So why do we need mosaic imaging sometimes? You always dreamed about a bigger telescope, right? But there are some beauties that just do not fit in the field of view with a big telescope. Even with a rather short focal length of 384mm with our small astro-camera, we cannot frame objects greater than about 1.5 degrees. In order to exceed that limit, we need to do some simple tricks.
Mosaic imaging
It is really simple: make several images of your target, with different pointing directions that cover the whole field. The trick is to have overlap between the images, so the software you use for stitching your individual frames together can do the alignment. The rest is done as a usual business, except that one should aim for similar conditions in the different parts, so the noise is not very different. This may be difficult to achieve if shooting spreads over several weeks and months. And that can happen in the Netherlands because having say 7 clear consecutive nights around New Moon rarely occur.
IC 1396, a massive star-forming region
Not as famous as the Orion Nebula, but just as fascinating is the emission nebula IC1396 in the constellation Cepheus. In fact, even though they appear in opposite directions in the sky, they are both located in the local spiral arm of the Milky Way galaxy, the Orion arm. Its physical size is about five times bigger than its more famous cousin. The entire mass of IC1396 equals 12000 Suns. Although it is located farther away, this complex extends over 3 degrees in the sky, which is equivalent to 6 Full Moons. Mosaic imaging is the only way to capture the whole region with big telescopes. The mosaic image made by the 2.5m Isaac Newton Telescope used some 400 exposures. Scientific data from this image alone revealed more than a hundred new stars in the process of making.
The Elephant’s Trunk, and our image
This region is famous for its bright-rimmed dark, dusty molecular clouds. The main culprit behind the scenes is the hierarchical triple star system HD 206267 (near the center of the image), which consists of a very rare type of O-stars. Additional help comes from the central star cluster of type B stars (similar to Regulus we shot before), known to astronomers as an OB2 association. Ultraviolet radiation makes the interstellar gas clouds shine as far as 100 light-years.
In addition, HD 206267 produces the fastest stellar winds of its kind, exceeding 3000 km/s velocities. This compresses dark clouds hosting young stars. Stellar winds from these young stars hold against the powerful wind from HD 206267, creating shock fronts on cosmic scales. This makes the edges of the clouds shine, while inside, a whole new generation of stars are born in the process. The largest and most famous of these is the Elephant’s Trunk Nebula (to the right of the image). What exceptional scenery!
Most photos focus on this famous structure, but we wanted to capture the beauty of the whole complex. That is why we did mosaic imaging, which took most of the clear nights in April and May 2021. Besides the bright-edge clouds, there are a number of foreground molecular clouds, but also structures like gas filaments and outflows from young stars can be seen in staggering detail. Excuse us for the color choice – this doesn’t attempt to represent the natural colors of the object, rather, it is an artistic representation of the dramatic scene.
Herschel’s Garnet Star
Oh yes, and before you go: μ Cep (Mu Cephei), or Erakis, otherwise known as Herschel’s Garnet Star still finds its place on this mosaic image, in the upper left corner. It was named as such because of its garnet red color (that is, in natural-color images). Erakis is just about the largest star there is in the sky visible to our eyes. It is a red hypergiant with a size of a minimum of 1000 solar radii and shining like 100-300.000 Suns. While normal stars are powered by hydrogen fusion, this evolved star started fusing its helium to carbon. Check this image here to compare μ Cep to other objects, to see just how big it is!
Higher-resolution jpg version (3.1 MB)
A stunning black and white version (2.7 MB)
References
Weikard et al. 1996, A&A, 309, 581
Barentsen et al. 2011, MNRAS, 415, 103
Sicilia-Aguilar et al. 2014, A&A, 562, A131
Wikipedia
Observing log
16-17 April 2021
It is getting slightly warmer. Beautiful clear skies. Moving over to a new target, IC1396 with the Elephant’s Trunk Nebula. Cepheus is still low in the North-East and partially blocked by the neighbors’ house, but could start at around 23:30 CEST. Had to upgrade the cushion-castle though, as another street light down the path next to our house was disturbing the view in this direction as well. This is all experimental, we have to find out later how to frame different parts of this nebula. I also want the part with the Garnet star, maybe a separate one on the Trunks, and a third one on the bit with small globules. Cepheus will be around for some time! Did 60x300s frames with the L-eXtreme, gain 240, at -20 deg.
