M13 Globular cluster in Hercules
This is my first "serious" image with the 150mm Esprit. Formerly, I thought globulars were easy targets. After all, this one is so bright, I was able to acquire the red data from my light-polluted back yard. But the level of fine detail in a globular is unrivalled by any other kind of object, and the color balance between blue, red, and yellow stars is crucial, so while they are easy to image, they are not easy to image well.
My smallest FWHM with this setup, achieved only during focussing, is 1.6 pixels; normally 2.0 is the best I expect from a 500s exposure. I compiled this image from B and G up to FWHM=2.5, and R up to FWHM=2.2. I found if I included R up to 2.5, red halos around stars were objectionably large and colors were inaccurate. I must presume this is because the region with less than half-maximum intensity in R extends further out than G and B, because it is impossible to focus long wavelengths as well as short ones. This would be irrelevant for nebulae, and relatively unimportant for galaxies. So respect the common globular!