Caldwell 21
NGC 4449
27’ x 18’ | 0.3”/px | 5500 × 3727 px
Canes Venatici
RA 12h 28m Dec +44° 07’ | 0°
Caldwell 21, also known as NGC 4449, is an irregular galaxy located approximately 12.5 million light-years away in the constellation Canes Venatici. It is part of the M94 Group, a collection of galaxies in the vicinity of the Milky Way. Caldwell 21 is notable for its active star formation, which is evident from the numerous young, blue star clusters and pinkish star-forming regions scattered throughout the galaxy. This high rate of star formation is unusual for a galaxy of its size and type, making it a subject of interest for astronomers studying galaxy evolution and star formation processes. The galaxy's irregular shape suggests that it has likely undergone interactions with other galaxies in the past, which may have triggered its current burst of star formation. Caldwell 21 is also known to have a complex structure, with a bar-like feature and a halo of older stars surrounding the central region.
source: Mistral.ai
Data Acquisition
Data was collected over 9 nights during the month of May 2025, using a 14” reflector telescope with full-frame camera at the remote observatory in Spain. Data was gathered using standard LRGB filters, as well as a Ha narrowband filter, to highlight specific areas of intense star formation. A total of approximately 28 hours of data was combined to create the final image.
Location Remote hosting facility IC Astronomy in Oria, Spain (37°N 2°W)
| Sessions | Moon% | Moon° | Hum% | SQM | T°C | Frames | Exposure |
|---|---|---|---|---|---|---|---|
| 20250503 | 46 | 53 | 70 | 20.7 | 10 | 37 | 1h 51m |
| 20250514 | 94 | 96 | 80 | 19.5 | 10 | 16 | 0h 54m |
| 20250515 | 89 | 105 | 70 | 20.0 | 10 | 27 | 2h 25m |
| 20250516 | 82 | 113 | 60 | 21.3 | 10 | 26 | 2h 49m |
| 20250517 | 73 | 122 | 55 | 21.0 | 10 | 45 | 2h 45m |
| 20250518 | 64 | 129 | 55 | 21.6 | 12 | 38 | 3h 49m |
| 20250519 | 53 | 135 | 45 | 21.5 | 12 | 68 | 4h 45m |
| 20250520 | 42 | 139 | 55 | 21.7 | 11 | 71 | 5h 05m |
| 20250521 | 31 | 139 | 50 | 21.7 | 15 | 34 | 4h 05m |
| Total | 362 | 28h 28m |
| Frames | Bin | Gain | Exp.(s) | Frames | Exposure |
|---|---|---|---|---|---|
| Lum | 1 | 0 | 180 | 191 | 9h 33m |
| Red | 1 | 0 | 300 | 40 | 3h 20m |
| Green | 1 | 0 | 300 | 37 | 3h 05m |
| Blue | 1 | 0 | 300 | 38 | 3h 10m |
| Ha | 1 | 2750 | 600 | 56 | 9h 20m |
| Total | 362 | 28h 28m |
Equipment
Telescope
Mount
Camera
Filters
Guiding
Accessoires
Software
Planewave CDK14 (2563mm @ f/7.2), Optec Gemini Rotating focuser
10Micron GM2000HPS, custom pier
Moravian C3-61000 Pro (full frame), cooled to -10 ºC
Chroma 2” LRGB and Ha (3nm) unmounted, Moravian filterwheel L, 7-position
Unguided
Compulab Tensor I-22, Dragonfly, Pegasus Ultimate Powerbox v2
Voyager Advanced, Viking, Mountwizzard4, Astroplanner, PixInsight 1.9.3
Processing
All processing was done in Pixsinsight unless stated otherwise. Default features were enhanced using scripts and tools from RC-Astro, SetiAstro, GraXpert, CosmicPhotons and others. Images were calibrated using 50 Darks, 50 Flats, and 50 Flat-Darks, registered and integrated using WeightedBatchPreProcessing (WBPP). The processing workflow diagram below outlines the steps taken to create the final image.
The automated stretching scripts caused a too much blown out centre of the galaxy which then gave some strange artefacts when combining with the RGB information. Therefore the Luminance was stretched manually using HistogramTransformation, and some selective contrast was added to the lower brightness areas of the galaxy using GeneralisedHyperbolicStretch.
To blend in the Ha signal, mainly the same continuum subtraction approach was used as for M51 recently, albeit with a few small modifications. This time the Ha signal was more intense and that required a few adjustments. First of all, the continuum subtracted Ha image was not stretched as part of the subtraction utility. Instead the utility kept the image in linear stage and stretching was done using the SmartStretch scriplet in PixelMathGUI. That way, the stretching would happen with the same parameters as the RGB image. During the blending process, a few adjustments were made as well. The saturation was set a bit lower (0.6) as the Ha was a very intense signal, and black-point was raised not to get a magenta colour cast in the background. Other than these few tweaks most of the process was the same, This approach to continuum subtraction seems to be working well.
The rest of the editing followed a standard processing workflow.
Processing workflow (click to enlarge)
This image has been published on Astrobin.
