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Help... diy planetry cam


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I found an old webcam in the draw so decided to convert it to a planetry cam, I removed the lens assembly, exposing the image sensor and put the shell back together then superglued a focused tube cap on the front.

As soon as a break in the cloud came I rushed outside, slipped it in the focused of my little 114 500 reflector and connected it to laptop running sharpcap.

Along the terminator (moon) the detail was great but everything else is over exposed. Have played with all the settings and even with brightness and exposure to the minimum the moon and Jupiter are overexposed.

Any suggestions on how to rectify this?

Thanks in advance...

Grant

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I've never used sharpcap, but I just did my first webcam astroshooting as well (as in an hour ago). I used a Linux system and guvcview. Camera was a Logitech C270.

I left the brightness and contrast at defaults, turned gain down to zero, and used an exposure of between 50 and 65 out of 10000 for the moon. No auto settings of course.

Hope that helps!

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Depends on what other controls the camera offers, I think.  If it doesn't allow you to control the gain for instance, and just uses automatic gain control, it might be deciding that your image of Jupiter is too dark because most of the sensor is dark and ramping up the gain, still ending up with an image that is mostly dark, but with an over-exposed planet in the middle.

James

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If there was a filter then it was probably an IR filter.  Removing that will tend to bias the exposure more towards the end end of the spectrum on some targets and will probably give you a brighter exposure for the same exposure time than with it, but the only way to tell if it is contributing to the problem would be to add an IR filter back in which is likely to be tricky I guess.

James

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I doubt that removing an IR filter would lead to such overexposure problems, as a webcam's IR filter doesn't cut out very much visible light. If you can find the driver specific to the webcam, perhaps that would enable SharpCap to deal with gain control and other fine-tuning? If it's a very old webcam then this might be tricky, but worth a look. 

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If you have no camera control you could try making a cardboard mask to go over the front of your scope.  Try cutting a smallish hole (50mm?) in it and try taking a picture with the webcam - you are effectively making your aparture smaller so should get a dimmer image.  its a bodge but may get you going.

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Thanks, my scope 114 aperture reflector has a smaller diameter in the front cover that you can use, but I was under the impression that the resolving power is related to aperture, so wouldn't I loose detail?

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I doubt that removing an IR filter would lead to such overexposure problems, as a webcam's IR filter doesn't cut out very much visible light.

The point is that CCD and CMOS camera sensors are often fairly sensitive to IR which can noticeably increase the apparent exposure, particularly for the red photosites in the case of a colour camera.  That's why the IR filter is there in the first place.

How much IR there is in astro images is dependent on the target, but people do for example create lunar images using only a fairly narrow IR band.

My suspicion is that the IR component is not the main cause of the overexposure though.  Without knowing what controls the camera offers however, it's difficult to make any particularly good guesses.

James

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Thanks, my scope 114 aperture reflector has a smaller diameter in the front cover that you can use, but I was under the impression that the resolving power is related to aperture, so wouldn't I loose detail?

In theory yes, but I'd not worry about the details until you get things working :)

What might be useful would be to plug the camera in and start SharpCap or whatever you're using for capture, take a screenshot of the available controls and post it here.  At least then we might be able to see what settings you are able to change that might help.

James

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For lunar imaging with a webcam that does not have gain control try increasing the magnification to the point where the image is larger than the field of view, this will give a more even illuminated background and reduce the selective overexposure.   :smiley:

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Looks like there's no gain control then :(  You might try reducing the brightness setting to see if that helps though it's not really ideal.  I'd also turn off the sharpening and see if changing the gamma setting helps.

James

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Played with all the settings, tried uping the fps to force shorter exposures but wont go above 15fps which isn't ideal anyway so might just buy another cheap webcam and start again...

Any recommendations?

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Depends how much you're prepared to spend, really.  The Xbox Live camera is cheap, can be fitted with a standard 1.25" nosepiece and will take an IR filter.  It's not stunning, but it can capture passable images of Jupiter, for instance.  There are occasional SPC900 cameras that pop up on ebay that are somewhat more expensive but considerably better.  After that you're probably into the realm of dedicated cameras such as the ASI034MC.

James

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In general it's not possible to say.  An HD webcam may have smaller pixels and potentially be less sensitive.  Or it may not.  What it will have is more pixels in total, which puts an upper limit on the maximum frame rate, though that may not be an issue if sensitivity is insufficient to sustain high frame rates.

James

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Have you got any colour filters that you could attach to the camera nose-piece/ adapter? 

As others have said, it's very difficult if the camera has automatic gain control with no manual override. If the camera sees a lot of dark sky in the image it will increase gain to compensate in an attempt to get what it thinks is a balanced average brightness across the whole frame.  That makes sense for daytime use, but it doesn't know you're only interested in the brightest part of the frame and not the surrounding dark area.

There's a limit to how much the camera can increase gain and If you could darken the image enough with a deep coloured filter (or more likely a combination of filters that together would cut most of the light), that might give you a usable image even when the camera has boosted the gain as much as it can.   You can disable colour to get a more natural-looking monochrome image. 

Adrian

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