The Cassegrain Telescope
- Reflecting telescopes, or reflectors, utilise parabolic (curved) mirrors to collect and focus light from distant objects
- The most common type of reflector is known as a Cassegrain telescope
- Reflectors work because of the law of reflection where the angle of incidence, i is equal to the angle of reflection, r
The law of reflection on a ray diagram
Reflecting telescopes work through the law of reflection
- Instead of two converging lenses, a reflector uses a primary mirror and a secondary mirror
- The primary mirror is large and concave
- Here, incident light reflects towards a focal point which is behind the secondary mirror
- The secondary mirror is smaller and convex
- Here, light reflects again to form a real, magnified image at the eyepiece
- The rays are directed through an aperture towards an eyepiece lens which is located behind the primary mirror
Ray Diagram of a Cassegrain Reflecting Telescope
Ray diagram of a reflecting telescope in a Cassegrain arrangement
- The important features of the ray diagram for a Cassegrain telescope are:
- The rays enter the telescope parallel to the principal axis
- The curvature of the mirrors does not have to be the same
- The rays do not cross before the secondary mirror, they only cross in the aperture of the primary mirror
- Shading (or the lines on the mirror) indicates the non-reflective side of the mirrors
Worked example
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
Examiner Tip
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.