17-18 April 2021
Still doing the central parts of IC1396, exactly the same way as before. Started around 23:40 CEST, finished before 5am, still in dark. Pity we cannot fit all the nebula in here!
20-21 April 2021
Unexpectedly, very clear night. I packed the telescope already because the forecast was clouds (at least partly) for several weeks. Moon is in the first quarter. The idea is to do shots around the center of IC1396. This time I moved to the NW direction, to get more of the base of the Trunk itself. Started 60x300s around 23:40 CEST. During the day it was up to 17 degrees, but at start, it was 10, and only cooled to ~7 by the morning. Could easily cool the camera down to -20 deg. Somehow this autorun lasted longer than the one before, stopped after image 56 (~5:15), because it was dawn.
23-24 April 2021
Not the most transparent night and the Moon is growing, but relatively clear otherwise. Did 57x300s shots on IC1396A (same position as a few days back).
25-26 April 2021
Very clear night, but almost full Moon. Doing the corner of the Elephant’s Trunk Nebula with the famous Garnet star. Succeeded in 57 shots (the last three got too much light at dawn).
27-28 April 2021
Last night was cloudy and cold but today we had a beautiful King’s Day (and Zoli’s 7th said anniversary). We went shooting tulip fields near Dronten. Crystal clear all day, initially quite cold, but warming up quite a bit (~15 deg. or even higher). I took some nice shots of the telescope and the cushion castle before the Sun went down, it was shining directly at the telescope. The plan for tonight is still the Garnet Star corner of IC1396. On source and tracking at 23:45 CEST. Clouds were rolling in in the morning. Only kept 47 shots (almost four hours, so the clouds must have arrived shortly before 4 am). The air was amazingly dry, the cushions were not even slightly wet. But the Moon was full.
10-11 May 2021
The SE corner of IC1396, 54 shots. Great quality, near New Moon.
18-19 May 2021
For weeks, we had close to winter conditions with dense cloud coverage, rain/showers, and low temperatures. Seems it never ends, according to the forecast it will last till the end of May. There are occasional gaps with a few hours of sunshine. We are really near to finishing the Elephant’s Trunk nebula, just one corner remains. Finally, today clear skies are predicted. Built the cushion castle in advance, but couldn’t get the telescope out early because it started raining at 8 pm. At around 9 pm all ready, but it is not dark enough to polar align (and there are still some clouds) before ~11 pm. The Moon is nearing the first quarter. Started a 60x300s run, but after 2 am clouds came so managed to take only 32 shots.
19-20 May 2021
Similar conditions to yesterday. The finishing night of IC1396. Run almost complete, however after 4:20 it is getting lighter so finished with shot 52.
Elephant’s Trunk Nebula
IC1396 mosaic – 39.6 h, from Hoogeveen, Netherlands
C: 16-17 April (60x300s), 17-18 April (60x300s),
NE: 20-21 April (56x300s), 23-24 April (57x300s),
NW: 25-26 April (57x300s), 27-28 April (47x300s),
SE: 10-11 May (54x300s),
SW: 18-19 May (32x300s), 19-20 May 2021 (52x300s)
Just to summarize: 475 x 5 min = 2375 min; center and four “corners”; most parts at two epochs, except for the SE corner that was shot during New Moon. All shot with gain 240, at -20 deg.
Telescope: William Optics GT81, f/5.9, with Flattener 6AIII 0.8x
Camera: ZWO ASI533MC-P
Guidescope: WO Uniguide 50/200 with ASI120MMmini
Mount & control: iOptron CEM40, ASIair Pro
Filter: Optolong L-eXtreme
Software: Astro Pixel Processor, Photoshop