PixInsight Blogs

The second RECTA / CAHA / DSA collaboration is a portrait of the Messier 57 nebula in Lyra.

These data were acquired mainly through the 1.23 meter Carl Zeiss telescope. It is a rather long exposure of the nebula taken during several nights from July to September:

- 18 hours of optical narrowband data in the H-alpha and OIII lines. This data was acquired entirely through the 1.23 meter telescope, and it gives an extreme deepness to the nebula’s halo. The H-alpha filter has a bandpass widht of 5 nm (it transmits part of the NII lines), while the OIII one has a bandwidht of 9 nm. Subexposures were all 20 minutes long.

- 3 hours of broadband data with standard photometric Johnson BVR filters. These data were acquired through the 3.5 meter telescope. It gives the required deepness to the background field, as well as the high resolution required to show all the detail in the halo and inner disk. Subexposures were 2 minutes in R, 3 minutes in V and 2 minutes in B. All the data were acquired in a white night.

- 45 minutes of infrared narrowband data, acquired through the 3.5 meter telescope and the Omega-2000 IR camera. The infrared image was taken through a filter centered in the molecular hidrogen emission at 2.12 microns. All subexposures were rather short (45 sec), as is usual in IR ground based observations.

The whole data set was calibrated manually with the ImageContainer and PixelMath tools.. The PixelMath formula is rather simple:

($T – bias)*Avg(flat)/flat

Where:

$T is the target image being calibrated,
bias is the master bias frame,
flat is the master, bias substracted, flat frame.

The image registration and integration was done automatically with the StarAlignment and
ImageIntegration tools, respectively.

The nebula, as seen through each filter, can be seen in the image below:

To combine the optical narrowband with the broadband one, we introduced the different emissions in the BVR images. The R filter transmits only the H-alpha line. But the B and V filter transmit in different proportions the OIII and H-beta emission lines. So we introduced the OIII and H-alpha images in both filters with varying weights.

Finally, the infrared image was processed separately and introduced in the processed optical image, as both images represent very different structures of the nebula.

This 6-channel mixing gave us the resulting emission line tones:

- The OIII emission has a teal tone.
- The combined H-alpha and H-beta gives a nearly violet emission. This gives us a good differentiation between optical hidrogen emission and the infrared one.
- Molecular hidrogen IR emission is represented as pure red.

All these operations were carried out with the PixelMath tool.

Color calibration was performed with the ColorCalibration tool, taking the whole light of the objects in the image as a white reference.

This image has two decisive steps: the deconvolution of the higher signal areas, and the dynamic range compression.

Deconvolution was done with the regularized Richardson-Lucy algorithm, and it was done only in the inner disk, where we have enough signal to noise ratio. The contrast between the ring and the halo of the nebula is very steep, so the mask to select the areas with enough signal is amlost a binary mask including only the ring and the central hole. This can be better understood if we make a 3D plot of the below linear H-alpha image:

The 3D plot (made with the 3DPlot javascript script for PI, ver 1.3, by Andrés Pozo, David Serrano y Juan Conejero) shows that even the hole at the center of the main disk of the nebula has a very steep illumination decrease respect to the ring:

To make a deconvolution to the outer halo (barely visible in the graphs due to its extreme faintness) would be impossible. Even if you deconvolve these areas with noise regularization, the wavelet-based algorithm would not detect any structure to deconvolve, as there isn’t enough signal strenght.

It’s interesting to compare our red image with the one taken by Hubble Space Telescope ten years ago:

Sorry, but the HST red image is heavily clipped. This side by side comparison shows the extremely good performance of this deconvolution algorithm, as well as the movement of the nebula structures during this 10 year period.

The inner nebula disk is extremely bright, compared to the outer halo. For that reason the dynamic range compression procedure is of crucial importance. Below we can see the stretched image prior to any dynamic range compression:

The dynamic range compression and processing was carried out with the HDR-Wavelet algorithm, as well as with the techniques described in the NGC 7331 processing notes.

A 3D plot of the above stretched image shows clearly the illusion we create when we stretch the data:

It seems that the center hole of the nebula has near the illumination level of the ring, and seems that there’s only a tiny part of the real contrast between the dist and the outer halo.

Please be sure to read the FAQs about this picture.

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:

www.caha.es

The new DSA website:

www.astro-photographer.org

And my website:

www.astrofoto.es

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.

NGC 7331 with the 3.5m Zeiss Telescope of Calar Alto Observatory and the LAICA camera, by Vicent Peris (PTeam/OAUV). Entirely processed with PixInsight version 1.2.

This image of NGC 7331 and its surrounding galaxies is my first astrophotographic work with Calar Alto Observatory (CAHA). First I must thank Joao Alves, the Director of the Observatory, because the data for this work have been acquired during his discretionary observational time. Also thanks to Gilles Bergond, the service astronomer who acquired the data, and to David Galadí-Enríquez, who wrote the image release published at Calar Alto’s website. Finally, thanks to Vicent J. Martínez, ex-Director of the Astronomical Observatory of the University of Valencia (OAUV), the institution where I work as a professional astrophotographer.

For me, this image is just a starting point. The work of scientific photographers is important for the public outreach of astronomical research. The CAHA and the OAUV have given a first step; I hope this cooperation will continue from now on as a contribution to the excellent science communication activities performed by both institutions.

To know more about the objects present in this image, you can read the official image release published at Calar Alto Observatory’s website.

This image integrates a total of 139 minutes of exposure time. It is composed of 1-minute and 10-minute exposures that I have combined to create a linear, high dynamic range image. I have written a processing example, available on PixInsight’s website, which describes the whole processing of this image.