Newtonian reflector Tube measurement

[justinshort010]

Hi  does anybody know the extra amount of tube past the secondary mirror i need to account for when cutting the Length my focal length is 2.46 

[Astronomist]

I assume you are talking about cutting the tube to length for a homemade newtonian? 

If so the length is not all that important as long as there is enough room for the spider fixings and the end stiffening ring. Within reason more extra length is better as it helps block stray light and increase contrast, but most commercial newtonians have between 1 and 3 inches in front of the secondary.

[Mandy D]

Is that 2.46 feet, so 750 mm? You also need the tube diameter as part of the light path crosses half the tube, then you need to know how muc back focus you require.

[justinshort010]

10 minutes ago, Astronomist said:

I assume you are talking about cutting the tube to length for a homemade newtonian? 

If so the length is not all that important as long as there is enough room for the spider fixings and the end stiffening ring. Within reason more extra length is better as it helps block stray light and increase contrast, but most commercial newtonians have between 1 and 3 inches in front of the secondary.

Man thank you so much 

[Captain Scarlet]

Skywatcher solid tube Newtonians generally stop the front of their tubes just beyond where you can’t see the face of the secondary. Which is, assuming a 45 degree secondary, just over half the tube diameter beyond the middle of the secondary.
Personally, I’d go a few inches longer giving more dew protection and shielding against stray light. That’s exactly how I specified my own 8” newt carbon tube. Where the focuser hole is, as @Mandy D says, a different but related consideration)

Cheers, Magnus

Edited August 3 by Captain Scarlet