ASTR 110L                                                                                                                          Name:                                               

Fall 09

In-the-Field Telescope Practice — Fall Semester




0.  Assemble your Dobsonian telescope.  Be sure to stow all caps and covers in their proper locations!  See your instructor or TA with any questions or problems. 





1.  Install a 32-mm eyepiece (no filter) — this will give the widest possible field-of-view (lowest possible magnification) of all four of our lab eyepieces.  Then choose any really bright object (any star or planet) in the sky as your first target.  To point the telescope, do the following 3 (or 4) steps: 

a.  Grab the knob at the open end of the telescope tube, or grab the sides of the tube, and point the telescope tube in approximately the correct direction.  “Sight” along the side of the telescope tube if it helps. 

b.  Look through the finder scope — you should see your bright object somewhere in the field.  Nudge the telescope tube until the bright object is centered on the black cross-hairs.  [If you don’t see the bright object, return to step (a).]

c.  Finally, look through the eyepiece — somewhere in the field-of-view you should see a small bright point of light (if the eyepiece is focused) or a circular blob of light (if unfocused).  Nudge the telescope tube (very slightly!) until the bright object is centered in the eyepiece field-of-view. 


The following step should only need to be done once, right after the telescope is assembled:

(d.  After centering the object in the eyepiece, quickly return to the finder scope — if the object is no longer on the center of the cross-hairs, then the finder scope is out of alignment.  This is normal when the telescope has just been assembled.  Quickly use the two small black plastic alignment screws on the top and side of the finder scope to align the cross-hairs to lie on top of the bright object.  This entire step must be done quickly, or else the object will drift away from the center of the field and the finder alignment will be inaccurate.  If you think you may have taken too long, return to step (c), and then try step (d) again.) 





2.  Using a 32-mm eyepiece in the telescope, point up the telescope on Vega(in the constellation Lyra).  Vega is one of the brightest stars in the entire sky, and is the northwest corner of the summer triangle.  If you need help finding Vega, consult the all-sky map for “September, Northern latitudes” in Ridpath (pp. 58-59), or you may ask your instructor or TA. 

a.  Once you have the telescope sharply focused on Vega, what does the star look like through the eyepiece?  Is the star shaped like a disk (circle), or just an infinitely small point?  Describe and/or sketch here: 









b.  Intentionally de-focus the eyepiece badly… take it waaaay out of focus in either direction.  Sketch what the star looks like now:  


2. continued:

c.  Re-focus and center the star in the middle of the field-of-view using the 32-mm eyepiece.  With the help of a partner, measure how much time (approximately) it takes for the star to drift to the edge of the field-of-view and disappear.   The telescope is sitting perfectly still, so why does the star drift in the field-of-view? 










d.  Switch to the 14-mm eyepiece.  Then, repeat steps (a) through (c)… how is your experience different for each step with the 14-mm eyepiece than it was the 32-mm eyepiece?  (Note:  There may be no difference for some parts!) 




















e.  If the star was in focus before you switched eyepieces, did you need to re-focus with the new eyepiece?  Try it again and see.  Comments: 












3.  Observe either Deneb or Antares with the telescope, and record which one you chose.  (Or observe both!  Again, use the all-sky chart in Ridpath to locate the desired star in the sky.)  Also note which eyepiece you used.  Does this star look any different than Vega did?  If so, how?  Describe and/or sketch: 


4.  Observe the Andromeda Nebula M – actually the Andromeda Galaxy. A bright moon may not allow this to be seen and this observation is best done later in the observing session.     Optional:  You may try to sketch what you see through the telescope if you wish… otherwise, just answer the questions below: 


a.  Observe the Andromeda Nebula with both the 32-mm eyepiece and the 14-mm eyepiece.  What are the differences, if any?  (Do the stars look different?  How?  Can you see fainter stars?  Does the nebulosity look different?  How does the field-of-view change?  Does the color change?) 












b.  Using the 32-mm eyepiece, install the nebula filter (also known as the Skyglow filter).  How does the nebulosity look different with the filter installed vs. without the filter?  How do the stars look different with vs. without the filter?  (You may team up with a neighboring telescope to quickly compare with vs. without the nebula filter.) 











5.  Observe Jupiter, using the 14-mm eyepiece.  Your instructor or TA will point it out to you in the sky — it is NOT on the chart in Ridpath.  (Thought question:  Why not?)  Sketch what you see.  You may be able to see some small, faint dots very close to Jupiter — those are some of its satellites (moons).  In your sketch, be sure to include any details you are able to observe on the “surface” of Jupiter (like cloud bands).  Make a note of any colors, etc.  

Date & time:



6.  Observe the Moon using either the 32-mm or 14-mm eyepiece (make a note of which one you chose).  (You do NOT need to sketch the Moon at this time — just look and enjoy.  We will return to the Moon for a future observing project.) 

a.  How does the Moon look different with the Moon filter installed in the eyepiece vs. without the Moon filter?  Describe: 










b.  Remove the Moon filter and center up on the Moon.  While you look through the eyepiece, have a friend slowly block up to 75% of the telescope’s main aperture with a notebook, clipboard, telescope cover, or other opaque object.  Describe how this affects what you see compared to an unobstructed main aperture, if at all: 












7.  Optional:  If you have spare time, you may use the telescope to observe anything else of interest to you.  Describe and/or sketch what you observe.