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Thread: Proper exposure for stars, no moon?

  1. #51

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    Re: Proper exposure for stars, no moon?

    I would add that since declination is the angle of a star north or south of the equator, then at the latitude you will be at and photographing a southern horizon, it is quite possible that you will be photographing stars with zero declination.
    This is not the same as your declination which is your latitude and from my understanding, it is a combination of your latitude and the star declination which determines how quickly a single star in the sky moves. But you are not photographing a single star. You are photographing a large chunk of the sky and each star is moving across your field of view at a different speed. So if you want astronomical accurracy, you must first work out which star in the sky is moving fastest. i.e. the one nearest to zero declination. But then you have to factor in your latitude which none of the examples given have mentioned.

    Hence the formula T = 343 / (F Cos d) is a fudge formula and assumes 35mm film format. Assuming 0 declination and 50mm lens, then that gives 6.86 secs. Thats for film. But use a 50mm medium format lens, then things change depending on the lens design. i.e. how much of the field of view is projected onto the useable camera sensor. So you must know what angle of view is projected onto the sensor area to make an accurate calculation. It all becomes a royal pain for the casual night sky shot. So unless you are serious about finding the correct star to work from, its declination, your exact latitude, the required level of accuracy for high density sensors, and doing all the calculations based on the actual field of view on your sensor, then a simple worst case scenario makes life very easy. Or the fudge formula adjusted to your field of view on the sensor but you will require trial and error to work that out.

    Having said all that, since your camera is digital, you should have zoom software in the back to see the images in fine detail. Therefore why can't you just take some shots and check the trails or not at the time, so you can be 100% sure you have no star trails. Thereby negating the need for any of this astronomical stuff.

  2. #52

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    Re: Proper exposure for stars, no moon?

    percepts,
    you're talking total nonsense. If you have no astronomical knowledge you should avoid making up answers that, as in this case, are complete rubbish. You mix up the celestial and the geographical equator, stars declination and the geographical latitude, declination and azimuth and mistake one for the other, etc.

  3. #53

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    Re: Proper exposure for stars, no moon?

    GPS suppose you give a worked example so we see that you are only quoting big words you found in a dictionary. And while you're at it, how about you explain the magic number 1000 or 343. And further more how about you explain:
    "It's useless to give him advice about f2.8 or f4... Stars being point light sources depend on the actual aperture opening dimension in their effects on the film. (a f4 on a 300mm lens is not the same as f4 on a 65mm lens, for a point light source.) "

  4. #54

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    Re: Proper exposure for stars, no moon?

    percepts,
    sincere at last, you give the indication of the way you make up your posts. For the numbers you're interested in ask those who post them. As for my quote, it is self-explanatory and basic astronomical knowledge for astrophotographers. Not for you, understandably enough.

  5. #55

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    Re: Proper exposure for stars, no moon?

    Quote Originally Posted by GPS View Post
    percepts,
    sincere at last, you give the indication of the way you make up your posts. For the numbers you're interested in ask those who post them. As for my quote, it is self-explanatory and basic astronomical knowledge for astrophotographs. Not for you, understandable enough.
    Just as I thought. You're not capable of backing up your claims. Only capable of dissing other people who are trying to help. Says it all.

  6. #56

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    Wink Re: Proper exposure for stars, no moon?

    Quote Originally Posted by percepts View Post
    I would add that since declination is the angle of a star north or south of the equator, then at the latitude you will be at and photographing a southern horizon, it is quite possible that you will be photographing stars with zero declination.
    This is not the same as your declination which is your latitude and from my understanding, it is a combination of your latitude and the star declination which determines how quickly a single star in the sky moves. But you are not photographing a single star. You are photographing a large chunk of the sky and each star is moving across your field of view at a different speed. So if you want astronomical accurracy, you must first work out which star in the sky is moving fastest. i.e. the one nearest to zero declination. But then you have to factor in your latitude which none of the examples given have mentioned.

    Hence the formula T = 343 / (F Cos d) is a fudge formula and assumes 35mm film format. Assuming 0 declination and 50mm lens, then that gives 6.86 secs. Thats for film. But use a 50mm medium format lens, then things change depending on the lens design. i.e. how much of the field of view is projected onto the useable camera sensor. So you must know what angle of view is projected onto the sensor area to make an accurate calculation. It all becomes a royal pain for the casual night sky shot. So unless you are serious about finding the correct star to work from, its declination, your exact latitude, the required level of accuracy for high density sensors, and doing all the calculations based on the actual field of view on your sensor, then a simple worst case scenario makes life very easy. Or the fudge formula adjusted to your field of view on the sensor but you will require trial and error to work that out.

    Having said all that, since your camera is digital, you should have zoom software in the back to see the images in fine detail. Therefore why can't you just take some shots and check the trails or not at the time, so you can be 100% sure you have no star trails. Thereby negating the need for any of this astronomical stuff.
    Stars declination does not change with observers Earth's latitude. Therefore mathematical equation that calculates the stars apparent movement on film for a given focal length and given star declination is independent of the observer's latitude.
    Last edited by GPS; 20-Jun-2009 at 10:34.

  7. #57

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    Re: Proper exposure for stars, no moon?

    Quote Originally Posted by GPS View Post
    Stars declination does not change with observers Earth's latitude. Therefore mathematical equation that calculates the stars apparent movement on film for a given focal length and given star declination is independent on the observers latitude.
    Yes that was error on my part, I meant longitude but everything else applies.

  8. #58

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    Re: Proper exposure for stars, no moon?

    Stars declination does not change with observers Earth's longitude either. Therefore mathematical equation that calculates the stars apparent movement on film for a given focal length and given star declination is independent of the observer's longitude.

  9. #59

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    Re: Proper exposure for stars, no moon?

    Quote Originally Posted by percepts View Post
    GPS suppose you give a worked example so we see that you are only quoting big words you found in a dictionary. And while you're at it, how about you explain the magic number 1000 or 343. And further more how about you explain:
    Go to the post #42, click on the link, find the page 10 and read the explanation of one of your "magic" numbers. Just don't expect this service on my side all the time...

  10. #60

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    Re: Proper exposure for stars, no moon?

    Quote Originally Posted by percepts View Post
    GPS suppose you give a worked example so we see that you are only quoting big words you found in a dictionary. And while you're at it, how about you explain the magic number 1000 or 343. And further more how about you explain:
    "It's useless to give him advice about f2.8 or f4... Stars being point light sources depend on the actual aperture opening dimension in their effects on the film. (a f4 on a 300mm lens is not the same as f4 on a 65mm lens, for a point light source.) " GPS

    Study more, my son, it's the post #...!

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