I'm presently constructing a dob for an f/4.5 17.5" mirror. I'm debating whether to use a 3.1" secondary or a 3.5" secondary. I've been checking out the design program at Stellafane:
http://stellafane.org/tm/newt-web/newt-web.html
I get this curious result. At least it's the opposite of what I expected:
Leaving all other parameters the same, the Stellafane program tells me that:
1) for a 3.1 secondary, the distance from primary to secondary (the program call it "mirror face to focuser hole") ought to be 65.5"
2) for a 3.5 secondary, the distance from primary to secondary (mirror face to focuser hole) ought to be 65.8"
Isn't a 3.5 secondary supposed to be CLOSER to the primary than a 3.1 secondary?
Perhaps Stellafane's idea of "mirror face to focuser hole" is more literal and includes the distance no only from primary to secondary but also from secondary to the opening of the focuser.
Any ideas on this?
Clears,
Joe
http://stellafane.org/tm/newt-web/newt-web.html
I get this curious result. At least it's the opposite of what I expected:
Leaving all other parameters the same, the Stellafane program tells me that:
1) for a 3.1 secondary, the distance from primary to secondary (the program call it "mirror face to focuser hole") ought to be 65.5"
2) for a 3.5 secondary, the distance from primary to secondary (mirror face to focuser hole) ought to be 65.8"
Isn't a 3.5 secondary supposed to be CLOSER to the primary than a 3.1 secondary?
Perhaps Stellafane's idea of "mirror face to focuser hole" is more literal and includes the distance no only from primary to secondary but also from secondary to the opening of the focuser.
Any ideas on this?
Clears,
Joe
In lumine tuo videbimus lumen.
8O Home-made 10” Dob / Home-made 4” refractor
EPs: Konig 32mm (1.25") / Zhumell WF 30mm (2") / Nagler 13mm T1 / Orion Sirius Plossls 25 & 10mm / Zhumell Plossl 9 mm / Meade MA 9mm