Using a smaller Secondary

[neiil phillips]

Hi guys. I want to try a Antares 1/30th wave secondary with my 245 mm 1/10th pv wave primary. F6.3  My current secondary is 50mm minor axis

Antares have two options A 1.83 minor axis. Or 2.14. I Would prefer to go smaller. I have calculated the size will be 46.482 mm  For the Antares 1.83 Flat. So a loss of about 3.6 MMs Over my current 50 MM Flat. The question is, will i lose any light from the primary with a 3.6 mm loss ?

https://www.antaresoptics.com/SEMirrors.php#:~:text=Secondary Elliptical Mirrors All Antares Optics secondary mirrors,GPI interferometers can measure highly reflective coated surfaces.

Edited October 18, 2020 by neiil phillips

[vlaiv]

That depends on distance between primary and secondary, and consequently focus position.

Here is quick scheme:

Secondary in position to the left will not reduce effective aperture of primary. Secondary on the right will reduce effective aperture of the primary.

If you want to know if your primary will be stopped down - just measure your focus position and diameter of the tube.

For example, you have 245mm primary, and let's say you have 255 tube diameter. You also have focus position at about 100mm above tube. This means that focus plane to center of secondary is 100 + 255 /2 = 227.5mm.

You have F/6.3 scope, so size of beam at secondary is 227.5 / 6.3 = 36.11mm

In this case, using 46mm secondary would not stop down your aperture, but it would reduce fully illuminated field in comparison to 50mm. Of course, for your exact case, you would need to measure distance between secondary center and focal plane as above is just an example.

[neiil phillips]

Hi Vlaiv

Thanks for the reply. Well i am not so worried about a slightly reduced fully illuminated field. It will be used for the moon and planets mostly. But of course losing aperture is something else.  Will be sometime before i can measure the distance from secondary center to the focal plane. Wonder if anyone knows this figure. Its a Orion Europa F6.3 

Cheers for the help

 

[Woj2007]

3 hours ago, neiil phillips said:

Hi Vlaiv

Thanks for the reply. Well i am not so worried about a slightly reduced fully illuminated field. It will be used for the moon and planets mostly. But of course losing aperture is something else.  Will be sometime before i can measure the distance from secondary center to the focal plane. Wonder if anyone knows this figure. Its a Orion Europa F6.3 

Cheers for the help

 

Neil, 

Take a look through your focuser without the eyepiece, set at about half-height.

Can you see the whole reflection of your primary in the secondary? How much room is left on the borders? If the secondary  embraces the whole primary and there is still room for further size reduction you could do it.

However, I'd go with the 2.14", as you already have a low-profile focuser and the secondary size is optimized for this. The difference between 1.83 and 2.14 will not degrade contrast visibly, the quality of the primary mirror is important here.

And you can fiddle with numbers here:

https://www.bbastrodesigns.com/diagonal.htm

 

[neiil phillips]

4 hours ago, Woj2007 said:

Neil, 

Take a look through your focuser without the eyepiece, set at about half-height.

Can you see the whole reflection of your primary in the secondary? How much room is left on the borders? If the secondary  embraces the whole primary and there is still room for further size reduction you could do it.

However, I'd go with the 2.14", as you already have a low-profile focuser and the secondary size is optimized for this. The difference between 1.83 and 2.14 will not degrade contrast visibly, the quality of the primary mirror is important here.

And you can fiddle with numbers here:

https://www.bbastrodesigns.com/diagonal.htm

 

Thanks for the extra info i will look further into it.