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I have read that most astro CCDs cannot typically record short term 
variability due to slow image download even if they can be set to 
short exposure times. To equal exposure times made with video, CCD 
systems need to record images every 1/30 sec for at least a 
minute.  And the system needs to know the exact UT for each exposure.

A possible way to use CCD cameras for transits might be too use the 
"trail" or "drift scan" method.

Basically, it might go something like this: Set the scope at a fixed 
position offset from the star.  Orient the camera with North up so 
star trails will run along rows of CCD images.  A few minutes before 
the event, begin imaging but then turn off your clock drive at given 
WWV time mark and continue to image the star's trail.  The trail will 
break during the occultation and reappear after the occultation 
ends.  After a few more minutes, turn the drive on at a given WWV 
time mark.  The beginning and end of the trail gives you time 
markers, and by finding your plate scale, the break in the trail will 
give you the time and duration of the event.  Counting pixels, once 
you have calibrated the plate scale in terms of pixels, can be used 
to gauge the event times and duration.  Of course, you will need to 
make sure your field width and the off-set position does not allow 
the exposure to run out of the field of view.  This is a must since 
the ends of the trail are use for timing calculations.

Howard

At 09:30 PM 1/23/2007, Tandy W. Carter Jr. wrote:
>Hi Chuck,
>
>    The minimum shutter speed for the SBIG ST-7XME is 0.11 second. I 
> don't know what the limiting magnitude of that shutter speed. I 
> know that a full frame transfer requires 1 second. I know that the 
> transfer speed can be improved by only transferring small portions 
> of the frame.
>
>    CCDSoft can process the data in two methods. First, on a 
> stationary object, such as a star, it will create a light curve. 
> Second, on a moving object, such as an asteroid, it will create a path.
>
>    I can understand you not wanting to spend money you don't need to.
>
>Tandy W. Carter Jr.
>[log in to unmask]
>http://home.att.net/~tandy.carter/
>----- Original Message ----- From: "Charles Broward" <[log in to unmask]>
>To: <[log in to unmask]>
>Sent: Tuesday, January 23, 2007 2:01 PM
>Subject: Re: Up-Coming Occultation
>
>
>>Hmmm,
>>The occultation events need to be recorded with at least a 1/10 sec 
>>resolution...I know my Neximage will do 30 frames a second....I 
>>will try and see if it will detect something on the order of a mag 
>>4 star....I know that CCD imagers will integrate to mag 13, but for 
>>occultation timing it would have to integrate at 1/10 sec 
>>speeds....can they do that, can the software do that?  And will
>>software let you do image processing and record time?
>>
>>Interesting issues....the IOTA scheme lets you record 30 frames a 
>>second with a time hack on the tape or disk or whatever...a simple 
>>solution if you are willing to buy the camera (super sensitive) and 
>>a recorder....I just don't want to spend a $160 for a camera I might use once.

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