RigTonight's compatibility checks explained, and how to fix each one

Tutorial 3 · 2 November 2026 · 4 min read

The Compatibility panel on the Rig tab lists problems first (red), then warnings (amber), then notes, then what's fine. Every problem comes with the fix. Here is what each check means, roughly in the order you'll meet them.

Back focus

Your flattener, reducer or coma corrector needs the camera sensor a set distance behind it, usually 55 mm. RigTonight adds up the camera's own back focus, the filter wheel or drawer, any off-axis guider, and the extra distance filter glass adds.

Add 18.2 mm of spacers (or similar). The train is short. The check lists stock spacers that add up to the gap, for example 16.5 + 2 mm.
Optical train is 19.3 mm too long. Spacers can't shorten anything. Use a thinner filter holder, drop a part, or use a camera with a shorter back focus. A DSLR with a filter wheel, or a filter wheel plus an off-axis guider, are the classic cases.
Back focus isn't published for this camera (or telescope). The manufacturer doesn't list it on its website. Check the manual or drawing.
Back focus is spot on. Nothing to do.

The back focus guide works through the sum by hand.

Corrector

Add a field flattener or Add a coma corrector. Your telescope gives stretched stars at the edges without one. The check names correctors that fit.
Isn't made for this telescope. Correctors are designed for specific optics. Pick one the panel offers for your scope.

Image circle and vignetting

Sensor is bigger than the corrected field. The sensor's diagonal is larger than the area the scope sharpens and lights evenly, so corners go dark or soft. Use a smaller sensor, or crop.
Filters are too small for this sensor. At your f-ratio the light cone is wider than the filter's clear glass by the time it reaches the corners. Go up a filter size (1.25″ to 36 mm, or 36 mm to 2″).
Filters may darken the corners slightly. A small overshoot. Flat frames correct it.
M42 thread will vignette this sensor. The narrow M42 opening clips the light for a large sensor. Use M48 adapters where you can.

Sampling

Compares your image scale with the seeing you set on the Your rig card.

Oversampled. Pixels are much smaller than the blur. You gain no detail, so the check suggests binning, for example 2×, and shows the binned scale.
Slightly undersampled or Undersampled. Normal for wide-field rigs. Dithering and drizzle integration recover some detail.

Payload

Over the mount's rated payload. Expect poor tracking. Use a bigger mount or lighten the rig.
Heavy for imaging on this mount. Under the box rating but over the realistic imaging limit (60% for worm-gear mounts, 70% for trackers, 80% for strain-wave). Short subs may still work.
Long focal length for a star tracker. Trackers are happiest below about 500 mm.

Guiding

Guiding recommended. Above about 300 mm, guiding lets you take long subs without trailed stars.
Guide scale is 7× coarser than imaging. The guide camera can't see the small errors that blur your image. Use a longer guide scope or an off-axis guider.
Add a guide camera or Add a guide scope or OAG if you have one half of the pair.

Filters

A mono camera needs filters, or Add R and G for full colour, if your set is incomplete.
R, G and B filters don't help a colour camera. It already has a colour filter on every pixel.
Single narrowband filters waste most of a colour sensor. A dual-band filter suits a colour camera better.
3 nm filters at f/3 need high-speed versions. In a fast light cone the passband shifts. Buy filters made for fast optics.
Add a filter wheel or drawer if you've ticked more than one filter but have nothing to hold them.

Controller and power

Peak draw is more than the power source can supply. Coolers starting up while the mount slews can trip it. Use a bigger supply.
Runs about 6 h, but tonight is 12 h. The battery won't last the night. Use a bigger battery, a warmer cooler setpoint or lower dew heater power.

Spotted a check that seems wrong for your gear? Use Spotted a wrong or missing spec? Tell us under the parts list. Corrections are fixed quickly.