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Converting the camera to prime focus

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Offline RaduP

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Re: Converting the camera to prime focus
« Reply #30 on: 09 / July / 2008, 12:29:32 »
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In order to do image stacking, you need to have something other than noise, so unless you have some huge aperture (say, 12+ inch), you are not going to get much data at all.
As for the drift, you need a tracking telescope for deep space imaging.
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Offline LjL

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Re: Converting the camera to prime focus
« Reply #31 on: 09 / July / 2008, 14:08:18 »
Sorry, I still don't understand. Sure, given a single image, it's entirely possible that you won't be able to discern anything but noise in it... but for each pixel, there's still a higher probability (not much higher, possibly) of it being lit if there's a light source in that position, no?

Which in turn means that if you take enough frames, you'll eventually be able to see the object. And if you take 600 exposures of 0.1s each, you should have the same (if not better) SN ratio as if you'd taken a single one minute exposure...

Of course, alignment will be exceedingly difficult if it needs to be performed, but you said you need a tracking telescope for this in the first place.

What's wrong in my reasoning?
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Offline RaduP

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Re: Converting the camera to prime focus
« Reply #32 on: 09 / July / 2008, 14:15:49 »
Well, one thing that's wrong is that with most of the webcams you might not get exactly a raw image, so you get artefacts at each frame.
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Offline LjL

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Re: Converting the camera to prime focus
« Reply #33 on: 09 / July / 2008, 14:36:32 »
Ah, I suppose that's true, but the older the webcam, the higher the chances it doesn't do fancy processing beore handing out the data to the driver, I guess...
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Offline RaduP

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Re: Converting the camera to prime focus
« Reply #34 on: 09 / July / 2008, 15:13:07 »
Another issue is that even if somehow you manage to capture the raw frame (and I don't think that's easy, and requires a special driver for each camera), you still have another problem you can't beat:
The sensor is an analogue device. If it gets, say, 100 fotons, then the ADC will give you a 1, if there are 200 photons it gives you 2, and so on. Obviously, those are made up numbers, just to ilustrate the problem.
Now, say you want to image a faint nebula. So if the frame rate of the camera is 1/s, and you only get 99 photons from the nebula, then you will get a 0 pixel. If you take 1000000 frames, you average the values: (1000000*0)/1000000=0
If you can get a long exposure, then you won't get dark pixels because the ADC will have more photons to convert.
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Offline LjL

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Re: Converting the camera to prime focus
« Reply #35 on: 09 / July / 2008, 15:25:25 »
Now that one's convincing ;)
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Offline dzsemx

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Re: Converting the camera to prime focus
« Reply #36 on: 12 / July / 2008, 18:50:06 »
yeah better to do long exposures with tracking telescope, and darkframe substraction to remove ccd errors
and canon powershots are limited to 65 sec... it isn't enough to capture that poor light
« Last Edit: 12 / July / 2008, 18:53:22 by dzsemx »
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