Student Page
Activity 6a Gathering Starlight, Experimenting with Aperture
Gathering
Starlight:
When you look up at a star in the night sky, you see it as
a point of light. If you get out
your telescope and look at the star, it still looks like a point of light. So, you find a bigger telescope and look
at the star. To your surprise it
still looks like a point of light. If
the stars are going to remain looking like points of light, why do we need
telescopes with bigger mirrors or lenses? What’s
the advantage?
Although starlight appears to come from a point, it spreads
out in every direction. If you stand some distance from a few friends and you
all see the same star, then you all must be receiving a portion of the light
that the star produces. Your eye
intercepts only a very small bit of the light that comes from a distant star. Think about using a telescope mirror that would
see all the light that you see from the star and all the light your friends see
and all the light in between. The size of the light collecting mirror or lens in
a telescope is called aperture. We will do an experiment to see what happens to
an image when we increase or decrease telescope aperture.
In the activity, F Box – Explorations, you did a lab
experiment with aperture. The experiment was to put masks over the lenses to
reduce the size of the lenses. Then you examined the image on the screen
to see how it changed with reduced aperture. (Remember, aperture is the term
used by astronomers to define the diameter of the primary lens or mirror.) In
this investigation, you will analyze images taken with varying amounts of the
opening of a telescope tube covered.
The Problem:
How does changing aperture size affect the images we get from telescopes?
The Experiment:
To create an experiment to examine the effect of aperture on telescope
images, we decided to take a series of images and change only one variable at a
time. In this case the variable we manipulated was the telescope aperture. We tried to keep everything else
about the telescope and imaging process the same.
The images were taken on the same night, one right after the other. Can you think of some reasons why?
 |
We used the Yerkes Observatory 24 inch telescope for this
experiment. It has a 24-inch mirror to
collect the light, so its aperture diameter is 24 inches. We have a
wooden cover that fits over the end of the telescope tube.
|
The wooden aperture cover has a center opening to fit over the mount
for the secondary mirror; no light gets through this center part.
There are two 8 inch openings. We cover one of these with
poster board. Now the cover blocks out all the incoming light except for
the light coming through the 8 inch diameter opening. The telescope’s
aperture has changed from 24 inches to 8 inches.
To reduce the aperture even more we add masks to create smaller openings
of 6 inches, 4 inches and 2 inches.
 |
How does changing aperture size affect the
images from telescopes?
To conduct an experiment to
answer this question, we collected observations for two different sky
objects. The first time we imaged m56, a globular cluster in the
constellation Lyra. The second time we imaged m57, a planetary
nebula, also in the constellation, Lyra. We took images without
the mask using the full aperture diameter of 24 inches (minus the 6 inch
central obstruction). Covering the telescope tube with the various
masks, we took images with a 2 inch aperture, a 4 inch aperture, a 6
inch aperture, and an 8 inch aperture.
|
Predict -
Analyze the Data - Explain the Results.
Predict the
difference in images taken when the telescope aperture was covered, leaving
smaller openings to collect the light. What will change? What won't
change? Record your ideas and your reasoning before you
analyze the images. Compare the images taken using the different apertures;
collect quantitative data with HOU-IP data analysis tools. Explain the results
of your analysis. Compare your results with your predictions
Open the HOU IP software. Open the folder 6a-Aperture. Choose
either the images of m56 or m57.
M56 is a globular cluster in the constellation, Lyra; M57 is a nicknamed the
Ring Nebula and is a planetary nebula, also in Lyra. Open all five of the images
in the set you chose. Each one was taken with a different size opening
over the aperture of the 24 inch telescope at Yerkes Observatory. The
smallest aperture diameter was 2 inches; the largest aperture was for the whole
telescope, 24 inches, which has a six inch obstruction over the center part of
the telescope because of the secondary mirror.
Observe, compare and discuss the quality of the images. Collect data for each image using image processing and
analysis tools. Here are some ideas for using the HOU-IP tools.
Compare brightness data
between images for individual objects.
-
We suggest Data
Tools, Aperture. Set the values to 7 for Radius and 14 for Sky
Radius.
-
Calculate aperture area
and compare that to brightness data.
-
Use graph paper, or a spreadsheet and
graphing program to record, display and analyze your data.
Compare the field of view
for each of the images.
Other tools you
could use to investigate the images.
-
Use the Find command
under Data Tools to locate the number of stars at a particular brightness
in each of the images. This tool is more appropriate for the images of
M56, the globular cluster. (For m56 locating stars with a
brightness of 100 counts above the sky works well).
-
Compare the Sky
brightness. If you plot sky
brightness and aperture area the curve will be close to that found when
you graph aperture area and the brightness of a particular star, though it
will not work as well. (Data
Tools, Sky)
Conclusions: Look carefully at the data and your analysis. Use complete sentences to:
- Answer
the problem question. How does changing aperture size affect the
images from telescopes?
- Explain why this aperture experiment was
titled, "Gathering Starlight"?
Application:
On a particular Friday night, when The
University of Chicago Yerkes Observatory was observing for the Science Museum in
Tokyo during their Science Live Show, UNIVERSE, the Japanese audience requested
to see the beautiful full moon. When the first image was taken, the moon
was so bright, no features could be seen. What would you recommend to the
astronomers taking the images?
When you look at the moon through a telescope
or binoculars, it is often so bright that it hurts your eyes. What could
you do to make studying moon features more comfortable for your eyes?
_______________________________________________________________________
Astronomers often apply for observing time on very big telescopes. What
advantage does a telescope with a large aperture have over smaller aperture
telescopes for doing research?
_______________________________________________________________________
Plan an experiment to test telescope aperture while observing with binoculars or
imaging with a telescope.
_______________________________________________________________________
_______________________________________________________________________
