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 Post subject: First steps in webcam imaging
PostPosted: Mon Jan 23, 2012 9:22 am 
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I've recently been in dialogue on this topic with one of our newer members, which I thought would be worth sharing, since others may have similar questions or be able to offer suggestions from their own experiences.

Here's the story so far..
Following my recent talk on imaging Jupiter, my friend had purchased a Philips webcam kit, including eyepice adaptor and IR-block filter:
http://www.sherwoods-photo.com/astrocam/astrocam.htm

We discussed how the camera is fitted to the telescope:
Quote:
You’ll find that the adaptor (and filter) which screw onto the front of the camera have the same 31mm diameter barrel as your eyepieces, so that you can slide it into the telescope draw-tube (in place of the eyepiece).
But it won’t be parfocal with your eyepiece, so you’ll need to re-focus when you put the camera in.
It’s easiest to start off practising with the Moon – it’s a large target and so much easier to keep in the field-of-view, even if you nudge the ‘scope when removing the eyepiece and fitting the camera!
Or you could even try in daylight with a distant terrestrial object (maybe a chimney-pot or TV aerial on a house across the street?)

and how big the image would be on the sensor:
Quote:
you’ll be using the telescope optics to form a real image onto the webcam’s sensor – just like a camera does with its telephoto lens. But in comparison with a 35mm camera using a 200mm lens, your telescope has a much longer focal length and the webcam sensor is only 1/3” across, so you’ll still get a reasonably-sized image (once you can get your ‘scope pointed accurately enough).
In order to get my large images of Jupiter I use a 2m focal length ‘scope and a Barlow lens, but this degree of magnification makes it extraordinarily difficult to point accurately enough – hence I need a cross-hair eyepiece with a flip-mirror for aiming!
And as the magnification increases, the image gets dimmer, and you’re even more at the mercy of atmospheric turbulence.

I recommended the free “CCD” utility from the Astronomical Society of Southern New England as a useful guide to image scale:
http://www.assne.org/ccd.zip
Here’s its output for my Jupiter imaging system:
Attachment:
ccdjupiter.png
ccdjupiter.png [ 9.58 KiB | Viewed 5916 times ]


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 Post subject: Re: First steps in webcam imaging
PostPosted: Fri Mar 21, 2014 4:58 pm 
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When planetary imaging at very high magnification with a webcam, one of the principal challenges is pointing the 'scope accurately enough to get the planet's image onto the tiny CCD in the first place. I normally use a flip-mirror and a cross-hair eyepiece, but as these can be expensive to acquire, I've just come up with a very simple zero-cost alternative.
I found an old translucent plastic canister from a Fuji 35mm film cassette (remember them?), which is a nice sliding fit into a 1.25" eyepiece holder. When this is fitted in place, a "real" image of the planet (or Moon) is projected onto the back of the canister, which allows the mount to be fine-adjusted to get the image precisely central (there are even concentric mouldings in the plastic to help gauge this):
Attachment:
File comment: Using the plastic canister for aiming the 'scope
SM90_finder.jpg
SM90_finder.jpg [ 279.83 KiB | Viewed 4921 times ]

It's actually easier using this to centralise the image than trying to estimate when it's in the exact centre of the field of an eyepiece. Then the webcam can be swapped in (taking care not to nudge the 'scope in the process):
Attachment:
File comment: Philips webcam fitted for imaging
SM90_toucam.jpg
SM90_toucam.jpg [ 271.85 KiB | Viewed 4921 times ]

:idea: If you can't find yourself a translucent canister, you could always cut the bottom off a plain black one (or just find a short length of tube of the right diameter) and stick some tracing-paper with a cross-hair ruled on it over the end?


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 Post subject: Re: First steps in webcam imaging
PostPosted: Sat Mar 22, 2014 10:42 am 
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Joined: Fri Sep 07, 2012 7:20 am
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A really useful tip Ross. I'll have a spare translucent 35 mm film case with me at the next meeting on 1st April which the new member can pick up or you can pass on. Bill


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