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Old 24-08-2008, 09:00 PM
thestarman
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Advice sought on lens f.l. for webcam to image whole Sun

Hi,

I would appreciate advice on the above.

I have a Phillips ToUcam which I want to webcast the entire disc of the Sun (I am getting ready for next years partial eclipse). Even with a focal reducer, the Schmit Cassigrain F10 is too long for the whole image of the Sun to be captured.

I figure I either get a new camera ($$ ouch..) with a bigger chip or perhaps I get a camera lens and adaptor for the webcam. I am assuming a camera lens would be cheaper but I don't know what f.l. to get. I have noticed that you can get a 80 - 300mm lens for a few hundred dollars. I don't want to waste my money so I thought I would seek some advice.

Regards,

Paul Floyd.
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Old 24-08-2008, 09:44 PM
Dennis
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Looks like a 300mm fl lens should just do it Paul. Here is how Ron Wodaski's ccd calculator sees it for the DMK21AF04 which has the same chip as the ToUcam?

Cheers

Dennis
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  #3  
Old 24-08-2008, 09:50 PM
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Jeff
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Hi Paul,

Assuming your webcam chip is 3.6mm x 2.7mm (640 x 480 @ 5.6um), the the captured FOV without a focal reducer on your 10" SCT F/10 would be pretty small ... only 4.9 x 3.7 arcmin.

To capture the whole moon (about 30 arcmin) in a single go, you would need to get your "effective" focal length down by a factor of 8.1 to about 308mm.

Cheers,
Jeff
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Old 24-08-2008, 10:10 PM
thestarman
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Would a 85 - 300mm lens be a good option then?

Good to hear from you Dennis.

What do you think of the above as an option then?

Paul.


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Originally Posted by Dennis View Post
Looks like a 300mm fl lens should just do it Paul. Here is how Ron Wodaski's ccd calculator sees it for the DMK21AF04 which has the same chip as the ToUcam?

Cheers

Dennis
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  #5  
Old 25-08-2008, 04:01 AM
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iceman (Mike)
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You'd never be able to reduce your SCT by a factor of 8.1.

You'll either need a 300mm camera lens (with appropriate adapter from MoggAdapters), or use afocal technique with a DSLR and a wider field eyepiece. But then it won't be webcast. So sounds like the 300mm lens is the go.
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Old 25-08-2008, 05:46 AM
thestarman
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Thanks

Thanks for everyones input.

I will start looking for a decent 2nd hand 300mm lens.

Paul.

www.paulfloyd.id.au
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Old 25-08-2008, 07:26 AM
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You could also use a small refractor and focal reducer. Say a second hand 80ED and a 0.5x (1 1/4") focal reducer which would give you a focal length of 300mm. By adjusting the placement of the FR you can control the focal length and so optimise your field of view to mach your webcam. It is then also easier to fit a solar filter and mount the optic. This way you will have a system with minimal chromatic aberration for relatively low cost. Any 'cheap' telephoto will have chromatic aberration that really shows up at the pixel size of a webcam.

You could also use a H alpha filter WITH a solar filter to minimise chromatic aberration.

Bert

Last edited by avandonk; 25-08-2008 at 07:36 AM.
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Old 25-08-2008, 07:41 AM
Zuts
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Hi,

I hope you are planning on using a filter, otherwise you run the risk of melting your CCD chip and or your eyeball.

Paul
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Old 25-08-2008, 03:43 PM
Dennis
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Hi Paul

A 300mm focal length prime lens (with a full aperture solar filter) should cover the entire disc according to Ron Wodaski’s fabulous and free CCD Calculator. However, it might be prudent to reduce the focal length to say, 200-250mm to allow sufficient empty space around the solar disc for alignment, tracking and aesthetic reasons, so a zoom lens may be the way to go?

You’ll also need an adapter to convert the lens thread/bayonet to a 1 inch fitting to accept the ToUcam.

You can practice on the full Moon as the angular size of the Sun and full Moon is almost identical.

Cheers

Dennis
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Old 25-08-2008, 07:17 PM
thestarman
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chromatic aberration ???

Bert (and everyone else),

I have tracked down a Canon EF 75-300mm F4-5.6 on Teds Cameras website for $200. Have a look at http://tinyurl.com/5kypmd

Any thoughts on how that lens would go with the webcam?

Regards,

Paul Floyd.


Quote:
Originally Posted by avandonk View Post
You could also use a small refractor and focal reducer. Say a second hand 80ED and a 0.5x (1 1/4") focal reducer which would give you a focal length of 300mm. By adjusting the placement of the FR you can control the focal length and so optimise your field of view to mach your webcam. It is then also easier to fit a solar filter and mount the optic. This way you will have a system with minimal chromatic aberration for relatively low cost. Any 'cheap' telephoto will have chromatic aberration that really shows up at the pixel size of a webcam.

You could also use a H alpha filter WITH a solar filter to minimise chromatic aberration.

Bert
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