Frage zur visuellen Grenzgröße

[Michael Hagen]

Habe gerade mal getestet was so die Grenzhelligkeit vor unserem Haus etwa 100m abseits vom Dorf ist. Ich war total erstaunt, denn nach http://www.seds.org/billa/lm/rjm14.html komme ich auf eine Grenzgröße von 7.1, mit etwas gutem Willen auch 7,2. Kann das sein oder ist der Test falsch. Wie ermittelt ihr die Grenzhelligkeit?
Vielen Dank im Voraus
Servus,
Michael

[jan lameer]

Hi Michael

I don't think this is possible unless you are over an elevation of say 2000 meters high, with no dust or water vapour in the air and almost no lights in your village.

Also for mag 7.1 you need much longer dark adaptation than the usual 30 minutes that is recommended. I'd say at least 2 hours under total darkness.
(Experiments of the US army show that even after two days in total darkness your eyes still get more sensitive, that usually is why morning zodiacal light is seen much more extended than evening zodiacal light)

To me limiting magnitude at zenit is not so important, limiting magnitude near the horizon is usually what counts.

greetings,
JanL

[Michael Hagen]

Hi Jan,
First of all thanks for answering my questions. But what do you think about the test at http://www.seds.org/billa/lm/rjm14.html ? Does it generally give you the real limiting magnitude or is there a mistake in the test. Just two hours ago I stepped outside one more time and I was able to count 57 stars within the square around the swan. According to the test-rules, that would mean I had a limitin magnitude of 7,3. I would say the test is wrong because neither the sky is extraordinarily dark here(10km outside Bamberg City) nor did I have a very long adaption time.(abt 5mins) So a value of 7,3 is just imposible. Do you know any other (more reliable) method to determine the limiting magnitude?
ciao
Michael

[jan lameer]

Hi Michael

I cannot judge wether the test is o.k. or not.
It would mean studying it in detail and comparing it to charts that are calibrated for visual brightness instead of photographic brightness.
I mainly use the small dipper (Uras Minor) to see if I have mag 5 (when you see all stars) or the stars inside the bowl of the big dipper (Uras Major) to see if I have 6.2 - 6.9 mag.
If I see three triplets and a few doubles with naked eye inside the bowl of the big dipper at least for a full hour easily, than I know I see stars of magnitude 6.9 - 7.1 .
At that time I usually start looking for other objects like extragallactic nebulae and Gegenschein or Zodiacal Lightbridge and I totally forget about the faintest star I might possible see.
I would however like to see Uranus with naked eye, or Vesta, Ceres or any other of those never seen naked eye objects.

I can propose a test: don't look at maps of the big dipper and wait until UMa is high in the sky.
Then draw a map everytime you observe UMa with your naked eye.
Do this for at least a few good nights before you check it with library maps.

JanL

[jan lameer]

eh Michael

Cygnus is a Milky Way region with many blue stars.
I would never use this region for visual estimates. I think the test is just wrong.
Go for "poor regions", see if you can follow Draco easily, Eridanus, Giraffus and all those hard to find constellations. Try to see M33 for the first time and similar objects.

I have an experimantal linkpage on which you might find more:

http://www.angelfire.com/movies/timelap ... kpage.html

regards,
JanL

[Michael Hagen]

You might be right about the blue stars. Thanks for all the tips and hints! *o.T.*