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NGC 1499 - California Nebula

January 1, 2022

Alovera-1A
- Takahashi TOA 150B (1100mm f7.3)
- Moravian C3-61000
- Hα 30x300"
- SII    30x300"
Alovera-2
- Sky-Watcher Esprit 100ED Super APO Triplet (550mm/f5.5)
- ZWO ASI 2600 MC Pro
- Narrow band, 30x120”

Introduction

The California Nebula is an emission nebula located in the constellation of Perseus. Its predominant color is red, due to the Hα emission line of Hydrogen, the dominant line in this nebula. The star responsible for the ionization of Hydrogen is believed to be Menchib. This nebula can only be seen with the naked eye on very dark nights.

In this post we bring two captures of the nebula made in parallel at Alovera 1A and Alovera 2 stations, during the last days of December (2021) and the first of January (2022). In Alovera 1 we have exposed 30 frames of 300 ”, with each one of the narrow band filters Hα and SII, with the Moravian C3-61000 monochrome camera. In Alovera 2 we have used the double band Optolong L-eNhance filter, coupled to the ZWO ASI 2600 MC Pro color camera. In this case, in addition, a mosaic has been made to cover the 2.5 degrees of the nebula field of view.

Alovera-1A

Takahashi 150/Moravian C3-61000

The image field for this combination of telescope and camera is 1.55 x 1.15 degrees, as can be seen in image 1, so the nebula cannot fully enter into the frame. The camera orientation used is not the best either, as it could have been tilted some 45 degrees to get the nebula along the large side. However we preferred current orientation in order to make Menchib appear in the image.

Image 1. Framing for NGC 1499 with the combination of Takahashi TOA 150B/Moravian C3-61000 (Alovera-1A). Zoom

28 shots were taken for each of the narrowband filter used: Baader Planetarium Hα, 6nm bandpass, and Baader Planetarium SII, 6nm bandpass. Exposure time fore each shot was 300 seconds, with a gain setting of 2800 (scale of the manufacturer). This gain is just a bit above the manufacturer's parameter 2750, where the read noise drops to 1.46 e-, while keeping a modest 16,900 e- of well capacity. Surrounding stars saturate the capacity anyway, while for the nebula is still enough. In turn we have less read noise. Bining was maintained at 1x1 and sensor at -20 degrees Celsius. The stacking was done with DeepSkyStacker, accompanying the shots with a total of 100 FLATS, 60 BIAS and 15 DARKS. The stacked images for each filter where stretched for inspection. They can be seen in images 2 and 3.

Image 2. Stretched image for Hα with Takahashi TOA 150B/Moravian C3-61000 (Alovera-1A). Zoom Image 3. Stretched image for SII with Takahashi TOA 150B/Moravian C3-61000 (Alovera-1A). Zoom

These two images have been post-processed using Pixinsight. RGB format was created according to the following channel relation: Hα -> R, SII -> G, SII -> B. The resulting color image is processed then in a rather generalistic way, with noise reduction and normal stretching of the histogram (e.g. stages and layers). Image 4 is the result of the processing.

Image 4. Final color image in RGB of NGC 1499 with Takahashi TOA 150B/Moravian C3-6100 (Alovera-1A). Zoom

Note that due to the way in which the images of each filter have been mixed, a fundamentally red image is generated (the SII inserted in the G and B channels contributes to increase the intensity in the areas with Hα signal, maintaining the red of (dominant) Hα ). In any case, the individual images (B&W) of each filter are the ones that best facilitate the understanding of the composition by zones of the gas in the nebula.

With this captures we have advanced a bit more in learning to combine channels.

Alovera-2

Sky-Watcher Esprit 100ED/ ZWO ASI 2600 MC Pro

The cover image of this article on NGC 1499 (also image 5 below) is a mosaic made up of two tiles that were obtained throughout different nights of late December 2021 and the first nights of January 2022.

Image 5. Mosaic of NGC 1499 with Sky-Watcher 100ED/ZWO ASI 2600 MC Pro (Alovera-2). Zoom

The framing plan, as well as the capture of the image sequences that make up the different tiles, was carried out using the N.I.N.A. 2.0 Beta017.

Each of the two individual frames was obtained from 50 narrow band 300" shots with the Optolong L-eNhance / ZWO ASI 2600MC Pro filter / camera combination. In all cases the sensor temperature was kept at -18 degrees Celsius, the gain was 105 and the binning 1x1. The overlap between both frames is 10%. The selected filter presents intense transmittance bands in the Hbeta - OIII lines (blue-green 480-520nm) and in Hα (656nm). This combination of transmittance bands allows to capture the majority of the light from the emission nebula NGC 1499, while filtering very effectively light pollution.

The stacking of the frames from the light shots and the different calibration shots was carried out for each of the tiles independently using Pixinsight. When joining the different tiles that make up the mosaic, special attention must be paid to correcting the distortion and adjusting the relative intensities of each frame. This process can be carried out in different ways. In this particular case, the background was subtracted and the linear processing of each tile was carried out and a first preliminary mosaic was created that allowed us to obtain the coordinates of the resulting image box using the PI Image solver script. From these coordinates, a canvas of artificial stars was created that covers the same image field of the mosaic on which the two tiles obtained independently with the StarAlignment process will be aligned. Finally, both tiles were joined with the GradientMergeMosaic process. The resulting image was stretched and processed in the usual way with PI.

As can be seen, the image obtained does not present artifacts, with well-corrected distortion and intensity in the overlap area of both tiles. This capture of NGC1499 has allowed us to become familiar with the process of obtaining wide field images by generating mosaics of at least two tiles. The next challenge will be to increase the number of tiles to join.

External links

Link to NASA/IPAC Extragalactic Database (NED) here. Summary in NASA Science here.

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