Re: [ATM] Lurie-Houghton (was) : Schmidt-Cassegrain

["vladimir sacek" <vlad@copper.net>]

Considering its off-axis image quality, it should be
very forgiving in regard to diagonal misalignment.
According to the raytrace, it is also quite insensitive to both, deviations 
in the corrector-to-primary separation, and
separation between the two corrector elements. Primary or
corrector tilt or decenter, similarly to that in a Schmidt system,
induces center-field coma. A 200mm f/4 (form Rutten&Venrooij) with slightly 
more than
1/10 degree (0.002 sin, or 0.2mm edge deviation) primary tilt develops ~20 
micron comatic blur (~1/1.7 wave of s.a. equivalent), which increases in 
proportion to the tilt. This means that for this particular system tilt has 
to be less than 0.1mm at the edge for
"diffraction-limited" level. This is still more comfortable
tolerance than that of an f/4 Newtonian, approx. by a factor of 2
(at the level of miscollimation sensitivity of an f/5).

1mm primary decenter also induces center coma of about
7 micron (still better than "diffraction-limited), which also increases in 
proportion to the decenter. The astigmatism changes little in both 
instances, and it is coma that limits image quality, both on and off-axis.

Tilt and decenter errors can induce opposite coma, and effectively cancel 
each other (one of those things easier said than done).

An f/6 Lurie_Houghton should be less sensitive to
primary tilt than an f/6 Newtonian, and much less
sensitive to the diagonal tilt.

Vlad


> Does anybody have any thoughts on how sensitive to
> collimation problems a Lurie Houghton might be, say,
> at f/6?
> Guy B
> --- vlad@copper.net wrote:
>
>> Jean-Guy,
>>
>> You are right, the three mirror systems,
>> especially with this fast primary, are sensitive
>> to anything one can think of. ............
>
> =====
> Guy  Brandenburg
> Washington, DC
> My home page:
> http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 


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