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    Avasilev's Avatar
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    Default F Stops Question



    I've been hearing more and more about speed in terms of the f stops, and was wondering something (been trying to read around but can't seem to find any direct answers to this question)....I want to do deep space photography more than anything and was under the impression that you use longer exposures, and yet a lot of people seem to be attracted to scopes that can do f/2 which is faster exposures right?

    Does this have to do with the idea of using a BUNCH of really short exposures and then stacking them to get a cleaner image because the short exposures don't show "smearing" nearly as much as a longer exposure might?

    I figured the faster exposures were more for planets and the longer ones more for nebula and galaxies? Or am I wrong?

    I am a bit confused on this.

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    Default

    You are going to have a lot of reading to do...

    Astrophotographers, and astronomers in general don't think in terms of "stops", or "f-stops" like traditional photographers. While there are dualities between the fields, it's best to put them out of your head initially.

    In astronomy and astrophotography, we think in terms of focal ratio, a convenient metric that expresses a telescope's performance as a single number, usually in the range of f/2 to f/15 - there are "faster" and "slower" scopes, but they are either in the research domain (faster than f/2) or are too slow to be of practical value (beyond f/15) to an astronomer or imager.

    The focal ratio is computed as:

    f = focal length / aperture

    Focal length and aperture are usually expressed in millimeters, although any convenient unit as long as both dividend and divisor are expressed similarly is fine.

    In the general sense, a "faster" (smaller) focal ratio implies a larger aperture, while a "slower" focal ratio implies either or both smaller aperture and/or longer focal length.

    The focal ratio is a convenient means of expressing the telescope's relative light gathering capability.

    By and large, aperture is king. The light gathering capability function of a telescope is based on the square of the objective's area. The larger the diameter (greater the aperture) the more photons the instrument can gather per unit time.

    The faster an instrument is (lower f/number) the shorter a given photographic (digital or film) exposure can be, which in turn translates into other benefits.

    These can include less demands on the mount precision, in terms of tracking and error rate, noise gain, including thermal and other noise sources, and so on.

    I recommend starting here for depth, and if you can afford them, obtain the CD-ROM based books:

    Catching the Light: Astrophotography by Jerry Lodriguss
    CGEM 800 HD, NexGuide,
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    Default

    K that clears up a lot.

    Thank you very much for taking the time to write that out for me.

    See you around the forum!

 

 

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