NGC2158 is the symbol that may ring some bells for most of astronomy amateurs, but probably not many of them can immediately say what it is. If you image the night sky then most probably you already captured this object, though not as main target. NGC2158 open cluster is this small companion of well known M35 in Gemini. It was discovered by Herschel. NGC2158 and M35 are not related in any way. This remote open cluster is located about 16,500 light years away, and M35 is four times closer. NGC2158 is about 2 billion years old, while M35 is ten times younger. Due to its advanced age NGC2158 consists mostly of old, yellowish stars, because all blue and hot objects already died long time ago.
M35 at the image above already started to appear in the top left corner of frame. Stars in the M35 are quite sparse. In NGC2158 stars are pretty dense – both clusters have similar dimensions (real, not apparent), but NGC2158 contains much more stars. The seeing was quite decent during the nights when I captured data to this image. Due to this fact stars in NGC2158 are resolved till the cluster centre.
Image technical data:
Date: 30.03, 01.04.2019
Location: Nieborowice, Poland
Telescope: Meade ACF 10"
Corrector: AP CCDT67
Camera: QHY163M, gain 100
Mount: SW EQ6
Guiding: SW 80/400 + ASI290MM
Exposure: LRGB 80:40:30:40 x 30 seconds
Conditions: suburban sky, seeing good, transparency good