basskep,

OK, here are what I get for a Celestron C8 used with a Celestron F/6.3 Focal Reducer and an Opolong L-eNhance filter.

I first looked at the optics. I could not find the optical design data for the focal reducer other than it has four elements. From other information I have seen, I think the focal reducer is organized as two groups of two element each. Plugging that information into my spreadsheet and assuming the scope is reasonably clean, I find the following:

*Calculation of Telescope Optical Transmission Properties.*

This result says that 81.4% of the light that enters the aperture of the scope is delivered to the next optical element, the focal reducer. After accounting for the central obstruction and light losses in the focal reducer, a total of 77.4% of the light that entered the telescope's aperture makes it to filter in front of the camera.

Now we look at the camera's response to that light. Here is a plot of wavelengths showing the transmission through the filter and onto the sensor. (I have ignored the effects of light loss through the filter glass and the camera window + sensor cover glass.)

*Net Effective Response Of Sensor With Filter.*

In this plot, the gray line is the response of the Opolong L-eNhance filter, the Red line is the response of the Red Bayer Pixels on the sensor, the Green line is the response of the Green Bayer Pixels on the sensor, and Blue line is the response of the Blue Bayer Pixels on the sensor. The dashed yellow line is the average response of a Bayer Quad (Red, Green, Green, Blue) of the sensor.

[Note that the spreadsheet graphs don't show very narrow values very well. Even though the H_{a} Peak of the filter looks too pointy, it is actually 5 nm wide in the data which is what gets integrated. Until I find a better way to plot the data, the graphs should be taken as approximations only.]

If we integrate the area under the yellow curve, we get the average percentage of light that is captured by the sensor / filter combination over the wavelength range of 400 nm to 700 nm.

That Total Effective Response is 4.07%. Now if we multiply by the Effective Throughput of the telescope, we get a result of 3.15%. This represents the percentage of photons that are actually captured per unit of time from the 8" aperture of your system.

Now we can enter this into Steve's spreadsheet. Use the following inputs:

- In Cell D17, enter 0.0315 -- This is the effective system QE of your full imaging train.
- In Cell D15, enter 150 -- This is the (half)bandwidth over which the System QE was calculated.

These should get you a result that is pretty close to real world. I have verified Steve's spreadsheet with real world results using this method. When I use a similar Effective System QE value for my own imaging setup and input data from a set of real images into the "Input Data from an Image" section, I get accurate Sky Brightness values that match my actual Unihedron SQM-L readings to within 0.1 mpass.

John