Re: [APML] 5.5" Schmidt Woes

[Aplanatic@aol.com]

Hi John:

Here's the alignment method I used on my 9" f/2.5 Schmidt camera.  After 
taking a couple of test shots, I subsequently sold the optics for this camera 
to a friend (it's a long story).  In the next year, I plan on making a 
duplicate set of optics and taking some photos.  Hope this helps.

1) Align the optical axis of the primary to the center of the corrector.

I diamond scribed an "X" at the center of the corrector to facilitate this 
step, but you could use a removable mark from a fine tipped "Sharpie" if you 
prefer.  Since the corrector plate is located very close to the radius of 
curvature of the spherical primary, the illuminated mark at the center of the 
corrector plate is imaged by the mirror and focuses very close to the mark.  
This aerial image is easy to see if you look from the front of the OTA down 
into the primary and move your head back and forth.  When the primary is 
correctly collimated, this mark will overlap the image of itself in the 
mirror.  

2) Align the center of the film holder with the optical axis.  

I first removed the film holder from the spider.  Looking into the OTA one 
sees the center mark on the corrector, the bolt hole in the center of the 
spider and then the center dot on the primary.  Move the spider until these 
three line up.  The film holder does not have to be perfectly centered, but 
do as well as you can.  ( A slightly offset film holder will only cause a bit 
of asymmetrical vignetting, and not coma, as long as the following step has 
been done correctly.)

3) Adjust the tip and tilt of the film holder.

I found this to be very time consuming and I tried several methods that 
didn't work before I rediscovered this old technique.  I have a 10" f/6 OTA 
that I made a few years ago.  Pointing this 10" Newtonian into the Schmidt 
camera, I first focused the Newt on the center of the film holder (on-axis) 
at high power (9 mm Nagler + 2X barlow).  I then rotated the Schmidt so that 
one edge of the film holder was imaged in the Newt.  I refocused the Newt and 
recorded the change in focal position between the center and this edge.  (I 
mounted a dial indicator on the Newtonian focuser in order to do this 
accurately.)  I then rotated the Schmidt so as to image the other side of the 
film holder, and refocused.  By comparing the difference in the three focal 
locations[1], I could calculate the exact amount that the film holder needed 
to be tipped in this direction.  I repeated this procedure for the orthogonal 
axis.  After making the necessary adjustments to the tip and tilt of the film 
holder, I iterated the procedure a couple of times until the focal positions 
were the same for all four off-axis locations.

4) Focus.

I took the Newtonian outside, propped it on a table, focused on Polaris at 
high power and locked the focuser.  Back in the garage, I pointed the 
Newtonian into the Schmidt (on-axis), as above.  With a piece of processed 
film in the holder, I adjusted the longitudinal position of the holder until 
the image of the film was focused as well as possible in the Newt.  
Surprisingly, this got me to within about a mil of perfect focus, as verified 
by several test images made with the camera.

Best of luck,
Dave Rowe

[1]  Let the focal length of the Schmidt be F1 and the focal length of the 
test telescope (Newtonian in the above) be F2.  If the focal position moves 
by dF in the Newtonian, then it moves by dF*(F1/F2)^2 in the Schmidt.

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