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I grew up stargazing through various telescopes of increasing aperture sizes, and now that I can afford it, I'm about to invest in a more serious telescope to suit me for decades to come. My primary goals are both planetary and DSO viewing, with some astrophotography, but one thing I'm keeping in mind is next month's annular eclipse and next spring's total eclipse, which I would like to also use my telescope for (with, of course, a certified solar filter IN FRONT OF the aperture lens). (And, also, possible future Mercurial or Venusian transits.)

What I'm wondering is, is there a maximum practical aperture size and/or focal length beyond which you cannot view the entire Sun in the field of view? I understand a good bit about telescope design and mechanics, but I don't pretend to fully understand the science, and I'm wondering if there's a point where the Sun is magnified so much that it more than fills up the field of view of the telescope. (It's possible, of course, that this aperture size and/or focal length is far beyond any sizes I'm considering, in which case it's not a particularly important concern.)

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    $\begingroup$ Make sure you do get the right kind of filter; there are filters for photography and filters for visual. I once looked through an 8-inch (20 cm) telescope with a photography filter, and the Sun was too bright for comfort (not to the point of damaging my eye, but still too bright). $\endgroup$ Sep 18, 2023 at 5:24

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If you want to see the entire Sun in a single field of view, your concern is with the magnification and field of view of your eyepiece, rather than overall aperture and focal length.

Magnification is the focal length of the telescope divided by the focal length of the eyepiece. Common eyepieces have fields of 40-50 degrees. The angular diameter of the Sun is about 1/2 degree, so you probably want to top out at about 70x magnification.

There's no real limit on how big the aperture of a solar telescope should be (the Inouye solar telescope in Maui uses a 4 meter primary) but since the sun provides plenty of light and the atmosphere tends to be pretty active during the day, atmospheric distortion rather than aperture tends to put a limit on resolution when viewing the sun (the Inouye telescope uses adaptive optics to counter atmospheric distortion.) Therefore people get satisfying solar observations with quite small telescopes-- a 60 mm or 70mm refractor for example. People with larger reflectors will sometimes use an off-axis aperture mask when observing the sun, to reduce diffraction from the secondary and to reduce the impact of distortion from moving air columns.

Editing to add something more specific to the question asked: if you are buying a telescope for deep-sky and astrophotography, you will probably be getting something with from 200 mm - 300 mm aperture, and 1200 mm - 3000 mm focal length (depending if you are getting a Newtonian or a Cassegrain). With 1200 mm focal length, it's straightforward to get to 70x magnification. With a 3000 mm focal length, you might need a focal reducer to get the entire sun to fit into an eyepiece's field. Either way, you may want an aperture mask when looking at the sun; it will make the required solar filter smaller and easier to handle.

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    $\begingroup$ (+n!) for more on off-axis aperture masks see answers to Why does this large Newtonian telescope's front cover have two or three holes in it? $\endgroup$
    – uhoh
    Sep 17, 2023 at 22:31
  • $\begingroup$ This was helpful, thanks. I'm still trying to decide between the Celestron 11" Schmidt-Cassegrain and the 9.25" EdgeHD. I have a lot of factors to consider, not just eclipse viewing. Interestingly, the default 40mm eyepiece that comes with the 11" makes it 70x, so I wouldn't need a different eyepiece for eclipse viewing, if I read your comment correctly. However, the default 23mm eyepiece that comes with the 9.25" renders it 104x, so I'd need an eyepiece with a larger focal length to bring it down to 70x. (I plan on buying several different eyepieces, so that's not exactly a problem.) $\endgroup$ Sep 20, 2023 at 14:04

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