The Astronomy Lab Report
Fundamental to the process of scientific investigation is the concise,
objective reporting of the results of the investigation. It is only through such
communication that various different observers can compare results and
develop a consensus for understanding a particular phenomenon.
You will probably not make any discoveries in this laboratory that will
have an impact on the world of astrophysics; however, you should not treat
the work you have performed in the laboratory as trivial. Pursue each of the
experiments with an attitude of curiosity and treat them as if they were your
own original investigations. You'll find that this will make the laboratory
more interesting and enjoyable and it will make writing the lab reports easier.
The communications skills you develop in writing good lab reports will be
useful to you in any field of work you choose to pursue.
Each lab report should contain the following elements:
Title Page
Introduction
Procedure
Results
Discussion and Conclusion
Title Page
The title page should include the title of the investigation, your name,
the date on which the experiment was performed, and the names of others
(lab partners) who may have worked on the experiment with you.
Introduction
Summarize in a few words the purpose of the experiment, including
any modifications as the exercise progressed. Give background information
on the phenomenon that you have investigated and discuss any theoretical
considerations that are essential to understanding the experiment. If any
references other than those given in the laboratory guide are used, list them.
Procedure
Describe the equipment and procedures you used to carry out the
experiment. Be specific but don't go into excessive detail. If any substantial
changes from the laboratory guide were made, note them.
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Results
This section will include your data, calculations, and graphs. Label all
data and whenever possible, construct a table to summarize your data. The
data should be presented neatly in rows and columns with appropriate
headings whenever possible. Don't forget that in science all measurements
have units. If possible, take several measurements for each data point and
find the average value. You may use a computer printout of your data if you
have one.
The evaluation of experimental data deserves a bit more explanation.
In order for observations and measurements to be useful, their validity must
be determined. The method used, the conditions under which they were
made, and the accuracy of the method must be specified. Data must be
recorded clearly and objectively. Others who refer to your data must know
whether they can depend on these data for useful information. This is
particularly true in astronomy where one individual's work is used decades
later by others.
The ability to evaluate objectively the reliability of information and to
be disciplined in recording your own observations are habits that are
important for research and study in all fields. These are among the more
significant reasons for the study of science by students majoring in other
academic disciplines.
Recording Data
Data should always be recorded neatly in vertical columns with
appropriate headings for each column. The units in which the measurements
are made (centimeters, degrees, seconds, etc.) should appear at the top of
each column. As often as possible, data sheets will be provided with the lab
handout for each experiment.
Never make mental calculations when recording data. These are
susceptible to error. Always record the raw data first and then make any
necessary computations. If you obtain a datum that you believe is greatly in
error, do not erase it. Draw a line through it, but leave it visible. Write a
brief note on the reasons why you suspect that particular piece of information
to be inaccurate.
At the conclusion of each experiment, you should write a brief analysis
in your notebook. This should be done while the experiment you have
performed is still fresh in your mind. This analysis can be expanded later
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when you write your report. The analysis should include a conclusion and a
statement about the validity of this conclusion based on your evaluation of
your data.
Evaluation of Data
Experimental data are always subject to uncertainty. This uncertainty,
referred to as experimental error, is imposed by the limitations of the
instruments, the method used for making the measurements, and the human
limitations intrinsic to the method. The error can be minimized by increasing
the precision of the instruments and by using more sophisticated methods, but
it will always be present. For a particular measurement to be useful, it is
necessary to evaluate the error intrinsic to the method used. For experiments
in this course, a simple and direct method for doing this will be presented.
This is best done by way of example.
Suppose that the apparent diameter of a planet is measured by
observing the time for the planet to transit a line in the telescope eyepiece.
Several measurements are made and recorded as follows.
