[ATM] Collimating a Lurie-Houghton Newtonian

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

Since I haven't found any information about this on the net, I wanted to 
share my first hand experiences with my 6" f/4.5 and see if anyone else 
could improve on it or offer suggestions.

Collimating a Lurie-Houghton Newtonian is a lot different than collimating a 
regular Newtonian in both the effects of miscollimation and the procedure.

There are four major areas to concern yourself with.

1) Miscentering of the two corrector lenses in the corrector cell.  The 
effect (I deduced through ray tracing) is that you get a 1/2 dose of 
chromatism and a 1/2 dose of astigmatism, on axis.  This chromatism is like 
from a prism, such that one side of a star is red, and the other is blue.

2) Tilt of the corrector cell.  The effect (I deduced again through ray 
tracing) is that you get a 1/2 does of prism-like chromatism and a 1/2 dose 
of something similar in appearance to coma.  This "coma" is not radial from 
a center of field like it is in a Newtonian.  This "coma" all points 
uniformly in the same direction across the entire field of view.  The 
direction it points is the direction of misalignment between the corrector 
optical axis and the primary mirror optical axis.

3) Just as in a regular Newtonian, tilt of the primary mirror.  The effect 
is the same sort of "coma" described above except that there is no 
chromatism.

4) Just as in a regular Newtonian, tilt of the secondary mirror.  The effect 
is that the optical axis of the optical system is not centered on your 
imaging system or your eyepiece.

My procedure was as follows:

I did not run into trouble with centering the corrector lenses.  I did this 
by eye, within 1/32", which seems to be sufficient.

I also did not run into trouble with the tilt of the corrector cell.  I 
squared my corrector to the optical tube by measuring the distance from its 
front surface to the end of the tube at the three collimation points 
(assuming that the tube was cut squarely).  Making all three distances equal 
seemed to be sufficient.

The remaing procedure for adjusting the tilt of the primary mirror and 
secondary mirror is not like how I do a Newtonian.  I usually put a film 
canister with a small hole cut in its center to center the image of the 
primary mirror in the secondary.  This is a good start for the 
Lurie-Houghton.  Then I find a star in my FOV that appears to be at the 
center of the coma and center that star.  The Lurie-Houghton is completely 
different this way.

The next step is to adjust the tilt of the primary.  Rack the focus in and 
out, on either side of focus for a star.  Watch carefully to see how the 
light focuses at the last second into the star.  Look to see if it seems 
lopsided, like coma.  The "coma" should converge and diverge on the same 
side of the star.  You may not see a miscentering of the shadow of the 
secondary mirror when the scope is well out of focus (at least if your 
secondary and primary are properly sized).  I would not use this cue.  
Adjust the tilt of the primary mirror in the direction of the "coma", such 
that the star moves in the direction of the "coma".  If it gets worse, 
reverse direction.

This can be done anywhere that in the FOV, not just the center.  The "coma" 
will all point in the same direction.

The final step is to adjust the tilt of the secondary.  Look at the edges of 
your FOV out of focus.  You should see ovals that all point radially to a 
center or, if you are on the opposite side of focus, around a center.  This 
is off axis astigmatism.  Find a star that appears at the center.  Use the 
collimation bolts on the secondary mirror to center that star.

I have never collimated an SCT, but I have heard you do the same thing.  You 
use the secondary to center your FOV, rather than the primary.  I find that 
interesting.

If at the end of this, you still have problems, rinse repeat.  Also, check 
for chromatism such that one side of the star is red and the opposite side 
is blue, like a prism effect.  This may indicate a problem with the 
corrector.

This procedure was actually performed on a CCD so I am not sure how it will 
look in an eyepiece.  I think some eyepieces induce their own off axis 
astigmatism so I don't know how that might confuse the process.  I spent 
most of my time adjusting the tilt of the secondary so I could get the 
optical axis centered right in the middle of that 10mm rectangle of my CCD.

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