What about Mars magnification?

Am I missing something? Is my arithmetic off? Presently Mars is some 20 arc
seconds wide. This implies that at 100 X's magnification, it should be 2000
arc seconds across...or just over 1/2 a degree, which is the diameter of a
full moon!! (1/2 degree = 1800"). Last night, the image at 100 x's is no
where near the size of a full moon. What am I missing here??

Mick wrote:
Am I missing something? Is my arithmetic off? Presently Mars is some 20 arc
seconds wide. This implies that at 100 X's magnification, it should be 2000
arc seconds across...or just over 1/2 a degree, which is the diameter of a
full moon!! (1/2 degree = 1800"). Last night, the image at 100 x's is no
where near the size of a full moon. What am I missing here??

You only think it is smaller than the size of the Full Moon. Planets
in the eyepiece look smaller (at least, at first) than you think they
might, given the magnification. It's a well-known effect, without (I
don't think) a well-known cause.

It's been discussed on SAA a number of times.

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

On Tue, 22 Jul 2003 18:39:21 GMT, "Mick" wrote:

Am I missing something? Is my arithmetic off? Presently Mars is some 20 arc
seconds wide. This implies that at 100 X's magnification, it should be 2000
arc seconds across...or just over 1/2 a degree, which is the diameter of a
full moon!! (1/2 degree = 1800"). Last night, the image at 100 x's is no
where near the size of a full moon. What am I missing here??

You were missing a full moon near the same line of sight ;-)

During one of my recent Mars observations (using a refractor without a
star diagonal) I couldn't help but notice how *small* (and bright!)
the moon was compared to my telescopic view of Mars. The scope's
magnification was between 200 and 300x.

Without the moon near the same line of sight it's *very* difficult to
accurately judge which would appear to be larger -- assuming you don't
"cheat" and use a little math!

An absent full moon isn't as large as most of our memories of it ;-)

Bill Greer

On Wed, 23 Jul 2003 01:33:26 GMT, "Mick" wrote:

Well, thats a bit much to imagine. I am speaking about 100 - 150 x's..at
200-300 Mars should appear large..

Hi Mick,

Perhaps with more specifics the situation can be clarified. I was
observing Mars at 280x when the moon was a few degrees away in the
sky. I was using a refractor without a star diagonal (an important
point, as this made it very easy to glance back and forth between Mars
in the telescope and the moon with the naked eye). The diameter of
Mars in the telescope appeared to be *several* times the diameter of
the naked eye moon. Getting even more specific, the naked eye moon
appeared to be less than one third the diameter of the telescopic
Mars.

A little bit of math would show that if Mars (at 280x) appeared to be
over 3 times the size of the naked eye moon, then Mars at 100x would
appear to be somewhat larger than the naked eye moon.

The two most likely reasons why you felt that Mars at 100x appeared to
be much smaller than the naked eye moon are (in order of probability):
1) You were unable to directly compare the telescopic view of Mars to
a naked eye view of the moon. 2) Your telescope/eyepiece combination
was yielding a magnification much less than 100x.

Your math was accurate in your original posting. A telescope at 100x
will show a Martian disk that is larger in apparent size than a naked
eye moon. If you make a direct comparison next month as I (more or
less accidentally) did this month you'll discover that the *tiny* disk
of Mars in a telescope at 100x is indeed larger than the apparent
naked eye diameter of the moon.

It's a *big* mistake to compare the real-time telescopic size of Mars
to the memory's naked eye size of the moon. It's far better to make
the comparison when both are in nearly the same line of sight, thus
allowing one to rapidly glance back and forth from one to the other.

Bill Greer

" Well, thats a bit much to imagine. I am speaking about 100 - 150
x's..at
200-300 Mars should appear large..

Hi Mick,

Perhaps with more specifics the situation can be clarified. I was
observing Mars at 280x when the moon was a few degrees away in the
sky. I was using a refractor without a star diagonal (an important
point, as this made it very easy to glance back and forth between Mars
in the telescope and the moon with the naked eye). The diameter of
Mars in the telescope appeared to be *several* times the diameter of
the naked eye moon. Getting even more specific, the naked eye moon
appeared to be less than one third the diameter of the telescopic
Mars.

A little bit of math would show that if Mars (at 280x) appeared to be
over 3 times the size of the naked eye moon, then Mars at 100x would
appear to be somewhat larger than the naked eye moon.

