In spite of my love for RGB images, I realize that it is often a very good idea to use the Hubble palette (SHO) when photographing nebulas. It is much easier to capture fine details in the nebula in the Hubble palette, and it is also much easier for amateurs to get a good image of a nebula in SHO than in RGB.
But I frankly don't approve of photographing
stars in SHO. Yes, I realize that Hubble often photographs globular clusters in weird-looking colors by using only two filters, often 606 and 814 nm, which is very bad for imaging blue horizontal branch stars. And just two filters is generally bad for bringing out a rich palette of colors in stars.
But there is a good reason for Hubble to use the 814 nm filter when photographing globular clusters: The 814 nm filter brings out the small red stars, which are extremely plentiful in globulars. And using only two filters saves time for Hubble, whose precious time is very much in demand.
But I don't see the point of using the SHO filter for bringing out the stars of an emission nebula. Because while these three filters bring out amazing detail in nebulas, they function as just two filters for stars. That is because the SII filter reacts to the wavelength of
658 nm, while the Hα filter reacts to the wavelength of
656 nm. These two wavelengths make a great difference in nebulas, but they make no difference whatsoever in stars.
Take a look at the Hubble picture of the globular cluster M30 at left. I could not find any actual information on the filters used for that image, but the pale, washed-out colors and the strange bluish cast of the image tells me that this is a 606 + 814 nm filter image.
Like the Hubble image of M30, today's APOD is effectively a two-filter image of the stars ionizing the Rosette Nebula. But the image of the surrounding nebula is indeed a three-filter image.
Ann
NGC 2244: A Star Cluster in the Rosette Nebula
