[ATM] Lurie-Houghton Newtonian collimation revised

["Chris Dalla Piazza" <dalchri@hotmail.com>]

Now that I have more experience, I've found a way to collimate all three of 
the corrector, primary mirror, and secondary mirror.  You'll need some sort 
of pinhole eyepiece.  I use a 35mm film canister with the bottom cut off and 
a small hole cut in the cap.

1) Center the image of the primary mirror in the secondary mirror, just as 
you would in a regular Newtonian while using the pinhole eyepiece.

2) Continue to use the pinhole eyepiece.  Find the reflection of the 
corrector cell in the primary mirror.  You should be able to see this as 
your primary mirror should be oversized relative to the corrector cell.  Use 
the collimation screws in the primary mirror mount to center the image of 
the corrector cell in the primary mirror.

3) Perform a star test.  Determine if there is any coma present in the star 
image by racking it through focus.  If there is coma, use the collimation 
bolts in your corrector cell to move the star in the direction that the coma 
points.  Note that this is opposite what you would do for a normal 
Newtonian.  The coma points to the center of field in a normal Newtonian and 
you would move opposite the direction the coma points to center the optical 
axis.  Not so with a Lurie-Houghton Newtonian.  Keep going until there is no 
more coma.

4) You have now messed up the secondary mirror collimation performed in step 
#1 since your corrector supports the secondary.  Repeat steps #1 and #3 
until satisfied.  The primary mirror collimation in step #2 should only have 
to be performed once but you can recheck it if you wish.

5) At this point, I had to square my focuser to get the focal plane parallel 
to my CCD chip.  Once I was done I found that the field was properly 
centered on the CCD chip.  However, if you find that the center of field is 
not centered on your CCD or eyepiece, use the secondary collimation bolts to 
center it.  The center of field is indicated by where the astigmatism points 
off axis when out of focus.

I still have not had to mess with the centering of the two corrector lenses 
relative to each other.  It seems that what looks good (within 1/32") is 
good enough.

The reason that I messed with the corrector collimation is that I found I 
had coma that flared to one side of the star image inside focus and then 
flared to the opposite side of the star image on the other side of focus.  I 
surmise that this is what happens when you try to compensate for the 
corrector's collimation with the primary mirror collimation only. The result 
on axis at focus seemed like astigmatism.  Properly collimating the 
corrector eliminated this.

The pictures that I am getting are pinpoint edge to edge.  I still do not 
get nearly the same performance in the eyepiece as I do on my CCD.

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