Image shift

[thommo10]

Hi all

Does anybody know how to reduce image shift when focusing by hand on a ETX. ?

[NGC 1502]

Hi Keith, do you mean the wobble caused when operating the focus knob ?

That can be helped by using a cable extension. The knob is removed, and

replaced by the extension. They are commercially available, or I've seen

home made ones using curtain spring. A bit like a cable release for a camera.

Or you may mean image shift caused by 'mirror flop', mechanical slack in the

focusing system that causes the image to move across the field of view as

you focus. It's fairly common with Maks and SCTs. Most learn to live with it

if it's not too bad. Correcting (or reducing it) means a strip down. Unless the

movement is very bad, I would leave it be.

HTH, Ed.

Edit - here is the focus cable http://www.astro-engineering.com/prod/Astro-Engineering/ETX-fleximate/AC343-AC344-AC345.html

[ollypenrice]

One thing is always to make your final focus one in which you are pushing the mirror up the tube, not down it. If you push it up it will tend to sit on its carriers. If you pull it down it will continue to settle after you have stopped. Don't dither between the two because you leave the mirror in a state of dither as well! I don't know which way round the ETX focus knob works but you should be able to suss that out.

On my old LX200 I have a Telescope House gadget that screws into the transport locking hole of the primary and pulls it permanetly back with a light spring. It is good but may not exist for the ETX, I don't know.

Olly

[thommo10]

Hi Ed & Olly

I mean the movement of the image across the field of veiw when focusing. I cannot find a transport locking hole on the tube, I guess I will take your advice and leave it alone, thanks anyway.