source_ (individual stars) magnitude limits are
perhaps 1-2.5 mags less than those listed above. You'll probably get
many different opinions based for every amatuer astronomer you talk
to.

I disagree with this also. Individual stars are as good as point
sources for this purpose, even taking into account diffraction effects.
Even with the same integrated magnitude, increasing magnification and
therefore increasing apparent size at the cost of decreasing apparent
surface brightness often *enhances* visibility rather than detracting
from it. See, for example, Mel Bartels's excellent site on this matter.

Each binocular or small telescope, when paired with its owner's
individual eyes, is unique. Practice with a star chart and your
personal binoculars will quickly give you a feel at what magnitude of
stars printed on star charts that you can ignore because they are too
dim.

I agree with this advice wholeheartedly--the empirical approach is
best for any individual person. For a large group of people (say, SAA
readers), a general rule is often more useful, but it should, I think,
be taken with a grain of salt.

[snipped lots of good observing advice and targets]

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

I'd be very curious what your numbers for H_t, Si_p, and St_p
are for a 24 (610 mm)and a 30 inch (762 mm) glass, if you have them,
or can extrapolate them from the data.
Thanks ahead of time,
Tom W.




Telescope Limiting Magnitude by Aperature Size
(in inches and millimeters)
Theoretical (H_t) and
Practical (Si_p and St_p)

D D
" mm H_t Si_p St_p
2 51 10.3 12.1 12.2
3 76 11.2 13.0 13.1
4 102 11.8 13.6 13.7
6 152 12.7 14.5 14.6
8 203 13.3 15.1 15.2
10 254 13.8 15.6 15.7
20 508 15.3 17.1 17.2

Sources: