Refractor baffling

[Glenhuon]

I'm considering inserting some baffles into my 102mm SS OTA. Been googling for some info on sizing/position etc without much luck. Anyone got a recommendation for a site ?
I can work out the inside diameters using simple geometry of the light cone, but is there an optimal postioning for these baffles.

Bill

[anj026]

Hi Bill

When you draw the light cone don't draw it to a point at the eyepiece end of the focal length. Make the eyepiece end of the focal length a line equivalant to the diameter of the fully illuminated field about 27mm for 1.25 inch eyepieces or 46mm for 2 inch eyepieces. Then baffles theoretically should be spaced such that any off axis light ray entering the objective end will not reflect of the tube side and on to the focal plane. You may find it only takes 2 or 3 or 4 baffles to achieve this. When I made my wooden refractors I didn't worry about the spacing too much as the baffles were acting as frames so I just spaced them equally about 200mm apart.
If you can get hold of a copy of Telescope Optics by Rutten and Venrooij (Willman Bell) there is a descrption of the theory in Chapter 19. Also Build your own telescope by Richard Berry (Willman Bell) has a plan for a 6 inch refractor.

[Glenhuon]

Thanks Andy. I'll look up the references. Just looking at the tube length (about 800mm) I'll plan for 3 at 200mm spacing. with the first one 200mm from the objective. Next project at the "Mens Shed" is on its way :). Most of the other guys are making toys or fixing up old furniture, I'm into the Astro Bits.

Cheers
Bill

[anj026]

I've been looking back over what I have and it seems that the bigger the tube diameter in relation to the objective then the less the number of baffles required in theory. As the tube diameter decreases with respect to the objective diameter then the more baffles will be required to prevent off axis rays reflecting off the tube wall and onto the focal plane.
On the other hand you can do what Televue do with their refractors and not have baffles at all. They use high quality flocking material to line the inside of the tube. This effectively becomes continuous baffles and allows the telescope to be as compact as possible for it's aperture.
One method of doing this is to glue ground up walnut shell or similar coarse material to some card sheet and then paint it matt black. Roll the card sheet into a tube and push it into the telescope tube, gluing it at each end to hold it in place. Or you could just buy self adhesive flocking paper from Protostar on the internet.

[citivolus]

I tried a similar technique when building my artificial star, using card stock and sand, and I also tried sand paper. My problem was that I couldn't find a paint that was really matt when viewed at the angle that the light will be hitting it at. I guess I need some carbon nanotubes.

[Glenhuon]

Yes, thats the problem with paint, it seems to be impossible to get a really matt black one. I've sprayed the inside of the tube but still think I'm getting reflections off the walls. This one is the same ID as the objective so will look at 3 or 4 baffles spaced accordingly and see how that goes. The tube is PVC so fairly light and the baffles will add very little to the overall weight if I make them from plastic material (lids of ice cream containers comes to mind :) )
Thinking I might try and mix up some sort of flocking using the small tin of flat black I have here and a texturising material. We have a walnut tree in the garden, must be dozens of old nuts lying under it.

Cheers
Bill

[PhilW]

For my 102/600 I flocked the tube and the dew shield with the Protostar hi-tack flocking paper. I also put a single baffle midway down the tube. It all seems to work & doesn't suffer from stray reflections.

Phil

[anj026]

About 12 years ago I made a an 8" dob and used black velvet to line the inside. It worked well. I've looked since to get more black velvet and it is not so easy to find.

There is a current thread on Cloudy Nights that mentions a type of black velvet that was tested to be superior to other types of paint and flocking. It may be possible to order this material online or find a local supplier.

[Kokatha man]

Was that black velvet or black felt? Spotlight (the big chain store) have various thicknesses of black felt in stock at low cost in wide widths.

[PhilW]

I've previously used the synthetic velvet they have at Spotlight. It is effective as a flocking material, but it is tricky to stick on. The good thing about the Protostar hi-tack material (apart from its optical characteristics), is that it is easy to work with & install. It's fairly cheap too, and one sheet would do several Surplus Shed refractors.