MONET Focus Tool

In support of MONET observers, the median and maximum values of the images and the median and mean FWHM of the star images are measured for each new image taken using the professional program SExtractor. The results appear at the link Image Summary. There is a direct link to this page in the tool collection available in the MonetObserving web under the icon "Images".

The results of the image analysis can be used to focus the telescope using several methods, e.g.

• ImageJ: Save the contents of the frame with the list of measurements in a text file (left-click on the frame and save the file originally called "median_counts.txt"). Import it into ImageJ using the "File > Import > Results..." command. Delete the rows not containing focus measurements then invoke "plugins > Astronomy > Find Telescope Focus", which fits a hyperbola to the corresponding columns without any further input needed. Unfortunately, SExtractor does not like highly unfocussed images ("donuts"), so don't go too far into the hyperbolic part of the focus curve: +/- 0.5mm in steps of 0.1mm is enough.

• UNIX: Two simple csh-scripts for downloading the focus series and fitting a hyperbola via gnuplot are available at MONET Focus and MONET Refit. The first script gets the summary and fits a hyperbola using all reported datapoints. Before applying the second script, one should remove outliers or irrelevant datapoints from the local copy of the image summary (median_counts.txt) and determine the focus value again. For testing purposes you can download a focus series here and use the refitting script refocus.csh. The script requests a start and end value for the focus and will use these as fit boundaries. If the fit does not work, change the start and end values. This method works especially well close to the last reported value, i.e. +/- 0.4mm and in steps of 0.1mm.

Use 1x1 binning to get the best results. Filter offsets can be estimated using the focus log (2009-03-05) for allfilters except for U. The precision of the reported values is roughly +/- 0.01.

Filter offsets

Measurements from 2009: From the existing focus values (between 2008-11-04 and 2009-04-08) we can determine the following offsets. All values are subject to correction.

# V - R 0.0146666666667 +/- 0.00673850186669 entries 3
# V - I 0.019 +/- 0.00804155872121 entries 3
# V - Clear 0.0275 +/- 0.0053033008589 entries 2
# V - Ha 0.0085 +/- 0.00671751442127 entries 2
# V - Sloan g 0.032 +/- 0.0 entries 1
# V - Sloan r 0.02 +/- 0.0 entries 1
# R - I 0.0035 +/- 0.0116672618896 entries 2
# R - Clear 0.005 +/- 0.00212132034356 entries 2
# R - Ha 0.0 +/- 0.0 entries 1
# R - Sloan g 1.0045 +/- 0.700389266765 entries 2
# R - Sloan r 0.9985 +/- 0.704631907452 entries 2
# I - Clear 0.0015 +/- 0.0137885822331 entries 2
# I - Ha -0.02 +/- 0.0 entries 1
# I - Sloan g -0.006 +/- 0.0 entries 1
# I - Sloan r -0.018 +/- 0.0 entries 1
# Clear - Ha -0.002 +/- 0.0 entries 1
# Clear - g 0.012 +/- 0.0 entries 1
# Clear - r 0.0 +/- 0.0 entries 1
# Ha - g 0.014 +/- 0.0 entries 1
# Ha - r 0.002 +/- 0.0 entries 1
# Sloan g - Sloan r -0.006 +/- 0.00424264068712 entries 2