From: "Dwight K. Elvey" 
To: atm{at}shore.net
Reply-To: "Dwight K. Elvey" 


>From: "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.

Hi Jack
 This is why I wanted you to describe your method. The peep hole
should not be offset relative to anything other than the secondary's
reflection( which it should be offset ). It still sounds like you are not
aligning correctly.
1. Center the secondary as seen through the focuser( not the reflection ).
2. Tilt/rotate the secondary such that you can see the
 edges of the primary, evenly in the field of view.
 ( repeat to step 1 & 2 if needed, they do interact ).
3. Do a preliminary adjustment of the primary such that
 the center spot of the primary matches the hole in a
 peep site.
4. On a star ( Polaris is good with no drive ), using a medium power
eyepiece, find where in the field of view the diffraction rings are best
concentric ( this is just slightly out of focus. If you can see the
secondary shadow you have gone way too far ). Once you've found this
location, without moving the telescope, move the image of the star to the
center of the field of view. Repeat step 4 with a high power eyepiece until
satisfied.

 When done, ignore reflections. If the first two steps
were exactly correct, everything except the returned reflection of the
secondary should be concentric. Even if things are not exactly concentric,
the telescope is still close to optimum. The only issue being concentric is
a slight change in the centering of the 100% illuminated area. If it is way
off, look to see how the first two steps can be improved. On fast
telescopes, having the secondary's image, as seen from the focuser,
reflected in the primary, be concentric is an indication of incorrect
alignment!

>
>>  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?

 The size of the secondary, the f/ratio and the distance between
the secondary and the focal plane determine the size of the 100% illuminated area.
 If we calculate to include the longer cone by the calculations
as follows:

( 18" - 5.5" ) / ( 4.5" - 1.8" ) = ratio of obstruction
cone = 4.63
    Note: 18" = fl = 4.5 * 4

The vertex would then be behind the secondary by

 4.63 * 1.8" = 8.33"

Extending this cone forward to the focal plane

 ( 8.33" - 5.5" ) / 4.63 = 0.61"

 In this calculation, I didn't include the slight decrease because
of the secondary offset. It is so small as to not be worth the extra
calculations. If you wanted to include it, the value of 1.8" would be
slightly smaller by the line across the ellipse with the 0.14" offset.
I also didn't include the slight decrease because of the fact that I used
the edges of the triangle and not the height ( changes the value by less
than .01 inch ).
 This number does not match my earlier statement of 7" for 0.5"
I'm not sure how I came up with that number since I didn't show my work as
I did here. It surely was in error. A 0.5" circle would allow the
focal plane to be about 7.8" from the center of the tube, not 7".
Dwight

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