Equivalent Exposure Calculator Details (a.k.a. Disclaimer)

The calculations are based solely on telescope focal ratio, CCD quantum efficiency (QE), pixel size, and binning.  Therefore the calculation is a bit oversimplified.

The quantum efficiency at 450, 550, and 650nm for each camera are used to determine equivalent RGB exposures.  These are about the middle wavelengths of typical red, green, and blue filters.  Some cameras have peak QEs within a certain filter bandpass (say, red) that exceeds the value at the chosen wavelength (650nm in this example).  But this approximation at least allows a direct relationship between various cameras independent of the filters used and works well for comparison purposes (which is what we're doing, after all).

Luminance exposure is based on the average of the red, green, and blue QE values.  The actual value should probably be the integrated value of the QE curve multiplied by the light transmission curve of all the telescope optics involved (including filters, focal reducers, etc.), and if you want to find and download all the QE graphs and transmission charts and do the calculus, be my guest.  I think the mathematical function used here is close enough given all the other factors involved.

Factors such as filter transmission and object elevation above the horizon are not taken into account, so the results are only approximate.  Remember, an object 30 degrees above the horizon suffers from a 2x atmospheric extinction factor, compared to an object at zenith.  An image of Omega Centauri from Australia will require a shorter exposure than one from Arizona, all things otherwise being equal.  Seeing conditions will also affect the outcome.  And of course, an image from an Arizona mountaintop and an "equivalent" exposure from suburban Boston aren't going to be that "equivalent".

Also, optical coatings can alter the light transmission -- so, your 27-year-old, weathered orange-tube C8 isn't going to transmit light like your friend's new CPC-800 with XLT coatings, I'm afraid.  Comparisons between refractors and reflectors and catadioptric systems can be tricky because of this.  Of course, some coatings and optical glass transmit more light in one color than another, altering the RGB comparisons.  Still, the differences are not huge.

Also, noise is not taken into account.  See the page of Optimum Exposure Theory for more details, but suffice to say that an "equivalent" exposure with a certain camera may not sufficiently overcome inherent noise (such as read noise) as well as another camera.

Pixel size and binning affect exposure as well, so cameras with similar QE but different pixel size will have different equivalent exposures.  (This is taken into account in the calculations.)

In the end, the major factors are taken into account and this should give you a good idea where to start.  Some experimenting may be required, but that's part of the fun, right?

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