ABOUT THE IMAGE

This image was shot by BAT-members during the winter and spring months. With over 265hrs of compiled data from various Bortle zones, this image marks one of the deepest views ever created on these two interacting galaxies! All the telescopes used are amateur equipment ranging from 4" refractors to 14" Schmidt Cassegrain systems. In addition, lucky imaging was used to sharpen the core of M82. A more detailed view of that project can also be found on our Astrobin account .

Many thanks go out to those, who decided to take their time and image this target over countless hours.Thank you @Lyaphine, @Jarinn, @Michael Meredith,  @Luka Poropat , @MrCrazyPhysicist, @reglogge , @astrobiscuit , @UlrichvonZottmann, @Philipp Weber, @@Alphan, @Dickyblowout, @ugapeyton, @CodyBrownBudgetAstro,  @Pam Whitfield.
This image was processed by: @stefan2499
Thank you @Philipp Weber for helping me out on the Physics behind this image!

INTERESTING DETAILS AND  FACTS WITHIN THE IMAGE

The most prominent subjects of this image are without a doubt the two galaxies M81 and M82. Although it may not appear this way, these two galaxies do interact with each other. In fact, this causes one of the most prominent features in this image: The starburst activity in M82!! In a starburst region, stars will form and age much quicker than usual in galaxies. This in return causes a high rate of supernovae in this galaxy. It is to be believed that those supernovae fuel the bipolar outflow of M82. 


Image 1:  Closeup of M82 and its starburst region (H-alpha region)

Taking a closer look at M82 however, yields a few more interesting results. Towards the top left of M82, a blue stream can be seen, emerging from the galaxy. Previously, it was often assumed to just be part of the galactic cirrus in our galaxy. However, we believe this might not be the case! Due to the high depth of this data, we can clearly determine the color of that area to be blue! Next to the different color, even the shape completely contradicts with any of the remaining cirrus around the galaxies. One of our theories is that this region could in fact be part of a huge shock wave, caused by the super-winds in the starburst regions of M82. However, without the ability of taking a closer look at this region, it will remain impossible to say for sure. 

One more interesting feature near M82 is a very faint H-alpha bubble towards the bottom right of the galaxy. Due to the lack of H-alpha anywhere in the galactic cirrus, we also believe that this area is—with high likelihood—part of M82 and its starburst region.


Image 2: M81 & M82 in HI-Emission (blue), marked out are various star forming regions within M81's & M82's gravitational pull, like the dwarf galaxy UGC 5336
HI-Data: 
Blok, W. J. G., “A High-resolution Mosaic of the Neutral Hydrogen in the M81 Triplet”, The Astrophysical Journal vol. 865, no. 1, 2018. doi:10.3847/1538-4357/aad557.
M81, M82, NGC 3077 | HI data of the M81 Triplet (M81, M82, NGC3077) (astron.nl)

M81 however also offers many interesting features! As previously stated, M81 and M82 do indeed interact with each other. This interaction can be visualized in the HI spectrum, basically a "map" of gravitational pulls between the galaxies. Interestingly, this gravitational pull also affects a third galaxy! NGC3077, which is just outside of our framing, has a well defined "bridge"of HI-Data connecting itself with M81. Due to hotter temperatures and more surrounding emission, star forming regions (like the dwarf galaxy UGC 5336) spawn stars like mushrooms along these gravitational pulls. Marked out in Image 2 are several star regions, which we believe to be part of the M81 & M82 galaxy group. Additionally, when comparing the shapes of the galactic cirrus with the HI emission (blue), a clear consistency can be seen in the pattern. Thus, we also believe, that some of the cirrus is indeed actually part of M81, and not part of our Milkyway. 

Next to the galaxies, the large abundance of galactic cirrus fills the image. This is indeed still part of our own galaxy, the Milky Way! The galactic cirrus mainly consists of dust that stems from supernovae explosions, or from star wind condensated carbon. Its particles are no larger than tobacco smoke. In the inverted view of Image 3, the extent of this galactic cirrus can easily be seen.


Image 3:  Inverted version highlighting the galactic cirrus 

Lastly, I want to highlight all the very distant and faint galaxies visible in the background of this image. They are often overlooked, but in my opinion one of the most spectacular features in galaxy images. The mere thought that there are hundreds, if not thousands of galaxies visible in this image blows my mind. In particular I want to showcase a distant galaxy cluster, to whom my eyes always get drawn to, when spectating the image. It is located just behind the brightest star in this image (HD 85161). Of course, I invite you all to take a close look through the image. Make sure to look at it in full resolution to dissect all of its beauty. I'm certain, that there are many many more features to be discovered in this image, that weren't yet covered here. 


