Optical Making a Dobsonian Telescope DIY

[Shreyas Shree]

I am going to make a Dobsonian telescope. Primarily a 6" f/8. I have no idea what the secondary mirror size would be an its distance from the centre of the primary. I have been told that if the secondary is too small then the effective aperture would be reduced. And i do not understancd these terms: central obstruction and secondary offset.
Thank you in advance

 

[jedishrfu]

Welcome to PF!

Here's a primer to get started:

https://stellafane.org/tm/dob/

 

[Chronos]

For a 6" f8 primary, the secondary needs a minor axis of about 1-1/4" for a fully illuminated [captures the entire light cone of the primary] field of view. Frankly, I would just buy the secondary. Figuring optical flats is more tedious than figuring a parabolic primary and the flat should be corrected to double that of the primary - i.e., a 1/8 wave primary needs a 1/16 wave secondary. I mentioned 1/8 wave because that is about as good as can be achieved on most home brew primary mirrors. You can get a secondary mirror like this for about $50 from Antares.

 

[Patzee]

We had a chance to make a 6" telescope in class run by Dobson himself. Lot of work, but so worth it. Not sure we saved any money, but really learned a lot.

 

[davenn]

...but really learned a lot.

Well done ! ... And that is the sole reason for building your own scope Dave