An Introduction to Astrometry

Red Barn Observatory MPC/IAU H68

Established 2006

A Practical Guide To Chasing Asteroids

For The Astrometrist

By: Steve E. Farmer Jr. & Marcelo Saavedra

Chapter 1: Introduction to Astrometry

It is very important to understand how to work well with all the pieces that allows a successful astrometry session. This includes our instruments, cameras, software, and knowledge. You would need to investigate and make some calculations to determine some of these points in detail. The following is a briefing of things that we must understand to have a "soft" introduction to Astrometry. Remember, the most important things to add to this list is patience, perseverance, and fun!

As a checklist, we could have the following points:

Completely understand what your instrument is capable of doing, considering the optical configurations that can be configured and their parameters. You can use many calculation formulas and software that is available on the Internet for this. Here is an application (eyepiece calculator) by Marcelo Saavedra that contains the most common formulas: http://www.snapdrive.net/files/323779/Astronomy/EPC.zip

In the same line, you will need to know your camera’s capabilities and characteristics, and how it (they) conforms a unit jointly to your instrument. To have this information always handy, it is a good idea to build an Excel spreadsheet with your cameras specifications as pixel size, pixel array size, total pixels, full well capacity, typical read noise among others and which image scales and FOV you can reach with different optical configurations. A very good and useful tool to know you CCD/Telescope performance is the free Ron Wodaski's CCD Calculator. You can find it here: http://www.newastro.com/newastro/book_new/camera_app.asp

Regarding the setup operation, you will need to practice how to locate a specific celestial field in your telescope/ccd. In spite of the use and connection using a computer between planetarium software and your telescope, tangent boxes and integrated firmware's into the telescope control boards can be used also to locate the fields. Also, you should have a good idea on how accurate your telescope/mount can aim to the field center. By using star charts or software you should be able to identify that what you are seeing with your CCD, is that same field showed in these charts. Once done, then the magnitudes you are reaching with your “seeing conditions” must be estimated. Astrometry procedures can be applied to define the limited magnitudes also but this should be explained in another guide. This will give you an idea of what you can expect to reach, follow and find.

Depending on your mount accuracy, you could need to do auto-guiding by using any known method in order to achieve the wanted exposure length. This will require some expertise and experimenting with your particular setup and we would recommend doing a mount test to define this. You can use any software with auto-guiding capabilities and use the guiding log to trace and analyze your mount behavior in an Excel graph. A very good couple of free software that you can use to do this are PHD (guiding software) and PEAS (periodic error analyzer). With both you can determine your mount characteristics very well.
PHD http://www.stark-labs.com/phdguiding.html
PEAS http://web.telecom.cz/elektro-metal/peas_a.htm

The last point of this very short and summarize list involves the use of your CCD and the related imaging and Astrometry software. You must be very clear on how and when to take Bias, Flats and Dark shots and how to do a good images calibration. This is very important because dark/cold pixels, cosmic ray hits and other artifacts can produce false identifications and detections, so you would be very sure to reduce these possibilities to their minimum. Many imaging software programs are capable to do astrometry reductions, blinking and stacking. It is important to read the software manuals and tutorials in depth. Play with them; investigate the terms and functions in your software that you are not sure about in order to feel comfortable and confident handling them.

This "load" of information may sound very complicated and confusing, but after reading through this guide and working with your equipment to produce your own astrometry, all of this information will become clear.

Chapter 2: Preparing the Observing Session

Introduction

Chapter 1: Introduction to Astrometry

Chapter 2: Preparing the Observing Session

Chapter 3: At the Telescope

Chapter 4: At the Computer - Data Reductions

Chapter 5: How to Obtain an Observatory Code

Chapter 6: How to Image Fast Moving Objects

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Last modified 05/31/2007 08:42 PM -0400

This web site has been active since March 1, 2006

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