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Thread: Large format Astrophotography

  1. #31
    Corran's Avatar
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    Re: Large format Astrophotography

    Using a 150mm lens this implies a bellows extension of well over 300mm (1:1) right?

    Seems...improbable. That also needs to be mounted and moving on a equatorial mount.
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  2. #32
    Steven Ruttenberg's Avatar
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    Re: Large format Astrophotography

    I haven't settled yet on a lens, but it can't have distortion typical of wide angle lenses.

    My equatorial mounts can handle up to 240 lbs with pinpoint tracking f I ur many hours at least 8-10 and much longer if night time lasted that long.

    My current test setup in pic, the camera is rock solid. 300mm wouldn't be a problem for just using camera. Even 300 or 600 mm. However, It will require an adapter to mount to scope directly for prime focus and a way to focus the required distance. Usually custom spacers are used. So it should not be a problem for long exposures of 8 hours.

    How would enlarger lens be for distortion, fast f-stop, ic, etc.

  3. #33
    Corran's Avatar
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    Re: Large format Astrophotography

    Steven, the previous post mentioned 4:1 ratio assuming a 4x enlargement of the image circle. Is that how much you will need? Bellows factor will make this nigh impossible. Drew Bedo mentioned not using a lens. What is the difference in techniques here? I think you are going to find a custom larger scope with corresponding IC is a better plan? All the astronomers who made glass plates had big telescopes right? Like BIG big.

    https://m.youtube.com/watch?v=yAfUBiWOjNc
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  4. #34

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    Re: Large format Astrophotography

    are you using strap heaters to keep the lens dew free ?
    notch codes ? I only use one film...

  5. #35
    Steven Ruttenberg's Avatar
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    Re: Large format Astrophotography

    Quote Originally Posted by Corran View Post
    Steven, the previous post mentioned 4:1 ratio assuming a 4x enlargement of the image circle. Is that how much you will need? Bellows factor will make this nigh impossible. Drew Bedo mentioned not using a lens. What is the difference in techniques here? I think you are going to find a custom larger scope with corresponding IC is a better plan? All the astronomers who made glass plates had big telescopes right? Like BIG big.

    https://m.youtube.com/watch?v=yAfUBiWOjNc
    They used refractors, acts and newts. The special ones were RC scopes. Most of them only threw 3-4 in circle maybe. Anywhere from 4-10 in refractor and all the way up to the 200in in CA.

    They had to use optics to expand ic to upwards of 11x14 and bigger. Prime focus is always best, but without a focal reducer to expand the ic you will be stuck with about 2-3 in at most even on large 24in plus scopes.

  6. #36
    Steven Ruttenberg's Avatar
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    Re: Large format Astrophotography

    Quote Originally Posted by Fred L View Post
    are you using strap heaters to keep the lens dew free ?
    Not needed most of the time. But I do use a few heater when needed.

  7. #37

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    Re: Large format Astrophotography

    Steven -

    Are you familiar with Astrophotography for the Amateur by Micheal Covington? It was written during the film era and is an absolute wealth of information. Below is an Amazon link. Good Luck!

    https://www.amazon.com/Michael-Covin...-2&tag=mh0b-20

  8. #38

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    Re: Large format Astrophotography

    Successful astronomical imaging requires understanding the aims of the project and the properties of the site and available technology - it's like what Bernice says about needing to understand what kind of photograph you want to make before asking what camera or lens is best.

    To give an example of this, if one had say a 8" diameter f/10 telescope (focal length 2030 mm), that illuminates a 1" circle, then the scale at the focal plane is (206265/2030) = 102 arcsec/mm. That means 2 arcsec is 0.02 mm on the detector. So if the site and system are capable of delivering 2 arcsec seeing, the stars will be 0.02 mm on film, which could be getting to where film resolution is the limiting factor. If one used reimaging optics to magnify the image, effectively making the focal length longer, then it will give a larger f-number. This will make the exposure slower for diffuse sources such as nebulosity, but point sources such as stars may improve because now the image quality is better matched to the film (assuming the reimager, alignment, etc can maintain the image quality). IOW if your telescope is fixed, then bigger film is sometimes but not always better.

    Large telescopes (like 1 meter and up) have the problem that the scale at the Cassegrain focus is very large, so it's not that well matched to photographic plates or electronic detectors. So to do wide field imaging, they often use a complex reimaging camera to do focal reduction and flatten the field. Or they image at prime focus, since the f-ratio of the mirror is faster. However, to image at prime focus they need a prime focus corrector to remove aberrations of the mirror and flatten the field.

    The most famous wide field imaging telescope is the 48" Palomar Schmidt - Schmidt telescopes use an aspheric corrector in front of the mirror and put the plate at prime focus. Plates for the Palomar Schmidt were 14" square, covered a ~6 degree field, and were held in a special holder that bent them slightly (I think). https://www.astro.caltech.edu/paloma...es/oschin.html

    Michael Covington used to post to the rec.photo newsgroups way BITD and that book is a great recommendation.

  9. #39
    Steven Ruttenberg's Avatar
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    Re: Large format Astrophotography

    I will check out the link. It is always good to understand ones objectives. Mine are two fold, science and art.

  10. #40
    Steven Ruttenberg's Avatar
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    Re: Large format Astrophotography

    The downside is I can't get started fir about 6-8 weeks as I just had my right shoulder repaired. The pain is by believable! Makes you wonder how the actor gets shot in the shoulder and keeps on lime no big deal! I'm calling bs on that one. My pain level is lije 50 on the 10 scale.

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