Attn. click above images for complex content
Images below show a dusty 6.25° × 4.25° large region in Cepheus containing NGC 7023 (Iris Nebula) and VdB 4141 (Ghost Nebula).
First figure shows a color composite from SDSS I' (mapped to red), R' (mapped green) and G' (mapped blue) filters.
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). 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. © Copyright Stefan Ziegenbalg
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.
Filter: G' © Copyright Stefan Ziegenbalg
Filter: R' © Copyright Stefan Ziegenbalg
Filter: I' © Copyright Stefan Ziegenbalg
G' R' I'
The following pictures are an attempt to compare the results from the SDSS filters with RGB shots.
Filters: RGB © Copyright Stefan Ziegenbalg
Filters: I'R'G' © Copyright Stefan Ziegenbalg
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.
FOV: 6.25° × 4.25°
Date: 12/2021 to 01/2022
Location Pulsnitz, Germany
Instrument: 2-3 × Nikon 300mm f/2.8 AF-S
Camera: 2-3 × QHY600L
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
All image processing steps are deterministic, i.e. there was no manual retouching or any other kind of non-reproducible adjustment.
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
Denoising and deconvolution
Dynamic range compression using non-linear high-pass filter
Tonal curve correction