NGC6888, Crescent Nebula in Cygnus.
1 x 30 minutes Ha, 2 x 30 minutes Oiii, processed as RGB: Ha synG Oiii using Carboni actions.
Apogee Alta U8300, APM115/805 APO Refractor, Astrodon 3nm NB filters.
Guided with DSI Pro II and 2x barlow, through Astrodon Monster MOAG, Stark Labs PHD.
Dark calibrated and aligned in Nebulosity, processed in FITS Liberator and PhotoShop.

Image shows significant field curvature toward the corners.