Red Barn Observatory MPC/IAU H68

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A Practical Guide To Chasing Asteroids

For The Astrometrist

By: Steve E. Farmer Jr. & Marcelo Saavedra

Chapter 6:  How To Image Fast Moving Objects

Fast moving objects (FMOs) are a little more difficult to image than slow moving objects (SMOs), but it’s still easily done with the proper procedures and techniques.

How do you define a fast moving object? Actually, FMOs are usually not moving any faster than SMOs but are merely closer to Earth – they only “appear” to be moving faster. In a long (or sometimes short) exposure of a field of stars, galaxy, or nebula, you will see a “streak” of light that appears. This could be a long streak produced by a FMO or a very short streak (if any) produced from a SMO. It's all according to the length of the exposure and how fast the object appears to be moving. Often you will notice bright streaks that will appear to move completely across the field of view and sometimes ruin your astro-photo. These are Very Fast Moving Objects (VFMOs) - or usually man-made satellites. The average asteroid moves at an absolute rate of about 20 kilometers per second. The motion/movement that we see in our images that appear as streaks of light or points of light from the asteroids is mostly determined by the “distance” between the asteroid and your position on the Earth.

Things you need to know to image a FMO.

1. What is the objects speed/motion? This is determined by checking the “/min or “/hour. Most commonly, “/min is used. An example of a FMO would be 10.2”/min. This means in one minute, the asteroid will appear to move 10.2 seconds (in distance) across the sky.

2. What is the P.A. (Position Angle) of the object? The P.A. is referenced to the direction in which the object appears to travel. This is determined by checking the P.A.

An example of the P.A. of an object would be P.A. 253.2

3. What is the magnitude of the object? This is usually listed as a “V”.

An example for magnitude would be V = 17.9

Below is how the magnitude and motion would be shown.

 V         Motion   "/min       P.A.

17.9                   10.06     253.2

Note: To convert 10.06"/min into "/sec use this formula.

"/min / 60 = "/sec 10.06"/min / 60 = 0.17"/sec

*Although much detail can be written here, it will be kept short and simple as it is written for the beginner.

The equipment used will determine how long the exposure length should be. Common equipment consists of an 8” to 16” computer operated telescope capable of guiding, and an astronomical grade CCD imager. The telescope can be set at a large range of focal lengths but most commonly a lower focal length is used (f/3 – f/8).

Once you determine the length it takes for an object to move across your telescope/CCD combination, you are ready to begin imaging the object.

Whatever software package you use to produce astrometry must be capable of stacking multiple images according to the motion of the object to get accurate measurements of the object (unless you are using a Very Large telescope and you are capable of detecting faint objects with very short exposures).

Example Observation:

Example Equipment: 14" Meade SCT working at a focal length of f/5 and an ST-8 SBIG camera.

SCT - 350mm f/5 = 1750 mm ST-8 CCD Pixel 9 x 9 micron 1530 x 1020

Arc-Seconds per Pixel 1.06 FOV in Arc-Minutes 18' x 27'

1 x 1 Binning (Un-binned)

Example Object: Object A123456 on the Near Earth Object Confirmation Page (NEOCP) with a motion of 10.06"/min (0.17"/sec) and a P.A. of 253.2 and a magnitude of 17.9.

1.06 * 2 / 0.17"sec = 12.46 second exposure

1.06 (Arc-Seconds per Pixel)

2 (movement across 2 pixels)

0.17"sec (speed of object in seconds)

This object is moving at a fast rate, but it is fairly bright for the telescope being used. Using the formula above, it shows that a maximum exposure length of 12.46 seconds can be taken. This will allow the object to move across 2 pixels (which is normally the "target" of movement).

Now you have your exposure length!

Assuming the positions from the NEOCP are precise, calculate a position to where the object will be in ~30 minutes. This is the point to where you will slew and center your scope. The reason for this: since this is an unknown NEO, you will want to report at least an hour of movement on it. If you center 30 minutes ahead of the target and image it for one hour, it should be centered in your field of view - midway through the session. Therefore, giving you positions - 30 minutes ahead and behind center field of view.

