My image shifts when I’m focusing my SCT and after I get a great focus, the focus will change. What is happening?
May 13, 2009
All Celestron SCTs and some Maks adjust focus by sliding the primary mirror along a set of nesting baffles inside the main tube. This is done by an arm to one side of the tube that is fixed to the mirror. Because of this design, some lateral shift may occur as you focus the scope. This will be seen as a jump in focus and is usually described as “image” or “mirror” shift.
While some of this is inherent in the design, excessive shift in new scopes can be caused by lubricating grease clumping on the baffles. Work the focus back and forth to evenly spread the baffle grease and reduce shift. This will also spread the lubricant greasing in the focusing bearings and on the threaded focusing shaft for smoother action.
If the focusing knob is off-center, it will pull laterally as you focus. So another fix is to loosen the three screws on the plate beneath the focusing knob and re-center the focus knob by trial and error. This re-centering repositions the ball bearings and the threaded focus shaft they carry in your SCT, improving the action of the focusing.
Aftermarket “zero-image shift” focusers that replace the visual back are another solution. They use the original focuser for rough focusing, then allow fine focusing without using the original focuser.
A more extreme version of image shift is mirror flop, seen especially in SCTs with apertures 11 in and larger. Gravity will cause the heavy mirror to actually move or flop a tiny amount as the scope tracks, causing focus to be lost. It often occurs when moving through the meridian.
You can always move the focusing knob left (counterclockwise or towards infinity on most scopes) to achieve final focus. This moves the mirror upwards (inwards) against gravity minimizing the possibility of image shift and mirror flop.
The best solution for flopping is to use mirror locks, a set of one or more screws that can be safely moved in contact against the primary’s back. This holds or “locks” it in place during astrophotography or a long observation of an object.
The EdgeHD mirror clutches apply tension to a flexible rod attached to the primary mirror support. With enough tension from both clutches, they are effective greatly lessening mirror shift or mirror flop when the scope is moved around in different orientations while in use.
Because they are flexible, the EdgeHD mirror clutches won’t prevent focusing while tightened. However, because they do put force on the mirror, they should be loosened when focusing. Otherwise the combination of their force and the focusing effort may put unequal pressure on the primary mirror, causing it to bind or unevenly shift while focusing.
Note: All C14 tubes have two regular mirror locks. (For the EdgeHD 14 in, they are provided in addition to the clutches.) These locks do just that: lock the mirror. They should not be used to lock focus. They are intended for scope transport only. They are properly used with the mirror in a fully backed position. Use in any other position may damage the mirror by placing too much pinpoint stress on it.
Finally, focus and collimation can change due to temperature changes which cause thermal expansion-contraction of your scope as it is used. This effect cannot be completely eliminated, but it is greatly reduced by use of annealed optical glasses and proper construction of the telescope tube.