JohnD wrote:Nonsense!
Or more politely, that may not be the explanation. A continuous exposure that showed such discontinuity could have that explanation, but this is not a continuous exposure. It is described as a 'composite', many individual exposures, each elongated into a dash by the duration of that exposure, superimposed on the same star field.
John
Greetings all,
I will explain the process I used to create this image and it’s accompanying animation to help alleviate anyone’s misconceptions.
My intent was not to do any precise scientific measurements, but to create an animation of one of the few astronomical events that could be captured in real time that shows something other than a static event. I captured the Solar eclipse of 04/08/05 (15% at my location). I captured 4 hours and 41 minutes of the Mercury Transit on 11/08/06, and I thought I would capture a fast moving NEO glide past M81 & M82.
I was out and had my equipment setup and balanced twenty minutes before dusk. I knew that 2006 VV2 was already passing M82 as the sky was darkening enough for me to polar align and synchronize my G-11. While it was still too light to start the image run, I was framing and focusing the camera.
The asteroid was moving at a tad over 0.5 arc/sec per pixel. This would allow about 12 seconds per pixel exposure time, as I binned the ST8-XE camera at 2x2 giving me 6.18 arc/sec per pixel image scale through the Orion ED80 I shot the images through. Since my intention was the animation I chose a 60 second exposure to give a five pixel streak per exposure. This time limit was also selected because the Moon was at about 82% full and I was shooting with the clear filter.
I started the actual saved images at 03/28/07 8:15pm or 03/29/07 03:15 UT. For the next 77 minutes the ST8-XE took 60 second exposures with a 5 second gap between exposures to allow the image to download to the laptop. The ST8-XE using it’s internal guide chip guide corrected the tracking of the G-11 to within one pixel for the entire 77 exposures using one second guide exposures.
The next day I used CCDSoft and “aligned” the images. This was mostly to correct for any field rotation that may have crept in from a hasty polar alignment. Then I used CCDSoft to median combine the 71 images. This allowed me to make a background image of the star field and galaxies, but the asteroid was not present because of the median combine blending method.
I then used PhotoShop, with the FITS Liberator plugin to layer all 71 individual frames on top of the background star field image. When I imported the 71 frames into PhotoShop, I set the histogram range to 5,000 for each frame. This gave the individual frames a uniform brightness level that removed the sky illumination caused by the Moon, yet left the stars and asteroid easily visible.
The “composite” image was the background frame and the 71 individual movement frames. To make the static “flat” image I just set all of the blending methods to “Lighten”. This resulted in the background frame remaining consistent, and the asteroid for each frame showing.
The animation was just the background image and one individual frame after the other saved as a separate GIF image. These where later used to create the animated GIF file.
There where no clouds visible during the exposures. All frames where the same exposure time. All image processing was done in an exactly consistent manner. When I started assembling the image I saw the asteroid’s brightness drop in the first three or four images, and wondered if I was doing something wrong. I checked to be sure that everything was done the same way each time. When I saw it was I just continued to process till I was through. I saw the light curve that this produced but was surprised. At the time I didn’t know 2006 VV2 had a rotation period of 2.43 hours. I knew most asteroids had much longer periods.
A few days later 04/02/07 I imaged 2006 VV2 again for 3 hours and 31 minutes (the entire FOV of the ED80 and ST8-XE) when the asteroid saw moving just below 0.4 arc/sec per pixel. I wanted to see if I could see the entire light curve, and if indeed it was a light curve I was seeing. You can see the image here:
http://www.budgetastro.com/web/2006VV2- ... 9-1240.jpg
or the animation here: (beware it is a 16 meg file)
http://www.budgetastro.com/web/2006VV2-04-02-07.gif
The same procedure was used for these images as the flyby image. The only differences are that they are 60 second exposures with a 60 second gap between exposures. Also the moon was full and fairly close, so the histogram range was different (I don’t remember what I used), but consistent for all frames. This image also clearly shows the light curve of 2006 VV2. I saved the PSD files, so if anyone would like them just let me know which one or both you would like and I will upload them.
If anyone would like to know anything else I haven’t covered, just ask.
Robert Long….
Vado, NM.