Alternate Telescope lab assignment
Name ____________________
This lab replaces the outdoor lab we did to compare how the different types of telescopes work.
In that lab we compared one of each type of telescope, and giant binoculars.
Use the OPT Telescopes website: http://www.optcorp.com/
The online catalog features many different types of telescopes and observing accessories.
Compare and contrast the 4 telescopes in the chart below having an aperture of 5 inches or
127mm. Many of the characteristics can be found on the Specifications tab on their description
page. Their exact names (so you can quickly locate them) are:
Refractor: Explore Scientific 127mm f/7.5 Achromatic Doublet Refractor
Reflector: Vixen R130Sf - 5" Newtonian Reflector Telescope
Maksutov-Cassegrain: Orion StarMax 127mm Equatorial Mak-Cass Telescope
Schmidt-Cassegrain: Celestron NexStar 5SE Computerized Schmidt-Cassegrain Telescope
1. Complete this chart of the four telescopes, selected for their comparable aperture:
Refractor Reflector Maksutov- SchmidtCassegrain Cassegrain
Aperture (include units)
Price
Focal length
Focal ratio
*Equivalent magnification
(25 mm eyepiece)
Highest useful
magnification
Limiting stellar magnitude
2. To calculate the equivalent magnification listed in the chart above, assume each telescope is
used with a 25mm eyepiece. Divide the focal length of the telescope objective (from the
chart) by the eyepiece focal length (25mm). Show your 4 calculation set-ups below and
enter the values in the chart above.
3. Use the completed chart to answer this question: What is the relationship between the
telescope focal length and the equivalent magnification?
The focal ration of the telescope is nicely explained here: https://www.astronomics.com/focalratio_t.aspx. Read through this explanation to answer the next two questions.
4. Which of the four telescopes would be best for deep space photography?
_____________________________
Which would be best for planetary viewing? _____________________________
5. What is the relationship between the telescope focal ratio and the field of view?
6. Since all the telescopes have essentially the same aperture, they should theoretically have
the same limiting star magnitude capability. However, the design of any telescope that
contains mirrors poses a problem because some percentage of the objective mirror is
always obstructed by the secondary mirror. Which of the telescopes in the chart has the
most light loss due to its design? ___________________________
7. Here are two pairs of giant binoculars found on the OPT web site:
Vixen 30 X 80 BCF Giant ARK Binoculars
Orion Giant View 25 X 100 Astronomy Binoculars
If you were going to select a pair to get the most light for deep sky searches such as for
comets, which would you choose and why?
Here’s a simple reflecting telescope diagram:
Here is a diagram showing how light rays pass through a Schmidt-Cassegrain telescope (SCT):
Light enters from the right through a thin glass corrector lens (1) then passes to the objective
mirror (2). It is then reflected back to a secondary mirror (3). The light is reflected from there
through an opening in the objective mirror and passes out the telescope back, where it enters a
star diagonal that contains a flat mirror (6). The purpose of this attachment is to direct the light
to a more convenient angle for viewing. Lastly, light passes into and through an eyepiece lens
(7). The telescope is focused by turning the focuser knob (8), which moves the objective mirror
forward or backwards very slightly to compensate for the focal length of the eyepiece.
The SCT design uses folded optics. The light rays bounce back and forth in the telescope tube
before passing out of the telescope. This allows the telescope tube to be shorter, greatly
reducing its weight and size and allowing it to be used as a portable telescope. Study the light
path in the two telescope diagrams, then answer the questions below.
8. How many mirrors are in the reflecting telescope diagram? ___
How many mirrors are in the SCT diagram? ___
A simple rule of thumb is if the telescope has an odd number of mirrors, the image will be
reversed right to left, but erect. If the telescope has an even number of mirrors, the image
will be inverted. Look at the license plates below for an example:
Inverted
Reversed
9. How will the image appear through a reflecting telescope? _______________________
How will the image appear through a SC telescope?
_______________________