Length of a newtonian.

[Robindonne]

Goodmorning.   Whats the reason that some fast newtonians have an overlength/useless length of the tube.  The sec. mirror and focuser are approx 15 cm deeper inside the tube, i assume.  Is there a good reason for this design? Balance?

[DaveL59]

I expect it acts as a dew and light shield

[Robindonne]

18 minutes ago, DaveL59 said:

I expect it acts as a dew and light shield

If thats the reason then they lost a serious option in cutting weight ( metal, paint etc)

[John]

21 minutes ago, DaveL59 said:

I expect it acts as a dew and light shield

Yep. And that's a good feature IMHO. I wish more newtonians adopted this approach.

 

[DaveL59]

4 minutes ago, Robindonne said:

If thats the reason then they lost a serious option in cutting weight ( metal, paint etc)

and after purchase you spend extra buying a shroud or making one, which is better...?

[Robindonne]

5 minutes ago, DaveL59 said:

and after purchase you spend extra buying a shroud or making one, which is better...?

Yes thats of course true.  Maybe it didn’t occur to me because i havent seen them in use on a newtonian

[GiorgioF]

Factory has to manage a single code for raw material. With same tube, make the focuser and spider holes in different position will also save time in assembly line....

[Louis D]

20 hours ago, GiorgioF said:

Factory has to manage a single code for raw material. With same tube, make the focuser and spider holes in different position will also save time in assembly line....

That's probably the real reason for most commercial Newts.  They aren't all that way.  Here's the Takahashi Epsilon 180 ED f/2.8 Newtonian Astrograph Reflector as a counter example having its focuser right at the end of the tube: