From: "Jack Schmidling" 
To: 
Reply-To: "Jack Schmidling" 


From: "Dwight K. Elvey" 

>  It doesn't sound like you are doing the star alignemnt correctly.
> When you do this, you should not see the secondary's shadow......
> This means you are running the focus too far out.
 >The focus should be just slightly out of focus for testing
collimation.....

That is all I meant to imply but it has to be out of focus enough to see
the diagonal shadow for the rough adjustment.  Once the diagonal shadow is
centered, I move closer to focus as I tweek the primary and keep the star
in the center of the field.  When it is precisely centered, the infocus
image is a point with no flare.

>  As I stated, a properly collimated telescope will show the
> secondary shadow off center for both sides of focus. This is
> normal and expected for a f/4.....

Are you saying this is not normal for a slower scope?

My 16" f4.5 barely shows any difference one either side of focus.

Where did I get the idea that a properly collimated scope must have the
diagonal shadow centered when out of focus?

>  Please describe your collimation sequence. I suspect that there
> may be an issue with some element of your sequence. Use your
> own words and start from the beginning of each step.......

It's easier to just refer you to Suiter's book but I will give it a whirl.

1.  Square focuser front to back ( I believe side to side is corrected by
diagonal rotation)
2.  Diagonal support/spider is offset away from eye by .14" per chart
in Suiter for 4.25 f4
3.  ET2 AutoCollimator inserted in focuser.  (Couldn't resist the plug) 3. 
Diagonal rotated to center primary in diagonal. 4.  Diagonal tilt adjusted
to center primary in that axis. 5.  Primary tilt adjust till all circles
line up and center dot on mirror is in the peephole of the ET2.

ET2 mirror and peep hole are offset to the front of the scope when viewing
through the hole.  All other reflections are concentric.

>  Also, could you give the distances from the center of your
> telescope to the focal plane?

That is one of the variables I have not quite settled on because of stuff
at the camera end.  I have a series of holes so I can move the primary over
a range of about 3 inches.    I would have to look at it to get the current
number but the tube is 5" and the FP is about 3" out from there now.


>The distance would need to be at least 7 inches to create a .5 inch circle.

What is the connection between the two?

js

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