Coma corrector
Also called: Paracorr, MPCC.
A lens group that slips in the focuser of a fast Newtonian and makes stars sharp right to the edge of the field.
A coma corrector (Tele Vue's Paracorr, Baader's MPCC and others) cancels a parabolic mirror's coma. On a fast Newtonian or Dobsonian (below about f/5.5) it belongs in the focuser for every eyepiece — the Moon and planets too, since on an untracked Dob the planet drifts towards the edge between nudges. The Paracorr also lengthens the focal length by 15%, raising every eyepiece's power.
Each eyepiece focuses at a slightly different height, so the Paracorr's tunable top has a setting per eyepiece.
At the eyepiece
Stacking an ordinary Barlow on a coma corrector changes the spacing it was designed for; if you need more power, use a shorter eyepiece (Tele Vue documents only its Powermates for this).
In AstroPilot
AstroPilot keeps your coma corrector in with every eyepiece on a fast Newtonian and includes its factor in every magnification.