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NUMBER
II.C. TITLE:
Astronomical Imaging
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PURPOSE:
To learn how cameras may be used
to collect images of celestial objects.
To make measurements of astronomical
photographs.
To determine plate scale by formula
and observation.
To get an introduction to
‘electronic imaging’ (CCD) devices.
DESCRIPTION:
Much information about the universe has been gathered using imaging devices, the most familiar of which is the film camera. More recently, electronic imaging has become prominent as data about celestial objects (physical appearance, light variation, motion, etc) is collected and stored digitally in a computer for faster and more efficient analysis. This exercise will address ‘emulsion’ imaging with an introduction to CCDs (Charged Coupled Devices, a type of digital camera).
PROCEDURE:
The instructor will demonstrate various ways of attaching a standard 35mm camera to a telescope to make both wide field and long focal length prime focus photographs. The concept of f/ratio will be presented. An application of an electronic (CCD) camera will also be demonstrated.
The instructor will derive the formula below and explain its application and use in this exercise.
F = 57.3o x S = 3438’
x S = 206265” x S (Eq. 1)
Ao A’
A”
‘F’ represents the focal length of the
camera lens or objective measured in mm.
‘A’ represents the angular space between
two points on the celestial object measured in degrees, minutes, or seconds or
arc
‘S’ represents the image size or measured
dimension on the film or imaging surface measured in mm.
To
determine f/ratio use the following:
f/ratio = FLobjective/Diameterobjective
= F/D (Eq. 2)
MEASUREMENTS/OBSERVATIONS:
1.
Determine the exposure time (minutes) for each of the 3 star
trail photographs. Enter results in
Table A.
2. Measure the lunar images.
a.
On the 35 mm slide
(made at prime focus of Meade or C-8)
b.
On the photograph
(made a prime focus of MDRC*)
Determine the focal length of both
systems ‘a’ and ‘b’ (use Eq. 1). Enter data in Table B. Assume angular size of
moon as indicated in Table E.
3. Measure the size of the image of
Jupiter on the slide provided. By the
given telescope focal length, and your measured image size, determine the
angular size of the planet (use Eq. 1, but solved for
A). Enter
results in Table C.
TABLE
A
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Photo # |
Trail Angle |
Measured Exposure Time
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Actual Exposure Time |
% Discrepancy |
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1 |
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2 |
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3 |
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TABLE B
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Diameter of Moon Image on Slide |
Computed Focal Length of Meade
or C-8 |
Actual Focal Length of Meade or C-8 |
% Discrepancy |
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mm |
mm |
2000 mm |
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Diameter of Moon Image on Photograph |
Computed Focal Length of Markowitz Dual Rate Camera |
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mm |
mm |
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TABLE
C
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Image Diameter of Jupiter |
Telescope Focal Length |
Computed Angular Diameter of
Jupiter |
Published Angular Diameter in
Table E |
% Discrepancy |
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17600mm |
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4. Examine the ‘CCD’ camera set-up in the front of the classroom. After the instructor explains its operation, make the necessary measurements to provide the information requested below.
TABLE D
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Telescope Focal Length |
CCD Chip Size (mm) |
CCD Field Size of object in mm |
Distance to Object in field of
CCD |
Computed Angular Field of CCD
Chip |
Published Angular Field of CCD
Chip |
% Discr. |
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2000mm |
2.5x2.5 |
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4.3X4.3 ARC MIN. |
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QUESTIONS/CONCLUSIONS:
1.
Give some possible reasons for discrepancies in Measurements
# 1 through 4
2.
Calculate the angular field that would fit on a single frame
of 35mm film at the prime focus of a 50 mm focal length lens.
Short dimension = 24 mm Field = __________ degrees
Long dimension = 35 mm Field = ___________degrees
3. What is the f/ratio of a Meade telescope?
Is this telescope ‘faster’ or ‘slower’
than an f/11 optical system?
4. Calculate the angular field along the short dimension of a 35mm frame at the prime focus of a Meade telescope.
5. Check Column A in table E those objects which would fit on the short dimension of a 35mm
frame at the prime focus of a 50 mm focal length objective.
6. Check Column B in table E those objects which would fit on the short dimension of a 35mm
frame at the prime focus of a Meade (or C-8) telescope.
7. Check Column C in table E those objects which would fit onto the ‘chip’ of the CCD camera
demonstrated in the classroom.
TABLE E CELESTIAL
OBJECT ANGULAR SIZES
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Celestial Object |
Size ‘or “ arc |
A |
B |
C |
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Sun |
32’ |
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Moon |
31’ |
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Jupiter |
40” |
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Saturn with rings |
42” |
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Venus |
20” |
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Mars |
8” |
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M-1 Crab Nebula |
6’ |
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M-8 Lagoon Nebula |
90’ |
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M-13 Hercules Globular Cluster |
12’ |
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M-27 Dumbbell Nebula |
8’ |
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M-31 Andromeda Galaxy |
160’ |
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M-42 Orion
Nebula |
60’ |
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M-44 Beehive Cluster |
90’ |
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M-45 Pleiades (7 Sisters) |
120’ |
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M-57 Ring Nebula |
90” |
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M-51 Whirlpool Galaxy |
12’ |
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8.
If the moon were photographed on various nights with the
same camera, would its image size change?
Explain.
Complete
summary in Bluebook and have instructor sign-off tonight.