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Thread: F ratio and imaging, someone check my math :)

  1. #1
    JimB1's Avatar
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    Default F ratio and imaging, someone check my math :)



    So I havenít seriously done any astrophotography yet, just messing around with equipment I have and thinking about what Iíd need if I want to get started. Looking at f ratios on scopes listed as astrograph as well as ones people generally use as astrographs even if they arenít necessarily listed that way. It seems that for reflectors f4 is pretty standard and for refractors f6 is pretty standard. Catadioptics go all over the place but usually f8 to f12 because of the long focal length.

    So as I understand it, each time you double the f ratio you have to multiply the exposure time by 4. Ie. If an image takes 20 seconds exposure time at f4 it will take 80 seconds at f8 to get similar light collection at the sensor. The practical point of it is that the higher the f ratio, the better the tracking of your Mount needs to be to prevent star trails or other abnormalities in the image.

    Hereís the calculation I found:

    To : Old exposure time
    T1 : New exposure time
    fo : Old resulting f/ratio
    f1 : New resulting f/ratio

    T1 = To / ( fo / f1 )2

    However if you are stacking shorter exposures like say 5 second images and you use 10x5 sec images from an f4 scope, you should be able to get similar results from an f6 scope with about 23x5 sec images or 40x5 sec images from an f8 scope right? The f ratio is 1.5x from f4 to f6 so the exposure time should be multiplied by about 2.25x and by 4x for the f8 if I am thinking of this correctly. Because of the short exposure time I donít think the tracking has to be as precise as long as the tracking is keeping the object relatively centered in the image view between images.

    Now for relatively bright objects like planets or the moon I am thinking higher f ratios may be better for longer exposure high contrast images but you may get motion blur where a fast f ratio will need a quicker shutter time to avoid over exposures but I guess you can also use lower iso settings and cut down exposure on bright objects at magnification.

    Am I on the right track here? I know a decent amount about general photography as I have been using DSLRs for years but I am picking up info about telescopes and astrophotography as a go along I am thinking about a new motorized mount in the spring so just trying to get my bearings on what I need to think about if I decide to go into astrophotography...

    Thanks
    -Jim
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    Default Re: F ratio and imaging, someone check my math :)

    Your math sounds right.

    For the Moon and planets, you want a long focal length, which is usually achieved with a slow focal ratio. SCTs at f/10, barlowed to f/20 are popular. Fortunately, those objects are so bright, that you can still use extremely short exposures. Most planetary astrophotographers shoot video at frame rates as high as they can manage, often up to 60 frames per second.

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    Default Re: F ratio and imaging, someone check my math :)

    Hi Jim,
    Its alright to do calculations, but when you do real Astrophotography, the maths is not very precise. I usually use the histogram or the image on the LCD of the dSLR to ascertain if the exposure is alright. The amount of exposure you can do depends on things on light pollution, the amount of imaging time you have. Polar alignment and guiding is also critical in allowing longer exposures for DSOs. For planets, it is video - higher frame rates the better. I think Damian Peach uses up to 300 frames per seconds and grabs 10000 images for processing planets !!!
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    Default Re: F ratio and imaging, someone check my math :)

    Your math is entirely correct, but your direction is not quite right.

    For long exposure Deep Sky Object AP (LEDSOAP) there are two main considerations you have to deal with.

    First, you fight to get the longest exposure time you can per sub-frame within the constraints the externals provide. The externals include but are not limited to light pollution, guiding capabilities, camera sensor physics, meteorology (seeing/transparency), etc.

    Second, how you compose your scene, your image, what we call framing. Framing is purely determined by focal length and sensor size and has nothing to do with the systems aperture or f/ratio. This does not mean that aperture and f/ratio are not important, it's just not important when talking about positioning the object within the frame.

    When you apply terrestrial photography thinking (the exposure triangle of aperture/shutter speed/ISO) to LEDSOAP the math starts to fall apart because our aperture is somewhat fixed and we find the best ISO for our sensor and leave it there, so our only real variable is exposure time.

    I strive to get at least 60 minutes minimum of total exposure time on nebular/galactic targets using an f/5 or f/6 scope, half of this for star clusters. I keep taking those exposures (what we call sub-frames or subs) till I get to my 60 minutes. If that 60 minutes of total exposure time does not give up the detail I'm looking for then I go back out and collect more subs. If I'm shooting at f/9 or f/10 then I'll add, at least, 50% more subs to get my stacked image. Now the length of each sub-exposure is variable from night to night. As Magnus stated we use a framing test image's histogram to determine how long our subs for that night will be. This is where we take the externals into account.

    So you can try to do the math to give you an idea of how you're going to expose each sub, but until you get out there you really have no idea how long that is going to be.

    Here is a picture of a histogram whose peak is in the correct quadrant.




    As Kathy stated above planetary/lunar imaging is an entirely different endeavor. Those objects are so bright that we can get away with videoing them. We take video to grab as many frames as possible to take advantage of an effect called "lucky seeing". The atmosphere from second to second may or may not be turbulent so we take several minutes of video to capture those frames where is seeing is very still giving a very clear image.

    Hope this helps,

    Cheers,
    JT
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  5. The Following 3 Users Say Thank You to jaetea For This Useful Post:

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    Default Re: F ratio and imaging, someone check my math :)

    Thatís close. Your example of going from f4 to f8 is correct but a little misleading.

    Most modern camera lenses use a standard f-stop scale, which is an approximately geometric sequence of numbers that corresponds to the sequence of the powers of the square root of 2: f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/45, f/64, f/90, f/128, etc. Each element in the sequence is one stop lower than the element to its left, and one stop higher than the element to its right. The values of the ratios are rounded off to these particular conventional numbers, to make them easier to remember and write down.

    As you increase the f number it cuts the diameter of your lens in half letting in half as much light. So for each stop double it. 10 seconds @ f 4 would be 20 sec @ f5.6, 40 sec @ f8, 80 second @ f11 and so on. I have a scope that is rated @ f6 and if I would compare it to a scope @ f8 it would be a little less than twice as sensitive. Not going to do the math as I hope you have the picture.

    When selecting a telescope you want to do the same thing you do for a camera lens. Buy the fastest, lowest f stop, your pocket book can handle. The reasons are a little different when comparing AP to traditional Photography but the standard is the same. Here is why.

    Itís all about the light. You can reduce the light gathered by the scope by decreasing your exposure time which you will need to do for a subject like the moon. @ f4 I might shoot for 1/125 or 1/250 or 1/500 sec. your camera shutter speed can get really fast.

    For deep space objects you will need to take shorter exposures for super bright objects like Orion Nebula and then stack a bunch of them to get the little details. A single exposure will reveal the nebula but stacking lots of them together with expose the little details. For faint objects you will need long exposures and then stack lots of then, hours of total time, to see anything at all. For these type of objects a lower f rating becomes very important.

    There are physical limitations to the equipment that allows you to track the sky. I have a decent setup and can get 5 or 6 minute exposures with round stars. Anything past this and the stars turn to eggs. Sometimes if I donít get everything just right I may only take a 3 minute exposures because my tracking is not as good. There are other factors that can decrease the length of time you can expose.

    Since your exposure length will be limited by your tracking mount having a telelscope that gathers a lot of light very quickly, low f stop number rating, you maximize the amount of light gathered in the amount of time you can keep the stars round.

    On a side note some classifications of telescopes are better than others For AP. Within each classification there are good telescopes for AP and good telescopes for visual observation. But this is also a different discussion.
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