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


Hi Scott
 You are correct!
 I should have mentioned earlier but forgot. One
should never make marks on their secondary. Some think that there is a
place that is hidden behind the secondary shadow. This is only true for the
light that goes directly to the center of the field of view. as you move
towards the edges of a reasonable sized field of view, any center spot
pokes out.
 Don't mark your secondary!
Just my opinion
Dwight

>From: "Scott Ewart" 
>
>|
>| Its best to put a small 1/2" diameter paper hole reinforcement ring on the
>| center of the diagonal and another on the center of the mirror before
>doing
>| the laser pointer alignment trick.
>|
>    I thought this was a good idea too.  But I was checking collimation once
>with a high-power eyepiece when I saw something odd just out of focus.  I
>went far out of focus to see what I was looking at.  All I had was a very
>small ring of paint marking the off-set center of my secondary.  But even
>slightly away from the center of the field (remember, high power, small
>field stop), I could see this dot.  And it looked big too, since it was
>actually a significantly large fraction of the size of the secondary.  The
>paint's gone now.
>    Sure, it makes it easier to do a rough alignment, but you do use ALL the
>secondary when you include off-axis objects.  It's not like the primary at
>all with its secondary shadow.  Consider what's happening. Let's say light
>comes in the tube at a one degree angle and the shadow of the secondary is
>say half an inch off center.  The whole light cone now hits the secondary
>one inch off center.
>    You WILL get added diffraction with a mark on the secondary.  If you
>really want to use a mark, make it absolutely as small as you can.
>
>Scott Ewart
>
>
>

--- BBBS/NT v4.00 MP