2.7. (2 pts) Press 'Reset' so it is concave. Change the curvature, and thus, the focal length of the
primary
mirror by clicking on the top of the mirror and dragging it down/left and up/right. What happens
to the
focal point (closest red tick mark) as you increase AND decrease the focal length?
2.8. (2 pts) Press 'Reset' again. Change the distance between the object and mirror by clicking
on the
object arrow tip and dragging it left and right. Explain how the image changes as you increase AND
decrease this separation.
2.9. (2 pts) Based on your results above, what is one advantage to using a convex mirror?
What is one advantage to using a concave mirror?
SECTION 3 ACTIVITIES :
3.1. (1 pt) List 3 common everyday objects – other than a telescope - which use optics (lenses
and/or
mirrors) to affect (reflect, refract, focus, etc.) light.
3.2. (2 pts) What are the two primary functions of the optics in a telescope?
3.3. (1 pt) A refractor uses __________, a reflector uses __________, and catadioptrics
use __________.
a) mirrors, lenses, mirrors and lenses
b) lenses, mirrors, lenses and mirrors and sometimes correcting plates
c) filters, mirrors, lenses and mirrors and sometimes correcting plates
d) lenses, mirrors, neither
3.4. (2 pts) Why is it important to design/build telescopes that produce few aberrations?
3.5. (2 pts) Give 2 reasons why reflector telescopes are used more by astronomers than refractors.
-------------------------------------------------
**EXPERIMENT**
Look at the two telescopes set-up in the front of the room.
One is a 3-inch Bushnell refractor. The other is
a 5-inch Celestron telescope
...it's a smaller version of the telescope we use during night lab (see lab text
Figure 8.)
3.6. (3 pts) Examine the appearance, structure, and usage of both telescopes.
List at least 3 differences
between the two telescopes.
3.7. (1 pt) The 5-inch Celestron
has a concave primary mirror, a secondary mirror, and an aspherical
correcting lens plate. According to the
lab text, what is the basic design type of this telescope?
3.8. (2 pts) Find the RA and DEC setting circles on the 5-inch class telescope. These are
used to help
you locate objects in the sky. The DEC circle is scaled so that one tick mark equals one
degree. The
indicator of where to read the coordinates is a raised white line. Remember, to determine
if it is positive
degrees (north) or negative degrees (south) as well.
The RA circle is scaled …
the bottom row of large
printed numbers correspond to the hour, and the smaller ticks marks represent
arcminutes. Each small
tick mark is equal to 5 minutes. The indicator of where to read looks like
a bullseye.
Look at the
setting circles and get
the present coordinates.
RA: __________hr __________m
DEC: _______________degrees
-------------------------------------------------
3.9. (3 pts) Look at this quantum efficiency plot for CCD detectors from http://www.ccd.com/ccd101/.
a) What is the peak wavelength for these front-illuminated detectors?
b) Are front-illuminated or back-illuminated CCDs seemingly better?
c) Is there more variation in CCD detector capability at shorter or longer wavelengths?
3.10. (3 pts) The WFPC2 CCD on the Hubble Space Telescope has an average visual
quantum efficiency
of 15% (q = 0.15) and average noise of s = 6 electrons
/ pix.
Let's say you then observe a certain object
for one orbit (t = 2500 s). On the resulting image,
you find a signal of F = 1194 counts / s · arcsec2 for the
object and a background of
B = 30 counts / s · arcsec2 in the visible over an area A = 1 arcsec2. With
this
detector each pixel is about 0.045 arcsec in area, so therefore, you use n = 22.22 pixels.
Calculate the
theoretical signal-to-noise ratio S/N for this system.
3.11. (3 pts) Detector A yields a source signal of S = 30 counts and has noise
s = 4 counts. Detector B
yields source signal S = 25 counts and noise
s = 5 counts. Based on signal-to-noise ratio calculations,
which detector is better?
Show the math.
-------------------------------------------------
**EXPERIMENT**
Now let's examine signal-to-noise ratio further with this "S/N for CCDs" applet © M. Richmond.
Before starting the exercises, open
the applet. Examine the list of entries...you will see that most of them
have already
been defined for you in this lab. The 'Filter' option detemines which wavelengths you want
to
observe :
U = ultraviolet, B = blue (short, visible), V = (visible), R = red (long, visible),
I = near-
infrared. The 'Mag Limits' option sets the magnitude (a.k.a.,brightness) range
of objects you'd like to
observe. 'Sky Brightness' is another name for background signal and
'FWHM' is the same as the 'seeing'
conditions. The option 'Radius for Photometry' indicates (in
pixels) how big of a circular area you want
to study.
