Extension tube? Focal reducer? Both?

[samtheeagle]

Hello all, I'm after some advice...

I'm trying to get my cheap and cheerful autoguiding set up together, but I have couple of questions about my guide scope / webcam combo... I'm using a 50mm finder scope for guiding with a SC1 modified 840K toucam as the guide camera.

The finder scope is a correcting / erecting beasties with a right angle, but I think I'll remove this and attach the webcam direct to the back of the finder. Is this the better approach? Or should I leave the gubbins in the mix?

Next issue is getting the webcam to focus. It appears that the focal point is a good couple of inches behind the finder (with the right angle removed). Obvious answer is to get an extension tube. But then I start thinking about focal reducers. I don't know alot about these, or optics in general tbh, but I know that they reduce the focal length of the scope to give a wider field of view. That sounds handy for a guide scope Will a focal reducer bring the focus point closer to the back of the finder?

I'm not sure what I need to buy, and I can't afford to go buying stuff I don't need!

All advice greatly appreciated!

Thanks.

[Psychobilly]

Extension tube definitely - the diagonal increases the optical path length so you will need to add that extra lenght back in back in to get focus again...

[Doc]

I don't know to much about optics as such but as you have no adjustment for focus on the finder wouldn't you need to work out how long the extention tube would need to be.

What I would do is connect your webcam to it's nosepiece then a focal reducer to the nosepiece and then make up a cardboard roll extention tube and by trial and error work out the exact lenght that brings everything into focus.

Measure it and then get an extention tube made up at the length required.

As I said above I don't know much about this sort of stuff so what I wrote might be a load of rubbish!

[Merlin66]

If the body of the finder takes 1.25" fittings, then a 50mm extension tube will do the job.

The focal length of your average 8 or 9 x 50 is only around 200mm so you definately won't need a reducer.

[samtheeagle]

Thanks all

I could not get the camera to focus with the diagonal in place either, I think it still needs a bit more back travel. Sounds like I just need to find the appropriate length extender then, as the finder body does take 1.25" fittings.

Any ideas as to the approximate FOV I'll get with this?

[Merlin66]

If the focal length is around 200mm, then according to CCD Calc ( which is freeware) a webcam chip (2.8mm x 3.7mm) should give you 47.5 x 63.4 arc mins ( about one degree by 1/2 degree)

[beamish]

You could always hold a white piece of card behind the finder and slowly move it out until you see the focused image ( in daylight obviously )

Karlo

[samtheeagle]

Anyone got a working link to CCD Calc? I've googled but the main download site seems to have gone

[beamish]

The New CCD Astronomy Home Page

just checked this one

[samtheeagle]

Weird, must have been temporarily down when I tried... Thanks

EDIT - DOH! Ok, so I got to the main page, but then the link to download CCD Calc from that page was wrong! :S After some trial and error I found that the correct url to the app is http://newastro.wodaski.com/downloads/ccdcalc/ccdcalcfull.exe

[samtheeagle]

I managed to pick up a 55mm extension tube which I thought would do the job... Now I have ever so slighty too much length for focussing GAHHH!!!

I can't get the camera close enough in to focus on distant objects. Things nearby are fine, but I don't think I'll get stars as it is. It's like 1 or 2mm off. Sigh. What I really needed was a 50mm I think. Doh!

Could be hacksaw time. It's only aluminium after all. Just need to figure out how I can get a new threaded hole for the retaining screw...

[Ant]

I went and bought a tap and dye set from my local hardware store... cost about a tenner.

The only issue is if the ali is thick enough to put a thread in it...

Ant

[Psychobilly]

if it isn't you can drill a clearance hole and fix a nut to the outside of the tube with araldite... or epoxy putty / chemical metal infact if you use chamical metal you can drill and tap that...

Peter

[beamish]

if it isn't you can drill a clearance hole and fix a nut to the outside of the tube with araldite... or epoxy putty / chemical metal infact if you use chamical metal you can drill and tap that...

Peter

That would be my approach too, like Ant suggested,I can't see the wall being thick enough to put a tap in.

Karlo