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M42 - Great Orion Nebula

27 December 2024-1 January 2025

Jubera
- TS Optics Photoline 115
- Moravian C3-61000
(More info later in technical sheet)

This is probably one of the most iconic nebula. Not only for astrophotography but also for researcher, since it continues providing information from the many features that it has.

This picture was taken under very good conditions: 6 nights of anticyclone period, with clear sky, low temperatures, zero wind, and very stable atmosphere without moon. It was taken in Jubera (Soria), between December 27th and the first of January, with a TS Optics Photoline 115 provided with a Moravian C3-6100. The cold night allowed me to set the sensor to minus 25oC, with just 60% of the cooling power. I had to wrap some fabric around the tube to prevent from accumulating frost, and I had to extend (rolling a carton) the dew cap of the guiding scope to avoid dew on the lens. Everything went perfect with the exception of some (unnoticed) defocus in part of the frames. The final image presents very low noise even without processing it with a NR tool. The filters used the complete set: R,G,B,L,Ha,OIII and SII.

TECHNICAL INFORMATION

Location        Jubera (Soria), Coord.=41.20170, -2.36595, Alt.=933 
                SQM=21.84 mag/arsec2 Bortle=Class 3
Main Tube       TS Optics Photoline 115 mm f/7 EDT Triplet Apo. 
                Flat 0.79x, F.L. eff. 632 f/5.5
Guider Tube     Homemade ThorLabs 2" tube assembly 
                F.L. eff. 200 f/4
Main camera     Moravian C3-61000, 9576 x 6388, 3.76 µm
Guider Camera   ZWO ASI290MM Mini, 1936 x 1096, 2.9 µm
Mount           iOptron GEM45
Filters
R Broad Band nm Δ= nm     1x1, 2800 gain, 0 offset, T=-25oC, 30s x 60
R Broad Band nm Δ= nm     1x1, 2800 gain, 0 offset, T=-25oC, 60s x 20
G Broad Band nm Δ= nm     1x1, 2800 gain, 0 offset, T=-25oC, 30s x 60
B Broad Band nm Δ= nm     1x1, 2800 gain, 0 offset, T=-25oC, 30s x 80
B Broad Band nm Δ= nm     1x1, 2800 gain, 0 offset, T=-25oC, 60s x 20
L Broad Band nm Δ= nm     1x1, 2800 gain, 0 offset, T=-25oC, 30s x 51
Hα  656.3 nm    Δ=6.5 nm  1x1, 2800 gain, 0 offset, T=-25oC, 60s x 60
Hα  656.3 nm    Δ=6.5 nm  1x1, 2800 gain, 0 offset, T=-25oC, 30s x 20
SII 670   nm    Δ=6.5 nm  1x1, 2800 gain, 0 offset, T=-25oC, 60s x 20
OIII 500  nm    Δ=6.5 nm  1x1, 2800 gain, 0 offset, T=-25oC, 60s x 60

Processing info

Processing images is an art on its own. Personally I am very prone to do as less processing as possible, trying to retain the information trustable in terms of color and relative intensity. In particular, for the combination of the narrowband shots I follow the proportions of their corresponding mapping values to RGB. As an example, the OIII signal is at 500 nm. 500 nm corresponds (in the standard wavelength-to-RGB mapping) to (0,255,146). Hence I add 0 of the OIII signal to the R channel, 0.64 to G and 0.36 to B. I do the same for the rest of the filters, taking care of normalizing everything in a way that keeps relative intensities in the correct ratios (neglecting in this process wavelength-dependent phenomena such as sensitivity of the camera, absorption in the tube or the atmosphere, etc…). The narrowband combination provides an image where both Hα and SII channels have been added to the same R channel. You may claim that in this way you do not distinguish between the two in the final color image. And that is correct: your naked eye would not be able to distinguish either, because the wavelength-to-RGB mapping gives the same (255,0,0) RGB code for both filters. If I want to distinguish between the two, I look at the individual filter frames (B&W): it is the only way to see where each type of gas is located. I add up the RGB image in order to get more realistic colors of the stars, and this adds also additional colors of the nebulae. This process alters a little the relative color ratios, as RGB also includes signal from narrow bands taken with HOS filters. The result is some difference in saturation, but colors remain realistic. You may find pictures of M42 in the internet with strong blue regions, this is something that does not happen with the aforementioned way of combining, as the B channel is not so populated as in the standard “HOS or HSO Hubble way”.

In any case, this is all for the aesthetic view of M42. If You want to examine where each type of gas is located, you have to look at the individual narrowband pictures. In the following, RGB and narrowband combinations are presented, together with the individual narrowband shoots.

You can find a lot of information about M42 in NASA’s page here.

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