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Old 03-01-2007, 06:35 PM
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really stupid question about webcams

HI everyone - hope you all survived the NYE period...

This is going to sound silly -

With webcam astrophotography, when you insert the webcam with its lens removed into the focuser (without the eyepiece inserted) - where does the magnification come from to take those close ups of the planets and the moon etc.

When i look down into the focuser all i see is the secondary mirror etc.....surely thats all the webcams going to see.....is there something else i have to do to the webcam other than take its lens off and stick a 35mm film canister on the front end?

I havent tried my new webcam in it yet because its been winter here for the past couple of days....but i thought i'd slip a newbie question in while i had the off time.

thanks in advance
Boxy
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Old 03-01-2007, 08:30 PM
Dennis
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Hi Boxy

Simplistically speaking, when used visually your telescope has two main components - the objective lens or primary mirror that gathers the light and your eyepiece which you then use to view the image produced by the objective. The objective actually magnifies the image so at the prime focus, there will be a magnified image of e.g. the Moon or Jupiter. Your eyepiece then “looks” at this magnified image and magnifies it even further.

This is a bit like taking a magnifying glass and focusing the Sun onto the back of your hand – OUCH – DON’T DO THIS. The magnifying glass is the equivalent of your primary mirror and the burning sensation on the back of your hand is the IR part of the Sun’s spectrum making things hot. If you had the mind of a Zen master and could tolerate the burning, you would see a small visible light image of the Sun on the back of your hand. This is what your webcam will be taking an image of.

I have read in several places that inserting a webcam in place of an eyepiece is like inserting a 6mm focal length eyepiece. So the image of the Moon or Jupiter on your computer will look similar to what you might see visually using a 6mm eyepiece.

In (terrestrial) 35mm film photography, a standard lens of 50mm focal length (fl) was said to have produced a photograph at x1 magnification, similar to what the eye sees. If you replace the 50mm fl lens with a 200mm fl telephoto lens, the 35mm film image will now be magnified 4x (4x50=200).

If you then fit a 1000mm fl telephoto lens to the camera, the image on film will appear 20x closer, or magnified 20x (20x50=1000).

So, when the telescope objective is focused correctly, the webcam will be “seeing” an image of what the primary mirror is pointing at, and won’t really be taking a photo of the secondary mirror that your eye appears to be looking at.


Cheers

Dennis

Last edited by Dennis; 04-01-2007 at 06:34 AM. Reason: "of" inserted in last sentence "photo of the secondary"
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Old 03-01-2007, 10:14 PM
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gee you are so good at these explanations, top stuff!
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Old 04-01-2007, 05:12 AM
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iceman (Mike)
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Dennis is king!

The magnification comes from the focal length of the telescope.. so increasing the focal length (by using a barlow), increases the magnification.
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Old 04-01-2007, 06:52 AM
Dennis
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Thanks for the nice complements guys.

There is some stuff I “know” from personal experience, practice and time in the hobby, then there is stuff I am just regurgitating from books that I have read, where I do not (yet) have any direct experience or knowledge.

I enjoy being able to reply to some of the questions, because they often force me to explore my own understanding, which usually leads me to realise that I am missing some stuff. My explanations usually represent my own journey in understanding something new. It’s a bit like finding a missing piece of the jigsaw puzzle in my knowledge gap, exposed by the question.

This is why there is no such thing as a dumb question!

Cheers

Dennis
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Old 04-01-2007, 07:53 AM
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thanks

Thanks very much for that Dennis - I am sure to have a lot more 'dumb' questions for everyone as I travel the path....


regards
Boxy

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Old 04-01-2007, 07:59 AM
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ving (David)
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well pass them on

onya dennis
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Old 04-01-2007, 09:09 AM
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A question is dumb only when you DON'T ask it.
I'm sure there were quite a few of us following this thread who were waiting to see your question answered.
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Old 04-01-2007, 02:16 PM
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I think I should add two cents worth more to this discussion :-)
Many people are confused with these concepts...
There is a fundamental difference between "magnification" and "scale"..
"Magnification" is the factor by which the apparent angle of the object is magnified by the optical system, and it is approximatelly given by the ratio between focal lengths of objective and eyepiece.... This is non-dimensional number and is applicable to visual observations only, since only virtual images are dealt with (by "virtual" I mean the image of the object which size can not be measured in terms of length).
In case of any sort of projection on the sensor (film, CCD) (like prime focus image) the real image is involved and we are talking about "scale", which is expressed in deg/mm or arcsec/pixel... To calculate the approximate scale of the simple telescope-camera system we can use expression: s=0.057/FL where FL is effective focal length in mm. The result will be in degrees per mm and will be valid for only small angles (like up to 1 degree).
I hope this explanation will be helpful :-)

Last edited by bojan; 04-01-2007 at 04:20 PM. Reason: clarification
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Old 06-01-2007, 05:46 PM
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very first afocal photograph of orion

Well, the journey has to begin somewhere ...... here is my first stacked (5 x 15sec exposures with a Canon A610 on a 130mm Skywatcher reflector) image of the Orion Nebula.....i didnt get the polar alignment right, so the stars have trails....almost feel embarrassed to put it up
I tried the webcam i purchased as well, works fairly well..... is it a case of stacking the best images selected from the avi file to get a crisp image of the moon?


regards
boxy
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Old 07-01-2007, 06:53 PM
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Great work Boxy.
I look forward to seeing more.
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Old 07-01-2007, 07:06 PM
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ballaratdragons (Ken)
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Boxy,

I was also informed that a Toucam/Neximage/LPI is equal to a 6mm EP. Look thru a 6mm EP and you will see what the Imaging Webcam sees. You can increase the magnification by attaching the webcam to an EP or Barlow, or decrease it with a Focal reducer, or take Widefields using the webcam without a scope. Just point it at the sky.
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Old 14-01-2007, 04:59 PM
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Quote:
Originally Posted by boxy66 View Post
...almost feel embarrassed to put it up
Hi Boxy never feel like that, Astrophotography is a refined art as I am finding out, and some of my pathetic initial attempts have some merit. To some of my friends they are amazed at what I have captured, regardless of the quality/focus etc.

Experiment and try everything you can. Post it up for comment and critisism. There is a great wealth of knowledge on this forum and some good guys who will point you in the right direction.

The moon is a great subject especially using filters. Its brightness is never the same value..

Some nights I have run off up to a 100 or more individual and differently timed shots and got only a couple I consider good. By keeping a log of condx and exposures one can refine the art.

The dumbest question is the one that isn't asked.
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Old 12-02-2007, 10:20 AM
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orion neb---getting a little better

Okay - this is a bit better than the last one....still with my Canon A610 at 15 sec exposure (so i'm pretty please with the result) - no stacking or anything - just the one pic, cropped and contrast altered a little - i'm happy i got some colour in it....
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