I had thought this was a first-attempt image of Sh2-171, but looking through my older files I see I did it 2 years ago in Hα, one of the last images I took with the DSI IIc acting as my main imager (before I upgraded to the ASI1600MM-Cool). At any rate, the current image took advantage of several consecutive nights of good skies just before the mountains southwest of me caught fire (with pretty good wind out of the southwest = heavy smoke), ruining any imaging opportunities for now. Sh2-171 is relatively bright in Hα, not so much in OIII, but shows interesting detail in SII that adds a lot to the image. I don’t usually do 600 sec subframes but this target seemed to need them. To top it off, I took some shorter-exposure RGB intending to just use that for the stars (and that’s exactly how I used it in this image), but then I realized we have just looked at the H atomic spectrum in my freshman chemistry class as build up toward the quantum mechanical atom. I pointed out the Hα line then, and intend to show several versions of this image (Hα only, RGB only, and Hubble Palette) in my next lecture to illustrate how much information you can get using narrowband imaging. Spectroscopy lets you know the chemistry of distant objects. So I’m having some fun with this.

Date: 8-11 Sep 2018

Subject: Sh2-171

Scope: AT8IN+High Point Scientific Coma Corrector

Filters: ZWO 31 mm diameter unmounted 7 nm Hα, SII, OIII, R, G, B

Mount: EQ-6 (EQMOD 2.000j)+PEC

Guiding: Orion Thin Off-axis Guider + DSI IIc +PHD 2.6.5 (Win 10 ASCOM)

Camera: ASI1600MM-Cool, -20 °C, Gain 139 Offset 21

Acquisition: Sequence Generator Pro 3.0.2.94

Exposure: 21x600 Hα, 23x600 SII, 24x600 OIII, 20x180 R, 16x180 G, & 20x180 B

Stacking: Deep Sky Stacker 4.1.1 (64-bit) dark+flat (no bias), κ-σ stacking with κ = 1.5.

Processing: StarTools 1.4.332: Cropped Hα but did not wipe, then stretched, applied HDR (optimize), masked out the “fat” stars, aggressively deconvoluted, then did untrack denoise. Followed essentially the same procedure for SII and OIII. Combined SHO in Photoshop using Annie’s Astro Actions, and played with the color balance to get it roughly how I thought it should look. Did several rounds of curves + levels and some Carboni deep space noise reduction. Combined R, G, & B in StarTools. Cropped but no wipe was needed, developed, HDR optimized, and applied default color module settings. Denoised without any deconvolution. Added the RGB stars on top via screen combine. Used a couple of rounds of “increase star color”, followed by a couple of rounds of “make stars smaller”. AstroFrame.