Last updated: 15 March 2002

Flip mirror correction attempt

Sent:	Monday, March 11, 2002 13:02:12
From: (Jack Merskin)
I tried the flip mirror correction to help the collimation of the ETX-90
RA.  You're right it's not a straightforward mod.  Number one, I would
caution against anyone attempting this mod without either a drill press
or they have to be darn sure they can drill a hole straight and true
hand held.  The position of the hole is not only close to the threads of
the rear mounting but also it is just about right on the junction
between the plastic and the aluminum.  I wasn't too successful.

Number two, be sure that you have the right size set screw.  I had to
use a #10/32.  The only size I could find in the local hardware stores
that was at least 3/8" long and not too large in diameter.

Well to make a long story a little shorter, I drilled and tapped the
hole, removed the sponge stop, and proceeded to try to get everything in
alignment.  I first used a Cheshire to line up the cross hair and the
center of the secondary, in the center of the primary, etc.  Tried it
out on a star....not good.  In focus image showed the inverted seagull. 
I started to screw in the set screw......seagull started to go away but
not enough set screw.  For a test, I substituted an 1 1/4" long, #10/32
screw.  I screw it in and the seagull go smaller smaller, but I started
to loose the image due to the secondary being too far from 45 degrees. 
The airy disk that was produced was still not correct.  The disks still
were a lot brighter to the bottom of the image then they were on the

So, I concluded that the scope is not collimated.

I have some heavy, firm felt that I cut to fit the tab on the flip
mirror and I'll glued it on the tab.  Currently, with the Cheshire, the
picture looks like it's close to being lined up.  Now I understand that
with a Mak this is not necessarily the case, the picture may look good
but the Mak is not aligned.  By the way, the heavy, hard felt gives a
consistant return position of the flip mirror, at least for now.

So now it's time to go after the screws on the back of the mirror cell. 
I understand that there are three sets of screws on the mounting cell,
one screw is to hold the mirror after the other screw was used in
collimation.  On my ETX, all six screws appear to have locktite on them.

Now I have collimated an Newtonian and a Celestron 5" SCT.  Is the
collimation process of a Mak similar to the SCT?  For the SCT, If you
adjusted one crew, you counter adjusted the other two.

Or do you back off the three screws that hold the mirror (how do you
tell which is which?), then proceed kinda like a Newtonian?  Make a
minor adjustment to ONE screw, star test, adjust, star test, etc.  Or is
it make minor adjust to screw one, counter adjust screws two and three,
star test, etc.?  Or is it something different?
Mike here: You should see the article "Collimating an ETX Mak" on the Telescope Tech Tips page.

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