EQ Newtonian Secondary Offset formula

[Astrofail]

Hi,

 

FPI-Protostar claims the following formula should be used for deriving a correct secondary offset for an equatorialy mounted newtonian:

(Primary diameter * Secondary diameter) / (4 * Focal lenght)

However, when examining this in a cad program, it looks more like it s the thickness of the secondary mirror itself that is to be used as the offset value to obtain alignment of the optical and mechanical axis.

 

The instrument in question is photographic only.

[wimvb]

This may help.

The thickness of the secondary shouldn't be part of it. Offset is always calculated relative to the reflecting surface of the secondary.

Btw

Diameter_primary / Focal_length = 1/F_number

edit:

the exact formula for offset is in the spreadsheet. Just enter the appropriate numbers into the yellow cells. There are approximations in earlier posts of the same thread.

Edited August 18, 2022 by wimvb