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Old 15-06-2013, 06:45 AM
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Astroman (Andrew Wall)

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Location: Paralowie, South Australia
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Can we work out Telescope focal length from a Camera FOV?

Hi all,

I was wondering if it is at all possible to work out the Telescope Focal Length from an image taken with a Camera. For instance, I used the website http://nova.astrometry.net on an image I took (below). On successful completion of the image, the website tells me I have a 2.77 x 1.84 FOV with a pixel scale of 4.98 arcsec/pixel and a radius of 1.66

Given the above information and adding the other known values such as I was using a Canon 400D, and the telescope I was using has an aperture of 80mm. Can we then work out the focal length the image was taken at?

Links to the annotated images using Astrometry.net
M8 and M20

NGC 4945

Images below...
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Old 15-06-2013, 08:07 AM
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Merlin66 (Ken)
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Yes, there are many way's of doing it.
The simplest is probably to use the freeware CCDCalc to enter your data and do some iterations of focal length until it matches...
(just plugged in your camera and FOV - close to 304mm focal length)
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Old 15-06-2013, 08:15 AM
cfranks (Charles)
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Focal Length (mm) = (Pixel Size (microns) / Arc sec per Pixel) * 206.3
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