The telescope is the part people research for weeks. The mount is the part that decides whether the telescope works.
What a mount actually has to do
Visually: hold the telescope still, and damp quickly when you touch the focuser. If the image takes three seconds to settle, you will use less magnification than the optics support and you will enjoy the session less.
For imaging: follow the sky to a fraction of an arcsecond for minutes at a time, repeatedly, in wind, with a camera and cables hanging off one end. Every error is recorded faithfully into every sub-exposure and no processing recovers it.
Those are very different jobs, which is why a mount that is excellent visually can be useless for imaging.
Payload ratings are optimistic
A mount rated for 10 kg will hold 10 kg. It will not track 10 kg accurately for a five-minute exposure in a breeze.
The working rule among imagers is to load an equatorial mount to no more than about 60 percent of its visual rating for imaging, and to count everything: tube, rings, dovetail, camera, filter wheel, guide scope, focuser motor and cables. An 8.5 kg optical tube is realistically a 12 kg load, which needs a 20 kg mount.
This is the single most common budgeting error in astrophotography.
Alt-azimuth, equatorial, and field rotation
An alt-azimuth mount moves up-down and left-right. Simple, intuitive, and — this is the part that surprises people — it rotates the field of view as it tracks. An object stays centred while everything around it slowly turns.
Visually, irrelevant. For a two-minute planetary video, irrelevant. For a five-minute deep-sky exposure, it smears every star except the one at the centre.
An equatorial mount tilts one axis to point at the celestial pole, so a single motor follows the sky without rotation. This is why deep-sky imaging is done on equatorial mounts, and why an equatorial wedge is the standard upgrade for fork-mounted Cassegrains.
A Dobsonian base is an alt-azimuth mount with no motors at all — excellent visually, and with no path to long-exposure imaging.
What to budget
For visual observing: a mount that damps in a second or two under your telescope. A Dobsonian solves this by construction, which is a large part of why it is such good value.
For imaging: at least as much on the mount as on the telescope, and often considerably more. The usual advice — buy the mount first and grow into it — sounds like a cliché and is simply true. People replace telescopes routinely; a good mount is bought once.
Signs your mount is the limiting factor
- Stars are consistently elongated in one direction across the frame.
- Guiding graphs show large excursions that correlate with wind, not with seeing.
- The image takes more than about two seconds to settle after touching the focuser.
- You have reduced exposure length until the images “work”, and are wondering why they are noisy.
All four are mount problems wearing an optics costume.