more about Mars magnification

At present Mars should be over 1/2 degree at 100 X's, which is a full moon
disc. At 200 X's, Mars should appear
as the moon would at 100 X's. This just doesnt happen because the moon
takes up the whole field in the EP at 100X's

So there is some optical trick occuring that eludes me...

At 200x Mars would appear as wide as two full Moons, not 100. You've only
doubled the magnification.


"Mick" wrote in message
. ..
At present Mars should be over 1/2 degree at 100 X's, which is a full moon
disc. At 200 X's, Mars should appear
as the moon would at 100 X's. This just doesnt happen because the moon
takes up the whole field in the EP at 100X's

So there is some optical trick occuring that eludes me...

Mick wrote:

At present Mars should be over 1/2 degree at 100 X's, which is a full moon
disc. At 200 X's, Mars should appear
as the moon would at 100 X's.

Surely you mean "At 200 X's, Mars should appear as the moon would at 2 X's."
Check your math.

This just doesnt happen because the moon
takes up the whole field in the EP at 100X's

So there is some optical trick occuring that eludes me...

At present Mars should be over 1/2 degree at 100 X's, which is a full
moon
disc. At 200 X's, Mars should appear
as the moon would at 100 X's.

Surely you mean "At 200 X's, Mars should appear as the moon would at 2
X's."
Check your math.

This just doesnt happen because the moon
takes up the whole field in the EP at 100X's

So there is some optical trick occuring that eludes me...


What I mean is, at 100 X's Mars is to appear as the moon does unaided. The
two are supposedly equivalent. (1/2 degree)
Now, if you up the anti to 200, Mars should be equivalent of what the moon
would appear to be at 100 X's....in the EP.

Mick wrote:

What I mean is, at 100 X's Mars is to appear as the moon does unaided. The
two are supposedly equivalent. (1/2 degree)
Now, if you up the anti to 200, Mars should be equivalent of what the moon
would appear to be at 100 X's....in the EP.

Hmmm, I'm not sure I get it. Ok, at 200, Mars should look like 1 defree, double
the size of the moon with unaided eye. I.e. it will appear the same size as
the moon would appear at magnification 2X in the eyepiece.

- Alex

Mick wrote:

What I mean is, at 100 X's Mars is to appear as the moon does unaided.
The two are supposedly equivalent. (1/2 degree) Now, if you up the anti
to 200, Mars should be equivalent of what the moon would appear to be
at 100 X's....in the EP.

No it shouldn't. What others have said to you is correct. Read and
understand it; don't just dismiss it because you don't like it. It is a
tad disrespectful to ask a question then just dismiss the responses you
get in this manner. Restating your original error in different words
will not make it correct, no matter how many times you do it.

At x200 Mars will have an apparent diameter of about a degree. At x100
the Moon will have an apparent diameter of about 50 degrees. There is
exactly no way in which these can be equivalent.

Best,
Stephen

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Stephen Tonkin wrote:

Mick wrote:


What I mean is, at 100 X's Mars is to appear as the moon does unaided.
The two are supposedly equivalent. (1/2 degree) Now, if you up the
anti to 200, Mars should be equivalent of what the moon would appear
to be at 100 X's....in the EP.


No it shouldn't. What others have said to you is correct. Read and
understand it; don't just dismiss it because you don't like it. It is a
tad disrespectful to ask a question then just dismiss the responses you
get in this manner. Restating your original error in different words
will not make it correct, no matter how many times you do it.

At x200 Mars will have an apparent diameter of about a degree. At x100
the Moon will have an apparent diameter of about 50 degrees. There is
exactly no way in which these can be equivalent.

Best,
Stephen


Mick,

I think I see where wour error comes from. Somehow it seems to me that
you *add* and *subtract* magnification numbers instead of *multiplying*
and *dividing*. When you bring the magnification from 100x to 200x
the things will get twice bigger, not 100 times bigger.

- Alex

"Stephen Tonkin" wrote in message
...
Mick wrote:

What I mean is, at 100 X's Mars is to appear as the moon does unaided.
The two are supposedly equivalent. (1/2 degree) Now, if you up the anti
to 200, Mars should be equivalent of what the moon would appear to be
at 100 X's....in the EP.

No it shouldn't. What others have said to you is correct. Read and
understand it; don't just dismiss it because you don't like it. It is a
tad disrespectful to ask a question then just dismiss the responses you
get in this manner. Restating your original error in different words
will not make it correct, no matter how many times you do it.

At x200 Mars will have an apparent diameter of about a degree. At x100
the Moon will have an apparent diameter of about 50 degrees. There is
exactly no way in which these can be equivalent.


Your first paragraph is far more "terse" than anything I have written so far
and unnecessary. You exaggerated your perception
of my communication...

However, your second paragraph exposes my error in logic...I really don't
care if I am wrong! I am just trying to get
at the answer..AND, the answer is this: My mistake is my perception of
magnification. To make Mars appear the same as a 100 X's magnification of
the moon I would have to take the 100 X's image of Mars and magnify the
image another 100 X's. This is not the same as 200 X's magnification. In
other words, I would need a 100 Xs ZOOM to achieve this.

"Mick" wrote in message ...
At 200 X's, Mars should appear as the moon would at 100 X's.
This just doesnt happen . . .
So there is some optical trick occuring that eludes me...

No optical trick - let's review the math:

The naked-eye angular diameter of the Moon at its mean distance is
1865 arcseconds, or ~31.1 arcminutes.

Magnification = apparent field / true field M = AF_deg / TF_deg

See http://www.twcac.org/Tutorials/apare...true_field.htm for a
diagram of the apparent and true fields.

