(IAAC) The blinking planetary NGC 6826 20" Dobsonian

Your skill:intermediate
Date and UT of observation:October 28/29 1997 01UT
Location & latitude:near Ayr South West Scotland lat 55 24'56"
Site classification:bright rural
Limiting magnitude (visual):6 (zenith) 
Seeing (1 to 5 - best to worst):4.9-5
Moon up (phase?):no
Instrument:20" f4.4 Dobsonian, 8" f8 Newtonian
Filters used:None
Category:Planetary nebula
Object data:mag8.8  27" X 24"
RA/DE: RA 19 44.2' Dec +50 28'(1975 coordinates) 
Description: A classic planetary nebula, it presented a nice circular and
featureless round disc in both telescopes. The central star was visible in
both scopes but was much more obvious in the 20". Since I was comparing the
two images in the two scopes I did not study the OIII image in the 20". Poor
seeing and wind shaking precluded the use of higher magnification in the 20".
This nebula is called the blinking nebula because of the way it fades when
looked at directly. Caused by differences in sensitivity between foveal and
peripheral vision , this effect was very conspicuous in the 20". When
concentrating on the central star the nebulosity fades to a truncated
conical remnant. Concentrating on the nebulosity it is at once clear but the
central star is less obvious.
Through a direct vision prism the nebula showed the very clear OIII line
image of the nebula with a continuos spectrum from the central star
superimposed but the continuum seemed to embrace the inner parts of the
nebula too.
Finding:- Go to theta Cygni and move upwards and east to the 5.3 magnitude
star 16 Cygni. The nebula is about one low power field (0.8 degree in right
ascension east) from this very pleasant 38" wide binary star. If 16 Cygni is
on the left of your lowest power eyepiece go a little right and the round
disc of the planetary should be easily visible.
|   nmartin@bonnyton.u-net.com    |
|"There are more things in heaven and earth ... than are dreamt of in your
philosophy"   (Hamlet, Act 1, Sc. 5)  |
Bonnyton House, Ayrshire,Scotland, UK. lat 55 24'56" N long 4 26' 00" W
Altitude 150 m.