My home-brew focuser.

Tim Auton wrote:

Linear plastic pot ?


Crappy resolution.

Plastic servo pot ? Very high resolution, though not accuracy.

Jo's criteria though is for a mechanism she can pull on and off as
needed. A feedback element on the focusser tube isn't going to be easy
to mount. It means her feedback and drive need to come from the focusser
knob.

A very good way of getting high resolution/precision linear feedback is
to use one of the cheap digital vernier heads (Axminster Tools sell
them) They're good to a couple of thou (sorry 50 u) precision and 12.5
micron resolution. I have just designed an interface for them based on
an 8052 microcontroller.There is a processor, dual comparator and serial
chip.

Steve

In ,
Tim Auton tim.auton@uton.[groupSexWithoutTheY] typed:

Crappy resolution. I imagine focussing must be accurate to a tenth of
a millimetre or better.


That part is solved, for me at least. With my latest focuser controller mods
the resolution of my homebrew setup is much better than 0.1 mm at the
focuser tube.

Encoders (absolute or relative) which are
specced to these resolutions are not cheap.
However, seeing £800
focussers mentioned here I was thinking of making one with a servo
motor drive and a decent absolute linear encoder. Decent electronics,
actuators and sensors can be had for the same cost as mechanical
4-speed focussing with a ferris wheel hanging out the side. A bit of
code could deal with temperature compensation and a single knob with
an encoder could serve as a multi-speed focusser (turn it slowly for
slow focus change, fast for fast focus change*). The difficult bits of
making it work strike me as time consuming rather than particularly
hard (changing numbers to make it feel right). Add a few friendly
preset buttons and an RS-232 interface you'd be away.


Let us know when you are done. I would love to see it.

What kind of positional resolution would you need I wonder ?

Good question. Is there "an answer" (like the Rayleigh criterion)? I
expect I could derrive one, but I also expect some better scientists
already have.


I don't know the theoretical solution but I know that my current version
can, in practice, drive through optimum visual focus very slowly at its
slowest motor speed.

Jo

"Jo" wrote in message
...
Here is some info about my DIY Meade LXD55 remote focuser.

I thought others might be interested in how I did it and what mistakes I
made.

http://www.nu-ware.com/Focuser/

Jo



That's cool.

Colin.