Image 4:  Unnamed galaxy cluster behind the star HD 85161

ABOUT THE "BAT"

Please check the link below if you are interested in joining the BAT. We are always interested in new members and do monthly group projects encompassing wide-field targets, high resolution projects all the way to seasonal mega projects like Hubble's Variable Nebula, as can be seen in one of our previous Astrobin posts. Membership is free and comes with no obligations other than being friendly and enthusiastic about astrophotography!
https://www.astrobiscuit.com/big-amateur-telescope

IMAGE PROCESSING

This image was mainly processed with PixInsight and some final touches in Photoshop. To avoid accidentally creating "fake" data, very careful background extractions where carried out. In addition, any AI was also completely avoided (exception: Starnet V2 to create star masks, used very restrained). Due to several people asking me on the processing steps taken to create such images, I decided to add a short write-up of them in this description! 

A. Data combination (Pre-Processing)
   1. Blink through the data, in this case already stacked files where combined. Make sure any outlier images (bad stars, extreme light pollution, insignificant data etc.) are removed. If you have regular data, stack them normally and ignore steps 2.-5.!
   2. The images might not cover the final FOV completely. Gradients can cause jarring edges when combined. For best results, gradients need to be removed prior to combining the data! In this image I chose to use the technique of Vincent Peris, covered in detail here: PixInsight — Multiscale Gradient Correction
His technique may take more time than the standard "Dynamic Background Extraction" (DBE), however in cases like this, a consistent background extraction is extremely important to guarantee an accurate representation of faint structures in the image like galactic cirrus. The reference image chosen to do the initial background extraction with DBE was Jarinn's data set. Coming from a dark Bortle 2, it was clean enough to guarantee a gradient free image after DBE.
   3. After applying the background extraction, make sure each image is registered to the reference image (reference image = the wanted FOV of the final image). It is important, that the reference image has enough resolution to not deteriorate sharpness in high resolution data. Turn on "thin plate splines" and distortion correction for best results in the Star Alignment process.
   4. Next,  analyze the images manually or with help of Subframe Selector. Do note, that subframe selector might fail to accurately measure the images. This is due to the different pixel sizes of the images, which in return changes the noise level. If Subframe Selector's results appear incorrect, I recommend manually writing weights into the fits header of the images. Use educated guess for this step, this will come with experience and is unfortunately a annoying process.
   5. Stack the images with Image Integration. Make sure to turn off pixel rejection. If you weighted the images, turn on the setting "weights: FITS keyword". With only two images, use PixelMath combine. 

