Telescope Resolving Ability

Joe Robertsjoe@rocketroberts.com

Telescope Resolving Ability

We will cover the basics of an optical telescope's ability to resolve detail on a subject

An often asked question: “Can Hubble see the Flag on the Moon?” Answer: not a chance, and we'll show why not!

Dawes Limit

The ability of an optical telescope to resolve detail is governed by the Dawes Limit: Resolution (in arc seconds) = 4.56/diameter of the

telescope mirror/lens (in inches) 1 degree = 60 arc minutes; 1 arc minute = 60 arc

seconds, therefore 1 degree = 3600 arc seconds

Example 1:Telescope Mirror Diameter

= 6 inches

Resolving ability = 4.56/6 = .76 arc seconds

Example 1:Telescope Mirror Diameter

= 6 inches

Resolving ability = 4.56/6 = .76 arc seconds

Example 2:Hubble Telescope Mirror Diameter

= 94.5 inches

Resolving ability = 4.56/94.5 =

0.048254 arc seconds

Example 2:Hubble Telescope Mirror Diameter

= 94.5 inches

Resolving ability = 4.56/94.5 =

0.048254 arc seconds

Can we see the Flag on the Moon?

First we need to make some basic assumptions about how big the flag is

We then do some basic geometry to determine how big (in arc seconds) the flag would look at the distance of the Moon We will assume that we want to just barely be able

to make out the stripes on the flag Once we figure this out we apply the Dawes

Limit equation to find out what size telescope is required!

B

A

American Flag Dimensions

A=1.0B=1.9 A

Stripes are 1/13 of A

Flag on the Moon: Assume it is 3 feet wide; therefore stripes are:

36 inches x [(1/1.9) x (1/13)] = ~ 1.46 inches tall

We'll round this up to 1.5 inches for our calculations...

So, we need a telescope that can just resolve something about1.5 inches wide on the Moon. So how do we figure this out?

Basic geometry is all that is required!

Earth - Moon Geometry(Sketch not to scale!!)

b

a = diameter of the Moon = 2160 milesb = mean distance from Earth to Moon = 239,000 miles

a

For our diagram: tan(A) = (a/2)/b

tan(A) = (2160/2)/239,000 = 0.004519

Therefore angle A = 0.259 degrees

The TOTAL diameter of the Moon (as seen from Earth) is:

2 x 0.259 = ~ .518 degrees

Angle A

Earth

Moon

Earth – Flag on Moon Geometry(Sketch not to scale!!)

b

a = height of a flag stripe (1.5 inches for our assumptions) a in miles = 1.5 inches / (5280x12) inches/mile = 2.3674e-5 milesb = mean distance from Earth to Moon = 239,000 miles

a

For our diagram: tan(A) = (a/2)/b

tan(A) = (2.3674e-5/2)/239,000 = 4.9528e-11

Therefore angle A = 2.838e-9 degrees

The TOTAL width of the flag stripe (as seen from Earth) is:

2 x 2.838e-9 = 5.675e-9 degrees

AEarth

Part ofOne Flag

Stripe

Apply Dawes Limit Equation

From previous slide The TOTAL width of the flag stripe (as seen from Earth) is:

2 x 2.838e-9 = 5.675e-9 degrees

There are 3600 arc seconds per degree; therefore 5.675e-9 degrees = 2.043e-5 arc seconds

Dawes Limit: Resolution (in arc seconds) = 4.56/diameter of the telescope mirror/lens (in inches)

2.043e-5 = 4.56/x; therefore x = 223183 inches = ~ 3.52 miles

Therefore it is not even close to possible to see the Flag on the Moon from Earth with telescopes available today!

So what can Hubble see on the Moon?

Hubble is in orbit ~ 375 miles above Earth, so basically it is no closer to the Moon than we are right now

Hubble's ability to resolve was determined to be 0.048254 arc seconds

Using geometrical methods previously used it can be shown that 0.048254 arc seconds is equal to an object about 295 feet wide at the distance of the Moon

This is the smallest object that could just barely be discerned!

What could Hubble see if it was pointed at the Earth?

Using geometrical methods previously shown it can be calculated that 0.048254 arc seconds is equal to an object about 5.56 inches wide at the distance of 375 miles

Basically Hubble could just make out the size and shape of a car license plate (assuming it was laying flat on the ground)

Reading the plate would be out of the question!