Earthshine blog

Earthshine blog

"Earthshine blog"

A blog about a system to determine terrestrial albedo by earthshine observations. Feasible thanks to sheer determination.

Simple Errorbudget Co-add

Error budgetPosted by Henriette Schwarz Dec 01, 2011 04:06PM
I have aligned and co-added the V-filter moon images from night JD2455864. 16 frames were rejected due to poor correlation when aligning (turned out they were overexposed on the bright limb). This left me with a total of 110 frames.
I have selected an 11x11 pixels area on both the bright and the dark side. The selected areas are shown in the figure below.

The errorbudget formula used is for co-add mode, so exposure time cancels out, and in this first simple version, scattered light is ignored.

For the earthshine I find S/N = 1.7, and for the moonlight I find S/N 128.2.

I have experimented with two different approaches to improve the S/N of the dark side. One way is to increase the number of mean counts, O, in the selected area. For night 864 V-filter O was in the range 398-405. Unfortunately the bright limb was overexposed in the images with higher values than this.
The other approach is to have more images from the same night in the same filter. I have investigated N from 1-1000.
Finding the right exposure time, where the bright limb is well-exposed near saturation, is very important. We can achieve a S/N higher than 3, if we have about 400 frames and a mean dark side count level of 405.
Hopefully we can achieve an even better S/N closer to the new moon. To be continued...

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