LBN 640 is evidently a very dim nebula in Perseus. Lynds has it is rated at a 4 on the color scale meaning only detectable on the Palomar red plate. It is rated as a 6 on the brightness scale which is the faintest. I imaged a couple rated at 4 from black skies and they barely showed up. Apparently an effort in futility from my house, I imaged this one in LRGB several years ago - https://www.flickr.com/photos/dcrowson/31606824138/sizes/l. I’m glad I circled back around and revisited this one with Ha.
Ha – 24x1800s – 720 minutes – binned 1x1
RGB – 8x300s – 40 minutes each – binned 2x2
840 minutes total exposure – 14 hours
Ha captured from Dardenne Prairie, Missouri (a red zone) December 11th, 12th, 13th, 16th and 20th, 2021. The RGB data was captured February 17th, 2023 at the Danville Conservation Area (New Florence, Missouri). Everything was captured with a SBIG ST-8300M on an Astro-Tech AT90DT at f/6.7 603mm.
HaRGB - https://www.flickr.com/photos/dcrowson/52699234088/sizes/l/