Enter the following data for the Hubble Space Telescope - WFPC2 detector system
into the applet
and press 'Submit':
Filter : B
Mag Limits : between 0 and 25 at intervals of 1.0 mag
Telescope Diameter : 240
QE : 'use value at right' 0.15
Pixel Size : 0.045
CCD noise : 6
Sky Brightness : 'use value at right' 24
Airmass : 1.2
Exposure Time : 2500
FWHM = 1
Radius for Photometry : 1
3.12. The applet should have posted a new web page entitled "Signal-to-noise Calculations" with
the results on it.
a) (1 pt) Have your TA verify that you have completed this activity to this point by giving
you a
check mark for this question worth 1 point.
b) (1 pt) Examine the results of your applet entry. Note that the smaller the magnitude, the
brighter
the object. Does the S/N increase or decrease as objects get fainter?
3.13. (2 pts) You may have noticed that in our applet we used the same parameters as in
#3.10 above. In
that problem, the object we observed had a brightness of 20 mag/arcsec2. Find this entry
on the results
page and read the signal-to-noise value. Give 2 possible reasons why this value is
different than what we
calculated in #3.10.
3.14. (5 pts) Knowing the definition of the items below, answer the following questions :
(Experiment with changing entries in the applet to help you find the answers)
a) Does signal-to-noise increase or decrease when you improve the seeing (i.e., decrease FWHM)?
b) Does signal-to-noise increase or decrease if we use a detector with more noise?
c) Does signal-to-noise increase or decrease when the background level is higher (i.e., decrease in Sky Brightness)?
d) Does signal-to-noise increase or decrease if the exposure time is more and/or we use a larger telescope?
e) Does signal-to-noise increase or decrease if no filter is used?
-------------------------------------------------
3.15. (2 pts)
Because light from celestial objects must travel through space and the Earth's atmosphere
before we observe it. A star has a true brightness of m = 12.5. If we observe it through an airmass of
X = 1.41 and the atmospheric extinction coefficient is k = 0.43, then what is the observed brightness
m0 of the star?
3.16. (2 pts) The light we see collect with a telescope can be altered from its original
state by many
factors. List at least 6 such factors.
3.17. (2 pts) Give 2 advantages to astronomers using modern detectors in their work.
SECTION 4 ACTIVITIES :
4.1. (2 pts) The aperture of the FIU telescope is 8 inches. Recall, 1 inch = 2.54 cm.
Calculate the telescope aperture in centimeters.
4.2. (2 pts) Use your answer in #4.1 above to find the light gathering power (LGP)
of this telescope.
4.3. (2 pts) Use your answer to #4.1 above to also find the limiting magnitude (LM) of the telescope.
Remember that the smaller the magnitude, the brighter the object. To understand then how good our
telescopes are we can
compare: the very nearby and bright Sun has a magnitude of m = –26.7, in full
phase the Moon has
m = -12.6, and the North
star has m = 2.0. Thus, our observatory telescopes are
able to see fairly faint objects.
4.4. If you observe the Sun through our 8-inch telescope (without tracking), the Sun appears to move
across the eyepiece. This occurs not because the Sun is orbiting the Earth or the telescope is moving,
but
because Earth is rotating. In 60 minutes it will rotate 15 degrees.
Measurements show it takes
approximately 2.5 minutes for the Sun to do this in our telescope.
a) (3 pts) Given this information, calculate the field of view (FOV) of our lab telescope in degrees.
b) (1 pt) The angular diameter of the Moon as seen from Earth is about 0.5 degrees.
In theory, can our
telescope completely contain the Moon in its field of view?
4.5. (2 pts) The focal length of our telescope is 203.2 cm. What is the F-Ratio?
4.6. In different units, the FIU telescope has a focal length of FL = 2032 mm.
a) (2 pts) If we observed Mars through a 10 mm eyepiece, how many times
is Mars magnified?
b) (3 pts) What focal length eyepiece (in mm)
would you need to use to magnify an object by 58 x ?
4.7. FIU's Astronomy Program is part owner of a D = 0.9 m telescope at Kitt Peak Observatory in AZ.
a) (3 pts) Let's say we observe light
(l = 5.5 x 10-7 m) with this telescope. Calculate the resolution.
b) (2 pts) FIU also owns a 0.3 m telescope. If resolution is our primary concern, which
telescope is the
better choice and why?
4.8. (2 pts) Based on all the topics discussed in this lab, list at least 2 reasons
why understanding optics
is important in astronomy.
* TURN IN THESE ACTIVITIES PAGES TO YOUR TA*
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