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In theory, telescopes have fairly straightforward designs, and even if they do not (such as with equatorial mounts) the instructions usually have some helpful information on how to use it. But over the years we have noticed that there are some features to telescopes that the instruction manual does not cover very well.

There is actually a bit of a historical reason for this. Telescope manufacture tends to be a bit imitative. So when one country tries to start up an industry it find the best thing to do is imitate the last manufacturer’s product. This means that features that aren’t really used much any more are continued, sometimes just for their own sake. But that does not mean they are useless!

1) Why does this telescope have two caps and openings?

AperturereduceMany telescopes, usually refractors, may have a double cap system on the cover for the lens. One cap comes off to reveal a small opening, while removing the entire cap exposes the entire lens for astronomical viewing.

So why this double system? Two reasons: the smaller opening can act as a “budget moon filter” by letting in less light from bright objects. The reduced aperture is handy for viewing the Moon as it can be very bright – almost painful to view! Reducing the aperture can also be handy in the telescope gets used for terrestrial viewing.

The other reason is probably not official. Sometimes lens caps can get stuck on the telescope tube, and by removing the small cap you can stick a finger into the cap and get a better grip on the cap.

2) What are those knobs on the outside of my reflector near the opening for?

secondaryvaneoutsideSo you are looking through the eyepiece on your reflecting telescope and you notice this shiny, attractive thumbscrew next to your focuser. Its so tempting! What it is for?

The answer is that it holds the vanes that hold your reflecting telescope’s secondary mirror in place. secondaryvaneinsideit is not a good idea to turn them or twist them as it may mess up the alignment of your secondary mirror.





3) What does ‘rich field’ mean when talking about a telescope?

This is a term that gets tossed around when discussing telescopes and the answer is fairly simple: As you increase the magnification of a telescope (how much it makes thing bigger) you decrease the field-of-view (how much you see).  Since magnification is determined by the focal length of the telescope’s mirror or lens divided by the focal length of the eyepiece you can see how the design of the telescope comes into play when determining magnification and field of view.

009Well, with Rich-Field telescopes the focal length of the telescope is kept deliberately short to keep magnifications low, but field of view high.  This usually means a short barrel telescope. Rich-Field telescopes may not be able to crank up high magnification but they give nice wide fields of view. Wide fields of view means it is easier to find objects in the night sky and is important for viewing the larger Deep Sky Objects (such as the Andromeda Galaxy)

Looking to buy telescopes?





Comments on: "The odder features of telescopes, or what does *that* do?" (2)

  1. Hello,

    So I have a quick question. I am beginning to become very interested in astronomy, and have been looking at buying a decent telescope. I am currently a student in college in NC and would like to buy something that will last a few years or longer. I have found a few models that I like after reading some reviews and such. I saw on http://www.besttelescopehq.com that the Orion SkyQuest XT8 Classic Dobsonian Telescope is a decent telescope and seemed to fit what I was looking for. My question lol. Do you have any experience with this telescope or could you recommend anything similar that I may could consider.


    • Hi Ben

      The Orion Skyquest XT8 is an excellent telescope. I used to own one myself and had a great time using it. The only downside is that these days Orion sells them ‘raw’ with very few accessories. Usually just one eyepiece and a red dot finder. Other features like the ‘Nav Knob’ and the eyepiece were stripped. This makes the telescope much less expensive but not hte best choice for beginners who lack their own eyepieces and other accessories. You might want to consider an upgrade to the XT8 intelliscope or the XT8 PLUS if this is your first telescope.


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