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Old 10-10-2021, 04:34 PM
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Stonius (Markus)
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Originally Posted by astro744 View Post
Similarity a 4x Powermate in a 1.25 barrel would not need the clear aperture it currently has since it only needs to accommodate a maximum field stop in an eyepiece of 27mm (max for 1.25 barrel) and not 46mm (max for 2 barrel).
DING! Lightglobe goes on - thanks! So I guess the size of the 2.5x powermate's field stop is the size it needs to be in relation to the maximum possible field stop of the eyepiece that can be inserted into it. Makes sense.

Originally Posted by astro744 View Post
The field stop diameter is simply the linear true field at the focal plane. The wider it is the more sky you see (for a given telescope). To get the true field in degrees you multiply the field stop diameter by 57.3 and divide by the focal length of the telescope.
<Lightglobe goes *pop*>

Okay, so if I were to remove the field stop from an eyepiece or a barlow, would the result be the same? ie, that I see more sky? (Just a thought experiment, BTW - I'm not *actually considering doing this!).

In my head I'm thinking I *would see more sky, but that extra bit would be outside of the design spec and be uncorrected / severely aberrated. Maybe I'd also get lots of internal reflections/scattering where light now reflects off things it was never meant to touch (lens edges, internal threads, whatever)? So, in theory could you enlarge the TFoV of a given Plossl by simply removing the field stop, even though the new bits of the image would be 'bluergh'?

Apologies for all the questions, I had never thought too hard about field stops until I was looking at getting a binoviewer.

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