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Forums Lobby MASTER YOUR VISION Astrophotography topic #416
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Subject: ""f" rating of a telescope" Previous topic | Next topic
autoexer Silver Member  Waukesha, US  Nikonian since 27th Jan 2011 Sat 20-Oct-12 04:46 AM
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""f" rating of a telescope"
Sat 20-Oct-12 04:49 AM by autoexer


i have a 114mm (4.5 inch) with 900 focal length for an f/8 (7.89) - 900/114= 7.89

i hade a shorter tube, i.e. 630mm it would be f/5.52. question - when i think of f-stops in a lens, the smaller the number the greater ability to gather light, a f/1.4 gathers more light than an f/3.5. This does not seem to be the case in telescopes. I can see the difference if i had a 900mm focal length and increased the mirror size to say 6 inch (150mm) then the f-stop would be 900/150=6, f/6.0. The 6 inch will gather more light in the same amount of time as the f/8 scope.

If it works the same, f/6 is faster than f/8, how can the focal length of 630mm gather more light given the same size mirror?

what am i missing here?

i did find this: http://www.chuckhawks.com/telescope_focal_length.htm

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Df, FA, FG, F

  

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lastdaylight Gold Member Laureate Ribbon awarded for winning in the Best of Nikonians 2013 images Photo Contest   Dallas, US  Nikonian since 09th Dec 2007 Sun 21-Oct-12 06:00 PM
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#1. "RE: "f" rating of a telescope"
In response to Reply # 0



I've given this some thought, too, and finally arrived at this:

For a given focal length, your statement is correct. The larger the objective lens or mirror, the more light it gathers.

When you change to thinking about constant objective lens or mirror size, think of it this way: As we increase focal length, less of the "image" falls on the sensor at any time. Start with a constant size object that you are looking at, say it's 3 x 2 meters in size, and exactly fills the camera sensor. If you double the focal length of the same diameter objective, you will only be looking at 1.5 x 1 meters of the object, still filling the camera sensor. Only 1/4 of the original light is now falling on the sensor, you will need to increase exposure by 2 stops.

Mark Smith
Just like I previsualized it, more or less...

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autoexer Silver Member  Waukesha, US  Nikonian since 27th Jan 2011 Mon 22-Oct-12 03:03 AM
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#2. "RE: "f" rating of a telescope"
In response to Reply # 1



so it would follow that being two stops faster, the image would be brighter with the faster scope?

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Df, FA, FG, F

  

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lastdaylight Gold Member Laureate Ribbon awarded for winning in the Best of Nikonians 2013 images Photo Contest   Dallas, US  Nikonian since 09th Dec 2007 Mon 22-Oct-12 03:54 PM
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#3. "RE: "f" rating of a telescope"
In response to Reply # 2



Correct, or in this case, the "shorter" scope. A 6-inch diameter (call it 150 mm) can be used to make a 600 mm f/4 lens or a 300mm f/2 lens (of course the design would be quite a bit different). The shorter one is "faster."

Another way to think of it is that if we were to double the focal length, all the light that used to be falling on one pixel in the camera is now split among four pixels, due to the higher magnification.

Mark Smith
Just like I previsualized it, more or less...

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nrothschild Neil is an expert in several areas, including camera support   US  Registered since 25th Jul 2004 Mon 29-Oct-12 03:40 PM
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#4. "RE: "f" rating of a telescope"
In response to Reply # 0



There a term used in astronomy called "Light Gathering Power" (LGP).

LGP equals the square of the diameter or radius of the lens (the surface area, and it needs to be adjusted for obstructed optics such as your newtonian, which has the secondary mirror blocking some of that light.

Lens speed (f/ratio) dictates the brightness of an object, regardless of the resulting image size.

LGP tells you how bright it can be at different sizes.

A bigger lens operating at the same f/ratio as a smaller lens will deliver the same brightness across a larger image.

Nothing above contradicts anything said previously here; just saying it a different way. It is a confusing subject for photographers turned astronomer because the idea of LGP is little discussed in general photography although it applies equally.

_________________________________
Neil


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