B. Post-Processing-Linear 
   1. If there is still some gradients left over, remove them with DBE
   2. Run Photometric Color Calibration (PCC) on the RGB image
   3. Run deconvolution on the Luminance image. There is many ways to do it, I usually do the following approach:
         a. Duplicate the Lum, rename it to "Original" (We will use it later)
         b. Stretch the Luminance with the script "Generalized Hyperbolic Stretch" (GHS). You can download it here: ghsastro – astrophotography
GHS is quite complicated. I recommend watching some tutorials on it first. Dave Payne – ghsastro
It is important to save an instance of your stretch parameters with "invert transformation" checked. This way we can return back to a linear stretch non-destructively.
[b]         [/b]c. Run Starnet V2 on the stretched image
         d. Create a range mask from the stretched image with the process "range selection", the mask should only contain the areas you want to sharpen with decon. These areas should usually be noise free or contain only low amounts of noise to prevent sharpening of noise
         e. Drag the saved instance of GHS over the starless image. If done right, it will now be linear again. This method is more work, but will work better than running Starnet V2 in linear mode.
         f. Create a star mask by using this formula in PixelMath:Original_image - Starless_imageSave the star mask as a new image
         g. Run the process "DynamicPSF" or run the script "PSFImage" by Hartmut Bornemann. You can find his scripts here: Herbert Walter PixInsight Scripts (skypixels.at). Make sure, that only stars of type Moffat are used for the PSF image. Around 20-30 stars from all areas of the image are enough to build a good PSF for our deconvolution
         h. Apply the range mask to the starless image. Open the process "deconvolution". Select external PSF and choose the PSF image you created in step g. I recommend playing around with different iteration counts to test the results. You may also want to look into wavelet regularization.-> If your decon sharpens noise, increase those. I also recommend using using previews on the starless image to save time. Once you found the correct settings, run the decon on the starless image.
I usually change the following settings:
Iteration between 10 - 20, Wavelet Regularization: 5 Layers, Noise threshold: 8-6-4-2-1 (from top to bottom), Noise reduction: 0.8-0.8-0.6-0.6-0.6 (")
         i. Add back the stars with PixelMath using the following formula:Starless_image + Starmask_imagePlease note that excessive deconvolution can lead to unnatural looking images. Handle with care.
   4. Linear Denoise according to Jon Rista's guide: PixInsights Tips: Effective Noise Reduction (Part 2 of 3) | Nature Photography (jonrista.com) (Apply to all images)
   5. This step I ALWAYS forget and it creates for a rather painful extra step later on. If you have H-alpha data, this is where you want to add it. I use the method explained here: Enhance HII regions in Spiral galaxies (arciereceleste.it)
In short: Do a continuum subtraction on the H-alpha image, so it only contains true H-alpha signal. Add this image to the RGB and the luminance. Use the PixelMath formulas from the website above. This may take a bit of trial and error to get it right. Therefore I recommend doing step 6. and 7. first and save them as instances. Then you can quickly stretch your image and easily see if the H-alpha addition looks natural.
   6. Stretch the RGB and Lum image with the GHS script. For best results, the Lum stretch needs to match the RGB stretch in brightness! I usually do a mix of "Col"and "RGB" stretch with GHS. 
   7. Combine the Luminance with the RGB image using "LRGBcombination". If the combined image looks desaturated or changed weirdly in brightness, then your stretches from step 6. didn't match. This can ruin an image if done wrong. Be careful here! Stretching is in my opinion the most critical step in your processing and you should take your time here!! 

C. Post-Processing-Stretched
This is the phase where you need to make your image look appealing to others. Depending on the data, you may need to do further stretches. In the case of this image, stretches might require masking to get the colors right. For the sake of this write-up, masking will be too in-depth. Below you can see my basic steps/ideas of what I do in the stretched phase. These steps will vary from image to image, from this point onward my steps will only be suggestions, following them blindly may not work for your image.
1. Further stretches, as described above potentially with masks. "Exponential Transformation" can be a very powerful way to boost galactic cirrus or similar faint nebulae in your image. I used this process several times. Run "SCNR" green if needed here. 
2. "Local Histogram Equalization" can be an excellent way to add contrast to your image. I like to run this method starless with masks. Your mileage may vary per image with this process.
3. "HDRMultiscaleTransform" will achieve a similar effect as step 2. It was also used for this image. BE CAREFUL!! Both these steps can quickly be overdone. Especially HDRMultiscaleTransform can quickly be overdone and the worst thing is, in the moment you may like it, but on a second look later you might hate it .
4. Saturation and Contrast can be added in many ways. I won't go into detail how I do it, as it is very basic to do. And yes, depending on the image, I will also use masks here .
5. The scripts of Hartmut Bornemann are excellent ways to fine tune your image. My favourites are "AdvSharpening", "BackgroundEnhance" and "DarkStructureEnhance". These scripts should be used late in the processing. 
6. You may also want to reduce star sizes in your image. Due to the background galaxies in this image, I chose not to. However, If I do, I use Adam Block's Star Deemphasis Method. Star Reduction (De-Emphasis) in PixInsight - YouTube
I recommend watching and learning this method to understand it. But for convenience, there exists a nice script: Star Reduction (De-Emphasis) in PixInsight: The Script! - YouTube

D. Photoshop
This is just for final touches. I usually remain within Adobe's Camera Raw feature. Things I always adjust is color denoise (usually with a mask). Sometimes I also do some minor luminance denoise. I think Photoshop has the best way to reduce color denoise. Use it if you have access to Photoshop!!
Depending on the image, I might also adjust the contrast, saturation, structure and some of the other sliders. Just be careful to stay non-destructive in Photoshop. Never override a layer, you may want to undo your steps from camera raw. 

This is basically all I do. I hope this helped a few of you and good luck processing your next images!!!

Thank you for proof reading @Thomas Fuchs
Written by @Stefan2499 for the BAT-Team.