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A Practical Guide To Chasing Asteroids

For The Astrometrist

By: Steve E. Farmer Jr. & Marcelo Saavedra

Chapter 4:  At the Computer

DATA REDUCTIONS

A common and very useful process for fast moving asteroid searching is the “track and stack”.

The track and stack technique allows you to align and then stack the images in two different ways. You can stack the images by aligning the stars, or you can stack the images by using the motion ("/min and P.A.) of the object - causing the stars to "trail". (not all astrometry software will align the images before stacking. In this case you can use your imaging software to pre align the images and use them to stack).

Normally you can “manipulate” the stacking speed and direction. Speed means how much one image is separated from the consecutive ones in pixels causing the “static” stars to appear as trailed lines. The Direction is known as “Position Angle” or P.A. for short. It allows you to set the movements directions of the images. If both parameters (speed and P.A) are shifted, you would see the stars as a trails and the minor planets as a static point. If Speed and P.A are set to 0 then the stars will appear as a point and the minor planets as trails. We use both cases together to detect the minor planets but giving a correct speed and P.A. to the image set will add (or average or median) the asteroids signal in our set and then possible unappreciable minor planet in one image can be seen in the “stacked and tracked” image. Normally, when a series of images are stacked using this procedure, the astrometry software will produce better measurements.

Here is an example of an Stacked images without tracking (Speed=0). Note the trace made for the 66MAJA Asteroid at the bottom left:

Image of Asteroid 66-MAJA by Marcelo Saavedra

Now, the same image with Speed=0,63 and PA=287,3. Note that 66MAJA appears as a static point and the stars as tracks:

Image of Asteroid 66-MAJA by Marcelo Saavedra
Both Images taken with a 0.2 m SCT Telescope
CCD ST237A at -7 Degrees
20 Subs 3minutes each.
Binning 1x1

In order to explain some things more in a practical way, suppose that we have taken 40 images in one hour with 1.5 min exposure each.

Considering our example, you have a total exposure time of 60 minutes. You would stack all 60 exposures with 0"/min and a P.A. of 0. Very faint stars will appear in the finished image, and faint trails will appear from any bright moving objects! Faint "moving" objects normally do not appear in stacked images using the average equipment.

NOTE: In the practice, you can detect any object at any exposure length depending on the object’s brightness and motion. However, very fast and faint objects will often not be detected in a single short exposure unless you have an extremely large aperture. 

Another way to detect moving objects is using the “Blink” feature. It is like the “old” movie composed by photograph frame, passed one after another with a given speed to create a movement effect. In our image set, If the moving object is bright enough to appear in each (or some) images, blinking them could be useful to determine the motion speed and angle. If not, track and stacked should be used with an empiric values for Speed and P.A., trying to locate the object.

In conclusion, if no trails are noticed in the images, then you can try re-stacking the images at various speeds and different P.A.s to search for faint objects. One possible method to avoid wasting your time doing a “try and error” on the Speed and P.A. could be start by stacking and shifting the frames based on the object’s known motion.  Take present that if you are trying the “Main Belt Asteroids” or MBAs, the movement on these objects could be very slow. In that cases, the detection can be done by just imaging an area long enough to detect the object and then measure it. If there is an MBA in the field and is bright enough to detect it you’ll find it without the images shift.

A good practice is to divide the total images in equaled slices for track and stacking. For our example, if 40 images were taken in one hour with 1.5 min exposure each then 4 stacks of 10 subs of 1.5 minute exposures would be stacked according to the objects motion so that it would "bring out" the light of the asteroid.

You would load and stack the first 10, then the next 10, and so on until you have four "staked", reduced images blocks with increased asteroid signal on each of them, made from our supposed 40 images collected.

Next, they would be blinked to re check and extract some useful information needed. In the blinking window, you would do the measurements and name the identified objects. If you found an unknown object or other than one on the NEOCP, you would assign your own name to it such as NAME015 as a temporary name but MPC will assign the name they considers correct. Note: If a new object was found, you should take measures in at least two different nights, covering no less than 30 minutes of observation on each one.  But, if the object appears to be an unknown FMO (Fast Moving Object), then a number of measurements should be taken and the object can be reported as a "one nighter".

NOTE: The name submitted to the Minor Planet Center must contain 6 or 7 characters for their computers to recognize it as observations.

Once your images reduction is done and you feel that a new object is on your images set, first check the MPCChecker.
Typically the MPCChecker is used when you have already detected something you think is new and are trying to eliminate a know object as being new. Also if your astrometry software has an option to make an overlay of your image with star catalogs you can use this option to look for unknown objects.

Finally, you would report the object to MPC by using the report option in your astrometry software or by sending an email in PLAN TEXT (no HTML) to the MPC using its normalized format (take a look here for more details: http://cfa-www.harvard.edu/iau/info/Astrometry.html).

 Hopefully this will produce new objects, and at the same time produce follow-up observations on known objects. A new discover while imaging other known objects could happen but some astronomers could think that if you want to find a new object then start looking in other unexplored fields.”

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Copyright © 2006 The Cometary Space Web of Steve E. Farmer Jr.

Last modified 05/31/2007 08:56 PM -0400

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