[meteorite-list] Aussie Photographs Meteor Through Telescope:NOT

From: Jason Utas <meteoritekid_at_meteoritecentral.com>
Date: Sat, 29 Aug 2009 13:28:45 -0700
Message-ID: <93aaac890908291328t3c5c5fc8s53f07634870cf06e_at_mail.gmail.com>

Hola All,
It's a nine-second exposure - why not a satellite? I don't know if a
long-term exposure of a satellite would result in a "wiggly" line, but
if it is as Elton says, possibly the result of the photographic
equipment used - well, any thoughts?
Regards,
Jason

On Sat, Aug 29, 2009 at 10:20 AM, Chris Peterson<clp at alumni.caltech.edu> wrote:
> Your understanding of how CCD's work is seriously flawed. (This camera uses
> a CMOS sensor, but that doesn't really change anything). The sensor
> passively collects photons during the exposure time, and then the value of
> each pixel is read. It's really not much different from scanned film in that
> respect. There is no scanning of pixels during the exposure, no temporal
> aliasing, nothing to generate trail artifacts.
>
> I note that somebody has done a plate solve, and determined that this is
> actually an image of beta Octans. Perhaps so... there aren't many stars for
> a match, but the plate scale works out correctly.
>
> In any case, this still looks exactly like images I've made where the
> exposure begins and then the scope starts moving- usually because the guide
> system fails. If you look are real meteor images, the trail doesn't wiggle.
> It usually varies in apparent thickness because of the changing intensity of
> the meteor, but that's quite different than what is seen here. You only see
> a wiggling trail when the image is actually showing the trail dissipating-
> not in a long exposure where the meteoroid's motion is what creates the
> _apparent_ trail (which is not a true trail). You often don't see any other
> trails when the scope moves during the exposure. If this is beta Octans, the
> next brightest star in the field is 100 times dimmer, and the trail will be
> below the noise threshold.
>
> If this image is actually beta Octans, than it is a fraud, pure and simple,
> since this contradicts the imager's statements. If it is around NGC 253 (and
> I note that there are no matching star patterns), then it is much more
> likely to be a simple guiding failure of some kind, and a misinterpretation
> by the imager. (And the "stars" look a lot more like hot pixels than they do
> stars.)
>
> In either case, there is darn little in this image that argues for a meteor.
>
> Chris
>
> *****************************************
> Chris L Peterson
> Cloudbait Observatory
> http://www.cloudbait.com
>
>
> ----- Original Message ----- From: "Mr EMan" <mstreman53 at yahoo.com>
> To: "Mike Hankey" <mike.hankey at gmail.com>; "meteoritelist"
> <meteorite-list at meteoritecentral.com>; "Rob Matson"
> <mojave_meteorites at cox.net>
> Sent: Saturday, August 29, 2009 10:40 AM
> Subject: Re: [meteorite-list] Aussie Photographs Meteor Through
> Telescope:NOT
>
>
>> I need to revise and extend my remarks from before. ?This probably is a
>> meteor in spite of our first judgments. I too piped in as an early naysayer
>> because I was thinking in the film paradigm. I've rethought the image in the
>> digital paradigm.
>>
>> Long and boring and technical rationale:
>>
>> I've looked at the photo through an image processing application where I
>> can zoom down to pixels and believe I can account for some things which we
>> casually dismissed before because it was not what we were used to seeing.
>> There is some actual color data in the head of the fireball when I adjust
>> the Gamma.
>>
>> 1. If we assume the bulbous tip was a terminal burst not just the end of
>> the exposure then there need not be an trail of equal diameter all the way
>> back up the trace on the image that is the width of the "bulb". I've looked
>> at the trail and it appears to have a uniform width save for the tip. The
>> wispy segmented trail is a result of a fast moving object crossing many
>> sectors of a Charged Coupling Device (CCD) while several passes of the frame
>> scan program are going on.(my interpretation) The color data should be
>> present all along the trail unless the bulb is a terminal flaring
>> representing a several-magnitude flash of energy as we see in a terminal
>> burst.
>>
>> 2. The tail is not as squiggly as we first thought, but seems to be an
>> artifact of the CCD array and how the image information is captured. ?Be it
>> remembered that while film collects the entire image the entire time of the
>> exposure, digital "timed exposure" imagery is the summing/melding of
>> thousands of passes over the CCD sector by sector, pixel by pixel. Each
>> pixel has to be given permission to purge information to make ready for the
>> next pass. even at computer Hz rates this can cause a pile up of information
>> as the data is read and written to storage. Some pixels simply will not be
>> ready to receive and hold light data as the fireball is passing.
>>
>> 3. To make a timed exposure in a digital camera, the data of one pass is
>> added to the data of all the cycles before it. The image data is also
>> processed by several algorithms to try to accommodate a range of
>> conditions--none of which are optimized for a high speed intensely bright
>> object on a black background. Likewise, we have no way of determining when
>> the meteor passed as all scan data is lost once added to the image file.
>>
>> 4. I won't delve into the full technical aspects of latency of signal and
>> how the microprocessor polls the signal from each pixel on the CCD, etc.
>> But- for an allegory we are all familiar with, think of how "wagon wheels"
>> in old westerns appear to spin backwards on film. ?It looks that way owing
>> to a difference in frame rate of the film and the actual speed of the
>> wheel's rotation. ?A timing discrepancy in a digital frame gives rise to a
>> "smeared streak" with black gaps at the spots where the data is being reset
>> so we can get a "-==_ --==-_ -==-_ -==--_ -==_" for what would have been a
>> continuous straight line to our eye.
>>
>> 5. When we look at the "mask" of a CRT TV that hides the edge of the
>> phosphors to make them appear uniform, we see a grid of black rectangles.
>> CCDs have a similar grid/blind grid. When an object crosses the screen
>> horizontally-- keeping on just that row the line is straight. ?Cross it
>> diagonally and it looks like a series of step downs or step ups. Add in the
>> aberration of lens curvature, a slight internal vibration from the drive
>> motor, process through a jpg compression algorithm and you get a "squiggly"
>> line even at normal zoom!
>>
>> All in all, in light of what I remember now about digital still-frame
>> photography, this is a righteous shot.
>>
>> Elton
>
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Received on Sat 29 Aug 2009 04:28:45 PM PDT