Locating a Tube

[Dachs (Dave)]

Hello all,

I am building a 6" Newt and am trying to locate a 200x1500 piece of PVC pipe. I am having a bit of trouble in doing so. Has anyone bought 200mm OD PVC before? I can find 225mm OD PVC and I think that would do fine. I will have to expand the mirror cell to fit the tube as the plan calls for 200mm OD. Would I be correct in that?

Thanks
Dachs

[gbeal]

If you come up blank, try what Mick did, roll a tube from sheet metal. I did it with an 8" f4, glued the overlap seam, and it worked a treat.
Gary

[Astrobserver99 (Rob)]

Don't use PVC pipe, it will eventually warp. Use sheet metal or formatube. Just my two cents worth...

[mswhin63 (Malcolm)]

Originally Posted by Astrobserver99

Don't use PVC pipe, it will eventually warp. Use sheet metal or formatube. Just my two cents worth...

I was trying to remember the name "Formatube" now I will consider purchacing some for a reflector.

PVC can come in heat resistant modes but usually smaller diameters. It is black in colour.

[Astrobserver99 (Rob)]

Links...

http://www.thetubeworks.com.au/index.htm

http://www.sonoco.com/sonoco/Product...rete_forms.htm

[mental4astro (Alexander)]

PVC piping may well warp, but it will serve as good experience if this is the first scope you build. You can always salvage the components for reuse.

200mm pipe would be sourced from builders' supplies or plumbers supplies. Most hardware stores would not carry this. But be prepared to buy a 6m length.

Have a look at some of the programs available from the net regarding designing newts. They will provide you with the parameters that need consideration after you punch in a few details such as mirror diameter, f/ratio, focuser size & height, etc.

The following is a link to a site of a fellow IIS member, 'ricci', and a collection of free downloadable programs. Scrolling down the list you will come to optics design:

http://www.pierpaoloricci.it/downloa...ftware_eng.htm

This next link will take you to another brilliant collection of links very useful to ATM'ers. It also contains info on every component in a scope, refractive and reflective, both as DIY and commercially made:

http://stellafane.org/misc/links.html

You might be inspired to alter your tube design for some other. Most are not difficult to make, we are talking ATM after all.

I picked up these two sites from here in IIS.

[Dachs (Dave)]

Thanks very much for the advice and sites. As a first timer I think I will stay with the PVC (when I find it). I just think it will be easier for me to work with. Rather than the 200mm OD PVC if I use 225mm OD, other than the larger rear mirror cell plate will this effect anything else?

[cristian abarca]

Hi Dachs I built a 10 inch Newtonian using PVC piping. It is easy to work with but one of the problems is that the PVC tends to hold the heat for a while and acts like a type of insulator. The air inside the tube tends to differ from the ambient temperature. This tends to make the images not as good as they should be. By all means do it but you will eventually change the PVC. By the way go for the bigger tube to let a bit more air into it so that your air currents inside the tube settle down a bit quicker.

Regards Cristian

[mental4astro (Alexander)]

Dachs, yes, it affects the position of the primary relative to the secondary, closer, in order to take up the extra distance between the mirror's edge and the inside of the tube. This will in turn mean that you will need a larger secondary mirror to capture the light escaping around it, which in turn means loosing light to a larger secondary obstruction! You need to get the light to come to focus somewhere just above the height of the focuser, with the draw tube in all the way. Unless the primary is closer, you won't achieve focus.

There is a balance between the size of the secondary and the %obstruction it produces and the distance the path of light needs to overcome. Too small a gap between the mirror and the tube, and you lose how wide a field the mirror can see. Too large a gap you increase the size of the secondary as explained above. And you need to add the focuser to the mix.

That is why I suggested you have a look at the scope design sites. These produce diagrams to help you visualize what goes on.

I'm having to be very careful with the 17.5" dob I'm rebuilding. I have an existing secondary I'm using, so I need to know what parameters I need to build around it and the primary to optimise the light that enters my focuser, and minimise the % obstruction of the secondary.

Simply put, it is maths applied to a direct physical project. Nothing more. It really is not complicated. Once you visualise the situation at hand, year 6 maths solves all your problems.

With respects to air circulation, if you leave some sort of gap system within the mirror cell, heat discipation would be quick for a 6" scope, even using PVC tubeing. People attach small 12V fans to their mirror cells to assist with this cooling.

[sasup (Stacey)]

Try this site ( I know its not pvc) if your keen on the ota pm me and ill give you the guys phone number that makes them here in sydney he is super cool.
http://www.acquerra.com.au/astro/equipment/Nemesis-16/

[Astro78]

you could use aluminium. Light weight, sandable, polishible, strong. excellent heat conductor (less dew)