Question about the focuser on my Dob

My Dob is an Orion XT-8, my first scope, owned it since January 2003 and am
very pleased with it.

Shortly after I got it, I bought the Tectron collimation tools (sight tube
with crosshairs, "auto-collimator," and Cheshire). I read up on collimation
and spent a couple of hours last February in my below-freezing garage
collimating the scope -- I was quite proud of the results as collimation did
improve the images in the scope.

I recently bought the Orion deluxe laser collimator and have figured out how
to use it.

For the past few weeks, I've been dragging the scope out to pubic Mars
viewings where I and the folks who look through the scope have seen the
polar cap, dark surface features, and the like.

This morning, I decided to check the collimation. The beam from the laser
was not hitting the donut in the center of the mirror and the returned beam
was visible on the reflecting surface of the mirror. I dragged out the
Tectron tools and started from scratch -- the secondary mirror was not
exactly centered in the sight tube so I centered it, checked and re-checked.
I fiddled with the secondary until it was a circle in the sight tube,
centered all the way around. I then installed the laser and adjusted the
secondary setscrews to center the laser dot on the primary mirror after
which I adjusted the primary mirror -- using the laser -- and the scope was
collimated.

I did all this with the focuser bottomed out -- that is, I ran the focuser
all the way in until the part that moves in and out was seated against its
frame.

HOWEVER -- I collimated the scope with the tube tilted at approx 30 degrees.
When I ran the focuser out to where it is when eyepieces are focused, and
elevated the tube to around 60 degrees, the laser beam slipped out of the
donut in the primary. I tried to figure out why this would be happening and
found that there is enough side-to-side slop in the focuser to cause the
laser collimator or an eyepiece to move ever so slightly from side to
side -- it's mighty slight but it's there.

Do I need to worry about this?

As I say, I've been happy with the scope, saw surface features on Mars with
no problems other than those caused by poor seeing; I cleanly split the
Epsilon Lyrae double-double; and stars always dissolve into round concentric
rings -- so -- it may not be a problem. Or, it may be.

What do you folks recommend?

--

----
Joe S.

In article , "Joe S."
writes:

When I ran the focuser out to where it is when eyepieces are focused, and
elevated the tube to around 60 degrees, the laser beam slipped out of the
donut in the primary. I tried to figure out why this would be happening and
found that there is enough side-to-side slop in the focuser to cause the
laser collimator or an eyepiece to move ever so slightly from side to
side -- it's mighty slight but it's there.

Do I need to worry about this?

With a 2" focuser and a 1.25" adapter, I can get very noticeable offsets
between collimating with or without the adapter installed. The 2" lock screws
offset the adapter, then the adapter screws offset the 1.25" collimator barrel.
Worst case is to have the scews on the same side, summing the offset. Maybe
180 degrees apart would minimize this source. Most of my observing is now with
2" capable eyepieces, so I collimate with the 2" section of the laser
collimator. That way, any slight offset induced by the clearance between the
walls of the focuser and the collimator are taken out when I use a 90 degree
offset with a sight tube in the 1.25" adapter. I have the 2" lock screws
running down the long axis of the tube, so I can allow for the up-down tube
variance with main secondary support shaft. Then I take out the adapter and
sight tube and do the laser collimating with the 2" section of the collimator.

I noticed a slight wobble in the laser impact point on the primary as I racked
the focuser from full in to full out, doubled in the return to the collimator
face, so now I set the focuser around where my main two eyepieces will be
focused and collimate from there. Just by taking out the adapter and fixing
the focus point before collimating has made a very noticeable difference in the
performance of my 10" Starfinder dob, 14" Starhopper (now departed to a
friend), and the new-to-me 18" Tectron truss dob. So, I'd say start from where
the focuser will get the most use, collimate at 2" diameter if you have a
choice, and if you need to use the 1.25" adapter have the 2" screws on the
opposite side of the barrel from the 1.25" screws. Collimate at 1.25" if
that's what you'll be using to observe.

Jim
"A Bad Night With A Telescope
Beats a Good Night Doing Anything Else"

Yes, if you have a 2", then pull the 1.25 adapter, and use the 2 inch
eyepiece hole, which usually is a tighter fit than the latter.
But if you must use the 1.25 inch hole, then set focuser where you
normally use your 1.25 ep's, and set the eyepiece set screw on the
collimator just as you would an eyepiece...and then complete the
collimation procedure. This should work, as it is the system I use.
Then follow up with a Cheshire collimator check, as IMHO, gives more
accurate collimation on the primary mirror. (I use laser for the
secondary alignment, and Cheshire on the primary)....then a quick trip
to defocused Polaris at high power for a final collimation check.
TW

Joe S. wrote:

My Dob is an Orion XT-8, my first scope, owned it since January 2003 and am
very pleased with it.

Shortly after I got it, I bought the Tectron collimation tools (sight tube
with crosshairs, "auto-collimator," and Cheshire). I read up on collimation
and spent a couple of hours last February in my below-freezing garage
collimating the scope -- I was quite proud of the results as collimation did
improve the images in the scope.

I recently bought the Orion deluxe laser collimator and have figured out how
to use it.

For the past few weeks, I've been dragging the scope out to pubic Mars
viewings where I and the folks who look through the scope have seen the
polar cap, dark surface features, and the like.

This morning, I decided to check the collimation. The beam from the laser
was not hitting the donut in the center of the mirror and the returned beam
was visible on the reflecting surface of the mirror. I dragged out the
Tectron tools and started from scratch -- the secondary mirror was not
exactly centered in the sight tube so I centered it, checked and re-checked.
I fiddled with the secondary until it was a circle in the sight tube,
centered all the way around. I then installed the laser and adjusted the
secondary setscrews to center the laser dot on the primary mirror after
which I adjusted the primary mirror -- using the laser -- and the scope was
collimated.

I did all this with the focuser bottomed out -- that is, I ran the focuser
all the way in until the part that moves in and out was seated against its
frame.

HOWEVER -- I collimated the scope with the tube tilted at approx 30 degrees.
When I ran the focuser out to where it is when eyepieces are focused, and
elevated the tube to around 60 degrees, the laser beam slipped out of the
donut in the primary. I tried to figure out why this would be happening and
found that there is enough side-to-side slop in the focuser to cause the
laser collimator or an eyepiece to move ever so slightly from side to
side -- it's mighty slight but it's there.

Do I need to worry about this?

As I say, I've been happy with the scope, saw surface features on Mars with
no problems other than those caused by poor seeing; I cleanly split the
Epsilon Lyrae double-double; and stars always dissolve into round concentric
rings -- so -- it may not be a problem. Or, it may be.

What do you folks recommend?