How do I align my SCT’s corrector plate and secondary mirror?

Alignment of your secondary relative to the corrector plate and to the primary is important to the proper optical performance of your scope. There are two alignments: corrector plate-secondary mirror assembly to primary mirror and corrector plate to secondary mirror. These alignments should be understood before attempting to take apart the optics.

Note: these remarks apply to most scopes made in 2009 and earlier. EdgeHDs and some newer optical tube assemblies (OTAs) may have marks with orientations varying from OTA to OTA.)

Corrector plate-secondary mirror assembly to primary mirror
Remove the retaining ring. With the tube horizontal, most recent OTAs will have a mark or etching on the edge of the corrector plate at the
3 o’clock
position (diagonally across the length of the tube from the focus knob).

For scopes like the NexStar SE, the focuser will be at the bottom
6 o’clock
position and the mark should be at the top.

The general rule is the alignment mark will be opposite the focuser. Since the focuser is attached to the mirror, this mark orients the corrector plate-secondary assembly relative to the primary mirror.

Corrector plate to secondary mirror

With the secondary mirror and cell removed and apart, the back of the aluminum mirror support plate should have a line on it extending halfway across the plate (radially outwards from the plate’s center). This line should be aligned with the mark on the corrector plate and on the same side of the secondary’s center as the corrector plate mark. If the line isn’t in this position, rotate the secondary housing until it is properly aligned.

In many cases, when everything is properly oriented, the serial number (if present) will be in a horizontal position. CPCs will have the triangle of collimation screws pointing straight upwards (one screw will be at the
12 o’clock apex position).

Important: If there are no marks found when removing the retaining ring, secondary cell, and secondary mirror and its support plate, mark everything carefully before removing it so the orientation of corrector-secondary-primary can be successfully reconstructed upon reassembly.

Note: Removal and adjustment of the optics in this manner may void your Celestron warranty.

Updated 12/21/13