Do I Really Need a Telescope

Few experiences with the night sky are more instantly rewarding than your first look at the moon, a nebula, or a planet through a telescope. Saturn, in particular, can look almost too perfect. One of us taught students (while in graduate school) who refused to believe that the planet that they were looking at through the telescope was real. This student insisted that Saturn was a sticker on the telescope lens. However, it is also true that such an experience can be singularly disappointing if that shiny new telescope you bought at the mall turns out to be a piece of wobbly, hard-to-use junk. If you are willing to invest in a good telescope (we'll talk about the magnitude of the investment in just a moment), and if you are willing to invest the time to learn how to use it, a telescope can be a wonderful thing to have.

But will you use it?

If you are an urban dweller who never escapes the streetlights of the city and are hemmed in by tall buildings, you may be better advised to spend your money elsewhere. Then again, owning a sufficiently portable telescope gives you a good excuse to pack up every once in a while and head for the country, where the skies are darker and the seeing is better.

You might consider an alternative to both the naked eye and the telescope: a good pair of binoculars. For handheld viewing, 7x magnification is comfortable for most people. If you have steady hands, 10x may work well for you, and if you have the hands of a (successful) brain surgeon, even 12x may work. Remember, the greater the magnification, the harder it will be to hold the binoculars steady because objects will wobble farther in your ever smaller field of view.

Magnification means less than you think when it comes to viewing stars. While pointing a telescope or binoculars at the night sky will make stars that are too faint to see with the naked eye visible, all stars are so incredibly far away (the closest beyond our sun, Alpha Centauri, is about four light-years away) that a given star at higher magnification will still be nothing more than a point of light. Magnification is also largely wasted if what you look at is too dim to see well. Get binoculars with the largest aperture (the diameter of the objective, or main lens) you can afford. An aperture of 50 millimeters is a good choice. Couple this with a 7x magnification, and you have a 7 x 50 pair of binoculars—a good all-around choice for handheld viewing.

If you want to successfully use binoculars with a magnification of more than 10x or 12x, you will need to mount them on a camera tripod equipped with a binocular adaptor clamp or a specially designed binocular tripod; otherwise, the sky will be a blur.

Binoculars have the advantage of being very portable, and whole guidebooks have been written about observing the sky with them (for example, Exploring the Night Sky with Binoculars, by Patrick Moore [3rd ed., Cambridge University Press, 1996]). However, at anywhere from $200 to $1,000 and more, binoculars with high quality optics are not cheap; if you're thinking about buying a pair of big, expensive binoculars, there are other possibilities you may want to consider.

r/rio

Astronomer's Notebook

Specially designed binocular mounts, created to hold the binoculars in place and allow precision aiming, are available from companies that specialize in equipment for amateur astronomers. Very good mounts range in price from about $150 to almost $600. The latter sum, however, is enough to buy you an entry-level Maksutov-Cassegrain telescope. Indeed, a decent pair of binoculars costs $200 to $400—considerably less than the cost of the best mounts. But keep in mind, the stability of your image comes down to the stability of your mount.

Astronomer's Notebook

Astronomer's Notebook

The magnification of your binoculars (or telescope) is calculated by a simple ratio of the objective (primary lens) focal length to the eyepiece focal length (see Chapter 5). If your objective has a focal length of 1,000 mm and your eyepiece has a focal length of 100 mm, then the ratio is 1,000 mm/ 100 mm, yielding a magnification of 10 times. There is a useful upper limit to magnification. The rule of thumb is that you get 60x magnification for each inch of aperture. Thus a telescope with a 4-inch objective lens (in the case of a refractor) or primary mirror (in a reflector) can usefully magnify up to 4 x 60, or 240x.

Before we leave the subject of binoculars, here are a few words of shopping advice— much of which applies to telescopes as well:

> Examine and feel the binoculars. They should strike you as a well-crafted precision instrument.

> Test the focusing mechanism. It should be smooth and offer steady resistance.

> Look for antireflection coating on all lenses. This thin coating will make the lenses appear blue, yellow, magenta, or purple when held at an angle to the light.

