Galaxy Blues Observatory

Astrometry

!!!!! UNDER CONSTRUCTION !!!!!

When doing astrometry, you are measuring an object's position in the sky. Since the targets that I'm interested in are always moving (asteroids), the position changes with time. By reporting these positions accurately to the International Astronomical Union's Minor Planet Center (IAU-MPC site), they can verify and correct the orbital elements they have to predict future asteroid positions (which is useful, since this helps NASA and other agencies plan the paths of their spacecraft sent to them for exploration for example, or determine if we need to send Bruce Willis in space!

Here are the steps (there are many, I'll break them into multiple pages when I have more time!) that I go through to collect astrometric data for asteroids.

• Go to the IAU-MPC's site (here) to find an appropriate target to study. For tonight, it'll be the trans-neptunian object 2005 FY9.
• Load the latest ephemerides of the asteroid into my favourite astronomy program. I use Hallo Northern Sky, or HNSKY for short. It's free! (HNSKY site)
• Point the telescope at the coordinates given by the program. Remember to get the coordinates for the moment you are slewing the telescope (now!).
• Turn on the program to record a video while I'm taking images. Since the laptop came with Windows Media Centre, I simply start the program and ScreenHunter Pro (a frame grabber, ScreenHunter Pro site). And because the MallinCam Hyper video camera integrates 128 fields (2 seconds), I ask it to take a picture of the screen every 2 seconds while I record the whole thing. The video feed from the camera goes through the KIWI OSD Time Inserter to give each field a time stamp that is accurate to 1/1000th of a second, thus ensuring that the minimum required accuracy of 1 second by the IAU-MPC is met.
• Reset the KIWI OSD so it goes through its cycle of getting a fresh time reading from the GPS satellites, start the recording in Windows Media Player and maximize the image so it goes full-screen, and start the frame grabber when the time stamps start appearing at the bottom of the images. Since Registax (Registax site) can only stack a maximum of about 275 BMP images at a time, I collect 250 images, sor a total time of 500 seconds (8 minutes 20 seconds approximately). If this confuses you, check out the 9-image animation I made here (10 seconds between images, with explanations).
• Use Registax to stack 250 images. It is important to know WHICH images are stacked to get an accurate time for mid-exposure, so I usually take images #2 to #251 which have a time stamp. I start with the second image because the beginning time of its exposure occurred when the first image was first displayed for 2 seconds. This has to be determined by watching the video and looking very carefully for the transition from one image to another. See the animation here for a clearer understanding (again, 10 seconds between images, total of 2 images, with explanations). For the end time of the exposure, I look at the appearance time of the last image, and go back one frame to obtain the time. Then I average both times to obtain the time of mid-exposure.
• The stacked image looks like this (no processing applied, Astrometrica will stretch the image automatically later). I then use Macromedia Fireworks to remove the bottom part with the blurred times that resulted from adding the images with the time display (like this), then convert to fits format with ImageMagick (ImageMagick site). I then name the image as follows: (asteroid name)_(date)-(time).fits. For example, in this case, the name of the file will be 2005FY9_20060421-01h20m24s493.fits since the time of mid-exposure occurred at 01:20:24.493 on 2006-04-21.
• I then start Astrometrica, the astrometry program I use (Astrometrica site). I won't go into the details of the settings of the configuration file or how the program works, suffice it to say that it measures the position of the asteroid and then prepares a precisely-formatted e-mail message that is then sent to the IAU-MPC when all the images have been measured. On the resulting images, it is not unusual for me to see magnitude 19.5 stars despite the severe light pollution of the site. The streched image (inverted to make stars and other objects black on a white background) looks like this. Conditions were OK for this evening, I could only go down to mag. 19.0, despite the 3 sodium lights shining into my backyard! (image of observatory with bright background here, handheld 2-second exposure, no flash necessary!) When I observe later at night when the lights are out, I can usually go down to about magnitude 20.
• Here's an animation of the movement of 2005 FY9. It doesn't move much in 2 hours because its orbit is beyond that of Neptune!

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