what is the light gahtering power of a 7 inch diameter telescope compared to the human eye?

Visible Light Telescopes

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Additional Reading from www.astronomynotes.com

• Types of Telescopes
• Powers of a Telescope

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Start Hither!

The Hubble Space Telescope Scientific discipline Plant squad has put together an excellent resource on the history of telescopes called "Telescopes from the Ground Upwards." It covers much of what I'm going to present below, then I recommend spending some fourth dimension looking through the textile provided under the link "Get to the root of it: Basic Science Concepts." Many of the links in the material below take y'all to pages from this excellent resources.

The last pace in studying the light from astronomical objects is detecting it when the light arrives hither on Earth. The standard instrument that astronomers use to notice light is a telescope, which collects the light and brings it to a focus, and a camera to record the lite from the object. Telescopes have three master functions:

1. to gather as much light from an object as possible,
2. to focus the low-cal into a precipitous epitome, and
3. to magnify the paradigm.

Magnification is probably the near familiar office of a telescope. Hither is a comparison of two dissimilar familiar astronomical objects. One appears as seen with the naked center, and the other is magnified. This presentation (with credit to Penn Land Astronomy & Astrophysics) allows you to click through slides that demonstrate the magnification you lot can experience with a telescope or binoculars.

Every bit you lot'll see at the Hubblesite pages about "What do telescopes practise?" and "What makes a skillful telescope?," though, magnification is the least important belongings of telescopes.

The most of import holding is a telescope's low-cal gathering power. The larger the discontinuity (the opening at the tiptop of the telescope tube), the more light the telescope will gather. To sympathize this, picture a telescope equally a "calorie-free saucepan." If you lot want to collect as much rain equally possible in a short time, you would go out during a tempest with a broad-mouthed bucket instead of a drinking drinking glass, because the opening in the bucket will collect more raindrops than the drinking glass in the same amount of time. Telescopes piece of work the same way. As photons "pelting" down on Earth, a telescope with a bigger aperture will collect more of them than a telescope with a smaller aperture. Thus, the low-cal-gathering power (which measures how bright an object appears, or, alternatively, how faint an object is that is just barely detected) of a telescope is determined by the area of the opening at the front end of the tube. For this reason, astronomers have built larger and larger telescopes since they were get-go invented four centuries ago.

Since most telescopes have round apertures, the light gathering power is proportional to the surface area of the aperture, or $\pi R^{\mathrm{two}}$ . So, given two telescopes of different apertures (say i that is 1 meter across, and the second that is 4 meters across), the light gathering power of the iv meter telescope is $\left(\pi \right)\left(2^2\right)/\left(\pi \right)\left({\mathrm{0.five}}^2\right)=16$ times better. Yous can interpret that statement in the post-obit way: y'all can say that if you look at the same object with the 1 meter and 4 meter telescopes, the object will expect xvi times brighter through the 4 meter than it does through the ane meter. Or, the 4 meter telescope will be able to observe objects 16 times fainter than the limit of the 1 meter telescope.

Endeavor this!

You lot can get an idea for how much more powerful a modern research telescope is than the man centre. That is, how much fainter an object can you find with the Keck telescope than with merely your eye?

Repeat the calculation above, but apply 10 meters for the bore of the Keck, and 5 mm for the bore of the educatee of your eye.

The primeval telescopes were uncomplicated—they had an objective lens at the terminate of an empty tube. The lens was shaped so that it would take parallel beams of light and focus them to an image inside the tube. An eyepiece (another lens at the other terminate of the tube), immune you lot to magnify the small-scale epitome in the telescope tube then that it appeared larger to your eye. Refractors (telescopes that utilise lenses to focus light) create sharp images; however, they endure from chromatic abnormality. That is, different colors of lite get focused to different points (recall how glass prisms disperse white light into its component colors), so the blue light from a star is not focused as well as the scarlet light, causing stars to be surrounded by a blue halo. For this (and other reasons described below), refracting telescopes were eventually superseded past reflecting telescopes (which use mirrors, not lenses).

Reflecting telescopes are empty tubes with curved mirrors stock-still to the bottom of the tube. The mirror will focus parallel rays of low-cal to a point inside the tube, near the top of the tube. Since the main mirror is fixed at the bottom of the tube, a 2nd, smaller mirror is set in the peak of the tube to direct light out through the side of the tube into an eyepiece. This is chosen a Newtonian reflector. If the second mirror instead sends calorie-free back down the tube and through a hole in the bottom of the tube, that is called a Cassegrain reflector. The images created by reflectors are non usually every bit sharp equally those created by refractors, considering information technology is difficult to get curved mirrors to bring all of the light from a afar object to a common focus (an upshot called spherical aberration).

In their article on ownership your starting time telescope, Sky & Telescope magazine compares and contrasts these ii types of telescopes and also has cutaways of the interior of a refractor and reflector, showing the path calorie-free takes to the eyepiece.

The largest refractor still in utilize today is the Yerkes refractor, which has a 40" (one meter) aperture. At that place are several reasons why larger refractors have not been built:

1. As lenses get larger, they brainstorm to sag under their own weight, degrading their ability to accurately focus light.
2. The largest lenses demand to be incredibly thick, only they can only be supported along their edges, where the glass is thinnest.
3. Lenses need to be completely transparent and allow as much light as possible to penetrate. Thick, massive lenses are expensive to smooth on both sides and absorb lite.

Want to learn more?

The American Constitute of Physics has written an excellent historical slice on how reflector telescopes became the standard inquiry telescope.

Reflectors don't endure from well-nigh of the problems mentioned in this Lesson. Mirrors tin exist supported along the entire back side, correcting for the sagging problems of large refractors. Only the front of a mirror needs to be polished, and if the coating degrades and begins to reflect less lite, information technology tin be recoated to restore its reflection efficiency.

A 2.5 meter (100 inch) reflector has been in utilize at Mountain Wilson in California since 1917, a 5 meter (200 inch reflector) has been in utilize at Mount Palomar in California since 1948, and the twin Keck 10 meter (400 inch) telescopes have been operated on Mauna Kea in Hawaii since the 1990s. All of the largest telescopes in the world are reflectors, and the next generation of large telescopes include proposed instruments with mirrors of 25 - 40 meters in bore like the 30 Meter Telescope, The Giant Magellan Telescope, and the Extremely Large Telescope.

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Source: https://www.e-education.psu.edu/astro801/content/l3_p7.html