> Look through the binoculars. Try focusing on a point of light (a distant bulb, for example). It should be absolutely sharp, at least until the point of light gets very near the edge of the field of view.

> Focus on a vertical straight line such as the corner of a building. Even with very good binoculars, the straight line will bend at the extreme edges of your field of view. However, if the line remains bent a third of the way from the edge, the quality of the optics is poor.

> The twin barrels of binoculars must be perfectly parallel with one another. If they aren't, you will see a double image. Your eyes will work hard to compensate and fuse that double image, but ideally, there should be no double image to fuse.

Close Encounter

Close Encounter

The "field of view" is the piece of the sky you can see through your telescope or binoculars. You can easily determine the field of view of a telescope-eyepiece combination if you know the apparent field of the eyepiece (this will be listed with the specifications of the eyepiece) and the magnification. Let's say you have a magnification of 10x. The field of view is equal to the field of your eyepiece divided by the magnification. If your eyepiece has an apparent field of 45 degrees (1/4 of the sky from horizon to horizon) with 10x magnification, your telescope will have a 4.5-degree field of view. If the magnification were 100x with the same apparent field of the eyepiece, the field of view would be 0.45 degrees. What should be clear is that as the magnification is increased, the field of view (or how much of the sky you see) decreases.

A typical amateur refracting telescope, Meade Model 395.

(Image from Meade Instruments Corporation)

1.

Tripod legs

17.

Focuser

33. Azimuth base

2.

Equatorial mount

18.

Focuser thumbscrew

34. Azimuth shaft bolt

3.

R.A. flexible cable control

19.

Eyepiece

3S. R.A. worm block assembly

4.

Dec. flexible cable control

20.

Diagonal thumbscrew

36. Dec. worm block assembly

5.

Counterweight

21.

Declination axis

37. Dew shield

6.

Counterweight shaft

22.

R.A. lock

38. Viewfinder bracket

7.

Counterweight lock

23.

Dec. lock

39. Objective lens cell

8.

Safety washer/knob

24.

6 x 30 viewfinder

40. Leg brace supports

9.

Latitude lock

25.

Telescope front dust cover

41. Tripod leg lock knobs

10.

Polar axis

26.

Viewfinder bracket thumbscrews

42. Accessory shelf central

11.

Latitude adjustment knob

27.

R.A. setting circle

mounting knob

12.

Optical tube assembly

28.

Dec. setting circle

43. Tripod leg Phillips-head

13.

Optical tube saddle plate

29.

Latitude scale

fastener screws

14.

Cradle rings

30.

Azimuth lock

44. Tripod leg bolt

15.

Cradle ring lock knobs

31.

Focus knobs

1/2" nut

16.

Diagonal mirror

32.

Polar shaft acorn cap nut

45. Accessory shelf

1. Tripod legs

2. Equatorial mount

3. R.A. flexible cable control

4. Dec. flexible cable control

5. Counterweight

6. Counterweight shaft

7. Counterweight lock

8. Safety washer/thumbscrew

9. Latitude lock

10. Polar axis

11. Latitude adjustment knob

12. Optical tube assembly

13. Optical tube saddle plate

14. Cradle rings

15. Cradle ring lock knobs

16. Viewfinder bracket mounting bolts

17. Focuser

18. Focuser thumbscrew

19. Eyepiece

20. Viewfinder bracket

21. Declination axis

24. 6 x 30 viewfinder

25. Telescope front dust cover

26. Viewfinder bracket thumbscrews

27. R.A. setting circle

28. Dec. setting circle

29. Latitude dial

30. Azimuth lock

31. Focus knobs

32. Polar shaft acorn cap nut

33. Azimuth base

34. Azimuth shaft bolt

35. R.A. worm block assembly

36. Dec. worm block assembly

37. Cradle ring attachment knobs

38. Tripod leg Phillips-head fastener screws

39. Tripod-to-mount attachment points

40. Accessory shelf

41. Accessory shelf central mounting knob

42. Tripod leg brace supports

43. Tripod leg lock knobs

Telescopes Mastery

Telescopes Mastery

Through this ebook, you are going to learn what you will need to know all about the telescopes that can provide a fun and rewarding hobby for you and your family!

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