webcam better than DSLR


by Cranfieldstar
Tags: dslr, webcam
sophiecentaur
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#19
Nov15-12, 06:40 AM
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Quote Quote by russ_watters View Post
A teleconverter is a complete optical system whereas a Barlow is not. This is your main mistake: not understanding the optics you're dealing with. Webcam astrophotographers do not use stock lenses on their cameras, they use telescopes. Step 1 in converting any camera to an astrocam is to remove the lens!
I'm not sure what you mean there. Good telescope optics are of exquisite quality and they are expensive. How much money are you talking here? Note, though, that mirrors are cheaper than lenses....
Thanks for that info (and the other posts). What you say makes a lot of sense; every field of study has its own wrinkles and you are setting me straight.
The birding telescopes I was looking at were less than 1k but not much less. I appreciate that birders are likely to be far less techie than astronomers so they may just not be so fussy - things like weatherproofing being possibly more important for (foul weather) birding than (essentially fair weather) astronomy.

I need to re-think my ideas about using cameras with telescopes. I was clearly 'raving' to suggest that the camera lens would replace an eyepiece as you are looking at a virtual image in an eyepiece and you need to project a real image with a camera. (though, why not use an eyepiece and then photograph what you would 'see'? I guess that would involve more glass in the way - not a good idea when contrast and low flare are seriously important in astronomy. However, I don't actually understand why the best solution appears to be just to use the native resolution. I think I understand what that means - just projecting the image from the objective (+barlow) onto the sensor?. Surely it must be possible to improve on final quality by processing images where the samples (pixels) are more dense (i.e. using more of the image array). I appreciate that the flux arriving at each pixel would be less but the processing would take care of that.
sophiecentaur
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#20
Nov15-12, 06:45 AM
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@russ
I looked at the web page with the CCD arrays and they are more like the sort of price I'd expect to pay for serious imaging. More than your average webcam!
So you astronomers really do have to be stinky rich, as with all serious pastimes! I pour all my money down a boat-shaped hole instead.
russ_watters
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#21
Nov15-12, 07:01 AM
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Well... Skill is important. There are guys using normal webcams who have gotten better results than me.

Native resolution is 1 pixel on the ccd = 1 pixel on the screen. You don't want the camera intepolating because it can mess up the data.
Cranfieldstar
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Nov15-12, 07:03 AM
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Cheers Russ, But when you say span 400 pixels, do you mean an area of 20 x 20 pixels, or do you mean a linear dimension of 400 pixels, so jupiter actually falls over an area of approx 400 x400 pixels (160,000 pixels).
russ_watters
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Nov15-12, 07:06 AM
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400x400
sophiecentaur
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Nov15-12, 07:11 AM
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Quote Quote by russ_watters View Post
Also: Jupiter is 40 arcsec across and the best amateur telescopes have a resolution of about 0.1 arcsec. So you really don't need Jupiter to span more than about 400 pixels.
Quote Quote by Cranfieldstar View Post
Cheers Russ, But when you say span 400 pixels, do you mean an area of 20 x 20 pixels, or do you mean a linear dimension of 400 pixels, so jupiter actually falls over an area of approx 400 x400 pixels (160,000 pixels).
Those sums, above, give you your answer.
But I can't help feeling that 400pxl linear gives you no chance to do really smart sharpening or any of the other clever enhancements that over sampling would allow. It must be a matter of optimum practice, based on lots of factors.
russ_watters
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Nov15-12, 07:19 AM
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Quote Quote by sophiecentaur View Post
Those sums, above, give you your answer.
But I can't help feeling that 400pxl linear gives you no chance to do really smart sharpening or any of the other clever enhancements that over sampling would allow. It must be a matter of optimum practice, based on lots of factors.
For Jupiter, it wouldn't hurt to double that since it is so bright. Oversampling may help a little since the pixels are square.

Other problems do creep in though: Planets won't stand still in the frame so you need to actvely track it. Saturn is dim enough you'll start needing longer exposures, which make atmospheric blurring worse.
russ_watters
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Nov15-12, 09:14 AM
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Looks like I misremembered the resolution: I just looked it up and my C11 has a max resolution of about 0.4 arcsec. I checked one of my photos and Jupiter is 300 pixels across, so I'm oversampled by a factor of 3.

