Telescope Basics
Goals of the Lab
To gain a familiarity with using the telescope. You should complete this lab before starting any other telescope labs.
To measure the field of view of the finder, the 25mm eyepiece, and the 10mm eyepiece. You will need these values for future telescope labs.
Requirements: Observation templates (printed out before lab), the telescope, both eyepieces, your logbook, and a stopwatch or a watch with a second hand. Before you do this lab, you must know how to use the telescope, and must have been taken through the basic telescope instruction by the TA's.
Part I: Looking Through the Telescope
Set up your telescope, and make sure the drive is on.
Find a bright star near the equator in the 25mm eyepiece on the telescope. In the fall, Altair is a good choice. In the winter, the central star on Orion's belt (ε Ori) is a good choice. Center it in the field of view, and focus the telescope.
Make some notes on what the star looks like. What color is it? Can you see any shape to it? Is the color constant, or does the color seem to flash or change at all?
Draw the field of view. Use an observation template (either the large version or the small version). You should be able to see at least another star or two in the field of view. Use the standard convention of drawing a bigger spot for a brighter star.
Figure out which way is North, South, East, and West using the procedures described in "Which Way is North? East?" in the telescope manual. Indicate the direction of North and East on your drawing from Step 4. Remember this procedure! You will need it in future labs.
Turn the Right Ascension knob very slightly clockwise (remembering to partially undo the clamp). Be very gentle doing this, or you'll lose your star out of the field of view and have to start over finding the star. Draw an arrow labelled with "6" on your drawing from Step 4 to indicate the direction the star moves when you turn the knob clockwise. Is this moving the field of view east or west?
Turn the Declination knob very slightly clockwise. Draw an arrow labelled with "7" on your drawing from Step 4 to indicate the direction the star moves when you turn the knob clockwise. Is this moving the field of view east or west?
Part II: Measuring the Field of View
In this part of the lab, you will be measuring the time that it takes for a star to drift across the field of view of your telescope with the telescope drive turned off. This will give a measurement of the field of view of the telescope. For an explanation of why this works, see Part II below.
Note: while you may do this lab entirely on your own, for this part of this lab only you may share data with your lab partner. You should take the data together, e.g., one person watches the telescope while the other watches the time. If you do this, switch off roles. You may not just copy data that your lab partner took by himself or by herself. Each person must log all of the data in his or her lab book and must perform his or her own calculations based on the raw data! Only the raw data (i.e., timings) may be shared between lab partners.
Locate a bright star near the equator (within 10° of the equator), and find that star in your telescope; you can use the same star as you used in Part I. Use the 25mm eyepiece. Be sure to note down which star you are using in your logbook!
Center the star as best you can in the exact center of the telescope field of view.
Turn off the telescope drive. Notice the star drifting? What is happening is that the Earth is rotating out from underneath the star; thanks to the magnification of the telescope, you can see the motion. (This made your professor feel heady with the speed of his motion the first time he observed this many years ago.) Whenever the telescope drive is not on, stars in the telescope will appear to drift in this direction. (Can you figure out if the stars appear to be drifting to the East or to the West?)
Turn the telescope drive back on. Adjust the right ascension knob on the telescope so that the star is just off the field of view, in the direction opposite the drift direction of the star. Do not adjust the declination of the telescope!
Turn the telescope drive off. Start timing (using a stopwatch or the second hand on your watch) as soon as the star appears in the field of view. Stop timing as soon as the star disappears off of the opposite side of the telescope. Turn the telescope drive back on.
Repeat the measurement three times, noting down how long it takes the star to cross the field of view each time. If one of your measurements is more than several seconds different from the other two, make a fourth measurement. Average together your three "good" measurements to get an "average crossing time" for this star.
Repeat steps 2-6 with the 10mm eyepiece in the telescope.
In the 25mm eyepiece, find and focus on a bright star between declination 40° and 60°. In the fall, Deneb is a good choice. In the winter, Capella is a good choice. Find this star in the telescope with the 25mm eyepiece. Repeat steps 2-5 with this star. (You need only do the 25mm eyepiece for the more northern star, and you need only make a single measurement— but be sure to do it carefully!)
Finally, return to the star near the equator. Center it in the finder scope. Because the finder scope has a much larger field of view, it will take much longer to measure it. If you have centered the star right on the crosshairs, you can just time how long it takes to go from the center to the edge once you turn the drive off. This will tell you half of the field of view of the finder, which is easy to double and get a full field of view. You only need to do the finder scope measurement once, although you are encouraged to do it more than once if you have time and if you want to check yourself.
Be sure to indicate whether you are measuring the field of view of a Celestron finder or a Meade finder, as they are different! Later, if you need the field of view of a finder scope for a telescope different from the one you measured, you can ask the TAs, or a fellow student who did measure the field of view for that finder scope.