For photographs illustrating the apparent and true fields, see the
bottom of web page:

http://www.astro.com.sg/chpt3.htm Apparent field is on the left;
true field on the right.

If the Moon just fills the eyepiece at 100x, it sounds like you are
using Possel eyepiece that has a 52 degree apparent field of view:

M = AF_deg / TF_deg
~100x = 52 deg / 31.1 arcminutes
= 3120 arcminutes / 31.1 arcminutes
= 187200 arcseconds / 1866 arcseconds
= ~100x

At its closest distance to Earth, Mars will have a naked-eye extended
apparent angular size of 25.1 arcseconds, or ~0.42 arcminutes.

At what power will Mars fill your Possel eyepiece with a 52 degree
apparent field of view?

M = 52 degs / 25.1 arcseconds
= 187200 arcseconds / 25.1 arcseconds
= 7458 power

This is considerably higher than observing Mars at 200x, as you have
been experimenting with.

It is probably impractical to try to construct equal sized images of
the Moon and Mars in the apparent field of your eyepiece.

Take a 36x eyepiece to look at the Moon - probably one of the lowest
powers in your eyepiece rack. In the eyepiece, the Moon would then
have a apparent angular size in your 52 degree apparent field-of-view
eyepiece of:

M = AF_deg / TF_deg
36 = AF / 31.1 arcminutes rearranging -
AF = TF x M
= 31.1 arcminutes x 20
AF = ~1120 arcminutes
= ~18.6 degrees

= 18.6 deg / 52 deg
= ~36% of the FOV

This means that the Moon takes up 18.6 degrees of your 52 degree
Possel eyepiece's apparent field of view or about 36% of the eyepiece
field of view.

With Matt Wier's online telescope simulator you can simulate how the
Moon would appear:

http://www.stic.net/mattwier/

In the online simulator, move the Eyepiece Focal Length bar in the
simulator to its maxmimum (55mm). This will simulate the view of the
Moon at 36x. The default apparent field of view in the simulator is 50
degrees. Move the FOV slider bar 52 degrees.

To view an equal-sized Mars in the eyepiece - that is to have the same
apparent field size (18.6 degrees or 36% of the FOV) in the 52 degree
apparent field of your Possel eyepiece - what magnification would you
use?

M = AF_deg / TF_deg
M = 18.6 degrees / 25.1 arcseconds
= 66960 arcseconds / 25.1 arcseconds
= 2667 power

This 2667 power probably exceeds the highest power eyepiece in your
rack by 4 to 10 times.

The online telescope simulator also contains the Object Data "Mars" to
simulate viewing the red planet. (But it cannot be made to simulate
2667 power easily, if at all! - ) Set the simulator to "Mars." Set
the magnification to 200x by moving the Eyepiece focal length to 20mm.

Viewing Mars at 200x using the 52 degree Possel apparent field of view
eyepiece gives:

M = AF_deg / TF_deg
200 = AF / 25.1 arcseconds rearranging -
AF = TF x M
= 25.1 arcseconds x 200
AF = 5020 arcseconds
= ~1.4 degrees
= 1.4 deg / 52 deg
= 2.7% of apparent FOV

This is about what is seen of Mars in the simulated (and your real)
eyepiece - a small distinct red disk with no detail.

You need about 800x to get a reasonably sized image in the Possel's 52
degree apparent field of view:

M = AF_deg / TF_deg
800 = AF / 25.1 arcseconds rearranging -
AF = TF x M
= 25.1 arcseconds x 800
AF = 20080 arcseconds
= ~5.6 degrees
= 5.6 deg / 52 deg
= ~11% of apparent FOV

At 800x and 11% apparent FOV, you get a blurry disk with detail
begining to resolve.

To set the online telescope simulator to 800x, hit the 2x Barlow
button. Then move the Eyepiece focal length slider to the left until
the magnification field reads about 800x (at about 5mm). You may have
to brighten the image up a bit by increasing the simulated telescope's
aperture size.

In another thread, message
... Peter Lawerence
has posted a number of pictures of Mars that his web site states were
taken using a Vixen FL-102 f9 refractor, a 20mm eyepiece lens, and a
2x Barlow - all coupled to a digital camera set at a zoom of 10x.

See his web site at http://www.pbl33.co.uk/

All put together, this gave Peter about ~920x:

fl_obj = 102mm x 9f = 918mm
M = fl_obj / fl_eyepiece
M = 918 mm / 20 mm = ~46x then add in the Barlow:
2M = 46 x 2 = 92 then the zoom camera
10(2)M = 10 x 2 x 46 = 920 power

You can use the simulator at http://www.stic.net/mattwier/ to emulate
920x. (Use the 2x Barlow button. In the Eyepiece focal length box,
type in 4.35mm. Brighten the image by increasing the aperture.) The
resolution of Mars in the online simulator resembles what Peter was
able to accomplish (with additional brightness being added by image
stacking instead of a larger aperture).

But I always get more of charge out of light coming from the real
thing than light received by "internet astronomy." -

Hope the above helps in better understanding how light moves through a
telescope. I know its confusing. Just stick with it.

Regards - Kurt

Some web sites for studying field of view:

http://www.twcac.org/Tutorials/apare...true_field.htm
http://www.spacegazer.com/basics.htm#field
http://www.astro.com.sg/chpt3.htm
http://www.stic.net/mattwier/

Mick wrote:

What I mean is, at 100 X's Mars is to appear as the moon does unaided. The
two are supposedly equivalent. (1/2 degree)
Now, if you up the anti to 200, Mars should be equivalent of what the moon
would appear to be at 100 X's....in the EP.

I believe your math is flawed.

Magnification is multiplicative, not additive ...

You double 100x mars magnification, you get 200x; you double 1x moon
magnification, you get 2x.