Right Angle Sweep Calculator

Introduction

The Right Angle Sweep Calculator is a spreadsheet document for OpenOffice.org and Microsoft Excel that calculates right angle sweep values from a pair of object coordinates in RA/Dec format, enabling users to find objects through the right angle sweep method. The right angle sweep method involves the use of an eyepiece with a known field of view to move from a known bright star to a target object by counting a certain number of fields in declination and right ascension. Coupled with the use of a star chart, this method makes it possible to find desired objects in just a few seconds.

Download

Click here to download the Right Angle Sweep Calculator in both OpenOffice (ods) and Microsoft Excel (xls) formats.

Determining an eyepiece's field of view

Sweep values are given in “fields”, where the view seen through an eyepiece represents one field. To determine the size of an eyepiece's field of view, find a star near the equator and center it within the eyepiece. Next, place the star at the eastern side of the eyepiece and measure the time it takes to move to the opposite side. The field of view of the eyepiece is given by the transit time (in minutes) divided by 4.

Entering values into the spreadsheet

  1. Enter your eyepiece's field of view into the box labeled “Eyepiece FOV”. If possible, choose an eyepiece with a field of view that is close to 1 degree.
  2. Obtain a list of object coordinates from a catalog, chart or software package and look up the location for each of your target objects.
  3. Find an empty row for each of your target objects. If you run out of rows you can expand the table by selecting the final row's cells and dragging downward on the small black square at the bottom right corner of the selection box.
  4. Enter the right ascension and declination values of your target objects into the H/M/S (for right ascension) and D/D'/D'' cells (for declination) under the “To” header. For negative declination values, type a minus sign into the +/- cell.
  5. For each target, look up the location of a bright, nearby star that can serve as the starting point for the sweep operation. If no bright stars are available near your target object, choose a previously entered target object as the starting point.
  6. Enter the right ascension and declination values of each source star into the H/M/S (for right ascension) and D/D'/D'' cells (for declination) under the “From” header. For negative declination values, type a minus sign into the +/- cell.

Finding objects through the calculated sweep values

With the appropriate source object centered in the eyepiece, first shift the view by the indicated number of fields in declination and then shift the view by the indicated number of fields in right ascension (the sweep must be performed in this order for the numbers to work). The direction of the sweep depends on the sign of the sweep value. Positive declination sweep values indicate northward movement, while negative values indicate southward movement. Positive right ascension sweep values indicate eastward movement, while negative values indicate westward movement.