[Aavso-photometry] Astronomical Seeing - How To Measure

[Tim Crawford]

Astronomical “Seeing” is a quantitative measure of how muchthe atmosphere is perturbing images (i.e., blurring/twinkling) through turbulence.  In essence it is a measure of the sizeof the seeing disk (angular resolution) in arcseconds.  In other words, it tells us to whatdegree we have great resolution or possibly just looking at blobs of light.


As we all know or soon learn, atmospheric turbulence isquite variable from night to night and at different times of the year.  However, for most geographical areasthe variability falls within a range that can usually be reasonably averaged. (FYI this issue also has some importance in terms of yourown local AVERAGE “seeing” when choosing a CCD for your existing scope orchoosing a Scope/CCD system.  See: www.aavso.org/observing/programs/ccd/manual/2.shtml#2for a discussion on his topic)


To Measure your local “seeing” you need to know two values:


1.     The FWHM of a current image (FWHM is thediameter of the star image at one-half it’s maximum pixel value)
2.     
      The Image scale of your Scope/CCD system in
arcsec/pixel


Seeing = FWHM * Image Scale


Your photometry program should have a tool that will allowyou to choose a star and see it’s FWHM value in pixels.  As an example, I use AP4WIN V2 for myphotometry processing and under the measure Tab there is a “Star Image Tool” thatwill let me choose a star to measure the FWHM.


To determine the image scale of your system you need a “CCDCalculator.”  I use a pgm writtenby Ron Wodaski to calculate mine. 
This is freebie and very user friendly with drop down lists for choosingyour scope and CCD: 


www.newastro.com/wodaski/pick_a_camera.htm


Also the AAVSO CCD manual (URL above) contains a CCD
calculator and there are other options available for those that want to google “CCD
Calculator.”


I just measured the seeing of a recent image at 3.57 pixels


Ron Wodaski’s Calculator shows my image scale to be 1.37
arcsec/pixel.


Therefore, on that night my “Seeing” was: 3.57 pixels * 1.37
arcsec/pixel = 4.89 arcsec.


Seeing in his range, by the way, would be considered to be pretty poor.  For the time of the year,however, it is typical of my location and has no appreciable negative impact ondifferential photometry but would not be suitable for “all-sky” photometry.


Good photometric nights would probably have seeing values =~1.5 arcsec  with less being muchmore preferable when doing “all-sky” photometry as opposed to differentialphotometry.   On Manua Kea Iunderstand they sometimes have seeing 
= .4 arcsec. 


In my location the seeing does not drop below 3 arcsec (whichI consider to be good - I am at sea level and only 100 yards form the Pacific)and I usually do not bother doing photometry when the seeing exceeds 7 arcsec(and those targets tend to be well spaced and brighter when the seeing is thispoor).  There are amateur inlandsites, around the world, usually at higher elevations, where seeing values =1.5 – 2.5 arcsec and some of those folks would consider their seeing to be poor whenthe seeing = ~3 arcsec.  


The Important thing to keep in mind is that with
Differential photometry a wide range of seeing values can be accommodated.


It is very important to watch your aperture and annulus
rings as the seeing worsens when you are doing the photometry.


Arne made some good comments about dealing with poor seeing
in an email from when he was still working at the Naval Observatory in Flagstaff:


www.aavso.org/pipermail/aavso-photometry/2003-November/000062.html




Per Ardua ad Astra


Tim Crawford
CTX 
Arch Cape Observatory