Telescope Basics
Goals of the Lab
To gain a familiarity with the telescope.
To measure the Field of View (FoV) of the finder scope, the 25mm eyepiece, and the 10mm eyepiece.
Requirements: Observation templates large version, a calculator and a stopwatch or a watch with a second hand.
Part I: Determining E/W and N/S in the Telescope
To put an observation made through a telescope into perspective, one must indicate at least North and East on the drawing. When you look through the telescope with any eyepiece, it is difficult to know which direction in the field of view is north, south, east, or west. You can observe this complication yourself by looking at a non-full moon. You will notice that the image of the moon in the eyepiece is flipped or mirror imaged compared to your naked eye observation. The eyepiece inverts the image (i.e. rotates it by 180 degrees). The image is then flipped right to left, i.e. mirror imaged, by the star diagonal. Since the star diagonal is not put in the exact same position each time you set up the telescope, it is difficult to know each night you make an observation if N/S and E/W are same as you determined the previous week. Therefore, each week you need to go through the necessary steps before labelling an observation. The procedure is written up on "Which Way is North? East?" in the telescope manual
Set up your telescope with the 25mm eyepiece.
Find a bright star near the equator. 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.
Sketch the field of view on an observation template. Draw any additional stars in the field of view. Distinguish between dimmer/brighter stars by using a bigger spot for a brighter star.
Note the star's appearance on your drawing. What color is it? Can you see any shape to it? Is the color constant, or does it flicker?
While looking through the eyepiece, very, very gently push "down" on the telescope near the end (where the corrector plate is). Make sure you push along the direction of the fork arms. This will not really be "down" if your star is in the Eastern/Western horizons. It will be down if your star is high in the Southern sky. By pushing downwards, you are moving the telescope south. This means the field of view of the telescope also moves south, so the north side of the field of view will get closer to the stars in the field of view. Or, another way of looking at this is to say that the stars in the field of view get closer to the North side, i.e. as you move the telescope down (or south) the stars you are observing appear to be moving towards the north side of the field of view. Use this to determine which way is North on your sketch. Note that if the moon is up, try this exercise on the moon. However, the moon will not always be up during lab and you will need to know how to determine directions for each lab.
Repeat the above exercise for East and West, by gently pushing the telescope to the right (i.e. West) and the the left (i.e. East). Make sure to push perpendicular to the fork arms. Pushing to the right will only be true West if the star you are observing is high in the Southern sky. If your star is closer to the Eastern/Western horizons, moving the telescope West will be pushing it at some angle. Appropriately label East/West on your sketch.
Turn the Right Ascension knob very slightly clockwise (remembering to partially undo the clamp). The Right Ascension knob moves the image East/West. Draw an arrow on your sketch 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, this will move the image North/South. Draw an arrow your sketch to indicate the direction the star moves when you turn the knob clockwise. Is this moving the field of view north or south?
It is important that you understand and remember the "nudge the telescope" technique for determining North and West. You will need to do this for each telescope observation you make in lab to correctly label your sketch. Ask a T.A.if you have problems with any of the above steps.
Part II: Measuring the Field of View
The following steps walk you through the measurements necessary to determine the Field of View (FoV). The FoV is the area of the sky that is visible through the telescope with a particular eyepiece. You will measure the FoV for the 25mm and 10mm eyepieces, as well as the finder scope. The FoV depends upon both the telescope and the eyepiece. The FoV for the Celestron is different from that of the Meade. The FoV of the 25 mm eyepiece is different from the 10mm eyepiece. The FoV is an angular size, measured in degrees, arc minutes, or arc seconds. You will measure a time, which you will convert into an angle next week. You will then have a number that represents how much of the sky you are looking at when using the finder scope or a given eyepiece.
Note: This part of the lab requires two people, one person to observe movement in eyepiece and the other to time the movement. For this part of this lab only you may share data with your lab partner. Switch off roles, so each person knows what to look for in the eyepiece. You will need to know how to both roles, as this measurement will need to be done again in future labs. Only the raw data (i.e. timings) may be shared between lab partners.
Choose a bright star near the equator (within 10° of the equator), and locate it in the 25 mm eyepiece. Center the star in the eyepiece. You may use the same star as in Part I. Note down the star you use.
Switch off the telescope drive. You should see the star drifting in the field of view. This is a result of the Earth's rotation. You can see the motion due to the telescope's magnification. Whenever the telescope drive is switched off, objects in the telescope will appear to drift. (Can you figure out if the stars appear to be drifting to the East or to the West?)
Switch the telescope drive 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.
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 five times, recording how long the star takes to cross the field of view each time.
Repeat steps 3 - 5 with the 10mm eyepiece in the telescope.
In the 25mm eyepiece, find a bright star between declination 40° and 60°. In the fall, Deneb is a good choice. In the winter, Capella is a good choice. Repeat steps 3 - 5 with this star. (You only need to do measurements with the 25mm eyepiece for the more northern star).
Now, return to the star near the equator. Center it in the cross hairs of the finder scope. The finder scope has a much larger field of view, which is why it is easier to locate an object with the scope rather than the eyepiece. However, due to its larger field it will take much longer to measure. Switch off the drive and measure how long it takes to drift from the center to the edge. This will give you half of the field of view of the finder. To determine the full time it takes, double this value. Do the finder scope measurement at least once. If you have time, repeat the measurement once or twice to check yourself.
Be sure to indicate whether you are measuring the field of view of a Celestron or a Meade telescope.
Question: Why do you think it is necessary to take more than one timing measurement?
Question: Were the crossing times you measured for the star near the Celestial Equator shorter or longer than the times measured for the star between 40° and 60°? Which star appears to be moving faster? Discuss a possible reason for this phenomenon. (Hint: Think about walking around the Earth at the equator versus walking around the Earth at Nashville, 36° latitude. Suppose it takes you a year to walk around the earth at 36°. Would you have to walk faster or slower to walk around the Earth at the equator in a year?)