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-- Main.jill - 12 Dec 2006

* NOTES on SKY VALUES IN ETC 12 Dec 2006*

Jill Bechtold
11 December 2006
Revised, Dec. 22, 2006


Summary: For U,B,V the sky used in the LBT ETC during proposal preparation for SDT in November 2006 should be very close to the sky observed on Mt. Graham with LBC. For the R filter, the LBT ETC had an error, and the sky numbers used were low. As of Dec. 22, the error had been more or less fixed, but the background in the real camera remains high, possibly because of a light leak in the fiber optics.

Quantitative measurements of background in the LBC await release of data.


We measured at night the stray light coming from within the closed dome and from the treehouses. Two 600s V-band exposures were obtained, one with the treehouse doors open and one with them closed. After bias subtraction, we note that the treehouses are responsible for ~0.32 DN/sec/pixel and there is an additional ~0.17 DN/sec/pixel from other light sources. Assuming the excess sky brightness is comparable to the ETC sky counts, excess light should be ~20 DN/sec/pixel in V, so in-dome stray light is only contributing at most ~2--3% of the excess background. I (dthompson) suspect the excess background is coming from scattered sky light (from a large solid angle) off the relatively dirty mirrors.

BACKGROUND During discussions of SDT time, it was noted that the sky in LBC frames is much higher than what is predicted by the LBC ETC. The sky values in the LBC ETC are based on La Silla, and it was hypothesized that the discrepency may be due to the fact that the sky is darker at La Silla, or that maybe we are near solar max. Alternatively, there may be a source of light in the dewar, or the dome.

The sky at Kitt Peak can be found on the NOAO web site, and is consistent with the sky at Tololo, and the sky at the MMT (looking straight up). Note that one of the reasons given for putting the LBT on Mt. Graham is that it's darker than KPNO or Mt. Hopkins.

Quote from the NOAO web site:

"What do we use for the sky brightness? The table below is taken from Alistair Walker's article on the sky brightness at Tololo in NOAO Newsletter No. 10. Photometry of the Kitt Peak night sky is consistent with these numbers.

Sky Brightness (mag/arcsec2)
lunar age (days) U B V R I
0 22.0 22.7 21.8 20.9 19.9
3 21.5 22.4 21.7 20.8 19.9
7 19.9 21.6 21.4 20.6 19.7
10 18.5 20.7 20.7 20.3 19.5
14 17.0 19.5 20.0 19.9 19.2

Here's what the LBC ETC gives for sky in Vega mags/arcsec^2 (on Dec 10, 2006):

Lunar Age (days) SDT_Uspec B_LBCBlue V_LBCBlue SDT_R

0 22.07 22.64 21.79 21.39
3 21.58 22.42 21.67 21.30
7 19.93 21.57 21.35 21.08
10 18.48 20.58 20.72 20.67
14 17.00 19.36 20.01 20.18

U, B, V skies for the LBT STC and measured sky at Tololo, KPNO or Mt. Hopkins are virtually identical.

The difference in R band was due to an error in the ETC, which will be fixed.

Adriano further wrote:
First, you may refer to the paper:
"The Large Binocular Camera Image Simulator", by Grazian et al 2004, 2004 PASP..

where it is described how it works. 
You may see that we took a high resolution spectrum of the sky (taken from a 
series of ESO observations) and normalized it to the standard LaSilla sky for 
different moon elevations.
The advantage is that you may estimate the sky magnitude also for non-standard 

About SDT_R, I found that there is an error in the AB-to-Vega conversion for 
this very filter: as you may see, AB mags and Vega mags are the same. If you 
decrease the sky mags by 0.15-.2 mags the disagreement decreases.
Second, if you use the R_LBCRed filter the predicted Vega mags of the sky are 
perfectly identical to the numbers given by Jill. 
The difference is that R_LBCRed is a standard kron-cousin filter, while SDT_R is 
indeed bluer, and might have a darker sky.
I therefore expect that, once we fix the AB-to-Vega conversion, the sky 
background will be correct. Please note also that all internal computations are 
done in AB, so the predicted performances are ok (barring other errors, of 

UPDATE Dec. 22, 2006:

The LBC ETC now gives the following for SDT_R sky:

Lunar Age       Sky (Vega mags)
0               21.26
3               21.17
7               20.95
10              20.54
14               20.05



An excellent summary of what you need to know about sky brightness and what it depends on was written by Alistair Walker and Hugo Schwarz, see

They say: "Astronomers often do not appreciate that the brightness of the night sky varies around the solar cycle, with a B or V-band peak-peak range of about half a magnitude, this is well shown in Figure 3, from Krisciunas (PASP, 109, 1181, 1997) for Mauna Kea. The data from Patat (2003,2004) would suggest that the amplitude, at least for Chilean sites, is not quite so large shown by Krisciunas (Patat states 0.4-0.5 mag. amplitude in B and V, from several studies). "

OK, then! -- where are we in the solar cycle?

For those of you who haven't been trying to show undergraduates sunspots in NatSci 102 lately, go to the Sunsplot Index Data Center

"The next solar minimum will likely occur sometime between December 2006 and October 2007 with cycle 24 peaking in 2011."

So the sky is as dark as it gets!


Update: Dec. 22, 2006. (from Olga Kuhn)

There is a scattered light problem which one of the Italians who was recently out here just suggested may be 850 nm light from the optical
fiber link. It is still under investigation... (see the ImageGallery in the wiki for a picture of this scattered light).

Olga adds a paper describing sky at the VATT. It can also be found under PartnerObserving Web: Site Info.

Info Sky Brightness and Seeing Measurements from data obtained at the VATT were published in PartnerObserving

[[][Conditions at Mount Graham: Vatican Advanced Technology Telescope UBVR Sky Surface Brightness Measurements from 1999 through 2003]] by Taylor, V. A., Jansen, R. A. and Windhorst, R. A. 2004, PASP, 116, 762.

-- DavidThompson - 28 Dec 2006
Topic revision: r3 - 21 Jul 2009, NormCushing
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