Quote:
Originally Posted by mains9
Hi All,
I am a 3 mth old novice.
Have an Canon 600D, ED80, HEQ5P, 2x barlow, 9mm ep.
Burning question .... Why does adjusting the focuser change the position of the target in the FOV?
One very annoying effect of this is when changing from prime to 2x barlow and having to refocus for the barlow the target disappears from the FOV although it remains centered in the guide scope. Then have to spend hours getting the SAME target back in the sensor bulls-eye.
Gets even worse when wanting to use the 9mm ep.
Any idea why this happens please. Suresh
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Hi Suresh,
The ED80 has great optics but most versions are supplied with very cheap mediocre focussers. What you are seeing is called image shift. There are a few causes.
There is some play in the focusser. When you tighten the focus lock you push the tube sideways shifting the field of view.
A similar thing happens if you change the eyepiece to one with a different mass, even without lock, the focuser flexes sideways under gravity by different amounts.
The other problem is slip, the wheel that turns to move the focusser doesn't have sufficient friction and the tube slips along the optic axis causing it to go out of focus.
When you use a barlow, you have to move the focus a fair way out, as you do, the focus tube shifts sideways as it moves and in the highly magnified field, the image shifts out.
There are tutorials on line how to adjust a Crayford focusser to minimize shift and play.
The best way is to replace the focusser. However, a good focusser can cost you almost as much as the telescope did.
Joe
with the tech there (..... very helpful mob). Don't want to splurge on a new focusser. I thought what I was experiencing was "normal" behaviour but you corrected that.
