Reflecting Telescopes (AQA A Level Physics)

Draw a ray diagram to show the path of two rays, parallel to the axis, through a Cassegrain telescope, as far as the eyepiece.

Answer:

Step 1: Start by drawing and labelling a principal axis, a primary concave mirror and an eyepiece lens

• Make sure to include the aperture (gap) at the centre of the primary mirror

Step 2: Draw and label a secondary convex mirror

• Use lines or shading to indicate the non-reflective side of each mirror

Step 3: Draw two rays parallel to the principal axis

Step 4: Draw the reflection of the rays by the primary mirror onto the secondary mirror

Step 5: Draw the reflection of the rays by the secondary mirror towards the eyepiece

• Make sure the rays do not cross until they reach the aperture

Step 6: Check your final image and make sure everything is included to gain the marks

• For a two-mark question, examiners will be looking for:

  • The correct curvature of the mirrors, i.e. a concave primary mirror and a convex secondary mirror

  • The primary to look like one mirror i.e. it should look like a continuous parabola

  • Two rays entering the telescope parallel to the principal axis

  • The rays crossing in the aperture of the primary mirror

Make sure to avoid these common problems when drawing the ray diagram of a Cassegrain telescope:

• Drawing the two halves of the primary mirror with different curvatures so it looks like two separate mirrors

  • Make sure to draw these with a pencil. You could always draw the full curvature of the mirror then just rub out a gap in the centre of the drawing

• Drawing the secondary mirror as a straight line, or even concave

• Forgetting to include the eyepiece lens

• Drawing the rays as crossing before hitting the secondary mirror

The Cassegrain arrangement is the only reflector you must learn about in this course.

You will notice that light rays from distant astronomical objects are always drawn as parallel. This is because the object is so far away, that by the time it reaches the lens, the rays are parallel, even if it's an irregularly shaped object.

There's a common misconception associated with the secondary mirror is that it blocks the light (which it does) resulting in the central portion of the image being missing (which is not the case). There will be a slight reduction in the amount of light, but the light rays from a distant source are parallel so that even light from a source on the mirror axis will reach the secondary and be collected.