Cahill Rooftop Observatory

User Guide: Questar

The 3.5-inch Questar is a mid-range telescope, easier to carry than the 8-inch but more effective than the Galileoscopes. The Questar comes with a Questar brand eyepiece and a Celestron reticle eyepiece, both of which provide roughly the same magnification (50×, not the 40× claimed on the Questar eyepiece barrel). The Questar eyepiece has a wider field of view and is needed for the finderscope, while the reticle eyepiece has better optics.

The telescope is quite short, and you may wish to provide a table for mounting it.


Telescope Setup

  1. Open the case by pushing both latches away from each other.
  2. Remove the telescope from the case.
  3. Loosen the declination brake by turning it clockwise.
  4. Tip the tube over by pushing on it, so that you can see the Cassegrain knobs.
  5. Unscrew the cap from the front of the telescope.

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  6. Remove the two identical side legs from their pocket in the case lid and insert them in the holes in the side of the telescope base. They go in on a radial vector. Make sure they go in fully – about 2 inches.
  7. Tip the scope up on the two legs, and insert the third (telescoping) leg into the socket on the bottom of the base.
  8. Adjust the length of the telescoping leg until the first pencil mark lines up with the edge of the outer tube, and then rotate the entire assembly to point north. Further adjustments to polar alignment can be made by sighting on Polaris.
  9. The telescope has a built-in drive that you can use to track objects if it's been polar-aligned. Simply plug the black cable into the socket on the bottom of the telescope base. Once plugged in, the drive runs automatically.

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The Questar has a number of different lenses and mirrors built in, for different observing modes. The modes are controlled by a combination of knobs on the back of the telescope.

F (focus)
Turn this knob to focus the telescope.
M (mode)
This knob selects whether the main scope or the finderscope mirror is routed to the eyepiece. Turn clockwise for the finderscope view. (If the field is dark, check the solar filter, described below.) Turn counterclockwise to use the telescope. The finderscope cannot be focused with the telescope's focus knob – instead, twist the top of the Questar eyepiece to focus. The finderscope cannot be focused at all with the reticle eyepiece in place.
B (Barlow lens)
Turning this knob clockwise inserts a Barlow 2× magnifier into the beam path.
S (solar filter)
Turning this knob counterclockwise puts a solar filter in front of the finderscope mirror. Note that solar observations with the finderscope not recommended; we have a separate page on the preferred procedure. Turn the knob clockwise to flip the filter out of the beam path for normal operations.


  1. Unplug the power cable if you were using the tracking motor and put it back in its bag.
  2. Remove the tripod legs, if they were installed, and return them to their pockets in the case.
  3. Return the barrel to zenith pointing (you may want to use the declination setting circle), and screw the cap back on. Be careful not to let the cap slip and scratch the lens; it's harder than it looks.
  4. Slide the telescope back into the case. Close and latch the case.