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    Default Prime focus question + Barlow question



    So as I wait for my telescope to return from Orion, and my webcam to show up, I'm reading up on prime focus AP. In running through this article, I don't follow how they specify Barlows can be moved to achieve different magnifications. What makes sense to me is that moving the sensor closer/farther from the Barlow would just bring the image out of focus, not change the magnification; similarly, moving the Barlow closer/farther from the optics would allow for changing the magnification. Can anyone explain this to me? I am specifically interested in high-mag AP, since my primary targets will be planets.
    Me: Jonathan. Working on webcam video system.
    Equipment: Orion StarMax 127mm EQ Mak (Oct 2014) | EPs: 25mm Orion Plossl, Zhumell Z Series Planetary 6mm & 14.5mm | .5 FR

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    Default Re: Prime focus question + Barlow question

    Some thoughts and terminology...

    Prime-focus is somewhat a misnomer for a catadioptric telescope, like a Mak or SCT. In this case, prime-focus would be mounting the camera/detector in place of the secondary mirror. The misnomer, prime-focus is usually referring to use of the Cassegrain focus of the telescope (i.e. downstream, or "behind" the primary mirror.

    A Barlow lens is a form of sub-aperture corrector, used in the light path, to cause convergence of the beam at a longer focal length than the native focal length of the optical system - it is similar to the tele-extender used with a fixed focal length telephoto camera lens.

    As an aside, Mak and SCT telescopes generally have a long native focal length in Cassegrain configuration - they are normally f/10 or f/15 systems - combined with a small area sensor, like a web-cam or video imager, the image scale (apparent magnification) and field of view (FOV) are generally already sufficient for planetary imaging without addition of a Barlow.

    With a Barlow in the optical path, the focal ratio is increased, the system throughput is decreased, and the FOV is decreased. The Barlow can be inserted in the path, and is somewhat insensitive as to the location and consequent "magnification" effect it has on the system. Moving it inside, or outside of the focal plane distance will change (decrease in the inside case, or increase, in the outside case) the image scale and apparent magnification.

    The other issue the Barlow can create is excessive flexure in the optical train, which can in turn create problems at the camera focal plane, including off-axis distortions, vibration effects, and other difficulty. Another concern is the effect relative to average seeing - there is no, or dubious value in a configuration that exceeds average seeing conditions.
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    Default Re: Prime focus question + Barlow question

    Ok, I think I understood all, or at least some, of that. I will now perform active listening to the delight and shuddering of all: Yes, I can move a Barlow back and forth to change the FOV/mag, and it's due to magic. Time for me to study more on optics.

    I was about to ask another question, but then I found this, which is a fantastic article and answered a few questions I had, and many I didn't! Starizona's Guide to CCD Imaging
    Me: Jonathan. Working on webcam video system.
    Equipment: Orion StarMax 127mm EQ Mak (Oct 2014) | EPs: 25mm Orion Plossl, Zhumell Z Series Planetary 6mm & 14.5mm | .5 FR

 

 

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