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Thread: Primary Mirror Focal Length: Meade ETX 125 (Maksutov)

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    Default Primary Mirror Focal Length: Meade ETX 125 (Maksutov)



    I am trying to determine the focal length of the Meade ETX 125. Does anyone know what it is please?

    I have not been able to locate that information anywhere.

    Also, does anyone know any sites I can visit for Cassegrainian telescope formulas please?

    Many thanks in advance.

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    Default Re: Primary Mirror Focal Length: Meade ETX 125 (Maksutov)

    According to the Meade website, it is f/15 with a focal length of 1900mm.

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    Default Re: Primary Mirror Focal Length: Meade ETX 125 (Maksutov)

    1900mm or whatever is etched on the front lens cell of the telescope is just an approximation for the "out of the box" condition

    for example a celestron/skywatcher/orion 127maksutov is only 1500mm with a prism diagonal , 1540mm with mirror diagonal , 1770mm with 2inch visualback and 2inch diagonal, 1900-2000mm with binoviewers and 1.25 diagonal
    focal length are determined by the exact spacing between the primary and secondary, and these will change between infinity focus and terrestrial view

    same applies to SCT telescope
    many users don't understand this and blindly advocate changing the C6 visualback to 2" the moment they get it

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    Default Re: Primary Mirror Focal Length: Meade ETX 125 (Maksutov)

    I lost myself between writing the thread title and writing the body of the thread.

    I meant to ask about the focal length of the primary mirror; what is its value please?

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    Default Re: Primary Mirror Focal Length: Meade ETX 125 (Maksutov)

    I doubt if anyone here knows the focal length of the primary mirror. You would have to find out from Meade, or else measure it yourself. The focal length will be significantly shorter than 1900mm.

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    Default Re: Primary Mirror Focal Length: Meade ETX 125 (Maksutov)

    The primary mirror of a Maksutov-Cassegrain usually has a focal length only about three times its diameter. For example, a 7" telescope would have a primary mirror focal length of about 21". This focuses the light to a point 21" in front of the mirror. By placing a convex secondary mirror between the primary mirror and this focal point, the light is reflected back toward the primary mirror but at a less steep angle. If you trace this light cone back to where it reaches the full width of the telescope's aperture you can see how long the effective focal length is. Most Mak-Casses have secondary mirrors that provide a magnification factor of around 5x. In this example that would make the effective focal length 105", or 15 times the diameter of the primary mirror, in a telescope only about 20" long


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    So, in your case it would be 125mm x 3 = 375mm


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    Default Re: Primary Mirror Focal Length: Meade ETX 125 (Maksutov)

    I'm really quite curious as to the reason for wishing to know the focal length of the primary mirror. It is not normally relevant to visual use of the instrument or to AP use either.

    I'm just hoping there is no idea about trying to Hyperstar the thing. I'm pretty sure that would not work out well at all.

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    Default Re: Primary Mirror Focal Length: Meade ETX 125 (Maksutov)

    Since you can't remove the secondary on a MAK, Hyperstar isn't going to happen...


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    Default Re: Primary Mirror Focal Length: Meade ETX 125 (Maksutov)

    Many thanks everyone!

    Actually, this Padwan Learner had planned to remove the corrector lens, replace it with some plane transparent material and locate a ccd camera at the now-available focal point of the primary mirror to discover what can be seen. I can then always return the corrector and agonize over whatever else comes up. It usually is somewhat fun!

    I have an ETX 125 ota that I de-forked... ( a long separate story)

    I still intend to get a C8 asap - Celestron is adamant the C6 is not Hyperstar compliant and the exchange rate of the NGN to the USD does not make a C6 purchase worth risking - but I am tempted to experiment with the etx125. My excuse is that I need to clean it. And I do have that Neewer lens holder removal spanner that I use for my filter cleaning tasks...

    I found great information about the optics of a Cassegrain here: http://www.handprint.com/ASTRO/ae2.html.

    There I found out that the primary is an f3 on the etx125 with a focal length of 414 mm.

    I do not intend to permanently alter the etx ota; I just want to discover.

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    Default Re: Primary Mirror Focal Length: Meade ETX 125 (Maksutov)

    It would be interesting to see if you can accurately calculate the focal length from comparing the diameter of the primary and secondary mirrors (and their separation) and determining the focal length by geometry. I suppose the problem would be knowing the exact position of the primary.

    However with such a fast spherical primary, I'd expect a lot of off-axis aberration in the primary image.
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