Sample focus run.

Two images have been acquired in the focus mode of the Cookbook camera. They come out in .PC format so they are first translated to standard FITS files with my PC2FITS software. I then display them with another of my routines which has a cursor and centroiding routine; this produces a file (called CURSOR.DAT) which is fed into the focus routine.

focus imageThe two focus images, one on either side of focus.

The separations of the two spots is measured as 14·54 and 14·96 pixels.  It is just an accident that the images have been taken roughly symmetrically about focus; it doesn't need to be so.

This is what the cursor file looks like. 

     56    42    55.82    41.90   9

     41    41    41.28    41.67   9

     64    42    63.86    42.04   9

     49    41    48.90    41.68   9

The first 2 columns are the x,y of the cursor; the next 2 columns are the centroided x,y; while the last column is the diameter of the box used in calculating the star position. Only the centroided values are used by the program.

Here is the focus calculation program running.The users responses are underlined. 

C:\CCD> focus 
 
                  F O C U S
                             v1.0  S.Lee
 
 
Enter "0" for manual entry.
How many star readings for each file? 1
Cursor data file? /CURSOR.DAT/  
First focus reading? 43.1
Second focus reading? 134.8
Aspect ratio? (1.1588)   

Separation on first frame  =   14.54
Separation on second frame =   14.97
 
Set focus reading to    88.3
 
C:\CCD>

If you have calculated separations but don't have them in a file, you may enter them directly into the program by first entering a '0' for the number of star readings. The program allows for several stars to be used for the calculations, although in practice I only use 1. The aspect ratio (rectangular pixels) is important if you don't line the stars exactly along either rows or columns. I have defaults set for the cursor file and aspect ratio so I just hit return at these prompts.

Having run the program, it is just a matter of setting the focus to the right value and observing. I must admit that I always check that a star shows just one spot before proceding, but it has never failed yet.

There is a rather obvious streamlining to these procedures and one day I may get around to doing it...

Page last updated 1996/05/02
Steven Lee