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A mad idea forming


geppetto

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Just been musing on a mad idea for a future Geppetto Towers project.

Cheap 6" mirror in one of my square MDF OTA tubes and secondary

replaced by a ccd webcam suspended at the focal point of the mirror.

A large astronomical camera 8)

I can of course be put off by you guys telling me it'll never work :D

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The spider might have to be larger to accomodate the heavier webcam plus the USB lead would impinge on the image but it should work. Not sure how much advantage it would give you over not using a secondary though but go for it. The tube would have to be longer as the focal point isn't "bent" away

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I toyed with this for the Skelescope project but the focusing demands put me off a little. Part of the Spec. was to have huge focus travel as the thing is designed to work with Barlow lenses and focal reducers (though not at the same time). I couldn't think of a reason not to if you only intend to use the webcam at prime focus. The SC3 box that I have is not much bigger than the secondary as it is and I've also got a wand type webcam which works reasonably well on bright things.

I got this from Maplin but they don't seem to have them anymore

http://www.gas.uk.net/webcam/ shows a picture of it mounted in a Zenit SLR body, the case is still on my desk and measures less than 20mm wide. The PCB would be about 15mm x 20mm so smaller than a secondary mirror.

Best of luck with it.

Captain Chaos

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ccd has to be on the board - *way* too much noise introduced even if you lift it 15mm away! Focus is easy if the board is fixed to a helical motor-type thing. Collimation would be as per normal scopes. Actually, SC and I thought about doing something like this as a dedicated polarscope/guidescope (obvipously lens-based though) a while ago. Too many other things to get designed and made though :D

Arthur

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Arthur, re the cable introduced noise, has anybody tried

using mini screened cable for each pin?

Yup. All ways tried back in the early days of QCUIAG once chips were being messed with. Trying to lift them up far enough to get a cold finger underneath was bad enough - but then people tried to put the chip further forwards into the nosepice to get more focus travel. No-go.

I see no reason why you have to take it out of the case though. If it's a webcam you're talking about it's already small enough, and I really don't think the cable is going to cause a problem either vis. diffraction.

Arthur

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Depencing on how big the camera is, you might want a bigger mirror. Have you considered having the camera still, and moving the mirror back and forth until you achieve focus? Once you're in focus, you wouldn't have to move it again.

Sounds like a good idea to me. The large observatory telescopes are set up this way, with a camera at the focal plane of the mirror.

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There are telescopes that do this. I believe they are called Herschellian. If they are of long enough focal length that you can get away with a spherical mirror, then you can use it as is, but otherwise you have to warp the mirror sllightly to get the right figure into it. Do a little googling, and you may be able to come up with some pages on it. I saw some a long time ago.

Hershcellian telescopes usually fold the light path in a Z shape, or N shape depending on your POV. :laugh: That way, the ep is on the back of the scope, where people expect it to be. I saw an even more convoluted design somewhere, recently, which folds the light path in ways not describable by any letter of the alphabet. These all require secondary mirrors of some kind, though.

Stick to your original notion, and use a curved spider to hold the camera.

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Ok

So far this is what the test bed will be.

madscope.gif

Going to borrow the mirror from my 130mmF5 reflector to keep the

test bed smaller.

Should give me an idea of what all the problems will be.

Re setting up the camera, I thought of placing a circle of white card in front

of the ccd and shooting a laser light at the mirror to get a return spot for

tweaking the camera position.

I reakon that hitting the ccd will be the biggest fiddle about bit.

There's still time to talk me out of this foolishness and of course if

the skies clear there will be a considerable delay in getting started :laugh:

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In a dark sky site, you wouldn't need much more than that. Put that box in a rocker cradle, and you've got a Dobsonian Camera. Use a circular piece of aluminium to act as a strut for the camera, and you should eliminate diffraction spikes.

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