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Old 22-02-2008, 04:47 PM
Bucky1379
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'Scope magnification with a webcam

Hi

I have a Philips 900CN with a 1.25" adaptor (which I haven't been able to use yet) having removed the lens as per instructions. I have a question though.

I know that I calculate magnification for my scope using the Primary focal length divided by the eyepiece focal length (eg 1200 / 9 = 133.3333) but how do I calculate it with a no eyepiece and a lensless CCD chip at the focal point.

I have searched the internet a found 1 reference saying the result is about equivalent to a 4mm eyepiece but is this true and how is it calculated?

Thanks for any help understanding this.

Steve
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Old 22-02-2008, 04:55 PM
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iceman (Mike)
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Hi Steve

A very similar question was asked the other day - check this out:
http://www.iceinspace.com.au/forum/s...ad.php?t=28666

Hopefully that answers your questions.. if not, keep asking!
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Old 22-02-2008, 08:18 PM
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bojan
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Hi Steve,
When using camera in prime focus, there is no such thing as "magnification".
Instead, there is "scale", expressed in "/pixel or pixels/" or pixels/deg.
What you really want to ask is "how big field of view I will have when I attach the camera?"
This will depend on focal lens of your telescope and the linear size of the camera sensor.
The formula is alpha=2*atan(d/(2*L)), where
alpha is field of view expressed in radians (if you want degrees, you have to multiply this by 180/pi)
d is linear dimension of sensor in mm
L is focal length in mm
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Old 22-02-2008, 10:06 PM
Bucky1379
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Thanks guys.

All the info is very understandable and makes sense. I also appreciate the formula's etc so I can work it all out for myself .

I could have just connected it all up and expected usable results because I see the photos on the web that other people get with similar equipment but I prefer to understand what I am doing and be able to estimate what will happen if I change something.

It's great being able to get answers so easily and quickly from people who have already worked this stuff out. Thank you.
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Old 23-02-2008, 06:26 PM
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You are very welcome :-)
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Old 27-02-2008, 10:03 AM
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Another way to look at things is to make a picture of the object. If it is on A4 paper and you hold it a tad more than 30 cm away, the magnification shown in the picture is (approximately) 40 divided by the angular width of sky shown on the short side, so for example, if the moon just fills the short side of the paper (20cm), the mag is 80 (40/0.5).

What I mean here by "magnification" is that if you hold the pic of the moon up to the sky at 30 cm from your eye, it will span 40 degrees, which is 80 times more than the real moon.
Geoff
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