David's Astronomy Pages CCD Image Notes - Session 89 (2005-06-10)

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Image Sharpness (Astronomical Seeing)

Atmospheric turbulence tends to cause the light from stars to jump around or blur instead of being pinpoint like. Astronomical Seeing is the measure of how sharp stars appear to be, and will vary with altitude, from location to location  and from night to night.  In CCD Imaging seeing is typically quantified by measuring the Full-Width Half Maximum (FWHM) of the stars in an image. FWHM is the apparent diameter of star at a cutoff value half way between the central peak value and the surrounding background. FWHM is generally calculated by software, though it can be measured 'manually' as an exercise.     When used as a measure of seeing it goes without saying that the image shown be at best focus (a poorly focussed image will have have more bloated stars and corresponding FWHM values will be larger and unrepresentative of seeing)

The best sites (mountaintop observatories) are able to produce short exposure images with FWHM of 0.3 arc secs, with more typical values of 0.7 arc secs. Amateurs, living at less desirable sites, are lucky to get FWHM of 2 arc secs, and will often have to deal with values of between 3 and 6 arc secs.

Previous tests have shown that at Kingcup observatory (Broadstone, Dorset) I normally saw FWHM values of around 4 to 5 arc secs.  At Clair observatory in NE Scotland,  I typically see FWHM values of only 5 to 6 arc secs. 

During observing sessions on 2005-06-06 and 2005-06-10 astronomical seeing was noticeably much better than usual , with FWHM values of 2.5 to 3.0 arc secs achieved in many images (see image below).

M92 Image, FWHM 2.6 arc secs

20 sec exposure, green filter  2x2 binning 2005-06-06 (#88032)

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Fine Focusing / Direct Read-Out of FWHM.   

The CCDSoft focus screen tracks either sharpness or maximum pixel value during focusing, however it does track both at the same time, and the sharpness value can not be used to tell exactly how sharp a star's image is, nor can not be used to compare seeing between one session and another. To fill this gap, I added a feature to my CCD Imaging Tool, that reports the FWHM of the brightest star in an image, along with its maximum pixel value and its S/N.

An example of the live output from the tool during S00089 session is shown below. It was able to tell me that at peak focus stars were being resolved with a FWHM (Full Width Half Maximum) value of 3 arc secs, which is pretty good seeing for my location in Scotland, directly adjacent to the North Sea coast.

  Take Focus Image (3s) ...    Max 1,145 FWHM=  7.2, Mag=  9.54, S/N= 107.3
  Take Focus Image (3s) ...    Max   930 FWHM= 10.3, Mag=  9.52, S/N= 109.5
  Take Focus Image (3s) ...    Max   554 FWHM= 13.1, Mag=  9.52, S/N= 109.2
  Take Focus Image (3s) ...    Max   897 FWHM= 10.0, Mag=  9.51, S/N= 110.2
  Take Focus Image (3s) ...    Max 1,011 FWHM=  8.4, Mag=  9.50, S/N= 110.4
  Take Focus Image (3s) ...    Max 1,224 FWHM=  7.2, Mag=  9.53, S/N= 108.7
  Take Focus Image (3s) ...    Max 1,498 FWHM=  6.2, Mag=  9.49, S/N= 111.3
  Take Focus Image (3s) ...    Max 2,116 FWHM=  4.6, Mag=  9.51, S/N= 110.0
  Take Focus Image (3s) ...    Max 2,355 FWHM=  4.0, Mag=  9.49, S/N= 111.0
  Take Focus Image (3s) ...    Max 3,508 FWHM=  3.0, Mag=  9.48, S/N= 112.2
  Take Focus Image (3s) ...    Max 3,241 FWHM=  3.0, Mag=  9.98, S/N=  84.0
  Take Focus Image (3s) ...    Max 3,482 FWHM=  3.1, Mag=  9.50, S/N= 111.0
  Take Focus Image (3s) ...    Max 3,426 FWHM=  2.9, Mag=  9.50, S/N= 110.7
  Take Focus Image (3s) ...    Max 3,446 FWHM=  3.1, Mag=  9.49, S/N= 111.7
  Take Focus Image (3s) ...    Max 3,323 FWHM=  3.1, Mag=  9.46, S/N= 113.3
  Take Focus Image (3s) ...    Max 3,599 FWHM=  3.0, Mag=  9.48, S/N= 112.3

The data from this focusing run is shown in the graph below. The insert pictures show star images at different focus positions, notice how the stars sharpen to a near point at best focus is reached.

