NGC 6960: The Witch's Broom Nebula: 29h integration with Chroma 3nm (Daniel Beetsma)

NGC 6960: The Witch's Broom Nebula: 29h integration with Chroma 3nm

Western Veil Nebula, captured in June 2020

25 x 1200s Ha

27 x 1200s OIII

36 x 1200s SII

Total exposure: 29h20m.

Images captured with NINA

Calibrated and preprocessing in Pixinsight (deconvolution, local normalization)

Colorcombining + postprocessing in Photoshop, including selective color.

Chroma filters weren't cheap, and especially for Ha and SII, alternatives like Baader and ZWO are very comparable. The big difference however is in the OIII filter which produces absolutely no halo or bloating or reflections or anything of the kind: after colormapping, the bright Cygnus star was just this absolutely dazzling pure white, and 'the star' of the picture, rather than a disturbing source of artefacts. Each of those 3 filters was as expensive as my mount or my telescope (both in the 600 euro range, second hand), but I find it worth it.

The mount performed particularly well after I dissasembled it, cleaned it, replaced all bearings and grease, and put it back together and tuned it again (I call it the Beetsma-Tuning). It now runs reliably at 0.50'' - 0.60'', sometimes touching the low 0.40''s on clear nights and with a favourable position regarding counterweights. This is all well within the margins of the 2''/p image scale that the 5.4 mu pixels of the KAF8300 sensor in the Atik 383L+ gives: there's not a hint of star elongation in these subs, which greatly helps the sharpness.

I think this image also answers the question: is a doublet good for narrowband? I think it's superior to a triplet for narrowband: there is less glass to go through, less cooling down time, less chance on reflections, less impurities encountered. With 3nm narrowband - who cares that blue has a slightly different focal point compared to red? With HFR of 1.10 and lower in NINA - I think we're fine. The problem with doublets however is that they are often inferior to triples in terms of glass used and especially mechanical solidity. This old Stellarvue 102ED, which is at least 10 years old, however is very well built, and the feathertouch 2'' focuser, of course, is a joy to work with. Collimation of the scope was done by Ido Oosterveld from the Netherlands, and later finetuned by myself.

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