NGC 7023 (Iris Nebula) and Molecular Clouds in Cepheus

Images below show a dusty 6.25° × 4.0° large region in Cepheus containing NGC 7023 (Iris Nebula) and VdB 4141 (Ghost Nebula).

SDSS I'R'G'

First figure shows a color composite from SDSS I' (mapped to red), R' (mapped green) and G' (mapped blue) filters.

NGC 7023 (Iris Nebula) and Molecular Clouds in Cepheus with SDSS I', R' and G' filters — Click on the image for a full scale version.
NGC 7023, also known as Iris Nebula, is the bright blue region top left of the center. The bright nebula that lies left to it, is VdB 4141 (Ghost Nebula). The deep red star between these two nebulae is T Cephei, a Mira variable star. These stars are dying a red giants characterized by strong infrared emission (that's why it appears red here). In near infrared T Cephei is about 100 times brighter than in visible light (-0.496 mag in J band vs. 4.644 mag in G band). Another bright red giant is AC Draconis in top right corner, a long periodic variable star.
The red and green artifacts below AC Draconis are probably caused by reflections on a part within the lens (due to the strange shape). The artifacts around the bright stars are residuals from the aggressive star reduction (by up to factor 40) and the fact that the PSF (point spread function) strongly varies across the field of view.
The color of the nebulae is influenced by scattering which makes dense region more opaque for blue light than for (infra)red radiation. The contrast between the colors is larger, the larger the bandwidth of the filter set is.

The difference between the color channel is also visualized in the following figure where the color components from the image above can be compared interactively.

NGC 7023 (Iris Nebula) and Molecular Clouds in Cepheus with SDSS G' filter — Click on the buttons to switch between the filters and view the different color channels as monochrome image.
Typical RGB filter sets for astronomy only cover the spectral range of the G' and R' filters, about 400nm to 700nm. The bandwidth of on-sensor color filter arrays of consumer cameras is even narrower. The information contained in the I' channel would be lost with RGB filters. Especially the denser regions of molecular clouds that absorbs shorter wavelengths are bright in infrared. These nebulae are red or orange in the color composite above. In a RGB images they are dark. This is also shown in the next figure.

NGC 7023 (Iris Nebula) and Molecular Clouds in Cepheus with SDSS R' filter — Click on the buttons to switch between the filters and view the different color channels as monochrome image.
Typical RGB filter sets for astronomy only cover the spectral range of the G' and R' filters, about 400nm to 700nm. The bandwidth of on-sensor color filter arrays of consumer cameras is even narrower. The information contained in the I' channel would be lost with RGB filters. Especially the denser regions of molecular clouds that absorbs shorter wavelengths are bright in infrared. These nebulae are red or orange in the color composite above. In a RGB images they are dark. This is also shown in the next figure.

RGB comparison

The following pictures are an attempt to compare the results from the SDSS filters with RGB shots.

NGC 7023 (Iris Nebula) and Molecular Clouds in Cepheus with RGB filters — Click on the picture to toggle between RGB and SDSS I'R'G'.
The image shows a 4.2° × 2.6° large region from the picture from top of this page for which RGB data was available. This data was captured with two D=200mm Newton telescopes and Nikon D800e cameras. The resolution of the larger instrument is higher, but it took about 114h of total exposure time in order to detect about the same amount of photons from the Object as with the smaller instruments within 26.5h. (Due to obstruction and vignetting each D=200mm telescope collects about 3 times as much photons from the object as a D=100mm lens. But because it's a 2×2 mosaic, only 25% of the light is used. Furthermore the sensitivity of the QHY600L with SDSS filters is about 4 times higher than the sensitivity of the Nikon sensor with its filter array.) That means, if low-pass filtered to the same resolution both data sets from RGB and SDSS filters have about the same SNR.
Nevertheless, the images here are a little bit darker than the one at top of the page, in order order to make more of the Iris Nebula visible. That is easier to achieve with the SDSS filters, because with them only the G' channel is highly saturated (also see the previous figure) while in the RGB image all channels are more or less saturated. This is also the reason why the Iris Nebula is more colorful with SDSS filters.
Apart from that, it is also not possible to produce similar results with both filter sets because nebulae that are dark in RGB are bright in infrared, i.e. red or orange in the I'R'G' composite. That was already shown in previous figure: The Information captured with the I' filter is missing in the RGB variant.

NGC 7023 (Iris Nebula) and Molecular Clouds in Cepheus with SDSS I'R'G' filters — Click on the picture to toggle between RGB and SDSS I'R'G'.
The image shows a 4.2° × 2.6° large region from the picture from top of this page for which RGB data was available. This data was captured with two D=200mm Newton telescopes and Nikon D800e cameras. The resolution of the larger instrument is higher, but it took about 114h of total exposure time in order to detect about the same amount of photons from the Object as with the smaller instruments within 26.5h. (Due to obstruction and vignetting each D=200mm telescope collects about 3 times as much photons from the object as a D=100mm lens. But because it's a 2×2 mosaic, only 25% of the light is used. Furthermore the sensitivity of the QHY600L with SDSS filters is about 4 times higher than the sensitivity of the Nikon sensor with its filter array.) That means, if low-pass filtered to the same resolution both data sets from RGB and SDSS filters have about the same SNR.
Nevertheless, the images here are a little bit darker than the one at top of the page, in order order to make more of the Iris Nebula visible. That is easier to achieve with the SDSS filters, because with them only the G' channel is highly saturated (also see the previous figure) while in the RGB image all channels are more or less saturated. This is also the reason why the Iris Nebula is more colorful with SDSS filters.
Apart from that, it is also not possible to produce similar results with both filter sets because nebulae that are dark in RGB are bright in infrared, i.e. red or orange in the I'R'G' composite. That was already shown in previous figure: The Information captured with the I' filter is missing in the RGB variant.

Image data

FOV:

6.25° × 4.0°

Date:

12/2021 to 01/2022

Location:

Pulsnitz, Germany

Instrument:

2-3 × Nikon 300mm f/2.8 AF-S

Camera:

2-3 × QHY600L

Orientation:

North is up (exactly)

Scale:

3 arcsec/pixel (at full resolution)

Total exposure times:

SDSS G':	10.5 h
SDSS R':	9.9 h
SDSS I':	6.1 h

Image processing

All image processing steps are deterministic, i.e. there was no manual retouching or any other kind of non-reproducible adjustment. The software which was used can be downloaded here.

Image processing steps where:

Bias correction, dark current subtraction, flatfield correction, noise estimation
Alignment and brightness calibration using stars from reference image
Stacking with masking unlikely values and background correction
Star subtraction
Denoising and deconvolution
Color composition
Dynamic range compression using non-linear high-pass filter
Tonal curve correction