Re: sct optical aberations/was [APML] more new pics

[Aplanatic@aol.com]

Colt,

> I did inadvertently pinch the optics while installing a Losmandy
>  dovetail plate. The screws were just long enough to act like a
>  lock down bolt on the primary and when I saw the focuser behaving
>  oddly, I saw the problem immediately and exchanged the screws. I didn't 
> recall this until you mentioned pinched optics, however I had no chips,
>  nicks or any discernible damage. I haven't taken anything apart.

>  The secondary seems tight, and aside from the description in the first
>  paragraph, the only other past trauma has been when the optical tube
>  baked all afternoon in the back of my car. Nothing seemed to have melted.
>  The optical tube is also subjected to the normal bumps and jolts of
>  driving to and from the site that I use.

Well, this is a complicated business, eh?  After drilling, screwing and 
baking the OTA, did you notice a change in the optical performance?  
If so, you may need to take the puppy apart and see if something is 
amiss inside.  This would include inspecting the primary and secondary 
cells to see if something is pinching the optics.
  
>  The problem isn't just coma I don't think. While I was attempting
>  collimation I got everything so out of whack that I saw just about
>  every example possible of miscollimation. I think.

Having fooled around with my CSCT for a year and a half, I understand 
how confusing it can get.  Here are the important points.  To eliminate 
focal plane tilt, the primary mirror's optical axis must be perpendicular 
to the film plane (assuming for the moment that the secondary is correctly 
aligned to eliminate coma from the on-axis images).  The primaries the 
optical axis is the line that passes through the center of curvature and 
the vertex (center) of the mirror.  If you goofed up the primary alignment 
when you screwed on the dovetail plate, then no matter how you adjust 
the secondary, off-axis images are going to be out of focus because of 
the relative misalignment between film and primary.  As I recall (and I 
have never owned a commercial SCT) there is no external way to adjust 
the primary, so if this is the case you will have to get inside the thing.  
(Someone correct me if I'm wrong).

Suppose the primary is OK.  Adjusting the secondary is the next step.  
I use two methods to adjust my secondary, one using a laser and the 
other using a Hartmann mask.  I suspect that you can't use a laser 
because of the closed tube and lack of visibility back to the focal surface 
due to the baffle.  The method using a Hartmann mask is quite simple 
and gives you a lot of information about the optics, including coma and 
astigmatism, as we shall see.

The Hartmann mask is just four openings in a piece of cardboard or 
thin plywood.  The holes should be about 1" in diameter, 90 degrees 
apart, and the outer edge of each hole should be about 1/4 inch inside 
the clear aperture of the scope.  Make the mask as symmetrical as 
possible and place it over the corrector plate.  A hole in the middle of 
the mask that fits over the little secondary adjustment cover might be 
nice.

To use the mask find a bright star and defocus the image a little until 
you see four distinct images of the star that are almost touching each 
other.  If the four images are in a perfectly square array then the 
secondary is correctly adjusted.  Rotate the mask 45 degrees and 
look again.  If the four images are still in a square array the scope 
is free from astigmatism.  If the four images look like a diamond, 
symmetrical but not square, then you have astigmatism.  If one of 
the images is farther from the center than the other three, you have 
coma.  To adjust out the coma, tweak the secondary so that that 
four images move in the direction of the spot that is farthest from the 
center.  Rotate the mask 45 degrees and do it again.  Keep rotating 
the mask by 45 degrees and tweaking until pattern is as symmetrical 
as possible in each orientation.  Bad seeing will cause the four little 
images to dance around, in which case you will have to mentally average 
their positions.

I have been having fun with the Hartmann mask and a little artificial star 
that I made.  The light source is a battery-operated super-bright LED 
that I ground flat.  I covered the LED with a 10 mil diameter pinhole made
 from aluminum foil.  The LED and battery are in a little box with an On/Off 
switch.  When I want to use the light source, I put some double-sided 
foam tape on the back, walk down the street and stick the box to a brick 
wall that is visible from my back yard, about a 100 yards away.  It's 
important to place the light source far enough away.  For a 10" f/8 optical 
system, the source should be about 100 yards away to make the spherical 
aberration negligible, but it could be closer if you are only interested in 
testing for coma and astigmatism, or trying to collimate your scope.  
Too far away and the light will be too dim to be useful.

I suggest that you test your SCT with the Hartmann mask and see 
what's up.

>  My problem is evident at low and high power, especially just outside of 
> focus. At low power there is just a little something that doesn't look 
right just
>  before focus. Stars come to a point, but things aren't as good as I'd like.
>  This problem will show up on film. At higher power it is very evident.
>  All of my attempts to fix this have been without a star diagonal.
>  
>  My eyes are bad, I need glasses to read, but I don't need glasses to see
>  the astigmatic star test pattern, or whatever it is. It's very obvious.

No, that wasn't the point.  If your eyes are astigmatic then you will see 
astigmatism in the image even when the scope is perfect.  You must 
wear your glasses during the test or all bets are off, especially at low 
powers.

>  I use a Ronchi screen for focus, and the line patterns remain remarkably
>  straight all the way to blink out, but I don't know how to interpret this,
>  or what, if anything, this means.

It means that the scope is well corrected for spherical aberration.  
Doesn't say anything much about coma or astigmatism.  Try using 
one edge of the grating as a knife edge at focus.  You should see 
all kinds of interesting things if you look closely.

>  I'm sure that tube currents play a big role in this, and with this time
>  of year, temperatures are dropping during the evening, and
>  equilibrium is probably never reached. I just don't remember
>  seeing this type of optical problem in the past.

That is a very common statement from people that I talk to.  You may 
never have looked for it, or maybe something has changed.  In either 
case, it's not satisfactory for you right now, so start trying to test the 
instrument as carefully as possible.  You will most likely find the problem.
  
This is a difficult matter.  I hope I've been of some help.

Dave Rowe.

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