Eagle Nebula NGC6616 (M16)

The Eagle is a good target to test telescope collimation and tracking with interesting splats and blobs. More enlightened people than myself have enthused about the nebula's beauty and cosmic significance.

This was a massive set of captures that combined the data from two telescopes (RC10A and RC8). Many many nights (at least 30 sessions). Generally I kept grabbing data whilst my favourite objects, galaxies, were at lower Hour Angles or the Moon was being a menace.

RC Collimation

For the main scope (RC10A) collimation I used the PixInsight FWHMEccentricity script as part of an iterative live method to align the RC secondary. The advantage is that it is an in-focus multi star-test technique (by using a suitably rich star-field). You need to use a guidescope and not an OAG. Also a word of caution: Don't drop your Allen/hex key on the primary mirror when adjusting the secondary as your scream will wake the neighbors.

Tracking

I finally figured out how to get a decent average PPEC into the CGX/L mount despite Celestron's best efforts to make this ridiculously difficult. Initially I was unsure if it made any difference but when the seeing becomes steady then the RA error was comparable to Dec error. Definitely helps.

Capture Info

The image was assembled using two scopes and four filters in an L+RGB approach.

Luminance (RC10A Ha₁(7nm)+ → L+)

A 7nm ZWO Ha₁(7nm) image is used as luminance — i.e., the Ha plus NII lines — using a RC10A (truss). The camera is a ASI293MM Pro with "native" 2x2 binning (1x1 binning has inferior SNR even taking into account the smaller pixels). Image scale was 0.67 arcsec/pixel with a compressor. I threw out lots of data (roughly 20 hours) before reducing to the 5 hours of data that was integrated for the luminance.

Chrominance (RC8 OHa₂S(3nm) → RGB)

I more or less emulated the palette used by Ignacio Diaz Bobillo in Zooming into The Pillars, which isn't Hubble but creates a sea of blue for the splats and blobs to reside. I used an RC8 with Astrodon 3nm filters and an ASI1600MM Pro with 1x1 binning and image scale 0.64 arcsec/pixel with a compressor. (So here the hydrogen alpha, Ha₂(3nm), is not the same filter/bandwidth as the Luminance channel Ha₁(7nm).)