Looking into the Iris. NGC7023

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timh
Posts: 515
Joined: Mon Aug 26, 2019 5:50 pm

Looking into the Iris. NGC7023

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Post by timh »

On the same nights as collecting data on the catseye I also took the opportunity to LRGB image the Iris.

Imaging from 050822 to 110322 at Bortle 6 after moonset (or with the moon nearly set)'

VX12 Orion Optics (UK) Newtonian (f = 1200mm, F4.0), SW parracor type coma corrector, CEM70 Ioptron mount, Baader steeltrack focuser, Pegasus Cube2 focus controller, PHD2 guiding using an ASI 120 mm guide camera and 80 mm SW startravel refractor at f = 400 mm.

ZWO 2 inch R, G, B, L and UV/IR cut off filters. ZWO AS1294 MM PRO mono camera at 2.315um pixel (0.4 arcsec/ pixel) at -5 C.

Capture in live stacking in Sharpcap with FWHM and brightness filters applied. Master flats and darks in Sharpcap. Subframe selection and preprocessing in PI. Guiding (usually < 0.4 arcsec RMS) in PHD2.

The RGB image comprised, 122 x 40s green, 133 x 40s red and 149 x40s blue all at gain 124 and at 0.4 arcsec/ pixel- some of these frames were rather dim and the integrations dominated by a lower number of brighter higher SNR frames. The L image was 177 x 10s at gain 124, had relatively good SNR and sharpness (FWHM ~ 1.4) and the bright centre carefully deconvolved with masking applied to stars and to the noisier outer regions.

The real challenges with the Iris are firstly the high brightness of the core star and its immediate surrounds. To bring out detail here the L image was compressed at the top end with range masking and application of the HDRM process in PixInsight. The second challenge was the fact that nowhere within the long focus image was there any dark background sky (all dust occluded) and thus it was important to be conservative with subtraction of background gradients. Otherwise processing was as usual in PI using DBE background removal, MLT smoothing and histogram stretching.

A real attraction of the Iris reflection nebula is its attractive subtle broadband colours. The blue arises from light scattering by dust - the same dust reddens the stars of the partly occluded NGC7023 star cluster - while the red and orange hues of the illuminated dust cloud edges may originate from the presence of the polycyclic aromatic hydrocarbons recently detected by spectroscopy.

Tim
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IRIS_VX12_05_110822_RED133x40s_BLUE159_GREEN122_DBE_MLT_TRAN_LRGBplus177x10s_90x40sLUM_HDRMmed8_curves_MLT_affinity16_PX_sm.jpg
IRIS_VX12_05_110822_RED133x40s_BLUE159_GREEN122_DBE_MLT_TRAN_LRGBplus177x10s_90x40sLUM_HDRMmed8_curves_MLT_affinity16_PX_sm.jpg (906.42 KiB) Viewed 1136 times
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