Angular Size??

[DJVege]

Hi guys,

I know how to calculate the angular size of an object (distance, diameter/length related), but how do I measure the angular size of an object when I'm at a certain magnification (when looking thru a scope or binos)?? I read something yesterday on the net that explained it, but I can't find that anywhere now. It showed a calculation using magnification...

Anyone know?

Thanks.

[Glenhuon (Bill)]

No mathematician, but imagine it would be the apparent angular size divided by the magnification factor.

Glenhuon

[sheeny (Al)]

G'Day DJVege,

For visual observing you will need something as a reference to compare to in order to estimate the size. Usually the field of view (FOV) is a good place to start.

I've give you an example:

I have a C8 with a focal length of 2032mm. Lets say I use a plossl EP with a FOV of 52 degrees and a FL of 15mm.

As you know the magnification is 2032/15 = 480 times.

The apparent FOV through the scope then is 52degrees / 480 = 0.108333 degs = 6.5 arc mins.

So somethig that takes up about 1/3 of the FOV is about 2 arc mins.

Clear as mud?

Al.

[ngcles]

Hi DJVege & all,

Sheeny had a good go at answering your question but the maths is not quite up to scratch.

The magnification you scope produces with a particular eyepiece is a function of the focal length of the 'scope and the fl of the eyepiece.

Magnification is 'scope fl / eyepiece fl.

As a simple example, a 2000mm fl 'scope used with a 20mm fl eyepiece produces:

2000/20=x100.

The true angular diameter of that x100 field is a function of the magnification the eyepiece is used at and the _apparent field_ that the ep has. The apparent field is the angle in degrees your eye must be turned to go from one side of the field to the one opposite. In many cases the manufacturer will supply this figure of _apparent field_. As a _guide_, a Kellner ep usually produces about 40 deg, an Orthoscopic 45 deg, a Plossl 50 deg, Televue Panoptic 67 deg, and Nagler 82 deg etc etc.

To find the _true angular diameter_ of the field of view in degrees, you should divide the _apparent field_ by magnification in use.

In the example above using a Plossl with an assumed AFOV of 50 deg at x100:

50/100=1/2 degree _true_ FOV -- or about the diameter of the Full Moon.

With say an 8mm Plossl you get as a worked example:

2000mm/8mm=x250

50/250=0.2 degrees or 12 arc minutes.

Therefore (using the first example), lets say you look in a catalog and see that an open cluster has a diameter of 15 arc-minutes. In the 20mm fl eyepiece which has a 30 arc-min field, it would appear 1/2 the diameter of the field.

Conversely, if you see a galaxy in your eyepiece and you already know the true diameter of the field you can then measure its visible extent by estimating what portion of the diameter of the FOV it occupies. If you are using the 20mm eyepiece whcih gives a 30 arc-minute true field, and the galaxy seems 1/5th the diameter of the field, then it is about 6 arc-minutes diameter.

Hope this is of some help.


Les D
Contributing Editor
AS&T

[DJVege]

Aaaah....awesome! Thanks guys!

Much appreciated, Les. I understand it now.