Re: (IAAC) Re: [Planetary_Nebulae] NGC 6826

In a message dated 8/29/01 9:53:53 PM Eastern Daylight Time, 
dedalus@latrade.com writes:

<< I suspect that a great deal depends on the sky contrast at the time
 in which your observation is made...  >>

Hi Lew,

I have observed NGC 6826 dozens of times over the years in scopes with 
apertures of 6", 8", 10" and 16". I have found the blinking effect to be more 
pronounced with each step up in aperture. While observing I also note that 
the nebula does its best disappearing act when you stare DIRECTLY at the 
central star (which is of course easier to see in the larger apertures), not 
just at the nebula itself. Then, when you "turn away" by using averted 
vision, it winks right on again, pretty much hiding the central star. A 
recent comparison between a 10" SCT and a 16" SCT on the same night confirmed 
the aperture advantage. Even though the magnifications used were similar in 
each scope (about 160X), the blinking was more pronounced in the 16", most 
likely due to the greater contrast it provides. Increasing the magnification 
to 300X or more reduced the blinking effect, though it was still there.

Also worth mentioning is that on our public nights (Frosty Drew Observatory), 
when 5 or 10 people observe this object in succession, we get many differing 
results; some observers find the blinking to be very pronounced, others 
barely see it at all. This is probably a factor of the observer's experience, 
and familiarity with using averted vision to good effect. But there may be 
other anatomical factors that are beyond my understanding at this point.

Doug Stewart 
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