RigTonightHow much can your mount really carry? Payload for astrophotography
The mount matters more than the telescope. A mount that is overloaded, or loaded right up to its rating, won't track smoothly enough for long exposures, however good the optics are.
The rating is for looking, not imaging
A manufacturer's payload figure is usually what the mount can hold and point. Imaging needs smooth tracking for minutes at a time, and long or heavy tubes add wind load and inertia. The long-standing rule of thumb is to image at well under the rating:
| Mount type | Comfortable imaging load |
|---|---|
| Worm-gear equatorial (iOptron GEM28 and CEM40, Celestron CGX…) | about 50–60% of the rating |
| Strain-wave (harmonic) mounts (iOptron HEM15, HEM27…) | close to the rating, often around 80%, because there is no backlash to balance against |
| Star trackers | well below the rating, and short focal lengths only |
RigTonight uses 60%, 80% and 70% for these three types, and marks anything over the guideline as heavy and anything over the rating as a problem.
Add up everything
Don't stop at the telescope. The payload includes:
- Optical tube, rings and dovetail
- Reducer or flattener
- Camera, filter wheel and off-axis guider
- Guide scope and guide camera
- A mounted controller (a Raspberry Pi or mini PC), dew heaters and cables
The builder adds about 0.8 kg for rings, cables and dew straps on top of the listed parts.
An example that changes the answer
An EdgeHD 8 with its 0.7× reducer, a cooled APS-C camera, an off-axis guider and guide camera comes to around 8.6 kg with accessories.
- On an iOptron GEM28 (12.7 kg rated, about 7.6 kg for imaging) it's flagged as heavy for imaging.
- On an iOptron GEM45 (20 kg rated, about 12 kg for imaging) it's comfortable.
Focal length matters as much as weight
A light rig at long focal length can still be too much for a small mount, because tracking errors scale with image scale. Trackers like the iOptron HST MiniTracker do best below about 500 mm; the builder warns above that.
Weights in the catalogue come from the manufacturers' own pages. Weigh your own rig with a luggage scale; it's the most accurate number you'll get.
For the rest of the imaging train, see How to build an astrophotography rig.