Concurrent Imaging / Guide Cam ..... Why not ??

[Craney]

Evening all,

Anybody give me a simple, non-too-technicalish answer to why the imaging camera cannot be used to guide the scope at the same time ??   

In this digital age, could  a slice of the electronic image could be dribbled away into a buffer to provide a ghost image to  'guide' from.

I was contemplating this whilst fiddling around trying to get my guiding to work...... so quite a while...  

The answer I came up with  was that.... (guessing).... the chip only offloads its digital values after the full exposure.  ie. you cannot get a snap-shot during the process

  Hence your guiding would be subject to the exposure time on the main camera.    Am I on the right lines.    Just curious......

 

[Davey-T]

You'd need to be able to download an image every couple of seconds so you'd need a sensor that doesn't exist as far as I know, bearing in mind that none of the sensors we use are actually designed for astro' imaging.

Dave

[Craney]

Yes, although they are used for astro work,  I suppose the technology is designed for regular 'light' photography and the market dictates the use.

Thanks for the reply.

[vlaiv]

Process of reading of pixels - destroys values - you need to let electrons to flow out of pixel well to "count" them. For CCD this is true, not sure for CMOS - one way to measure it is via electric charge, other is via current. CMOS has circuitry for each pixel, so in theory, if measuring voltage / charge (keeps electrons in well) - value could be "read off" in the middle of exposure, but I don't think needed circuitry is implemented in electronics of CMOS devices.

What could be possible is to "dedicate" certain portion of chip and read for example N top rows - giving you sort of off axis guiding - main imaging area would be smaller, but you could have guiding that way. Not sure if it can be done with current sensor design in firmware, or some changes in design would be required.

[Shibby]

SBIG have dual-sensor cameras, so can guide and image with a single unit.

[Craney]

Thanks @Shibby  , interesting to see how they work the magic.