Source for 12" Newtonian tube

[Tommohawk]

Hi all.

I have a project underway for which I need to extend the front of a 10" Quattro tube by about 15cms. The tube is actually about 288mm diameter, or the extension could go over the round end part of the existing tube which is a tad larger at 290mm.

Anyone have any ideas how  I might do this, preferable both simply and cheaply? I could start with a whole new tube but then I also have to think about how I mount the mirror cell.

One thing I considered - don't laugh - was using a 12" round cake tin. Its 1mm steel so would probably just do it though I do need to mount a spider inside it so some tension involved. Could maybe rig a reinforcing ring for the end. Trouble is this is not quite long enough - probably only 100mm and would need some overlap so only say only 70mm actual extension.

I dont have much in the way of engineering kit so cant machine anything - needs to be something I can adapt. Any other ideas? some kinf shory pipe/tube/drum which I could adapt? Or maybe some ATMer out there has something?

[Gina]

3D printer?  If you have one and it's big enough.

[JamesF]

Are you planning to attempt to mount a camera at prime focus in the centre of the tube or something like that?

James

[R26 oldtimer]

24 minutes ago, JamesF said:

Are you planning to attempt to mount a camera at prime focus in the centre of the tube or something like that?

James

Taking this a step further. If this is what you're trying to do, then there is no real need to extend the tube. You can just make a spider vane that extends outwards like this:

[JamesF]

1 minute ago, R26 oldtimer said:

Taking this a step further. If this is what you're trying to do, then there is no real need to extend the tube. You can just make a spider vane that extends outwards like this:

That was certainly one possibility I had in mind

Another, which is throwing money at the problem really, might be to buy the 250 flextube tube that's on astroboot at the moment and remount the mirror in that, though I don't know if the mirror cells are similar enough to allow the switch without modification.  The fact that the 250PX has a slightly longer focal length might well mean the upper cage is in pretty much the position the OP was after.

James

[Tommohawk]

Thanks for the swift replies!

52 minutes ago, Gina said:

3D printer?  If you have one and it's big enough.

Unfortunately I don't have one - and I'm not sure that plastic would be rigid enough. To be fair I know nothing about them though -  I imagine the plastic to be a relatively soft thermoplastic?? Maybe not.

I seem to  remember you making gears or other parts with a 3d printer so they obviously work fairly well. 

40 minutes ago, JamesF said:

Are you planning to attempt to mount a camera at prime focus in the centre of the tube or something like that?

James

Good guess.... and perfectly correct!

11 minutes ago, R26 oldtimer said:

Taking this a step further. If this is what you're trying to do, then there is no real need to extend the tube. You can just make a spider vane that extends outwards like this:

Yeah I wondered if that might be possible. And it leads to the other design issue I have which is I need to convert the spider so it has no mirror, but instead a 1.25" EP holder to  mount the camera in.

It wouldnt be easy to tension a forward projecting spider in the same way that a conventional spider is tensioned though, so I wanted to avoid that. Also the bracketing to the tube would be a bit clumsy. Worth further consideration for sure.

14 minutes ago, JamesF said:

Another, which is throwing money at the problem really, might be to buy the 250 flextube tube that's on astroboot at the moment and remount the mirror in that, though I don't know if the mirror cells are similar enough to allow the switch without modification.  The fact that the 250PX has a slightly longer focal length might well mean the upper cage is in pretty much the position the OP was after.

Well that's a thought, good spot! It would have the additional benefit that the tube length could be varied for experimental purposes. Thats said, I dont know how the flex tube locks - can it be clamped other than at full stretch?

As you say the longer FL would be perfect - but would need to be sure the mirror cell would transfer. The Quattro tubes might be larger as the scope is faster but they certainly look similar and they both have a 254mm mirror.

The other thing is that the 250 Quattro is decorating the lounge a the moment, but the aesthetics of this are lost on my wife. I suspect having a matching pair will do little improve matters. 

 

[R26 oldtimer]

10 minutes ago, Tommohawk said:

instead a 1.25" EP holder to  mount the camera in.

