Baffles

Hi hope someone can shed some light on this.

Have used the newt program to calculate for a 300mm mirror.
It says I need 350mm id tube with lots of baffles.

Is it usual to fit so many baffles in a Dob.?


Do truss rod Dobs have Baffles.?

Any help.

Regards

Paul.

Personally I've never seen baffles in a Newt, only in cats and refractors. The typical solution I've seen in a Newt is to flock the tube and truss Dobs don't even have that.

You can't have baffles in a Newtonian.
The light has to travel the entire length of the tube before it reaches the primary mirror.

Cat amongse the pigeons time...

ALL telescopes need to be baffeled to eliminate stray light from reaching the eyepiece - including Newtonians, both solid and truss tube type.

With a well designed solid tube Newt the baffeling scheme can be as simple as making sure that there is enough spare tube ABOVE the focuser/secondary to eliminate light from entering the focuser draw tube without first being reflected off the primary AND secondary mirror in the desired manner. It's amazing how many commerical Newts fail this criterion! The simple solution is to wrap a semi-circular piece of black card or plastic around the top of the tube directly opposite the focuser. Fixing this common defect will yield the single largest increase in image contrast in your system.

All Newts also need baffeling at the primary mirror end to prevent light (reflected from the ground) from entering the bottom of the tube and reaching either the secondary or the focuser. This may only account for a percent or so in contrast, but again, it is amazing how many commercial Newts fail here also!

Baffeling down the length of a solid tube Newt is also beneficial... though problematic. Correctly spaced baffels do yield an improvement in contrast but they also create pockets for warm air to accumulate coming off the primary mirror. So, if you go with tube baffels then you need to add ventillation slots as well.

Truss-tube Newts need careful attention when it comes to baffeling. There are a number of programs out there to help you design a good system, but they are much the same as for a solid tube, with the addition of baffels directly opposite the focuser and in the focuser draw tube. A full-length cloth shroud is not necessary contrary to popular belief!

Phil.

Paul,
I have added some simple baffles to my solid tubed 222mm Dobsonian. One behind the mirror to block the stray light from the ground as Philip said above and a couple up along the tube. I used Newt to ensure that they were out of the light (desired light) Path.
I just used some self adhesive foam draft excluder which goes rigid when painted matt black (blackboard paint). There certainly was a noticeable increase in contrast when trying to tease out details on the planets or when looking for faint fuzzies.
I also have a 12" StarSystems (thanks Phill) truss tube dob that has a shroud. As Phil said the shroud has little effect on the contrast when at a dark site but is great for keeping the dew away.

Thanks for the info lads.

Is Vignetting of 75% ray at front aperture a big problem.

I have a tube which is producing this alert when calculated in newt.

Paul