Newtonian secondary size calculation

[Mr Spock]

I've been looking at online calculators for Newtonian secondaries but wondered if anyone here had some experience in this.

I'm looking at sizing up a planetary only scope with an undersized secondary. It will be 254mm dia. f.l 1600mm in a standard size tube. I reckon I only need it to cover 0.4° maximum - a 65° eyepiece at x150.

Any thoughts welcome. I know we have quite a bit of technical expertise on SGL 👍

[vlaiv]

You also need to consider position of focus plane above the tube.

It is best to use low profile focuser in this case.

If you want to cover only 0.4° with full illumination - that is about 23mm fully illuminated field. A bit of trigonometry similarity of triangles should then be used to solve the problem:

Y is equal to tube radius + focus position above the tube and

X is smaller radius of secondary (or X * sqrt(2) = larger radius)

You can use proportion that goes:

(X-11.5) : (254 - 11.5) = Y :   1600

(X-11.5) * 1600 = Y * 242.5

X = 11.5 + Y *242.5 /1600 = 11.5 + Y * 0.1515625

(X-11.5) : (127 - 11.5) = Y :   1600

(X-11.5) * 1600 = Y * 115.5

X = 11.5 + Y *115.5 /1600 = 11.5 + Y * 0.0721875

Voila!

(someone please check my math - I did this very fast without careful analysis - back of the napkin sort of thing)

Edited January 18, 2022 by vlaiv
and of course I made an error ...

[vlaiv]

Sure thing I made an error - need to use half of primary diameter rather than diameter.

[Laurieast]

This help?

https://stellafane.org/tm/dob/ota/design.html

https://stellafane.org/tm/newt-web/newt-web.html

I once designed a telescope with a DOS version of Newt 21 ( A while ago 😉 )

[Astrobits]

I prepared my own calculator many years ago. FWIW here's the file:

You can change any of the yellow cells. I didn't include the fov but that could be added easily.

Nigel

 

flatcalc.xls

[Mr Spock]

There are only standard sizes of secondary available. I used an online calculator to determine a 44mm MA gives 100% illumination over 30' and an obstruction of 17%. The next size down (35mm) covers 8' at 14% obstruction and clearly isn't suitable. So, 44mm it is 

I believe in performance terms an obstruction of only 17% has a negligible effect on image quality.