It corrects the curved focal plane that most telescopes produce, so that stars are sharp at the corners as well as the centre. Like a reducer, it demands one exact distance to the sensor.
Most telescopes bring light to a focus on a curved surface, and a sensor is flat. That mismatch is what a flattener addresses.
What the defect looks like
Sharp in the middle, progressively softer towards the corners, with stars that bloat rather than stretch in a direction. It gets worse with a larger sensor, because the corners are further from the axis.
Why it is worse on fast telescopes
A steeper light cone curves the focal surface more. A slow refractor may not need a flattener at all with a small sensor; a fast one almost certainly will with a large one.
The spacing, again
A flattener corrects properly at one distance from the sensor and nowhere else. The whole train has to be measured before ordering, and a spacer of the right thickness is part of the purchase rather than an accessory to it.
What a flattener leaves alone
Tilt, which produces one bad side rather than four bad corners, and coma in a Newtonian, which has its own dedicated corrector.
If this page and the manufacturer’s own documentation disagree, follow the documentation. Every aperture, focal length, payload rating, field, eye relief, transmission figure and thread size on this page comes from the maker’s own published figures, with the unit as printed and the source named. None of it is measured here, and no figure is converted into another convention, because a converted number is a different number. And for any sensor: a pixel size means nothing without the focal length it sits behind. The same camera is well matched to one telescope and wasteful on another, and a frame rate quoted for the full sensor is not the rate achieved over the small region a planet occupies. This page reports the maker’s figures as published and computes no sampling from them.
The questions that keep arriving
Do I need one for visual observing?
Almost never. The eye accommodates a curved field without noticing, and the effect only becomes obvious on a flat sensor.
Is a flattener the same as a reducer?
Not necessarily. Some products do both and are sold as reducer-flatteners; some only flatten and leave the focal length unchanged. Which one you have changes the field of view and the spacing.
Last reviewed 17 September 2026