IC 1805, commonly known as the Heart Nebula, is a vast emission nebula located approximately 6,500 light-years from Earth in the Perseus Arm of the Milky Way, within the constellation Cassiopeia. This intricate structure of ionized gas and interstellar dust owes its name to its striking resemblance to a human heart, a poetic manifestation of astrophysical processes at work in our galaxy.

At the core of IC 1805 lies the open star cluster Melotte 15, a young assembly of massive O- and B-type stars that sculpt the surrounding nebular material with their intense stellar winds and ultraviolet radiation. These forces give rise to the nebula’s intricate filaments, dark dust lanes, and luminous ionization fronts, making it a striking subject in both astrophotography and scientific study.
Highlights of IC 1805
The Heart Nebula is a textbook example of a star-forming region, where gravitational collapse within dense molecular clouds has given birth to a new generation of stars. The interplay of radiation pressure and stellar feedback shapes the nebula’s dynamic morphology, revealing bubble-like structures, elongated pillars, and delicate tendrils of dark dust.
The dominant red hue of IC 1805 is characteristic of Hα (hydrogen-alpha) emission, produced as ionized hydrogen recombines with electrons, releasing light at a specific wavelength of 656.3 nm. Within the SHO-palette used in my image, the red hue is allocated to Green whereas S-II is responsible for Red. The dominance of hydrogen is best recognised in the following image, which shows the H-alpha emission only:

The nebula is also interspersed with regions of dark nebulosity, where dense accumulations of dust obscure the background glow. These regions are potential sites for future stellar genesis, marking the continuous cycle of star formation and destruction that defines the evolution in our cosmos.

Of particular interest is the presence of Melotte 15, which contains some of the most massive stars in the region. These stars, many exceeding 50 times the mass of the Sun, have significantly short lifespans, eventually ending their lives as supernovae. Such events will enrich the surrounding interstellar medium with heavy elements, contributing to the chemical evolution of future stellar generations.
Photographic Intro
Imaging IC 1805 is mostly done using narrowband filters, which enhance contrast by isolating specific wavelengths of light emitted by ionized gases. The use of Hα, O-III and S-II filters allows for a detailed study of the nebula’s structural composition, revealing regions dominated by hydrogen, oxygen, and sulfur.
First 2 Sessions (12.5 hours) of January 11 & 12, 2025
It’s still unbelievable to me how much detail is possible with the narrow-band shots in just 12.5 hours of integrated exposure time.

Outlook – Starizona Nexus 0.75
The next attempt to capture the Heart Nebula will be with the Nexus 0.75 reducer from Starizona:

Starizona describes the function:
„Newtonian telescopes require a coma corrector in order to give sharp stars over a large field of view, but most coma correctors keep the telescope’s native focal ratio or even make the scope slower. The Nexus instead acts as a coma corrector and also provides a 0.75x focal reduction factor. This turns a common f/4 Newtonian into an f/3 system, giving a larger field of view and reducing exposure times by a factor of 1.8x.
The Nexus incorporates extra-low dispersion (ED) glass in a sophisticated optical design that gives performance equal to the best coma correctors while decreasing, instead of increasing, the focal length. The Nexus is also designed to be parfocal, so it does not change the original focal point of the telescope. The corrected image circle is 28mm, accommodating camera sensors up to APS format. While nominally designed for the popular commercial f/4 imaging Newtonians, the Nexus can work with Newtonians as fast at f/3.
Backfocus requirement for the Nexus is 55mm, matching the common standard for most coma correctors, field flatteners and focal reducers. This distance allows for the use of DSLRs as well as mirrorless cameras and most astronomical CMOS and CCD cameras. The Nexus has M48 threads on the camera side. The barrel of the Nexus is a standard 2″ diameter to fit most focusers. On the telescope end are M48 threads for a standard 2″ filter.“
By shortening the focal length to around 750 mm, the f/4 Newtonian becomes a real „light engine“ with f/3. This is because the required exposure time in astrophotography is inversely proportional to the square of the f-ratio. When switching from f/4 to f/3, I will need approximately 1.78 times less exposure time to achieve the same signal strength (SNR) – see description above!
In practical terms:
- My current single exposures at f/4 is 300 seconds. I will need only about 170 seconds at f/3 to achieve the same signal level.
- Collecting 10 hours of total exposure time at f/4 is therefore equal to only 5 hours and 37 minutes at f/3 to reach the same depth.
This significant gain in efficiency allows for faster data acquisition, which is particularly beneficial when capturing faint nebulae or deep-sky objects. Also the larger field of view is ideal for the Heart Nebula. I’m really looking forward to the results!