Secondary mirror size

[nitram100]

Hi guys, does anyone know what difference it makes when it comes to secondary mirror size in a 10" newt? I read on the skywatcher site that the 10" newt has a 58mm secondary, whereas my 10" OO europa has a 63mm. What difference does this make?

Cheers!

[Kaptain Klevtsov]

The secondary has a lot to do with the f/ ratio of the 'scope. The light cone of a fast Newt. is steeper than a slow one, so following the light cone back from the pointy end where you put your eye or camera, the diameter is less in a slow Newt. by the time the cone has reached the centre of the tube. The main 'scope tube is longer in the slower Newt. to accomodate the more pointy cone. I know it works the other way round, but its easier to descibe this way.

Look at the design page of these guys, there's a downloadable spreadsheet to do the sums for you.

http://www.oldham-optical.co.uk/

Kaptain Klevtsov

[nitram100]

Great explanation mate, cheers

[SteelRat]

Am a little curious as to why the secondary on the Skywatcher f5 150 newt is 47mm where the secondary on the Celestron f5 150mm newt is 44.45mm Are their mirrors not the same distance apart?

But then Skywatcher don't even mount the secondary central to it's stalk on theirs (second pic);

Link to Skywatcher f5 150mm Newtonian

[Paladin]

For anyone wanting to design a Newtonian reflector, or simply check the design of a proprietary one, should check out Dale Keller's Newt freeware, link below. Its always highly recommended by the Amateur Telescope Maker fraternity, and will answer your query.

Paul

http://www.dalekeller.net/ATM/newtonians/newtsoft/newtsoft.htm

[The Warthog]

Am a little curious as to why the secondary on the Skywatcher f5 150 newt is 47mm where the secondary on the Celestron f5 150mm newt is 44.45mm Are their mirrors not the same distance apart?

But then Skywatcher don't even mount the secondary central to it's stalk on theirs (second pic);

Link to Skywatcher f5 150mm Newtonian

That secondary isn't as displaced as it appears to be. The appearance is a matter of the angle of the shot. All secondaries in Newts should be displaced a little towards the focuser, and there are formulae for calculating how much they should be displaced.

It is instructive to do a scale drawing of your mirror and its cone of light, then intersect it with a 45 degree line. You will quickly see why the secondary has to be a certain size in the major axis, and why it has to be displaced towards the focuser to capture all the light cone.

The difference in the size of the secondaries of the Skywatcher and the Celestron may be due to the distance of the secondary from the primary, or other reasons the clever chaps at the factories know about.

[themos]

Visualise the light cone in a Newtonian

(click to enlarge)

Perspective makes the secondary look almost as big as the primary, this is a Skyliner 300P 12" f/5.

[SteelRat]

That secondary looks quite central.

[Astroman]

The difference may be the tube diameter. A larger tube will necessitate mounting the secondary closer to the primary, and make the secondary larger. Don't know if that's the case here, but it's true.

[SteelRat]

The other night I read about something called secondary offset. Apparently the secondary mirror gets set a certain ammount off centre away from the focuser and by the same ammount toward the primary. If I read correctly the ammount of offset in newts of 200mm and up might not look very much but the skywatcher f5 150mm newt with it's 47mm secondary would probably have an offset of over 3mm. That would mean that the secondary would be mounted on it's stalk somewhere around 7mm nearer one edge than the other which explains why it looks like this? :shock: ;

[themos]

I guess we need to think of the secondary surface as the intersection of a plane and a cone, not a plane and cylinder.

http://demonstrations.wolfram.com/ConicSectionsTheDoubleCone/

[SteelRat]

We want a refractor :oops: