Image circle and sensor compatibility

Almost every “my stars are bad in the corners” problem reduces to one of two numbers being wrong. Both are checkable before you buy anything.

Number one: image circle against sensor diagonal

A telescope’s corrected image circle is the diameter over which it produces acceptable star shapes. Compare it directly with your sensor’s diagonal:

A telescope quoting a 30 mm corrected circle is fine for APS-C and will show soft, elongated corners on full frame. There is no processing fix — the photons were never corrected. If you own a full-frame camera, or expect to, this number should filter your shortlist before anything else.

Note the wording. Some manufacturers quote an illuminated circle, which is larger than the corrected circle. Illumination without correction gives you light in the corners and misshapen stars in it.

Number two: backfocus

A field flattener or corrector specifies a distance from its rear shoulder to the sensor plane — 55 mm is the most common convention, but Cassegrain designs often specify something quite different.

Your stack has to add up to that number:

camera sensor depth + filter drawer + adapters + spacers = specified backfocus

Camera manufacturers publish the sensor depth (often 12.5 mm or 17.5 mm for dedicated astronomy cameras). Everything else you measure or read from the part specification.

Add glass and the arithmetic changes: a filter in the path shifts focus by roughly one third of its thickness, so a 2 mm filter adds about 0.7 mm of optical path. Most people include this and forget it once.

Reading the failure mode

When spacing is wrong the corner stars deform in a characteristic direction:

• Too much spacing: stars elongate tangentially — stretched along the circumference, like arcs around the frame centre.
• Too little spacing: stars elongate radially — stretched along lines pointing at the frame centre.

The correction is typically one to three millimetres. Change it, shoot a short test frame, look at the corners, iterate. Two rounds is usually enough.

When it is not spacing

If the corners are wrong on one side only, or the pattern is asymmetric across the frame, it is tilt, not spacing. A tilt adjuster or a shim fixes it. Diagnosing tilt as spacing, or vice versa, is how people lose a season.

On a Ritchey-Chretien specifically, asymmetric stars are collimation far more often than tilt. Check collimation with a defocused star before buying adjusters.

The way to avoid all of this

Buy a telescope with correction built in — a Petzval, a RASA, or an aplanatic Cassegrain. There is one configuration, it is documented, and it is correct.

You pay for that. Whether it is worth it depends on how you feel about spending your first season diagnosing corner stars instead of collecting photons.