What is image shift and its effects?

[pat]

My f4.7 Orion standard focuser has a slight play in it. I've taken every measure to max its performance.

When racked in and out the laser beam moves/jerks about 4mm on the primary.

Does this have an ill effect on stars, planets, moon?

I've heard of image shift, but don't know too much about it.

I use TV eyepieces, if thats makes a differance. Pat

[hemihaggis]

Image shift is caused by what is commonly referred to as "mirror flop." As the primary mirror moves to and from the secondary mirror to effect focus, the focus mechanism could cant the primary mirror minutely in relation to the OTA and secondary mirror. Depending on the severity of the mirror tilt, the image will shift accordingly. In most cases, the shift is noticeable but very small. However, in severe cases, or when viewing under high power, the image can shift enough so that your target object is completely lost from FOV.