I’m glad to present the first image from the RECTA / DSA / CAHA collaboration: the planetary nebula Messier 97.
To download higher resolution images, and to know more about this project and the image, please visit the press release at CAHA website:
The new DSA website:
And my website:
The image has been done with the three CAHA telescopes: 1.23, 2.2 and 3.5 meter mirror diameter. The image has isolated light contributions from H-alpha and H-beta, O-III and reflection component. This can be achieved through the use of the correct filters. The different light components can be seen in this image:
To see the light reflection from the central star inside the nebula, we employed Strömgren b and y filters. These filters at 463 and 547 nm, are centered into spectrum areas without any significant emission lines. This makes possible to completely erase the light emitted by the nebula excited gas. Through these filters, we’re looking only the light from the central star and reflected (not emitted) by the gas itself.
The other three isolated line emissions are H-alpha, H-beta and O-III. H-beta emission was derived from the 3.5 meter telescope images. With this telescope, we employed a 30 nm wide filter centered at 489 nm from the ALHAMBRA Survey. The transmission curve of this filter allow to register at the same time the O-III and H-beta contributions. Thus, to isolate the H-beta from the O-III contribution, we substracted the O-III image from the 1.23 and 2.2 meter telescopes to the 3.5 meter 489 nm image. You can see the resulting H-beta image in the above picture. The result shows a clearly different emission distribution from the H-alpha. The H-beta, being a higher excitation level, is more concentrated to the center of the nebula; but also it is visible outlining the main disk.
For color representation, we chose at color model where the primaries are situated at 450, 550 and 650 nm, with linear transitions between them, as shown in the graph below:
The first step for converting this five band image into a RGB one is to calculate the reflection contribution at the wavelength of color primaries. As we have the continuum flux at 463 and 547 nm, we can derive fluxes at 450, 550 and 650 nm following the black body radiator function as a reference. Dr. Fernando Ballesteros (OAUV) designed for us the formulas to calculate the fluxes at the desired wavelength, wich were applied through PixelMath. With these forlumas, we make a synthetic color image where each R, G and B channel is calculated from the same Strömgren b and y images. The resulting image shows that the reflection component has the same bluish hue as the central star, typical for a hot white dwarf star.
Finally, line emission contributions are represented following the linear primary transitions defined by our chosen color model: bluish for H-beta, cyan for O-III and pure red for H-alpha.
Emission isolation is a key when photographing planetary nebulas to maximize content communication through the image. This kind of work let us explain different physical processes in one image: light reflection versus light emission nebulas, as well as gas excitation levels that tell us about the nature of the object.