One of those cheap 'n cheerful helical focusers would be even better!

[Tommohawk]

Just now, R26 oldtimer said:

One of those cheap 'n cheerful helical focusers would be even better!

I'm still debating whether to use a helical focuser as you suggest or convert the primary to a focusing type using ultrafine collimation screws and replacing the adjusting knobs with toothed pulleys linked by a belt.

I think it will be a nightmare to adjust a helical focuser -  it wont be easy to motorise it either. But I could cannibalise one just to mount the camera in. I'm just rummaging though some old stuff now!

[JamesF]

The flextube clamps can be fixed in any position I believe, but for the "normal" observing position they have a little locating mechanism to make sure the tubes are all extended by exactly the same amount.  Once the perfect length had been determined however, I think it would be possible to shorten the other end of the truss tubes to allow that locating mechanism to be used again.

James

[JamesF]

2 minutes ago, Tommohawk said:

I'm still debating whether to use a helical focuser as you suggest or convert the primary to a focusing type using ultrafine collimation screws and replacing the adjusting knobs with toothed pulleys linked by a belt.

I wonder if it wouldn't be better to leave the collimation system alone and have an additional set of screws to move the entire cell for focusing.  Of course if you're limited as regards engineering facilities then some of those decisions may be made for you...

James

[Tommohawk]

Brilliant - quite tempting then!

[Tommohawk]

Just now, JamesF said:

I wonder if it wouldn't be better to leave the collimation system alone and have an additional set of screws to move the entire cell for focusing.  Of course if you're limited as regards engineering facilities then some of those decisions may be made for you...

James

some time back I sourced some ultrafine brass threaded inserts with screws which could just replace the collimation screws. So the belt would be removed to collimate and then replaced to focus. I did the maths and it worked out about the same as a 1:10 DS focuser. In theory it should be an easy mod. In theory....

[R26 oldtimer]

Now that I think about it more, you don't even have to take the spider center outside the tube. You could make a new spider, as the original, and have a 2" tube in it's center, extending outwards to a helical focuser and the camera, like a reverse RC baffle tube. This way you can still use pure tension to center it.

[JamesF]

3 minutes ago, Tommohawk said:

some time back I sourced some ultrafine brass threaded inserts with screws which could just replace the collimation screws. So the belt would be removed to collimate and then replaced to focus. I did the maths and it worked out about the same as a 1:10 DS focuser. In theory it should be an easy mod. In theory....

Perhaps it wouldn't even be necessary to remove the belt.  On my modded PST I have a motor drive for the focuser, but the pulley on the actual focuser spindle is effectively only held in place by friction.  There's a nut that goes on the end of the spindle clamping the pulley into place.  Once the nut is released a little the pulley is free to spin and the focuser can be used manually.  Something a little more robust might be required in your case, obviously.

Does the Quattro have push and pull screws for collimation, or a single set of screws and springs?

James

[Tommohawk]

1 minute ago, JamesF said:

Does the Quattro have push and pull screws for collimation, or a single set of screws and springs?

Single set of screws with springs.

7 minutes ago, R26 oldtimer said:

Now that I think about it more, you don't even have to take the spider center outside the tube. You could make a new spider, as the original, and have a 2" tube in it's center, extending outwards to a helical focuser and the camera, like a reverse RC baffle tube. This way you can still use pure tension to center it.

Thats a possible solution, but there will be more sideways force on the spider vanes - not much  perhaps because the camera has no filterwheel. But I want to keep the central obstruction minimal -  no larger than 41mm -  and unfortunately I think I'll get vignetting if I extend back that far

[Tommohawk]

Well I went mad and bought the flextube. Hopefully the cell is straight swap - if not I'm in trouble! 

I've figured out how to convert the primary to do focusing and will use my Quattro vanes to support an Ep holder which will take the camera, or a powermate with camera. If my sums are good the flextube adjustment will allow coarse tuning adjelustment between camera only or powermate with camera. 

This is kind of a dummy run for a larger version where the CO reduction will be more significant. Fingers crossed!