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Old 29-04-2009, 01:40 PM
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allan gould
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Quote:
Originally Posted by celstark View Post
I hate to burst the bubble here, but the answer is no, you cannot. A few bits of info on this:

1) When you read off a CCD you dump the charge that has built up in the "wells". It's only when dumped that it goes into the ADC and you can read it off. Pretend you have one measuring cup and you want to know how much water is in some buckets (and you can't just look at the level in them). Dump and measure... But, the charge is gone and you must start over anew.

2) What you can do is guide between frames. So, see the drift starting to happen, figure the error, send the guide command, and go again. Meade will do this as will others. SBIG was doing this long before Meade.

3) Drizzle has nothing to do with this. Drizzle is a combined alignment / stacking technique developed by Fruchter for the Hubble. Many programs will let you align with Drizzle in an effort to reconstruct a higher resolution than you have in your original frames. Drizzle needs some motion between frames and if you had perfect guiding, Drizzle would not work.

4) There are 2 schemes used to say that "yes you can" and neither get around the fundamental problem. The SBIG solution uses a separate guide CCD. They read off it while the main one still builds up charge. The Starlight Xpress SX-2000 system would use the odd lines for guiding and the even for building up the image. They'd dump the charge from every other line on the interlaced Sony CCDs. But still, they can't dump the charge for guiding and image from the same CCD wells. (They'd flip which are the image and which are the guide every few frames).

5) It doesn't matter if you're using CCD or CMOS

If anyone still thinks the Meade system does this, try the following. Go out and setup a 10 minute exposure. Or a 30 minute one for that matter. You'll show the same exact error you'd see if you didn't guide and ran a 10 minute exposure. There is just no way around the problem that the same CCD wells can't be used to guide (read the image off) and image (keep building the image up) at the same time. Alternate? Sure. But if you start to trail at 20s of exposure you'll still be limited to 20s of exposure. You'll just not have the image drift off the frame as you'll bring it back b/n frames.

Craig
Sorry but you can use the same camera to guide and image at the same time. But it has to be a video camera ie Watec 120N+ (on a TVGuider) or Gstar will do it also. Have a look at Steve Massey's new book Deep Sky Video Astronomy. Ive used my Watec and TVGuider to guide and image at the same time using Virtualdub. You do need a good mount and keep the integrations at 10 sec or below.
You cant use a DSI, DSLR or LPI etc.
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