Re: [ATM] Secondary Offset

[waldo kitty <wkitty42@alltel.net>]

Alexis Cousein wrote:
> waldo kitty wrote:
>> Mitchell R wrote:
>>> Hm Okay. Normally I sort of just eye ball it, there is no way for me to
>>> truly center it under the focuser so I just adjust it till it looks 
>>> centered under the focuser, hopefully that is offsetting it enough.
>>
>> the real question is... /does it work for you?/ if the image thru the 
>> eyepiece looks round and the image doesn't have any deformities or 
>> coma and such, it is probably ok ;)
>>
> Lack of offset never causes aberrations, just a smaller fully 
> illuminated field, or one not centred around the on-axis object.

ahhh... maybe i'm misunderstanding the term offset as being used in this thread...

the way i see it, there are three "angles" to work with as far as the secondary 
is concerned...

1. the distance (up and down the tube) from the primary

2. the distance (left and right) from the eyepiece

3. the angle at which the secondary is mounted.

AIUI, #3 should be 45 degrees...

#'s 1 and 2 would depend on the focal length of the setup as well as the size of 
the secondary and the desired FOV... in all my calculations, i've always assumed 
that the secondary would be mounted dead center of the tube (#2 above) and that 
it was the distance from the primary that was critical in determining the 70% 
FOV and the FI FOV...

i think i said all that properly ;)

FWIW: i generally strive to work out a FI FOV with no loss of light pathway as 
calculated in NEWT... now, i dunno if this is the proper viewpoint on this or 
not but it has always seemed to be that one would rather have a FI FOV vise a 
70% FOV...

have i missed something and gotten totally lost in left field??

[FI == Fully Illuminated]

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