This application for MaximDL allows one to easily measure the atmospheric seeing at an observing site, using easily available equipment. When using in conjunction with my Sky Brightness plugin it is a useful way of quantitatively measuring the quality of a given observing site.
The algorithm I used for measuring the atmospheric seeing is described in a paper by Bally, Theil, Billawalla, Potter, Loewenstein, Mrozek and Lloyd, available online here.
Measurement of the seeing is useful for determining the effect of short term atmospheric turbulence on long exposures, as defined by the achievable FWHM of those images. The method used herein is DIMM - Differential Image Motion Monitor, this is used by many major observatories, e.g. CTIO and ESO.
is useful because it measures the relative (or differential) motion of
two images of the same star, which reach the detector through two
slightly different paths through the atmosphere, thus they are
affected differently by the pockets of turbulence through which they
pass. Since the two images are of the same star and they are captured
simultaneously, they are not affected by telescope tracking errors or
vibration, but only by the atmospheric distortions.
Atmospheric seeing typically changes in the order of 10-20ms of elapsed time, so individual exposures must be in that duration range to be able to "freeze" the seeing to measure it. To achieve this, a high speed camera is required - guide cameras or planetary imaging cameras are typically ideal. The camera should also ideally support hardware subframes, since the software will use these to increase the image capture rate.
In order to generate two images for a single star, one must put a dual aperture mask on the telescope where the two holes in the mask are separated by a distance of at least double their individual diameter. There are two main methods for causing the resulting dual star images to diverge:
1) Place a wedge prism in
one of the apertures, such that it diverts the incident beam by a small
about (typically a fraction of an arcsec)
2) Defocus the telescope such that the resulting beams have not converged by the time they reach the sensor.
The downside of Option (2) is that if the telescope is defocused too much such that the Strehl ratio drops below about 0.5, the result can be somewhat biased. Nevertheless, it provides a simple way for the advanced amateur to rate the quality of their site on a night by night bases, perhaps to help decide on the type of imaging to be performed that night. A reasonable way to determine the amount of defocus which is acceptable is to take a series of 10ms exposures with the mask in place but with the telescope at normal focus, measure the average FWHM, then gradually defocus, periodically measuring the resulting FWHM and stop once it becomes more than 20% or so larger than when at best focus. Once you have determined a workable focus offset, you can quickly move the focuser by that amount and mount the Hartmann mask whenever you wish to measure the site seeing.
Whichever optical divergence method is used, the operation of the software is the same - fit the aperture mask, point the telescope at a bright star near the Zenith, then after the user identifies the initial location of the two star images, the software captures a repeating sequence of very fast exposures and calculates the longitudinal and transverse differential motion between the dual star images in each, using the fomulae in the above cited paper to then calculate the effective seeing value over the sample period.
Upon launching the Seeing Monitor application, it will automatically connect to MaximDL and the user is presented with the main screen:
top section of the display presents the results of each seeing
measurement - this is the effective FWHM (in arcsecs) which can be
expected from a long exposure, along with the "r0" value which is the
telescope diameter (in cm) below which the seeing will have no effect
and the telescope will effectively be diffraction limited (ignoring any
other optical aberrations).
The standard deviation of observed longitudinal and transverse differential motion is also displayed, along with a percentage of individual exposures which were too badly distorted by turbulence to be able to measure the star centroids. Higher failed detection rates indicate possible under-estimation of the resulting FWHM, since there were moments of exceptionally bad seeing which were not able to be measured. These are normally obvious from watching the individual exposures - the stars smear out or vanish completely.
The middle section displays the dual star locations and movement amounts for each individual exposure as it is taken, in pixels.
The lower section contains an auto-scaling history graph plotting the measured seeing FWHM values over time.
At the bottom are the control buttons:
Setup Button: This displays a setup dialog:
Here the user can specify the distance between the hole centers (in mm), the hole diameters (in mm), the wavelength of light being captured (in nm) (typically 500nm is used for full range visible light), system noise (as described in the paper cited above, default 0), plus the exposure settings.
The user can specify a longer exposure for the initial star image selection process, which is useful to allow it to "average out" the seeing and to get a good average location the software will then use for monitoring the star centroids. The actual seeing measurement exposures should be in the 10-20ms range (0.01-0.02 seconds). The number of exposures to take to generate each seeing reading depends somewhat on the time resolution required and the speed of the camera used, a value of around 20 should be reasonable.About Button: This displays the software version number and author contact email address.
v1.0 - Initial release. Installer now updated to work on Windows XP.
v2.0 - Fix seeing computation algorithm, original release generated incorrect result
v2.1 - Fix startup crash
Seeing Monitor v2.1
If you download this to give it a go, please post feedback on the MaximDL
Yahoo group, or email me directly at: winfij_AT_gmail_DOT_com
I have also set up a new Yahoo group specifically for announcing new apps like this one, as well to offer better support/feedback. Please sign up if you want to keep up with new releases and updates: New Yahoo Group
One final note - if you have any errors, please note the exact error messages
and let me know!