Center the scope on the R.A. and Decl. - begin auto-guiding - begin your series of images taken at 12.46 seconds.

**Recommended settings during exposures --- have the imaging camera pause for 5 to 10 seconds between image download and the next frame (if your camera has a built-in auto-guiding camera). This will allow the auto-guider time to realign the telescope (if needed) after the shutter is closed and the light image has been downloaded. **Also, DOUBLE CHECK the computer clock to verify it has been synchronized and accurate!!! If your clock is off, your measurements will be incorrect!!

If you calculate about 3 exposures per minute times 60 minutes, you will have a total of about 180 - 12.46 second exposures of this new NEO (the more the better). Now it's time to process and stack the images and measure the object.

When a new fast moving object has been discovered, it is a good idea to report 5 to 6 measurements of that object over a period of at least an hour. This "example object" should have traveled around 600" (10') in this hour of imaging. This is plenty of movement across the 18' x 27' field of view of the SBIG ST-8 CCD and reporting 6 positions will not be too many measurements.

With stacking techniques, we can detect fainter objects whether they are stars, comets, asteroids, galaxies, nebula, etc... When stacking images on stars or galaxies (objects that do not appear to move), you simply align the stars and stack the images. When stacking images on asteroids, the images have to be aligned on the asteroids movement - then stacked. There are a number of programs available that can do this.

Example Stacking:

You have successfully collected 180 - 12.46 second exposures of this object over a period of an hour and you want to obtain 6 positions. To allow for any possible error, obtain 9 positions and eliminate the 3 worse. Divide 180 images by 9 positions and this comes to a total of 20. So, with this you will make 9 stacks of 20 images.

Using your astrometry program, load the first 20 images to stack and prepare the data reduction. You will be prompted to enter information for data reduction (R.A. and Decl – object motion). Enter the R.A. and Decl. for the center of the field of view and enter the motion of this object which is 10.06”/min and P.A. of 253.2. The astrometry program will then query the star catalog for that particular R.A. and Decl. and get a data reduction on your image. Once it does this, it will align all 20 images according to the motion of the object (10.06”/min – P.A. 253.2) and create a “master image” of the stacked 20 images. Finally, you will see an image of trailed stars, and the asteroid will show as a single point of light. Do not close this image and repeat this step for all 180 images until you have the 9 “stacked images”. Now you are ready to measure the positions of the object.

The easiest way to measure the object in all 9 stacked images is to blink the set of images so you will be able to see the movement of the object and also have the capability to “pause” the blinking process. Proceed to the first stacked image and click on the object. The position of the object, and the time (center of exposure time) will show. Now, you can list the name of the object as it is titled in the NEOCP – A123456. This will be entered when requested by the program. Blink to the next image and do the same, remembering to list the objects name A123456 for each entry until it has been measured in all 9 stacked images. When you finish, you will have a complete report to send to the MPC that contains measurements for the new NEO.

To check the positions, load an orbital configuring program that is capable of calculating residuals from a set of measurements. Once the program is loaded, have it to check your measurements for accuracy. Perfect measurements will have residuals of 0.00 but this is nearly impossible. Select your three measurements with the highest residuals and remove them from the list. This will leave you with 6 positions over the period of an hour. Reload your 6 positions, and you should now have a set of measurements with low residuals that will be accepted from the Minor Planet Center.

Image of Potentially Hazardous Asteroid (86039) 1999 NC43

6.30"/min  P.A. 092.7

Image Information:

Taken by Steve E. Farmer Jr.

Red Barn Observatory H68

Date and Time:  20070308 (01:21:01) UT

Image Center: R.A. 04:59:22 Decl. +15:43:07

Telescope:  0.30m Meade LX200 SCT @ f/5

Camera:  SBIG ST7

50 - 30 second exposures stacked in Astrometrica.

CCD temperature: -10c
Exposure Time: 30 seconds
Binning: 1x1

NOTES:

Pixel Scale is a measure of the amount of sky that fits on a pixel (in angles) The amount of sky falling on each pixel will depend on how big the pixel is (pixel size) and how much the object was magnified before it got to the pixel. Since the focal length of the telescope determines magnification, the focal length and the pixel size together determine the pixel scale.