Naked-Eye Visual Magnitude Limit

On 1 Jul 2004 08:42:32 -0700, (Tom Polakis) wrote:

Chris,

It's easy to measure seeing, but is there a way to measure
scintillation? I agree that there is some correlation between strong
scintillation and poor seeing through the telescope, but it hasn't
been strong in my experience.

I don't know of any instrumental way of measuring scintillation- maybe something
with a video camera and small aperture. The degree of correlation between
scintillation (observed subjectively) and seeing (measured) seems to be site
dependent. I'm at a high altitude (2800m) in an area of uneven terrain. My
seeing effects are dominated by both jet stream winds and turbulent surface
flow. Scintillation seems to be a high altitude effect; it is strong when the
jet stream is overhead, but may be absent even in the presence of strong, gusty
winds. By observing scintillation, I can predict instrumental seeing with better
than 75% accuracy. In part, that may simply be because the seeing is so often
poor here- like a California weatherman saying that there is a 75% chance of Sun
tomorrow; he's going to have a good track record even if he never checks a map!


I have seen very good seeing with scintillation quite often at Arizona
sites.

In the last five years, there have probably only been three or four nights here
where this was the case. While lack of scintillation is no guarantee of good
seeing, high scintillation virtually guarantees bad seeing (again, I am speaking
only of my site, not general principles). For me, very good seeing is 2.5",
typical seeing is 3-4", and bad seeing is 5"+. The prospect of 3" seeing always
gets the observatory open.


When I have tried to detect the faintest star through the eyepiece of
a telescope, it is certainly affected by seeing. I think it's as much
of a reason for seeing the faint star "25% of the time" or whatever as
physiology of vision.

Right. I think the central star of M57 is the classic example of this. There is
no doubt that in a smallish scope, say 12-14", the ability to pick it up depends
primarily on seeing.


I can't say I have tried to estimate naked-eye
limiting magnitude on a number of nights under different degrees of
scintillation.

I haven't done this scientifically. But there is no doubt that for me, stars at
the threshold of detection are lost when the scintillation is high.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com