Telescope Reviews: Re: Ground Truth for the Bortle Scale

Now some of may experience about the correlation between the SQM and the Bortle degree (as said the SQM fairly matches the maps of "total sky brightness, not the "classical" one, except for the screening effect).

The point is that the SQM measures the TOTAL light coming from the sky. On the other hand the Bortle grade means visibility of object, that is the ratio between object's light and background. The former (object brightness) depends on the extinction. Therefore I think that what matters is the SQM minus Extinction (which is the contrast).
In pretty dark skyes I have experienced that, often, transparent atmosphere produces somewhat brighter reading of the SQM. In one night the atmosphere was extremely clear. RThe SQM marked 21.55 and IC 1296 was easy seen (16"). In other occasions, even with darker SQM (21.6-21.65) IC 1296 was difficult.

Although the Bortle scale is carefully described, I still have difficulty in assessing the grade based on the descriptions. M33 is always difficult for me and it seems I am not able to go above mag 6.5 whatever the darkness: I have little progress starting from 20.8. What happens is that the region of uncertain detection expands somewhat, but the direct vision sticks at somewhat 6. So, some my sky has some lemengts that seems to be grade 2 (darkness and difficulty to see objects, black clouds) and others that appears to be grade 3 or even 4 of the scale (difficulty to see M33).

To summarize, here are my indications:

Location Casera Razzo (Italian Alps) http://maps.google.com/maps?q=46.4792+12.6097

1. DISTANCE TO METRO AREAS.

To the south east 64 km to Udine (some million people urban area). To the south south east the Padana plate at 80 km (a dozen million people).
Lightdomes: SE only some mild brightness in the distance between the mountains. Some diffuse light from N and W in very wet nights.
Elevation: 1740 meters.

2. COLOR ZONE IN LIGHT POLLUTION ATLAS

Green in the !"classical" Cinzano's maps. Light green (21-21.5 in the total sky brightness maps).

3. SQM.
21.4-21.6 on average (see also http://visualsky.blogspot.com/2007/11/airglow-fluctuations.html )

4. VISIBILITY OF MILKY WAY. Winter MW is visible. Summer MW is strutured with many many grades of gray (and visble eve in spring when low at North). Arm into Ophiucus obvious. North america si naked eye.

5. VISIBILITY OF NAKED EYE DSO'S. M33 is visible with adverted vision. Limiting magnitude 6.5 (16.1 or q16.6 at the telescope 16"). M13 visible. M31 is easy (shape clearly visible).

6. CLOUDS BRIGHT OR DARK? Low clouds are black voids. Only very high clouds faintly glow illuminated by the Padana plate 80 km apart. See also http://visualsky.blogspot.com/2007/10/linquinamento-luminoso-di-cr.html

Edited by Mauro Da Lio (08/17/08 04:09 PM)

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