Serious modern telescopes use CCDs to capture the results. This makes it convenient to process or display the results halfway around the world. However, there is something visceral about looking through an eyepiece and seeing the results live.

My telescopic experience has been with smaller (<=8") devices in not-great-seeing conditions, and I've generally found the results dim and fuzzy. I've imagined what it would be like to look through an eyepiece on a serious telescope, showing a nebula bright and clear. Imagine stereo eyepieces on the Large Binocular Telescope!

Hence my question: what is the largest aperture telescope that was ever fitted with an eyepiece so that someone could look through it in real-time? This should include situations where a normally-digital telescope was temporarily fitted with an eyepiece.

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    $\begingroup$ I can't beat either of the other two answers, but I am pretty sure I have looked through an eyepiece on the 74 inch Mount Stromlo telescope (sadly consumed in a bush fire in 2003). We pointed it at the moon during an eclipse back in the 90s and showed around a few visitors to the observatory. $\endgroup$ – Rob Jeffries Nov 28 '15 at 11:51

Realize that a large telescope would have a large prime-focus image, that would in turn necessitate a very extraordinary eyepiece to fully utilize.

This said, if you look at this image from the 200 inch Palomar telescope (apparently published in Life magazine):

enter image description here

You see an astronomer inserting a plate in the camera at the 200 inch prime focus. Close to his right thumb you see a vertical tube. This is an eyepiece that is used to examine a star at the edge of the prime focus image outside of the plate dimensions. The astronomer will watch this star through the eyepiece and manually adjust the tracking of the telescope to keep the star stationary. For hours. In the dark. With no source of heat. And no plumbing.

So we have an eyepiece being used in parallel with a photographic plate. It may be a stretch, but...

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From this answer, it seems that the Harlan J Smith telescope located at the McDonald Observatory, in Texas, with a 107-inch (2.7 m) aperture is the largest aperture telescope that was ever fitted with an eyepiece.

Some independent evidence is there to indicate that this indeed had an eyepiece.

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  • $\begingroup$ The linked answer specifically cites "available for public viewing", which makes sense because an eyepiece wouldn't be useful for serious work. However, I do wonder if there are larger instruments which have had, for perhaps frivolous reasons, an eyepiece temporarily attached. $\endgroup$ – Daniel Griscom Nov 26 '15 at 19:46
  • $\begingroup$ I can confirm the information about the 2.7m telescope. I don't think that they do it very often, though. However the Observatory organizes special views at the the Otto Struve Telescope (2.1 m): it's impressive. $\endgroup$ – Francesco Montesano Dec 1 '15 at 9:28

A recent Twitter thread included a post by someone who had looked through an eyepiece on the 8.2 meter Melipal ("UT3") telescope of the ESO VLT.

Here's the original post:

Astro friends: anyone know the largest telescope in the world which has [can have] a real mountable eyepiece through which you can look? Palomar 200-inch is the biggest I know. Others? I know Subaru can project images to a screen, but I mean real look-through-it eyepiece.

... and here's the reply:

In 2003, I had the privilege to see Mars through an eyepiece installed on UT3, almost at its opposition (25" apparent size), with a 0.5" seeing. It was marvelous!

(Another reply cited a blog post about an eyepiece on a 6.5 meter telescope.)

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