Building a Newtonian Reflector

[InvaderXan]

Hello

I was wondering if anyone here had ever built a Newtonian from scratch, and if they could perhaps offer me a little advice.

My idea was to use a big shaving mirror (10-20") with a truss rod assembly to make a budget light bucket. Would this even be feasible?

Thanks in advance.

-X-

[barkis]

I have built a few, one 8.75 f6 a 12" f6 a 14" f6 and others from 4" to 8" from f3 to f8.

The first two were tubed scopes, and equatorially mounted. The rest were Dobsonian, on fairly simple structures. I have no experience of Truss designs at all, and I detect that that is probably the area you are most interested in.

I trust the shaving mirror bit, was a light hearted quip.

Ron.

[SteelRat]

I trust the shaving mirror bit, was a light hearted quip.

Yeah - I mean who's got a face THAT big?

[barkis]

I trust the shaving mirror bit, was a light hearted quip.

Yeah - I mean who's got a face THAT big?

:laughing3: :laughing3: :laughing3:

[InvaderXan]

Well to be honest, I was only half joking about the shaving mirror... For the sole reason that they're about the cheapest type of concave mirror you can buy. I'm guessing that means this would be a dumb idea.

(I know how to use 'em, but I've no idea how to build 'em!)

[barkis]

I apologise, I had no intention to ridicule you in the slightest. I just thought you were having a laugh knowing full well we would treat is as such.

The truth being a shaving mirror, although having a spherical curve, would not serve as an astronomical objective. The focal length of it would be quite short. and to place a secondary mirror in order to divert the light out to a focuser, would not be practical. Having said that, the old Schmidt Cameras had the film placed at the focus, but it had to be on a curved support to ensure everything was in focus across the field.

If you have a shaving mirror, and reflect the sun on to a wall, and move it back and forth to get the image as small as you can, it will give you an indication of the focal length of the mirror.

Ron.

[InvaderXan]

Don't worry, no offense taken. The idea seemed like a long shot, but I was just wondering if it was possible...

So I know focal length is approximately = r / 2, where r is the radius of curvature. This presumably means that for a good focal length I'd need a mirror with a low "magnification". And isn't a secondary mirror required anyway, for an eyepiece?

Sorry if these are naive sounding questions.

[Doc]

Yes a secondary mirror will be needed to direct the light into the eyepiece.

The light will enter your scope hit your "Shaving mirror" and bounce back upto the secondary mirror.

Your focal length will determine where your secondaty mirror is in relation to your primary mirror.

[barkis]

Invader, if you have a pen light, and hold it to your temple, it don't matter which, shine it towards the mirror and catch it's reflection. Slowly;y walk backwards until the light starts to fill the mirror with light. You can bob your head up and down and the light will do the same. If it goes the opposite way to the up movement, then you have gone too far back. Go foreward again so that the light stays full in the mirror. Then measure that distance, it will have to be the average between your eye, and the lightsource , so if the light was at 10" and your eye at 12" the average is obviously 11"

Your formula R/2 is correct, so dividing by 2 would give the focal length. 5.5"

If you make your light source into a pinhole by putting the torch behind a bit of foil with a very tiny hole in it. Attach the torch to a makeshift stand for steadiness. Manoeuvre the lightsource to catch the reflected light, then intercept the beam with a straightedge such as a Stanley Blade, (be careful) As you cut the beam you will see the shadow of the knife in the Lightpath. That would be inside the focus. Move slightly back until when the knife enters the beam, the mirror darkens instantly. That is spot on the Radius of curvature of the mirror . The distance when measured divided by 2 is the focal length of the mirror.

The likelihood is, that at the centre of curvature, when the knife enters the beam, you will see all the problems that would make it totally unsuitable as an astronomical mirror.

Ron.