A mad idea forming

[geppetto]

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

[GordonCopestake]

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

[geppetto]

Might be a chance of seperating the ccd from the webcam board

and hanging just that on a short ribbon cable at the focal point.

No spider, just a single support arm

[GordonCopestake]

How would you colimate the CCD? Should work though, i have a CCD circuit board if you need one

[geppetto]

Cheers on the board Gordon but I've got some spare stuff hanging around 8)

Collimation would be close from word go as using a square OTA means I can very accurately

get everything on the same plane by some careful initial woodworking.

[GordonCopestake]

I look forward to seeing your progress!

[Kaptain Klevtsov]

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

[Ambermile]

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

Arthur

[geppetto]

Arthur, re the cable introduced noise, has anybody tried

using mini screened cable for each pin?

[FLO]

Only one reflecting surface 8)

[Ambermile]

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

[geppetto]

Ok Arthur

Thanks for that.

I know noise is a problem because my amp off mod caused a lot of banding.

[The Warthog]

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.

[geppetto]

WH

Giving the mirror a bit of forward and back is a good idea 8)

Be helpful to get the camera set and left alone as you say

[geppetto]

Another thought :?

Would there be any reason for not having the camera offset

with the mirror tilted so that there was no obstruction in the light path

or is that just silly

[Kaptain Klevtsov]

If you made the mirror to suit, no problem. There are specially made 'scopes out there with strange oval mirrors, but the standard one would give odd shaped stars like you get with a poorly collimated Newt. (which is what it would be).

Captain Chaos

[geppetto]

I see what you meen CC

Scrap that one :whip2:

[The Warthog]

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. 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.

[geppetto]

Ok

So far this is what the test bed will be.

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

[The Warthog]

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.

[geppetto]

Rough design for focus adjustment

[The Warthog]

Yep, but just one big spring, with the screw going up the centre.

[The Warthog]

Hey! Instead of a square aluminium box tube, give it the same section as a dovetail, and insert it directly into an EQ mount. Put a red-dot finder on the box, and you're away to the races!

[geppetto]

Keep them ideas coming WH 8)

[The Warthog]

If your webcam has a zoom feature, would that allow you to vary the power of the setup? Or, are you just focusing on the CCD array without an intervening lens?