Re: [ATM] Secondary Cage Calculations

["Arjan te Marvelde" <Arjan.te.Marvelde@hetnet.nl>]

That's indeed my rule of thumb, you can hardly go wrong then.
I was thinking of higher FoV with faster and shorter objectives. I did not 
make the calculation though, maybe the lower tube length/diameter ratio 
compensates somewhat?

   Arjan


> Arjan, Richard:
>
>>>>>>>>>>
> For a given aperture you'd expect the ring ID to grow with decreasing
> focal
> length...
>
>> DW> ring ID = aperture + 1% of focal length
>>
>> Without knowing the focal ratio and the design field of view that
>> statement is meaningless (like a lot of "rules of thumb")
>>>>>>>>>
>
> I wonder, Arjan....
> Assuming the distance mirror-ring is about one focal length (often it
> is a little less), that 1% would buy you a 0.01 radian or about 0.6 deg
> field free of vignetting of the primary. It would be centered in the
> FOV only with the correct offset from the focuser of the secondary (the
> old Obsession wisdom, I believe, was not to offset the diagonal at all,
> meaning the field free of vignetting from the *secondary* would not be
> even nearly centered!).
> Then again, vignetting from the cage is so mild as to be negligible,
> but if you want a "rule of thumb" with some safety factor built-in,
> instead use ID = aperture + focuser diameter.
>
> Nils Olof 

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