Stop me reinventing the wheel...

el_draco (Rom) · #1 02-10-2016, 08:43 PM

Does anyone have an equation for calculating the off-set of a secondary mirror in a fast newt. I can see it clearly on my CAD drawing but would like to verify with the math. I am a bit busy, (that's code for lazy

) at the moment to workout the calculation...

billdan (Bill) · #2 02-10-2016, 11:42 PM

Hi Rom,

This is the formula
Sec offset = Sec size / (4 * Focal ratio)

This is how much the secondary is offset toward the primary mirror and also how much it should be offset away from the focuser for fully offset collimation.

Anyway if the secondary is centred in the sight tube and you can see the edge of the primary the offset takes care of itself.

Cheers
Bill

el_draco (Rom) · #3 03-10-2016, 12:51 PM

Bewdy. Thanks Bill.

dave brock · #4 03-10-2016, 02:36 PM

Quote:

Originally Posted by billdan

Anyway if the secondary is centred in the sight tube and you can see the edge of the primary the offset takes care of itself.

Cheers
Bill

At least the offset towards the primary is taken care of. The offset away from the focuser must be done by measurement.

Dave

el_draco (Rom) · #5 21-10-2016, 06:04 PM

Eh, ended up doing this as an activity to satisfy myself. I've created a spreadsheet which will calculate separations and off-sets for a Newtonian. Simply whack in Primary and secondary diameters and an F ratio, and Bobs your cross dressin aunty... so to speak. I'm pretty sure it will work for any Newtonian configuration. I put in specs of the scope I am building and it correlates with my CAD design. (No warranty given or implied, primarily due to the empty shot glass on the desk next to me...

)

Perdy self explanatory...

Merlin66 (Ken) · #6 21-10-2016, 06:45 PM

Just a comment.....
Secondary mirrors are not exactly elliptical mirrors as required by the theory and formulae.
They are usually cut cylindrically from blanks.
The difference causes some additional light loss in the system.

glend (Glen) · #7 21-10-2016, 07:22 PM

Does not the available newt design app on the Net do this stuff already? When i built my 10" I used it to check my offset calculations.

el_draco (Rom) · #8 21-10-2016, 07:23 PM

Quote:

Originally Posted by glend

Does not the available newt design app on the Net do this stuff already? When i built my 10" I used it to check my offset calculations.

Never seen it? Where?

dave brock · #9 23-10-2016, 11:43 AM

Quote:

Originally Posted by Merlin66

Just a comment.....
Secondary mirrors are not exactly elliptical mirrors as required by the theory and formulae.
They are usually cut cylindrically from blanks.
The difference causes some additional light loss in the system.

Sorry but there is no difference. A cylinder cut at 45° (or any angle other than 90°) gives an ellipse just like a cone does. That's why when looking through the focuser the secondary appears circular.

Dave

Merlin66 (Ken) · #10 23-10-2016, 11:57 AM

Dave,
Sorry... an ellipse by definition is a section from a cone...not a section through a cylinder.
If you do the drawings of similar major axis sized outlines of an ellipse and superimpose the cylinder section you'll see that the outline shapes are different. Based on a 100mm diagonal I used in a 12" f5 design the total loss of "efficiency" was about 10% when a cylindrical section was used.

dave brock · #11 23-10-2016, 12:14 PM

A cylinder is basically a cone that is infinitely tall.
Check here
http://mathforum.org/sanders/geometry/GP18Ellipse.html

Dave

Merlin66 (Ken) · #12 23-10-2016, 12:36 PM

Hmmm
Just try to match a conical ellipse for an f5 cone with a cylindrical section.....

multiweb (Marc) · #13 23-10-2016, 04:22 PM

Click here to load the video

glend (Glen) · #14 23-10-2016, 04:27 PM

Quote:

Originally Posted by el_draco

Never seen it? Where?

Newt for the Web at Stellafane is one. There are others. A simple Google search is all that is required to find them.

Merlin66 (Ken) · #15 23-10-2016, 04:33 PM

Marc,
Neat video....
But it doesn't show the comparison between the conical (say f5 beam) section surface and that of a cylindrical section (based on the diameter being the same as the minor axis of the conical section.)
Believe me, the surfaces are NOT the same.....

I'll try to find the "drafting 101" tutorial on cutting sheet metal sections - like cutting and joining a conical funnel compared with cutting and joining a cylindrical tube.
(The major axis length of the conical section isn't the same length as a 45 degree cut through a cylinder with the same diameter as the conical minor axis)

multiweb (Marc) · #16 23-10-2016, 04:38 PM

Quote:

Originally Posted by Merlin66

Marc,
Neat video....
But it doesn't show the comparison between the conical (say f5 beam) section surface and that of a cylindrical section (based on the diameter being the same as the minor axis of the conical section.)
Believe me, the surfaces are NOT the same.....

I'll try to find the "drafting 101" tutorial on cutting sheet metal sections - like cutting and joining a conical funnel compared with cutting and joining a cylindrical tube.
(The major axis length of the conical section isn't the same length as a 45 degree cut through a cylinder with the same diameter as the conical minor axis)

Hi Ken, I first drew the cone at f/5, did a plane roughly at 45 degrees then offset and scaled a cylindre manually to match the edge of the slice. It's very very close close. Close enough not to impact the light path dramatically IMHO.

Merlin66 (Ken) · #17 23-10-2016, 04:56 PM

Marc,
Understood...
What about doing the same with a cylinder diameter matching the minor axis (as normally the case of secondaries supplied to the amateur)?
You shouldn't have to ""offset and scaled a cylinder manually"" The minor axis defines the cylinder
Can your program over lay the surfaces??
I found a 10% difference in effective area - some light loss due to undersize at some points and some light loss due to oversize at other points.
I did this work back in 1976 when designing a 12" f5 - I just wish I could find my notes/ drawings of the time.

multiweb (Marc) · #18 23-10-2016, 05:10 PM

Quote:

Originally Posted by Merlin66

Marc,
Understood...
What about doing the same with a cylinder diameter matching the minor axis (as normally the case of secondaries supplied to the amateur)?
You shouldn't have to ""offset and scaled a cylinder manually"" The minor axis defines the cylinder
Can your program over lay the surfaces??
I found a 10% difference in effective area - some light loss due to undersize at some points and some light loss due to oversize at other points.
I did this work back in 1976 when designing a 12" f5 - I just wish I could find my notes/ drawings of the time.

I see. I'll try what you said, match the diameter of the cylinder to the minor axis of the cone slice.

PS: Here's the new volumes sliced as per your suggestion. So cylinder diameter is the same as the minor axis of the elliptical slice on the F/5 cone. The cylinder axis matches the cone axis.

Top view show two circles as expected.
Side view shows the cone/cylinder slice offset. The volumes are coaxial.
Front view shows the same offset.
Last view is an orthographic projection normal to the slices so you can see the true difference of the surfaces.

There's not much difference. That's why I thought if you offset the cylinder then you can get a better coverage.

Wavytone · #19 23-10-2016, 08:43 PM

Ken, I'm sorry to say this but Dave is correct, this is high school maths.

Merlin66 (Ken) · #20 23-10-2016, 09:06 PM

Hmm
I still stand by the analysis I did between the f5 conic surface and a cylinder section.....