The two most likely reasons why you felt that Mars at 100x appeared to
be much smaller than the naked eye moon are (in order of probability):
1) You were unable to directly compare the telescopic view of Mars to
a naked eye view of the moon. 2) Your telescope/eyepiece combination
was yielding a magnification much less than 100x.

Your math was accurate in your original posting. A telescope at 100x
will show a Martian disk that is larger in apparent size than a naked
eye moon. If you make a direct comparison next month as I (more or
less accidentally) did this month you'll discover that the *tiny* disk
of Mars in a telescope at 100x is indeed larger than the apparent
naked eye diameter of the moon.

It's a *big* mistake to compare the real-time telescopic size of Mars
to the memory's naked eye size of the moon. It's far better to make
the comparison when both are in nearly the same line of sight, thus
allowing one to rapidly glance back and forth from one to the other.



Thanks Bill...it makes more sense now...but seriously, we look at the full
moon unaided and see significant
light and dark areas ie: "the man"...(I've always thought of it as a woeful
female myself) and presume that dark and light features on Mars will be
equally apparent...so far this is not the case under good seeing. What I
get is a faint speck of a south polar region and a "hint" of syrtis major at
20.5". My scope is exactly 100 X's with a 9mm OR.

..I find it hard to believe that my memory, or impression, of the unaided
full moon disc (within reason) is that affected by "illusion". I will have
to compare the two next month as you describe.

Don't confuse size with detail. The enlarged image of mars would have much
less detail than the unenlarged moon (unless your telescope had an aperture
100x the apature of your eyes).

-- Zan

Thanks Bill...it makes more sense now...but seriously, we look at the full
moon unaided and see significant
light and dark areas ie: "the man"...(I've always thought of it as a
woeful
female myself) and presume that dark and light features on Mars will be
equally apparent...so far this is not the case under good seeing. What I
get is a faint speck of a south polar region and a "hint" of syrtis major
at
20.5". My scope is exactly 100 X's with a 9mm OR.

.I find it hard to believe that my memory, or impression, of the unaided
full moon disc (within reason) is that affected by "illusion". I will
have
to compare the two next month as you describe.

Your 100x is also magnifying all the atmospheric effects -- which you
will also see when looking at the moon with the same power.

I've had variable results with Mars, depending on "seeing".

Phil

Mick wrote:

Thanks Bill...it makes more sense now...but seriously, we look at the full
moon unaided and see significant
light and dark areas ie: "the man"...(I've always thought of it as a woeful
female myself) and presume that dark and light features on Mars will be
equally apparent...so far this is not the case under good seeing. What I
get is a faint speck of a south polar region and a "hint" of syrtis major at
20.5". My scope is exactly 100 X's with a 9mm OR.

.I find it hard to believe that my memory, or impression, of the unaided
full moon disc (within reason) is that affected by "illusion". I will have
to compare the two next month as you describe.

"Zan Hecht" wrote in message
t...
Don't confuse size with detail. The enlarged image of mars would have
much
less detail than the unenlarged moon (unless your telescope had an
aperture
100x the apature of your eyes).

-- Zan


Are you saying that due to optical effects you can't compare an enlarged
(magnified) Mars to the moon as we see it unaided?

Hi there Mick. You posted:

.I find it hard to believe that my memory, or impression, of the unaided
full moon disc (within reason) is that affected by "illusion". I will have
to compare the two next month as you describe.

The dark and light areas of the moon show a little more contrast than those of
Mars, so detail is a bit easier to see with the unaided eye. In addition,
even fine seeing variations tend to blur the detail's edges, so it becomes
even harder to see detail of the same scale as that on the moon as viewed with
the unaided eye. Try using a red filter with your telescope and the dark
markings should be somwhat easier to see. Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

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"William Hamblen" wrote in message
rthlink.net...
In article , Mick wrote:
Am I missing something? Is my arithmetic off? Presently Mars is some 20
arc
seconds wide. This implies that at 100 X's magnification, it should be
2000
arc seconds across...or just over 1/2 a degree, which is the diameter of
a
full moon!! (1/2 degree = 1800"). Last night, the image at 100 x's is
no
where near the size of a full moon. What am I missing here??

The full moon is smaller than you remember. Look at it through sunglasses
or a paper towel core to reduce the illusion of size.


Ok..will do