Pier Analysis

...and now for our daily dose of controversy!

I developed somewhat of a cheat sheet (see attachment). Some properties are indisputable like field-of-view and CCD resolution, and others are open to interpretation. I'd like to get feedback, if any, on the latter part, that which I have highlighted in red.

The column labeled "G300s SNR" is a measure of light grasping power but it is much more than the conventional square of the ratio of apertures that we often see which only looks at aperture. My measure by contrast looks at nine properties of the telescope and camera. Later on in this post I'll give an exact definition.

If you scan the column you will see that Pier 3, the ASA 12n f/3.6 Newtonian, is the clear winner at SNR 7.88. The loser, if you will, is Pier 9, the TMB LZOS 152mm F8 APO, at SNR 1.42. If you look at it dispassionately it makes sense. If your objective is to capture dark nebulae then I would not choose the TMB but I would choose it for something bright like a globular cluster. All other scopes fall between those extremes.

Now let's calculate the mean of the SNR values: 4.69. So any Pier whose SNR value is greater than 4.69 is deemed to be suitable for faint DSOs and all others are suitable for bright DSOs.

Next let's move to the two "Phot" columns. These are related to photometry. It uses the same bright vs faint classification described above but adds a check for CCD resolution. If resolution is less than 1.35 then it is deemed to be suitable for photometry. The number 1.35 I measured during G2V analysis which put seeing conditions at 2.7"/px, so half of that is 1.35. This just means that the star's disc must be spread over a 2x2 pixel grid at least.

The controversial part is the classification of Pier 12 as not being suitable for faint DSOs. This is an edge case where its SNR value of 4.47 is just below the mean of 4.69. It's a fine scope but it has fallen victim to Boolean logic. Perhaps a better classification system would be based on percentages so that Pier 12 would fall just below 50%. Then it would be up to the user to decide.

Now to the definition of the column "G300s SNR". Imagine pointing the scope at the zenith and capturing a 300s image using the green filter. (I chose the green filter due the fact that most if not all CCDs are strongest in the green band.) Now, calibrate that frame and stretch it so that deep space is a shade of gray. You will see that space is noisy. Next, do this for all piers and compare the results. You will notice that deep space at Pier 3 is less noisy than Pier 9. That is because the optical system at Pier 3 is able to capture more photons, and therefore electrons, than Pier 9. Finally, divide the number of captured electrons by all sources of noise: Read Noise, Dark Noise, and Shot Noise. This then is the signal-to-noise ratio (SNR). Pier 3 provides a higher SNR than Pier 9.

Brian