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Old 29-05-2018, 07:48 AM
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Atmos (Colin)
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Mewlon 250-S CR back focus

I bought a second hand Mewlon 250-S late last year and I came with a corrector which currently isnít installed. Iím planning on doing some high resolution imaging with it and cannot quite figure out where the back focus is calculated from.

What Iíve found is below:
Metal back distance** 115.5mm


** Optimum imaging BACK FOCUS as measured from the rearmost surface of the optical device used that will make contact with the next adapter in the imaging train. [This is a critical distance for optimum imaging]

Considering how accurate the back focus needs to be, does anyone how where this 115.5mm is calculated from?
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Old 29-05-2018, 07:57 AM
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multiweb (Marc)
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This is for a 210 but it might be of use.
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Old 29-05-2018, 08:33 AM
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RickS (Rick)
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Are you a member of the UncensoredTakGroup on Yahoo? That would be a good place to ask...

Cheers,
Rick.
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Old 29-05-2018, 09:55 PM
LewisM
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Metal back is from the rear most metal of the corrector (not counting threads) to the image sensor
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Old 29-05-2018, 10:07 PM
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Atmos (Colin)
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Quote:
Originally Posted by LewisM View Post
Metal back is from the rear most metal of the corrector (not counting threads) to the image sensor
So when the back focus of 180mm is mentioned, it is referring to the maximum back focus?
Where as the Metal Back of 115.5mm is just the imaging back focus?
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Old 30-05-2018, 10:02 AM
LewisM
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I believe when they mean back focus, it is the FULL travel of the drawtube.

Metal back is in reference for optical train adjustment items like reducers, flatteners etc. So, yes, the PRIME position for the sensor with this corrector will be 115.5mm back from the rear most metal face of the corrector unit. Remember, you need to subtract your sensor spacing from the front face plate (whichever this is on your CCD/DSLR) from this distance. So, say my CCD sensor was 15mm behind the front plate (not the glass window, but the front flat face), the adaptor from the rear of the corrector needs to be 100.5mm in length.

Capisce?
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Old 30-05-2018, 10:24 AM
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Atmos (Colin)
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That’s for that Lewis!
Considering that the corrector is within the baffle and screws into the mirror, it may only end up being about 70mm back focus from the rear 71mm thread. I’ll have to install it measure it all up.
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Old 30-05-2018, 10:36 AM
LewisM
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You could shoot Art Ciampi at Texas Nautical/Tak USA an email, as he is VERY knowledgeable about this stuff, or will ask his technician Fred Garcia.
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