Another issue to consider though: Image scale for focusing and viewing on a monitor.

There are a lot of resolutions to consider:
-Sky
-Telescope
-Camera
-Monitor
-Eye
sophiecentaur
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Nov15-12, 09:36 AM
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Whaaat?
I can't quite read that word on the bottom. It's gone all blurrry.
digitaldave
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#28
Nov16-12, 01:23 AM
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About the 2 image sensors, webcam vs DSLR.

The telescope generates a full image format roughly the same size of the webcam, filling the entire surface of the chip with light. The DSLR sensor cannot be filled with the image the telescope creates because its sensor is too big.

Thus a mismatch, much like trying to mount a 110 CAMERA sized lens (0.51 inch 0.67 inch negative/sensor format size) on a medium format camera (2 1/4 inch x 2 1/4 inch negative/sensor format size).

The light transmitted by the telescope couldn't fill the DSLR sensor with light, only filling a very small area of the sensor thus creating an extremely tiny image.

*There are many size formats on DSLR's made today.

http://en.wikipedia.org/wiki/Image_sensor_format.
sophiecentaur
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Nov16-12, 03:56 AM
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I have a feeling that this effectively boils down to 'f number', used all the time in photography. For a bigger sensor area, you need a bigger aperture for the same field of view and the same 'exposure' if the sensor elements have the same sensitivity / noise performance. Obviously, the purpose designed astro cams will have been optimised for sensor area, compatibility with available apertures and focal lengths and there is a huge advantage in not having colour filters there when you don't actually want them. Also, they are incredibly neat little things that fit on the telescope very comportably.
I would love to have a good telescope - even if only to sit there and admire / stroke it. But it is in a queue, after a new marine diesel engine and a few other expensive bits of kit.
Drakkith
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Nov16-12, 05:41 AM
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Quote Quote by digitaldave View Post
About the 2 image sensors, webcam vs DSLR.

The telescope generates a full image format roughly the same size of the webcam, filling the entire surface of the chip with light. The DSLR sensor cannot be filled with the image the telescope creates because its sensor is too big.

Thus a mismatch, much like trying to mount a 110 CAMERA sized lens (0.51 inch 0.67 inch negative/sensor format size) on a medium format camera (2 1/4 inch x 2 1/4 inch negative/sensor format size).

The light transmitted by the telescope couldn't fill the DSLR sensor with light, only filling a very small area of the sensor thus creating an extremely tiny image.
If the two devices have the same pixel sizes, then the image of Jupiter will be the same. What does change is how many pixels just see black and don't contribute to the picture at all.
russ_watters
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Nov16-12, 05:46 AM
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Quote Quote by digitaldave View Post
About the 2 image sensors, webcam vs DSLR.

The telescope generates a full image format roughly the same size of the webcam, filling the entire surface of the chip with light. The DSLR sensor cannot be filled with the image the telescope creates because its sensor is too big.
There are plenty of people who use DSLRs at prime focus for astrophotography with little or no vignetting. How a Barlow changes the light cone I'm not entirely sure, but it should widen the cone unless the internal structure of the barlow cuts part of it off.
russ_watters
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Nov16-12, 05:47 AM
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Quote Quote by sophiecentaur View Post
I would love to have a good telescope - even if only to sit there and admire / stroke it. But it is in a queue, after a new marine diesel engine and a few other expensive bits of kit.
[shrug] Sounds like a problem of improperly conceived priorities to me.

Still, a local star party through an astronomy club or college would at least allow you to see what all the hubbub is about.
Drakkith
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Nov16-12, 05:48 AM
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Quote Quote by sophiecentaur View Post
I have a feeling that this effectively boils down to 'f number', used all the time in photography. For a bigger sensor area, you need a bigger aperture for the same field of view and the same 'exposure' if the sensor elements have the same sensitivity / noise performance. Obviously, the purpose designed astro cams will have been optimised for sensor area, compatibility with available apertures and focal lengths and there is a huge advantage in not having colour filters there when you don't actually want them. Also, they are incredibly neat little things that fit on the telescope very comportably.
Actually the F ratio only really affects the exposure time. Big sensors are generally used equally as well on fast and slow scopes. However vignetting can become a major issue in some types of scopes when you use large sensors. Field of view is completely dependent on focal length, not aperture.


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