3 sec exposures  Clear Filter, 2x2 binning

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Sky Brightness (June - January Comparison)

Sky Brightness Measurements during observing sessions  between Jan 2005 & June 2005
Notes Notice how observing sessions in June (without moon) had brighter skies than the May observing  sessions which had a moon  Notice how shortness of June night forces observation sessions to be taken around local  midnight (+/-1 hour) compared to January-March when early evening sessions are available)
Date / Session		Moon	Night Hours (sunset to sunrise)	Dark Hours (non twilight)
2005-06-10, S0089		No Moon	6h 00m	None
2005-06-07, S0088		No Moon	6h 14m	None
2005-05-17, S0087		Moon at midnight,  Phase 65%, Altitude 19˚	7h 15m	None
2005-05-13, S0086		Moon at midnight, Phase 20%, Altitude 7˚	7h 31m	None
2005-04-06, S0084		No Moon	10h 21m	5h 23m
2005-03-13, S0083		No Moon at midnight (Phase 15%, Altitude 5˚ at 21:00)	12h 19m	7h 58m
2005-01-05, S0082		No Moon	17h 05m	12h 13m

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Sky Brightness (June - January Comparison)

Comparison of summer & winter CCD exposures  showing brighter sky in June exposures a) January exposure (2005-01-05, S0082), Sky Mag +19.9/sq arc sec b) June exposure (2005-06-07, S00088),  Sky Mag +17.4/sq arc sec c) June exposure (2005-06-09, S00089),  Sky Mag +16.9/sq arc sec all 60 sec exposures, clear filter, identical black/white ranges

Comparison of summer & winter CCD exposures  showing impact of brighter sky on noise levels in June exposure a) January exposure (2005-01-05, S0082), Sky Mag +19.9/sq arc sec b) June exposure (2005-06-07, S00088),  Sky Mag +17.4/sq arc sec c) June exposure (2005-06-09, S00089),  Sky Mag +16.9/sq arc sec all 60 sec exposures, clear filter, 300 ADU black/white ranges

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Sky Condition Graphs

As part of a quantitative analysis of the Sky Brightness and Seeing Conditions in June observing sessions (2005-06-06 and 2005-06-10),  a tools were developed for determining and analysing observing conditions. These comprised scripts to analyse the the stars and background levels in recorded images and a spreadsheet for presenting the results graphically.  Graphs showing sky conditions against time for different observing nights is shown below [ Explanation of these graphs ] (Note that for each data item the best observing situation is towards the top of the chart).   

Sky Condition Graphs for Observing Sessions S86 to S89

Notice  1) surprising dark skies for mid May, even though it is meant to be still Twilight conditions at midnight  2) Continued darkening of sky even after local midnight (00:15 hUT). Dark sky even below Airmass=3 3) Progressive deterioration in sky transparency (thin, non reflective cloud ?)

See explanation of these graphs

Notice  1) slight brightening of sky either side of local midnight (00:15 hUT) 2) difference is sky brightness with change in altitude, with brighter sky below Airmass=2.  3) poor seeing or more likely poor focusing in the first part of session with progressive drift in FWHM 4) better FWHM in later part of session,  whilst re-focusing may have been performed, there was a noted change in sky conditions, with development of apparently less transparent but stable atmosphere.

Notice  1) marked brightening of sky either side of local midnight (00:15 hUT) 2) passage of very dark, low transparency thin cloud at around 23:40h.   3) low FWHM values down to around 3 arc secs (good seeing from my site) 4) gaps in data at 23:15 to 23:35 is due to images (Globular Clusters) which did not give astrometric solutions using software.

Notice  1) marked brightening of sky either side of local midnight (00:15 hUT) 2) difference is sky brightness with change in altitude, with brighter sky below Airmass=2. 3) apparent improvement in sky transparency towards after 00:20h, but with apparent deterioration in seeing, however fall in FWHM could equally have been due to a drift in focusing.

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