Transit Time (sec) Uncertainty (sec)
3.1
3.3
3.2
3.3
3.1
Avg. = 3.2
0.1
0.1
0.0
0.1
0.1
Avg. = ± 0.08
The average value of the measurements is 3.2 seconds. The column
labeled “Uncertainty” gives the absolute value of the difference (the sign of
the difference is ignored) between each of the measurements and the average
value. The (±) before the average of the uncertainties indicates that, as can be
seen from the data, individual measurements may be greater or less than the
average value of the measurement. For this example, the final value of the
transit time would be reported as
T = 3.2 ± 0.08 seconds.
These methods for the analysis of data are applicable when systematic
measurements are made of a well-defined quantity. In some of the
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experiments in this course, a more qualitative evaluation of the results must
be made. For example, visual observations of planets always involve a degree
of subjectivity on the part of the observer, however, observers must be
careful to record what they observe and not what they think they should see.
Factors that may affect the observation, such as sky conditions, telescope
diameter, etc., must also be recorded so that others may evaluate the
observation.
Calculations should be presented neatly and should be well organized.
The equations used for the calculations are to be given. If one method of
calculation is repeated several times for different values, show one sample
calculation in detail and tabulate the final results for the repeated calculations.
You may also use a computer printout of your calculations if you have one.
Give calculations in a logical order down the page. Indicate the equation
being used or the mathematical operation being done for each step--keeping
close track of the units along the way may often help you to avoid errors.
If graphs are required for the analysis of the results, the must be neat
and clearly labeled. Plot each graph to as large a scale as is practical. Title
each graph. Label the quantities plotted on each axis and label each curve if
you have more than one. Don't forget to give the units on each axis. Where
appropriate, draw a smooth curve or line through the data points. DO NOT
CONNECT THE POINTS IN A DOT-TO-DOT MANNER. The actual
data points do not have infinite accuracy, and thus may not lie exactly on the
"proper" curve. Draw a smooth curve such that positive and negative
deviations are about equal and such that the curve matches the general trend
of the data. This process averages the experimental fluctuations and the
results deduced from the curve are usually more accurate than those deduced
from the individual measurements.
If a standard value is available for the quantity you have measured (or
calculated), compare your experimental value to the standard and compute
your percent of error. The percent difference (or percent of error) is given
by:
Percent Difference =
Experimental value – Standard value × 100%
Standard value
Note that this error in your derived value is not the "experimental error"
resulting from uncertainty in measurements. Percent uncertainty or
discrepancy between two values is also calculated in a similar manner.
Note: A graphing program for the Macintosh is available on the computers
in the college's library as well as on those in the physics lab. This program is
called "Cricket Graph". It allows you to enter your data in a spreadsheet
format (similar to Microsoft Excel or Lotus 1-2-3). The program will then
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give you the option of graphing your data where one column will be plotted
on the horizontal axis and one or more columns can plotted on the vertical
axis. If your data is graphed as a "Scatter Graph", then a straight line can be
fitted to your points with the slope automatically calculated. Your instructor
and your classmates will help you get started with Cricket Graph. Make sure
your columns are in "decimal" format before you try to graph anything. To
make sure that this is the case, first highlight all of your columns and then pull
down the Data option from the pull down menu and select Column Format.
Finally, click on the Decimal button and all of your data will be converted to
decimal.
Discussion and Conclusions
Give a brief summary of your conclusions and final results. In this
section, you are to evaluate the significance of your result within the context
of the objective stated in the introduction. For those investigations that
require systematic measurements of a well-defined quantity, you may find it
helpful to answer the following questions in narrative form.
1.
How do my results compare with other published values for
the quantity I measured (if there is one)?
2.
What is the average uncertainty in my measurement?
3.
What factors contributed to the uncertainty? What could
be done to reduce the value of the uncertainty?
For those investigations that require a more qualitative approach, for
example, visual observations of the constellations or planets, this section need
not be separate from the Results part of the report. However, factors that
affected your observations, such as sky conditions, should be discussed.
This final section of the project report should also be where you answer
(as concisely and completely as possible) any Additional Questions posed to
you in the project handout. THINK before you write.
REMEMBER:
The basic purpose of the lab report is to give sufficient enough
(but not excessive...) detail that someone else could repeat the
experiment just by reading your report.
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