how to lengthen the tube on a newtonian

[alacant]

Hi everyone

I need to lengthen the primary mirror end of a Newtonian by 2cm. Anyone ever tried this? Any help or advice?

It's a pn208 with aluminium tube and the primary cell is held by 4 off m5 screws.

Cheers, clear skies and TIA.

[Cosmic Geoff]

I note that this is a 'Foto Newton'. Perhaps you could explain why you want to extend the tube. Most people wanting to mod their Newtonians are aiming to shorten them to get their cameras to focus.

[alacant]

22 hours ago, Cosmic Geoff said:

explain why you want to extend the tube

Hi. We have fitted a smaller secondary. We can focus with over 20mm to spare. Any ideas?

Cheers

[Paz]

My VX14 has an extension to the ota which was done by the previous owner. The purpose is so that I don't need an extension at the eyepiece for visual which means less work putting bits together when setting up and more focuser stability due to less of a moment working on the focuser.

I only do visual so I don't need the in focus that the VX is normally set up for. I think this may mean I could drop down to the next smaller secondary but I've never got around to doing this.

[alacant]

1 minute ago, Paz said:

extension to the ota

Hi. Thanks.

Yeah, that looks perfect. Any ideas where the extension bit was sourced?

Cheers

[Paz]

It would either have been done by the previous owner who was very handy with mechanical engineering, or it might have been specified and then given to Orion Optics UK to do (the manufacturers of the scope).

[NGC 1502]

12 hours ago, Paz said:

It would either have been done by the previous owner who was very handy with mechanical engineering, or it might have been specified and then given to Orion Optics UK to do (the manufacturers of the scope).

 

This modification would not be difficult with basic DIY skills and a visit to a sheet metal supplier for a strip of aluminium the same thickness as the tube.    The aluminium could be cut by hand with tinsnips, but a far neater job would be to ask for it to be cut on a guillotine at the suppliers.    

Sheet aluminium is easily bent by hand and bolted to the tube. With care a strong and neat job could be done.

The way that OO Newtonians are now made means that an extension tube on the focuser is needed for visual use, or removed to add a camera etc for imaging.   But for purely visual use a focuser extension tube looks a bit ungainly especially with a large eyepiece.   With the primary mirror further from the secondary that’s sorted, and also the optical edges of the secondary are not used.  It’s at the edges of the secondary where sometimes it’s less than optically flat, especially at the ends furthest and nearest to the primary.    A simple optical diagram will reveal all this.

Ed.

 

[alacant]

On 14/04/2019 at 23:08, NGC 1502 said:

ask for it to be cut on a guillotine at the suppliers.    

Hi. Getting there. Here we are after the chop. It's from a 200mm wide roll; 0.3mm thick. The only other stuff was -remarkably non bendable- 0.5mm rectangular sheet. I only need a 2cm tube extension. Do we reckon it'll do the job wrapped around the tube?

Cheers