Adjusting focuser changes posn of target in sensor

[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

[OzEclipse]

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

[mains9]

Thanks Joe for taking time to respond.

You have given me specific keywords to browse the net. I will now get on to Bintel and see speak "knowingly" :) 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.

Thanks again. Suresh

[OzEclipse]

It is more or less normal behaviour on those cheap focussers. I have one too. You can adjust them to reduce the problem but not to eliminate.

[raymo]

There is more to this than what Joe has told you. Even with a top quality
focuser that has little or no image shift, you can have trouble locating a
small object such as a planet. Your scope at prime focus gives a
magnification of about 12x, when you put in the barlow that becomes 24x,
and the field of view is halved, and when you put in the 9mm instead of
the barlow, the field will be 5.5x smaller than at prime focus. If you put the 9mm and the barlow in together, the field will be 11 times smaller than prime focus. This means that if your finder is not precisely aligned with the main scope, the target can easily be outside the field of view at the higher
magnification. A good way to minimise this problem is to put a distant
object such as a street light precisely in the middle of a reticle eyepiece
in the main scope, and then adjust the finderscope so the object is smack in the middle of the crosshairs.
raymo

[mains9]

Thanks for that raymo,

Been there. I had precisely aligned the main scope and the guide scope and thought that would make it easy to switch to barlow or the 9mm but didn't help. Changing to barlow, then to then start refocusing I line up with the appropriate mark on the draw tube but target is nowhere in sight in the main scope.

I had noticed when adjusting focus at any time the target in fov moves off-centre!

Suresh

[raymo]

If your scopes are aligned, and the object is out of the field of view when
a barlow is inserted, you must have the worst image shift that I have ever heard of, even worse than a $20 child's toy scope would have.
To be out of view when a barlow is added means that the image shift is more than half the prime field of view, that's unbelievable.
First stop the supplier, I reckon.
raymo

[mains9]

Going to do some terrestrial testing and be methodical about it. Nice day for it :-) Suresh

[RAJAH235]

Hi,
If you find that the focus tube has a lot of play in it, you might be able to pack it with some adhesive-backed Teflon tape.
It's available in various thicknesses.
or, as mentioned by Raymo, a visit to the supplier may be necessary & a replacement focusser obtained.