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Focal length in a frame is different from the telescope

Harry Zampetoulas
in PixInsight
Hello 
After running the Image Solver on an image and looking on the fits header find out that the focal length of the image is different from the telescope's focal length. 
What that means

Comments

  • BlockHeadBlockHead
    Hi Harry,

    I just want to point out I think you are asserting that a published value "different from THE telescope focal length" is different than what Image Solver calculates. However, if the calculation is valid and there are no issues... the calculation based on the stars in the image is much more precise then a published value. Do you see what I mean? You can have confidence in the measured value... the fact that the published value does not agree is where the discrepancy is. Typically published values are calculated from the system specifications such as mirror aperture and f/number. But given a specific telescope the *actual* aperture or f/number likely is not known precisely. So the published focal length is just an estimate. What you measure is "real."

    -the Blockhead
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