ETX TECHNICAL TIPS Last updated: 11 April 2004

Making a cross hair eyepiece

Sent:	Saturday, April 10, 2004 06:33:53
From:	"MMIC EOD" (mmic.eod@libertysurf.fr)
A cross hair eyepiece is, as you say, very useful for motor training and
star alignment.

Rather than buy one it is simple to adapt an existing eyepiece.

Unscrew the lens carrier from the eyepiece tube and find the focal plane
by focussing on your thumbnail through the lens.

If you can figure out a way of making a carrier to sit at this point
inside the tube then do it!

I used a plastic bottle top that was a push fit in the eyepiece tube.
Drill a hole in the middle to about half the diameter.

If you don't have a lathe most plastic mouldings have a pip of some sort
in the middle to give you centre.

Don't bother creeping round the rose bushes at dawn to harvest spider
silk, just beg, borrow or steal a single 'thread' from a fibreglass mat
used in car body repair.

Working in a good light against a dark surface select a couple of single
fibres from the thread.

Put four spots of neoprene glue ('contact adhesive') on the face of your
carrier, take your fibres and, while gently stretching each one, settle
them into the glue. Don't use superglue  it doesn't go off.

Cut off the excess fibre close to the carrier. The 'open time' of the
glue will let you push the fibres about a bit to get them central.

Once the glue has set put the carrier in place at the lens focal plane
and there you are.

To check how good a job you did focus the telescope on something with
vertical and horizontal elements like a telegraph pole, set the cross
hairs on the intersection then rotate the eyepiece through 180.

For motor training it doesn't matter if the cross isn't exactly in the
middle of the field of view.

For star alignment there may be some clever way of compensating for off
centre but, as far as I can see unless you are wildly out it doesn't
matter.

No point in photos since the diameter of the fibres is less than my
digital camera can resolve

Best wishes,

Steve Sales,

Return to the top of this page.

Go back to the Tech Tips page.

Go back to the ETX Home Page.