Telescope Mount Payload Calculator: Will Your Mount Cope?

A mount that is carrying too much will still track, it just tracks badly: stars smear in long exposures, the image wobbles for seconds after every touch, and guiding graphs look like a seismograph. Add up everything you bolt onto the dovetail and this works out what share of the rated payload you are using, whether that is sensible for what you do, and how much counterweight you need to balance it. The figure on the box is a ceiling, not a target: for long exposure imaging the usual rule of thumb is to stay at about half of it.

Why imaging needs so much more headroom than visual

Looking through an eyepiece, a slightly overloaded mount is an annoyance: you nudge the focuser, wait for the shake to die down, and carry on. A camera taking a three minute exposure records every wobble and every tracking error, and the longer the focal length, the more each of those errors is magnified on the sensor. That is why the advice tightens as focal length rises. Wind matters too: a long tube or a dew shield acts as a sail, and the same kit that is fine on a still night can be unusable in a breeze.

The calculator uses widely repeated rules of thumb rather than a manufacturer formula: for visual work, up to about 80 per cent of the rated payload is comfortable; for imaging at short focal lengths, about 60 per cent; for imaging beyond 500mm, about 50 per cent. Mounts differ, and some newer strain wave designs are rated in a different way, so treat the verdict as a starting point and check your own manual.

How to weigh your kit properly

• Use bathroom or luggage scales with the telescope set up as you would use it: rings, dovetail, finder, dew shield and camera all attached. The tube weight on a listing usually leaves some of those out.
• Do not forget the small things. A filter wheel, an electronic focuser and a mini PC can easily add up to a kilogram between them.
• Long tubes count for more than their weight. A long refractor or a big Newtonian puts its mass further from the axes than a compact scope of the same weight, so the mount works harder. If yours is long, treat the result as one step stricter.

Getting the counterweight right

Balance is a simple lever: the weight of the load multiplied by its distance from the RA axis should equal the counterweight multiplied by its distance on the other side. Fill in the three optional boxes and the calculator works out how much counterweight you need and where to slide it. The counterweight bar has some weight of its own, so in practice the weights usually sit a little closer to the mount than the figure shown. For imaging, many people leave the setup very slightly heavy on the east side so the gears stay engaged; final balance is always done by hand with the clutches loosened.