Truss Dobsonian project

[jetstream]

5 hours ago, Stu said:

I have a focal length spec but it might be worth checking this for accuracy. I guess this is doable?

You can temporarily install the mirror and secondary, collimate (doesn't need to be that great) and then use a light source (I've used the moon) to find the focal position. You can use a piece of bristle board mounted on a moveable bracket- or with help, someone holding it- just hold it way from the focuser until the moon (or source) focuses on the board and measure this from the focuser.

Then measure from focuser to sec, add this number and then measure from sec to primary- add it up and there it is.

When doing this it helps to install the mirror higher than normal- pushing out the focal position away from the focuser. I'm sure there are more fancy options to do this but its worked for me.

[Stu]

Right, I've had a complete change of plan on this, and decided to do it properly from the start.

The Zambuto mirror can be ready in a matter of weeks, so I am going to press ahead and get that sorted as soon as I can. I need to confirm a focal length to Carl, so need to get everything else sorted so I can measure it accurately.

I'm going to go for a Baader Diamond Steeltrack focuser as it is a reasonable cost and I like the quality and feel of them.

Secondary support will likely be Astrosystems although I'm still thinking about this. I had toyed with the idea of a curved secondary from Protostar but this would need more substantial mods to the upper tube assembly so I will stick with a good 4 vane support.

The cell will be from Aurora Precision, specifically designed to properly support the thin (20mm) quartz mirror. Hopefully this will cool quickly and the cell will ensure no astigmatism. I need to get this ordered as soon as they are back from an astro event, early August I think.

More updates as things progress.

[PeterW]

Interested in where you get bits from.... the exact focal length shouldn’t matter, you don't cut the poles till you have the mirror. Good luck

 

peter

[Stu]

1 minute ago, PeterW said:

Interested in where you get bits from.... the exact focal length shouldn’t matter, you don't cut the poles till you have the mirror. Good luck

 

peter

Poles are already cut to the right length for f5 and I would rather not mess around with them Peter. I'll get the focal length where I need it and there will be a bit of adjustment in the cell position to get it right.

[Piero]

Good luck with this project, Stu!

[furrysocks2]

Shame the 12" mirror set never saw light, but for the sake of fitting it all to the length of the light path, as you say better to do it properly from the start.

Good luck!

Matt

[Chriske]

Zambuto...!!!!

Wooooooooow....!😱....💰💰💰💰💰💰💰💰