As some of my fellow astrophotographer colleagues probably know, I'm permanently looking for objects or areas of the skies which aren't imaged very often. In tradition to this, I've captured a region in Cygnus which is most of the time only shown on wide-field images... obviously, my title is exaggerating.

It's also my first mosaic image. Given my equipment, I could have used my shorter focal length William Optics RedCat51 but it would have given up too much resolution for my taste.

I should send ahead, before folks have scratched their hair off about the colours of the Hydrogen regions, that the colour selection of the Hydrogen regions follows some logic instead of a brute-force blend into the red channel. More about this later.

First, what is shown here?
The prominent nebula regions have designations LBN 307,316 and 321. I admit, I haven't tried to get into the details on the number scheme of the LBN catalogue and so the same region can apparently be found under different numbers. As far as I could research, there's no distance information available on those regions and so we cannot "exactly" know how distant these structures are. A common problem of interstellar medium.

Located in the image centre, we can see the open cluster Dolidze 45, located about 2kpc away from Earth with a subsequent diameter of approximately 10pc (based on SIMBAD data). The stars are fairly subtle and the yellowish bright star does not belong to the cluster (it's HD 201777 at 300pc). If you cannot identify the cluster immediately, I've uploaded an image revision outlining the objects discussed here. Do45 is the smaller white, dashed circle.

Do45 is not the only open cluster in the image. We can see ASCC 113, located North (which is to the right, East is up) to Do45 with a distance of 450pc w.r.t. Earth and 7.3pc diameter (this information is from SkySafari, which has a surprisingly extensive object database. Although it's sometimes inaccurate and so, I'd be careful about trusting this data too much). In the annotated image, it's the large circle.

For deep sky objects, I could find some galaxies registered in the PGC which are visible in this image as well (yellow circles). Interestingly, all distance information notes about 100 to 120Mpc (SIMBAD) and they are considered as group candidates. I'm wondering how those galaxies are "connected"?
I'm fairly surprised to see those galaxies at this distance, given that we're looking through a larger part of our own galaxy. Maybe our own galaxy is emptier than full

If you belong to the visual observers as well, I find the double star 61 Cyg a pleasing object (notice that the image annotation is incorrect. It's the double at the bottom right of the image). While my rig was capturing the data for this image, I pulled out my wide field Takahashi FC76 at 570mm focal length to take a look at it. Due to the superb colour correction of this telescope, one can clearly identify both stars as yellow. Actually, if you are living under rather dark skies, you can see the double star with the naked eye (of course without splitting it).


About the image. As indicated, it's a mosaic, consisting of two panels (from North to South). For the star colours, I've collected RGB data which I combined with a luminance channel. For the Ha data, I've created a starless image which allowed me to screen the nebulosity onto the LRGB without altering the star colours.
With respect to the star colours, I hope I have met a good spot (not too much saturation, not too little).

Now to the colour of the Hydrogen data. Certainly, the Ha filter does record a narrow band of wavelengths which appears as pure red to an observer. From that perspective, it's absolutely viable to blend it into the data as a pure red information. However, through the Ha data, I wanted to identify the density of ionized Hydrogen gas. As such, the gas does not only emit the famous Ha line at about 656nm but also higher energy photons, known as H-beta (blueish), H-gamma (violet) and so on. If we ionize Hydrogen in a lab environment to temperatures comparable to the interstellar Hydrogen gas (8000K to 10000K), the superposition of the emission lines will make the gas appear pink, considering the intensities of the lines as well as our specific wavelength sensitivity.

As it's my goal to give a "natural" look into space, I wanted to set the Hydrogen data to the colour that we can expect it to have if we could see it with our naked eye. Since I could not find any indications for substantial absorption or diffraction through other interstellar medium between us and the emission nebula, I can safely assume that the relative intensities of the emission lines are close to what the gas emits. Interstellar dust for example does diffract shorter wavelengths stronger than longer wavelengths and therefore, the higher frequency gas lines wouldn't be visible and the emission nebula would appear much redder. This doesn't appear to be the case for this region of space.
I hope I could provide a reasonable and satisfactory explanation for the composition of this image.

As always, let me know what you think!

Björn