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 3:  At The Telescope

AN EXAMPLE/PRACTICE SESSION

1.  To prepare the telescope checking if necessary, the perfect polar alignment and a clean optic. Windex cleans the lens well and dries with no smears, but check with your telescope manufacturer for specifications on this.

2.  Make sure to have all the tools to make a good image calibration, taking Bias (if you normally use them), Flats and Darks frames. You will need the best possible images and this step is a must.

3.  Check the correct telescope slewing and centering of known objects within the FOV.

4.  Slew the telescope to the first selected field to be studied and verify the scope is in the correct location. This can be done with data reduction with astrometric programs or compare with your planetarium program.

5.  Take a moment to get perfect focus in both, guiding and imaging setups.

6.  Calibrate the guider system and begin auto-guiding (if needed). If the object is bright and your telescope system is capable of 30 to 45 second exposures without auto-guiding, then an auto-guider may not be needed at this point. Most faint objects will require auto-guiding when using the "average" amateur based telescope and equipment.

7.  Select the maximum exposure time to avoid any star trailing or asteroid trailing in the resulting images. You must achieve maximum exposure time appropriated to obtain the desired magnitude.

8.  Once determined and the exposure time is set, begin the exposures - saving the images in a pre-determined folder of your choice.

9.  After a 15 minute time frame and by using you astrometry software, blink the obtained images and try to locate the target(s) by their movement in the FOV (Field of View).

NOTE: 15 minutes is taken as a reference time and be aware that imaging time will vary depending on the objects speed and the instrument being used. The main idea here is that some exposing time has passed and motion can be seen from the object.

10.  You are now sure that the current field of view is properly aligned and the object is near centered. If the object(s) of interest are near the image center and away from any bright stars, continue exposing, guiding, and auto-saving the images for a period of at least one hour (unless the object is moving very fast). Checking the targets path with planetarium software will help avoid star interference also. If this interference exists, come back to the object once it has cleared the star.

11.  Now you have captured the light from the object. These images can be stacked in your astrometry software according to the objects motion (xxx"/min and P.A. yyy). At this step, you can be on presence of two different cases depending on what you are looking for: slow moving objects or fast moving objects (normally NEOs - Near Earth Objects).

a. Slow moving objects: Since this object is moving so slow, you may (or may not) slew to and measure other objects, coming back to the original target for more images making an observation/imaging cycle and waiting for a significant movement from the object. If the object is slow, this leaves a lot of time to leave the field and return only after the object has moved a significant amount of arc. It would take a particular object an hour or more to move even a few pixels for a given setup. You could image a lot of sky somewhere else in that amount of time. It's normally more efficient to image other objects and come back to it.

b. For a fast mover: it might be better to get all the frames at once since the object has moved enough arc to get 3 good positions. Take them and move on to the next object. You can still stack these altogether and look for other fast movers streaking across your field but at least you have 3 good positions to submit.  We will see this case later because it is considered as an “advanced astrometry” procedure.

12.  Finally, VERIFY darks, flats, and bias - these are a must!!! Darks, flats, and bias images may be taken and saved prior to the observing session to save time while actually working at the scope. Calibration frames:  flats and darks are a must.

13.You are now ready for the data reduction.

Chapter 4:  At the Computer - Data Reductions

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

Last modified 05/31/2007 08:49 PM -0400

This web site has been active since March 1, 2006

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