Re: [ATM] Help in moving my secondary

[Mike Lockwood <melockwo@uiuc.edu>]

RP,

RP Sutton wrote:
> I would like to upgrade my focuser, but to do that i will need to move my 
> secondary 1" closer to the primary. MY scope is a 10" f/7 and has a 1.53 " 
> secondary. I measured the distance from the center dot of the primary to the 
> center dot of the secondary and it is approx 61" +/- 1/4". My question is this - 
> will I be messing up anything in the light path by moving the secondary 1" 
> closer? will the light cone overflow the secondary if it is 1" closer to the 
> primary? I do not know much about these things and am using terminology I have 
> gleaned from reading this list so I hope and am stating things correctly.

For your telescope, for every inch you move towards the primary, the 
light cone expands approximately 0.14".
  ( = 5"/70"*2 = mirror radius / focal length * 2)

The cone of light that illuminates the central spot of the field at 
61" away from the primary is 1.29" in diameter.  ((-5"/70"*61"+5")*2) 
  From this we can observe that the fully illuminated field is 
probably only about 0.2" in diameter before the secondary is moved. 
(Note: Since the secondary is close to the focal plane, we can just 
think of expanding the cone by 0.2" and then we'd hit the edge of the 
secondary.  It is an approximate amount, not exact.)

If you move the secondary 1" closer, the cone of light needed to 
illuminate just the center spot of the field is then 1.43" 
(1.29"+0.14") in diameter, and that is only 0.1" smaller than the 
secondary.  Therefore, the center of the field is fully illuminated, 
but only a ~0.1"-diameter field is fully illuminated now, IF 
everything is centered exactly.

The point here is the secondary will work fine for higher 
magnifications (smaller fields), but there may be some observable 
vignetting if you're using wide field eyepieces for low-power 
observing.  This probably would have been observable WITHOUT moving 
the secondary.  So, if you didn't notice it or don't care, it probably 
won't matter if you move the secondary an inch closer to the primary. 
  But if you move it more than that you will effectively start 
stopping down the scope!

FYI, my 10" F/8.8 scope has a 1.5" secondary.  I think I set it up for 
a 0.4"-0.5" fully illuminated field.

Mike Lockwood
http://bi-staff.beckman.uiuc.edu/~melockwo/index.html

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