Secondary mirror size?

[pepston]

Hi, I`m building my 1st telescope on a Dob mount and using a10 inch tube all from PVC as I`m a plastic fabricator so material is not an issue. It`s housing an 8 1/2 mirror dia (weird size) or is that classed as 8 inch?

Just setting up its focal length tonight. Should be an F8. About 66 inches.

Can anyone advise me as to the size of secondary mirror to use? 1 inch?

Also spider for it and where I can obtain these items. Advice on where to get the mirror silvered would be appreciated.

Suppliers link would be real good.

Any ideas on size of optics to use would be great. The greater the lens size the better.

Many thanks for any comments.

KInd regards, Steve.

[Gina]

I would have thought you'd want to go for 2" optics and therefore a secondary mirror at least 2" x 3" but I'm no expert on scope making.

[pepston]

I would have thought you'd want to go for 2" optics and therefore a secondary mirror at least 2" x 3" but I'm no expert on scope making.

I really don`t know Gina but I`d have thought that a 2 inch secondary in a 8 inch newtonian would really restrict the incoming light! Thats why I suggested 1 inch. I can`t find any info on line. As you can guess I`m no expert either!!

Regards, Steve.

[asteria]

Hi - this should give you all the parameters:

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

It's a very interesting site, that calculates all the maths for newts.

Ed

[skybadger]

Draw a triangle out - fat end at the width of the mirror and narrowing by the f/ratio angle to the focal point.

Work out the distance of the prime focal point from the central axis of the scope at the height of the secondary - then you can determine the cross-sectional width of the light cone at that distance along the triangle which is what the maximum width of your mirror needs to be. If you care less about full field illumination then you can make this mirror smaller and there are software tools out there like the Nicholl optics spreadsheet on their website. .

If you make the mirror smaller, then the obstruction compared to the area of the mirror is smaller and results in a higher contrast image, hence they are sometimes selected intentionally for planetary viewing. Short f/ratios or wide fully iluminated fields will drive you to wide mirrors. My guess is a 65mm diagonal will do a 8" f/6. my 10" f/6 is a 73mm .

Edit: Althougfh the Newt-web calculator suggest both of these are large, Maybe my recollection is wrong.

regards

Mike

[pepston]

Thanks for that Ed. I`ll take a look..

Steve.

[pepston]

You guys are really on the ball. I didn`t expect such a response so quickly. Cheers Gina,Ed and Mike. I have work to do. Talk soon.

Rgs Steve.

[angusb1]

As long as your secondary minor axis is less than 20% of your aperture you will gain virtually no greater contrast (that you can see) by having a smaller secondary minor axis. Or put simply try to get your secondary minor axis at just under 20% for optimum illumination/contrast.

[Glasspusher]

Steve,

I generally use this calculator: http://www2.arnes.si/~gljsentvid10/diagonal.htm

For imaging a larger secondary is needed for full illumination.

John