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can you use a webcam with a reflector?

amyscott

By amyscott
in Imaging - Discussion

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Mikea

Mikea

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Hi Amy, it's not dumb - it's actually a very good question. One which I am not expert enough to answer fully except to say that there is a simple relationship between the scope focal length and the sensor size. You can effectively double the mag for a given focal length by using a x2 Barlow. One of the imagers will be along shortly with a better answer!

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kentronix

kentronix

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I am not really an expert but you can most certainly use a webcam, direct in the focus tube on a reflector to great effect.

The magnification is from the curved mirror at the bottom of the scope, this is basically the equivalent of the lenses in a refractor (I think !)

I have stopped there because thats basically all I understand about it :)

If you follow the link from my sig there are some examples of my images using a spc900nc webcam in a reflector.

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sixela

sixela

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"Magnification" is something that is linked to afocal systems (a scope plus eyepiece). But for a telescope plus sensor, the field of view of a sensor of size X i a given direction is given by:

Field (in degrees) = sensor size / focal length telescope * 57.3.

And yes, with a barlow the telescope plus barlow system has its focal length increased by the barlow factor.

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sixela

sixela

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FWIW, the ubiquitous SPC900n has a sensor with a diagonal of 4mm and a long side of 3.2mm. 4mm is the field stop size of a 6mm ortho, so what one of those webcam sensors showis indeed a rectangular field that just fits in the field of a 6mm ortho. Obviously, if it's a 6mm Ethos it's quite different :).

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catchem

catchem

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just to add to that, can the webcam be used without the barlow lens because i was able to focus with a 3 times but removing it i wasnt able to. Also im not sure how i could process with Registax as a one minute exposure was moving over the moon like i was flying over it!!

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Nillchill

Nillchill

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I have some pics posted in here of the moon I took with a webcam, without a barlow, it should focus.

you can either hand track the image by adjusting the RA (assuming reasonable alignment), or take a video stream , then pick a feature to stack the video around, Registax does this very well.

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catchem

catchem

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sorry i didnt make myself very clear, yes i took a video stream but the moon was being covered very quickly unlike a photo i could track roughly the same area but on video i didnt have the same sort of image to stack as it was covering a wide area, that was a 3times barlow i wouldnt have thought it would be any different with a 2times.

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themos

themos

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sorry to sound dumb but before i buy a webcam to use with my reflector id like to know where you get the magnification from if you are using an adaptor and no eyepiece?

Have you ever seen a pinhole camera? That thing doesn't even HAVE optics (mirrors and lenses), just a small hole in a screen and yet it makes an image that you can see if you hold a piece of paper behind the pinhole. The further away you hold the paper, the bigger the image!

Unfortunately, it also gets dimmer. If you make the pinhole bigger, you let more light in so the image becomes brighter.

Unfortunately, it also gets blurrier.

And that's where optics come in. By shaping a piece of glass and placing it in the pinhole you fool all the light that enters the enlarged pinhole to behave as if it came from a single point and the image becomes clearer!

Unfortunately, optical tricks like that only work at one distance from the hole. That's why lenses and mirrors have a "focal length".

So, the lens and mirrors don't create the magnification, the geometrical arrangement creates the magnification. The lenses and mirrors just make our image bright even far away from the hole (and remember, further away means bigger image).

Now you may ask, what does the eyepiece do? Well, the primary lens or mirror creates an image inside the scope and the eyepiece is just a glorified magnifying glass that we can hold close to the image so that it is magnified, same as when we hold a magnifying glass to a piece of writing on the page.

So why not use a magnifying glass directly on the moon?

Because of the geometrical arrangement of the magnifying glass, the distance of glass-to-object and glass-to-eye must be comparable. So, we could use a magnifying glass on the moon but someone would have to hold it in place halfway (or so) to the moon... not practical.

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rorschach

rorschach

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finally...something that makes sense in my head....I actually understand!!!! :)

so where does the f1-f6 fit in with releation to scope lengths and EP's? is there a distance chart or something? (sorry if this is off topic, but its related!)

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naked"I"

naked"I"

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Have you ever seen a pinhole camera? That thing doesn't even HAVE optics (mirrors and lenses), just a small hole in a screen and yet it makes an image that you can see if you hold a piece of paper behind the pinhole. The further away you hold the paper, the bigger the image!

.

wow, very good post there themos, now I understand all well some any way :)

Dave

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Stephen

Stephen

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finally...something that makes sense in my head....I actually understand!!!! :)

so where does the f1-f6 fit in with releation to scope lengths and EP's? is there a distance chart or something? (sorry if this is off topic, but its related!)

f number = focal length of tube / diameter of mirror

So mine is 1200 / 203 = f5.91

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rorschach

rorschach

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Sorry my head doesn't translate well to the page! lol

when we talk about the "f" value of things what does it actually mean?

an OTA can have a value of "f5" , and say , the web cam has an equivalent value of "f6"

does the "f" value reflect distance? or magnification? both?

please excuse my ignorance, I have never really played with optics up until now...much as I am loving looking through the scope , i feel an understanding of the "f" value will improve my viewing , and aid future lens purchases!

thanks for the help

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themos

themos

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Ok, the pinhole camera is f-infinity, theoretically, because the aperture is very small and the focal length, well, whatever you want it to be! Imagine we start making the hole bigger and sticking some glass in the hole to make light bend and make sharp images. As I said, that trick works only for a certain distance between hole and image, the "focal length". Since the hole opening is still small, this would be a f-100 or f-200. As we make the hole bigger, we need more glass and let's say we keep the focal length the same. Then we need to bend light more and more as we make the hole bigger. So we get down to something like f-8, the hole is one-eighth the focal length. After that it gets harder and more expensive to make the right glass shape but f-0.7 lenses have been made (that's a lens that has greater aperture than focal length, a monster that was used to shoot Kubrick's Barry Lyndon scenes in natural candlelight). So, that's as bright an image as we can hope to get and we are really bending light here.

Now, the webcam is not "f6", f-ratios don't make sense for sensors. The only thing that makes sense is how big they are. What people said is that if you put it at the focus of a particular scope, the rectangular sensor captures a part of the image that corresponds to what you would see in a particular eyepiece of a particular focal length (of 6millimetres). Not all 6mm eyepieces are equal. They all produce the same magnification but some show you more than others. Some feel like you're looking down a long tube and some feel like you're looking through a window. In the former, you would see only two stars of a constellation, in the latter you would see all 12, say. So it's not a great way of specifying the size of a field of view but it's adequate because the cheap-and-cheerful 6mm eyepieces are pretty much the same and many people have such an eyepiece and are familiar with them.

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themos

themos

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There must be some maths somewhere which will define this in relation to focal length, aperture and sensor size for a Newtonian reflector.

To calculate how big the field of view is that is captured by a sensor you only need to know the size of the sensor and the distance of the sensor from the aperture (the focal length), just as in a pinhole camera.

It's a right angle triangle. Call the distance to the sensor "f" and the sensor size "s" (they are not square so you have two sizes to worry about but let's keep it simple). The right angle is between f and s/2. The ratio of those is s/(2*f). You then find the angle whose tangent (not sine, mistake corrected) is equal to s/(2*f) and double it. For small angles that's just s/f. So, a 5mm sensor on a 1000mm scope corresponds to 5/1000. What units? Radians! To convert to degrees you divide by pi (3.14) and multiply by 180, to get 0.3 degrees which is about 17 arc-minutes (multiply by 60) or about half a full moon.

PS: it's Themos, not Thermos!

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