[privong]

> In other words, it's got a giant steel mounting that needs an entire building foundation to support it.

This is still the case for the Alt-Az telescopes. For stability, telescopes are built on a foundation that is independent from the building's foundation and, ideally, that goes to bedrock.

> Modern (giant) telescopes use alt-azimuth mounts (similar to a normal camera mount), which require two axes to be adjusted simultaneously with computer control. This causes the projected image to rotate and is not suitable for photographic plates but can be compensated with digital image processing.

Actually, there's usually a third rotation axis often added to instruments (or their mount points) for Alt-Az telescopes, called a field rotator. As one might expect, this counteracts the apparent rotation of the field and maintains the same image orientation on the detector. Note that this is also important for spectroscopy and not just imaging. For spectroscopy, you may also want to ensure the position angle of your slit is constant relative to your source of interest, particularly if the source is not point-like.

In principle, one could correct for the field rotation with image processing. But for many observing programs, the individiual exposures can be in excess of 15 minutes. For crowded fields or extended sources, it would be non-trivial to deconvolve the field rotation. In practice, it's much easier to just add a third axis to the instrument which counteracts the field rotation.

[spdustin]

There's also a model-specific wedge that can be used to "convert" to Equatorial. My SCT has one available, though I don't use it, since most of my imaging involves stacking much shorter exposures at lower magnifications than would be problematic with field rotation.

[privong]

Good point. Those are available for amateur telescopes. I should have noted I was referring to professional University or national-level observatories.

[exDM69]

> This is still the case for the Alt-Az telescopes. For stability, telescopes are built on a foundation that ...

Yeah, most certainly. But you can build much bigger telescopes with Alt-Az mounting, right? I have a hard time imagining a 10 or 30 meter telescope on an Eq mounting.

> Actually, there's usually a third rotation axis often added to instruments (or their mount points) for Alt-Az telescopes, called a field rotator.

Thanks for this snippet of information, I had not heard about a third axis. Is the third axis on the instrument only? Ie. the optics are on Alt-Az mount with two axes and the CCD is mounted on a third?