Re: ATM cass coring

[Anthony Stillman <atmer@flash.net>]

>...any reason for coring a Cass primary larger than 1.375"
>...10" primary, 3" secondary, eyepiece ID of 1.25"
>Glen WB4ZNW


Actually, yes.  I'll make some assumptions about your system.

f/15 efl
f/5 primary
m=3
0.75" of unvignetted field
10" bfl


2.25" of the secondary obstruct the on-axis f/5 primary light cone
Secondary to prime focus is 5*2.25 = 11.25"
m=3 so the secondary to the effective focus is 3*11.25 = 33.75"
bfl = 10" so the seperation between the primary and secondary is 23.75"

For an f/15 light cone, 10" from the focus the un-viggetted diameter is
(10/15)".  To this we add 0.75" of useful field and we come up with 0.67" +
0.75"   1.42"

Of course at this point you need to include twice the wall thickness of the
tube you will use to help stop stray light.  Say 0.125" total.  But he tube
will extend 1/3 to 1/2 the way to the secondary, again to help with stray
light.  So now instead of 10/15 we have 22/15  + 0.75 or about 2.23" +
0.125" or 2.355"

OK, now we have to consider the effect of air currents around the inner
walls of the tube.  Most ATMs settle for about 1/2" of clearence all the
way around.  It not really enough, but....you know, trade offs.  So this
takes us out to about 3.4" of hole through the primary.

As it happens, this is about the size of the beam block you will need to
put on the back of your 3" secondary to stop direct light from flooding
your eyepiece.

Hope this helps.

Anthony

Sorry, about grammer, spelling, etc.  I'm home sick with a fever.



"Computer Science is no more about computers than astronomy is about
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