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


>From: "Jack Schmidling" 
>
>From: "Dwight K. Elvey" 
>
>>  I assume you are talking about a Newton. If so, the shadow
>> should be off center for both inside and outside of focus.
>> This is called offset. An F/4 will have noticeable offset.
>>  Can you be more specific? What type of telescope are you talking
>> about......
>
>This is a 4.25" f4 Newt.  It has a 1.8" diagonal and I am
trying to fully
>illuminate the chip on my MX716 camera which is about .5".
>
>I have positioned all the components as they should be including a .14"
>offset to compensate for the foreshortening effect.  Looking through the
>peep hole it looks exactly like the drawing in Suiter's book.
>
>Problem is that when I tweek the collimation on a star centered in the
>field, it is not collimated when I go to the other side of focus.  I ended
>up sort of offsetting the view by the same amount on each side but this is
>rather disconcerting.
>
>A short image of the a star field slightly out of focus shows the central
>stars with the diagonal in the center and all the rest sort of point to the
>center.  This is as it should be but I do understand why a central star does
>not look collimated on each side of focus visually.
>
>js
>

Hi Jack
 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 or in focus for an eyepiece that is pushing
the 50X limit.
 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.
 Assuming that you have properly collimated from a star, this
could indicate that you have not matched your secondaries angle to the
focuser or that the focuser is not tracking in a straight line.
 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. F/4's can be much harder to get
right since they are so fussy.
 Also, could you give the distances from the center of your
telescope to the focal plane? It sounds like you have enough but just to
check. The distance would need to be at least 7 inches to create a .5 inch
circle. Somewhat less if you were concerned about the corners of the CCD.
Dwight

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