The Telescope Lab
The
purpose of this lab is for you to measure and investigate the optical
properties of your telescope. In so
doing you should gain a better understanding of the concepts involved.
Telescope,
meter stick, ring stand, red clamp, index card (i.e. a ÒscreenÓ)
q Determine the focal length
of the objective and the ocular (eye piece). This is done by
projecting the image of a very distant object onto a screen (index card). Secure the lens in question to the ring
stand and align it with a distant object such as a tree out the door or a light
across the room. Place the screen
on the side of the lens opposite the distant object. Adjust the position of the lens and/or
screen until a focused and clear image is formed. Use the meter stick to measure the
distance from the center of the lens to the image on the screen. This distance is the focal length. Repeat for the other lens and record the
results for both on the data table.
Part
B – Theoretical Magnification
q Using the measured focal
lengths determine the theoretical magnification of the telescope by dividing
the focal length of the objective by the focal length of the ocular. Show the calculation that you make. Record the result on the data table.
Part
C – Observed Magnification
q Now determine the
magnification of the assembled telescope using a more direct method. Secure the telescope to the ring stand
and focus on the large grid pattern or orange and white meter sticks. Method 1 (Direct observation) Look
through the scope with one eye while keeping the other eye open. With a little patience you should be
able to see both images at the same time – the image of the grid seen
with the naked eye and the magnified image of the grid seen through the
telescope. Adjust the telescope so
that the corner of one of the magnified boxes overlaps the unmagnified grid at
the origin. Then ÒmeasureÓ the
magnified boxÕs width in terms of the unmagnified boxes. This ratio of apparent
size is the magnification of the telescope. Method 2 (Photographic observation): use cell phone camera to take pictures
with and without telescope. Compare
and/or measure the pictures to determine the magnification. Record the result in the table.
Part
D – Field of View
q Secure the telescope to the
ring stand and focus on the large grid pattern or meter sticks. Look through the scope and ÒmeasureÓ the
width or height of your view through the scope by simply counting the number of
units across. Note that each square
on the grid has a width of one inch.
Use the distance to the pattern and the skinny triangle approximation to
calculate the angular width of the view through the telescope. This is called the Field of View (FOV).
Part
E – Theoretical Resolution
q Measure and record the
diameter of your telescopeÕs aperture with and without the washer in place. Use this value to determine the
theoretical diffraction limit on angular resolution for your telescope in units
of arc seconds. Assume a wavelength
of light equal to 600 nm. Show the
calculation that you make. Record
the result on the data table.
Part
F – Observed Resolution
q Determine the angular
resolution of the assembled telescope with and without the washer. Secure the telescope to the ring stand
and focus on the pattern of lines. For
each case – with and without washer – record the minimum separation
visible through the telescope and record the distance from the telescope to the
pattern. Note: the telescope must
be a sufficient distance so that not all of the pattern can be resolved –
which is to say you must ÒpushÓ the telescope to the limits of its resolution.
q Using the measurements from
above, determine the angular resolution of the telescope in arc seconds. Show the calculations that you
make. Record the result on the data
table.
q Try projecting an image with
and without the cardboard washer in place.
Carefully remove the washer and replace the lens with the flat side
facing outwards. Remove the
eyepiece and replace it with a piece of paper held taut across the end of the
tube by a rubber band in order to form a projection screen. Try observing a light bulb projected
onto the screen. Hold the cardboard
washer over the end and observe how this changes the image. Compare with and without the
washer. Is there any effect on the
brightness of the image? Is there any
effect on the clarity or resolution of the image? Simply describe what you see and record
your observations on the data table in complete sentences.
q Measure and record the
diameter of the aperture with and without the washer and determine the ratio of
the amount of light gathered without the washer compared to the light gathered
with the washer in place. Show the
calculations that you make. Record
the result.
1. Write a brief paragraph
comparing and contrasting the theoretical and observed values for your
telescopeÕs magnification. Things
to consider: Are the results the
same? If not, which do you think is
more accurate? Why?
2. Write a brief paragraph
comparing and contrasting the theoretical limit on angular resolution with the observed
value. Things to consider: Are the results the same? If not, why not? Which value is larger
and why?
3. Briefly discuss the use of
the cardboard washer. Discuss
advantages and/or disadvantages to its use.
4. Briefly discuss how this lab
exercise has helped or not helped your understanding of the telescope. Are there any suggestions for
improvement? Better ways to obtain
the data?
Telescope
Lab Report consists of responses to the above questions and the completed data table
showing all measurements, observations, and calculations.
|
|
Focal Length |
|
Objective Lens |
|
|
Eyepiece Lens |
|
B. Theoretical Magnification (Calculate by
focal lengths)
Calculation:
|
Theoretical Magnification |
|
|
C.
Observed Magnification (Compare image to object)
Describe
method and/or show any calculations:
|
Observed Magnification |
|
|
D. Field of View (Skinny triangle method)
|
Width of view through
telescope |
|
|
Distance from pattern to
telescope |
|
Calculation:
|
Field of View |
|
|
E.
Theoretical Resolution (Diffraction Limit)
Calculation:
|
|
Aperture Diameter |
Theoretical Resolution |
|
With Washer |
|
|
|
Without Washer |
|
|
F.
Observed Resolution (Skinny triangle method)
|
Distance from pattern to
telescope |
|
|
Minimum separation
resolved with washer |
|
|
Minimum separation
resolved without washer |
|
Calculation:
|
|
Observed Resolution |
|
With washer |
|
|
Without washer |
|
Observations:
|
Aperture diameter without
washer |
|
|
Aperture diameter with
washer |
|
Calculations:
|
Factor of light gathering increase without washer |
|
|