SIERRA COLLEGE ASTROPHOTO/IMAGING

LABORATORY EXERCISE

NUMBER II-Ba TITLE: Field of View

DATE- PRINT NAME/S AND INITIAL EACH GROUP

DAY-

LOCATION

OBJECTIVE:

Be able to:

Compute the magnification power of a telescope.

Calculate the true field of a telescope, given the apparent field of the eyepiece and with a camera attached.

By observation, measure the true field of a telescope using the Direct Measure method

DESCRIPTION:

Celestial objects come in a variety of sizes and shapes. When observing the moon, a planet, star cluster, or any celestial object, the size of the field of view (True Field) of a telescope will determine how much of the object will fit into the area of sky viewable through the eyepiece or camera. This activity will provide the opportunity to predict and measure the area of sky accessible by a Celestron-8 telescope using a variety of eyepieces and a camera. The field of view of the TELRAD finding device will also be examined.

PROCEDURES AND OBSERVATIONS:

The instructor will explain how to calculate the True Field referring to equations #1 and #2. Enter notes, if necessary next to equations.

True Field and Apparent Field are measured in degrees.

M = Fo/Fe M is magnification Fo is objective focal length Fe is eyepiece focal length

TF = AF/M TF is true field of telescope AF is apparent field of eyepiece.

EQ. #1 EQ. #2

DATA:

TABLE A – Calculated ^†C-8 True Fields Published Telrad True Fields and Camera
Eyepiece	M	AF^o	TF^o pred.
____mm		50
____mm		50
40 mm		43
Camera* 24mm	xxxx	xxxx	0.7^o
Camera* 35mm	xxxx	xxxx	1.0^o
inner mid outer	xxxx	xxxx	0.5^O
	xxxx	xxxx	2.0^O
	xxxx	xxxx	4.0^O

†Fo for C-8 telescope is 2000mm.

DIRECT MEASURE METHOD

Place a strip of paper about 2 meters long on a distant wall. View it through the telescope using the smallest focal length eyepiece, making certain that an edge of the paper passes exactly through the center of the eyepiece. The optical tube of the telescope should be level. Do not bump the telescope throughout this method of TF measurement. Have someone mark the edges of the paper that appear at the outer edges of the telescope field of view. Repeat this method for all eyepieces, camera and the Telrad. When you are finished, remove the paper from the wall and return to the laboratory for measurement. Enter data in the table below. Note: Instructor may provide student with pre-marked strip of paper to measure Fields-of-View.

TABLE B – Data for Direct Measure Method (S)
Distance (‘D’) to Paper Strip = _____meters =_________mm
Eyepiece Meas. Size of Field (‘S’ mm) TF^o (Eq. #3)
____mm
____mm
40 mm
Camera* 24mm	xxxxxxxx	xxxxxxxxx
Camera* 35mm	xxxxxxxx	xxxxxxxxx
inner middle outer

The observed true field (TF^o) may be calculated in this method by using the following equation, which will be derived by instructor. It is reasonably accurate for small angles.

TF = 57.3^o x S/D

EQ. #3

Examining finder scopes:

If they are available, select one of the FINDER scopes. Determine by visual estimation how much larger the image through the finder scope is as compared to the naked eye.

TABLE C – Finder Scope Magnifications

Finder Scope	Measured M M = size(telescopic) / size(naked eye)	Given M (from scope)	Field of View (measured)
A
B
C
D
E

Measure the magnification power of each finder scope combination by a method described by instructor. Enter these results in the above table for comparison with given values.

Celestial Object	Size ‘or “ arc	Camera* 35mm	40 mm	13 mm
Sun	32’
Moon	31’
Jupiter	40”
Saturn with rings	42”
Venus	20”
Mars	8”
M-1 Crab Nebula	6’
M-8 Lagoon Nebula	90’
M-13 Hercules Cluster	12’
M-27 Dumbbell Nebula	8’
M-31 Andromeda Galaxy	160’
M-35 Open Cluster in Gemini	28’
M-41 Open Cluster in Canis Major	38’
M-42 Orion Nebula	59’
M-44 Beehive Cluster	90’
M-45 Pleiades (7 Sisters)	120’
M-57 Ring Nebula	90”
M-51 Whirlpool Galaxy	12’

Questions (Answers to 1 and 2 to be completed in the Blue Book)

1. If the eyepieces in this exercise were used with a telescope of longer objective focal length, how would the true field (TF) be affected?

2. Given that a standard lens of a SLR camera has a focal length of 50 mm, what is the expected FOV along the long dimension when using that lens?

3. The longest angular dimension of several celestial objects is listed in the table below. Using the calculated values of TF in Table A, indicate (by an ‘x’) which objects would entirely fit into the C-8 field for each eyepiece or camera setup.

TABLE E CELESTIAL OBJECT ANGULAR SIZE

Print a star chart from TheSKY and view some of these objects through the C-8 to confirm your findings.