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question about dob construction // secondary mirro

Started by BABOafrica, 10/09/2012 01:29AM
Posted 10/09/2012 01:29AM Opening Post
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:

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?


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
Posted 10/09/2012 06:09PM #1
I think you just made a oversight somewhere. The distance from mirror surface to focuser does not depend on the diagonal size, at least for this program. It will depend on stuff like the focuser height, extra focus distance, tube thickness and tube diameter.

As you change the diagonal minor axis from 3.1 to 3.5, you should notice this value "0.xx" in inches right below the "Calc" button changing:
"100% Illuminated Diameter: 0.xx inch"
It should be about 0.4 inches larger for the 3.5 inch diagonal (assuming both values are larger than zero).