Astrophotography Tutorial: Creating Seamless Mosaics in Photoshop by Martin Cohen


This article gives a general outline of the techniques I use in Photoshop CS3 (CS2 and CS4 will work as well) to put together seamless mosaics of the moon and other celestial objects. Since I work with a PC myself, shortcuts for Photoshop will be given for a PC. For those who use a Mac the commands are very similar, but in general the Apple key replaces the Ctrl key.

Moon Mosaic- Credit Martin Cohen

To start off, I strongly recommend shooting RAW files. The advantages are clear: greater exposure latitude and more ways to enhance the image than simply shooting JPEG files.

I use a Nikon D300 DSLR, but many other DSLR’s are perfectly suitable. The Nikon D90 shoots video as well, which is wonderful if you want to stack images and process them in Registax before putting the mosaic together. Of course, when shooting video, you will not use the RAW format. The CCD-imagers often used for astrophotography are great, too, but I don’t own one so my personal experience covers DSLR’s. One of the very best on the market (expensive!!) is the Canon 5D Mark II, described in detail in the current (October) issue of Astronomy magazine.

Let me list the tools and shortcuts in Photoshop needed to create a seamless mosaic. A basic understanding of layers is required, as well as some Photoshop experience.

  • The Move Tool
  • The Rectangular Marquee Tool
  • The Lasso tool, with a feathered edge
  • The Eraser tool (various brush sizes, opacity usually 100%)
  • The Sharpen Tool
  • The Burn tool

From the menu:

  • Fill Edit – Fill (Shift + F5), mostly use black
  • Free Transform Edit – Free Transform (Ctrl + T)
  • Rescale layer Edit – Transform – Scale
  • Rotate layer Edit – Transform – Rotate
  • Warp layer Edit – Transform – Warp
  • Levels Image – Adjustments – Levels (Ctrl + L)
  • Curves Image – Adjustments – Curves (Ctrl + M)
  • Color Balance Image – Adjustments – Color Balance (Ctrl + B)
  • Hue/Saturation Image – Adjustments – Hue/Saturation (Ctrl + U)
  • Canvas Size Image – Canvas Size
  • Merge Layers Layer – Merge Down (Ctrl + E)
  • Auto-Align Edit – Auto Align Layers
  • Unsharp Mask Filter – Sharpen – Unsharp Mask
  • Smart Sharpen Filter – Sharpen – Smart Sharpen

Part 1

OK, let’s get started. I have used the described techniques for mosaics of the moon as well as the Milky Way, but they work with any object that you want to cover in hi-res and at the same time in a wider field than the individual images allow.

Essentially, it is like putting the pieces of a puzzle together. I always like to start off with a shot that covers the whole area, so I can use that as a blueprint for where the individual hi-res shots will go. Take a good image of the moon, whatever its phase is, showing the whole image. Hopefully, it will be detailed enough to still see the larger craters and of course, the Mares.

This will be our first layer in Photoshop. Create a nice large background to start with, maybe 20 x 24 inches or so at 300dpi, large enough that all the hi-res images will fit in easily (depends on the size of the individual shots). So click File – New (or hit Ctrl = N), and set the Width, the Height and the resolution of your file. Color mode is RGB color, 8-bit. The background contents should be black: you can simply open it in regular White, then hit Shift +F5 (or Edit – Fill from the menu), choose Black for the contents and hit OK. You should now have a nice black rectangle. Make sure the layer window is open and drag the “blueprint” full image of the moon into this black field, using the move tool. Save it as a Photoshop file, titled Composite_Moon_date.PSD. Note that the image will be much smaller than the field; we will stretch it to the right size later.

The individual hi-res images must of course cover the whole moon. A lot of overlap is a good idea, since not every photo will have the same resolution. Typically, I cover the moon with twenty to thirty different shots, also depending on the phase. Every image I shoot at least twice, so I can pick the best. (If you use video and stack them in Registax, of course you will be using hundreds of images). I like to shoot at f30 or f45, using eyepiece projection – Barlow lenses can be used as well to create a longer effective focal length. With larger telescopes, you may end up with a mosaic of 50 images or more to get the highest detail. It just means more work but the technique is the same.

After our basic blueprint is saved as a PSD file, we open a few of the individual hi-res shots and drag them into our composite. Don’t worry about the placement yet, they can be against the black background for now. The scale of these shots is much larger than the scale of the whole moon, so now we need to stretch the whole moon to match the scale. We click on the layer of the whole moon and hit Ctrl + T (or Edit – Free Transform in the menu). This will put a rectangular outline around the layer. Now we bring the mouse to one of the corners of that frame. A double arrow will appear. Holding the Shift key down (to maintain proper dimensions), we click on the mouse and drag that corner outward, thus stretching out the image. We keep stretching it until it is the same scale as the hi-res close-ups and then hit Enter.

Of course the large image will be fuzzy and pixellated, but we just need it for positioning. If the background turns out to be too small, we can make it larger by choosing Image – Canvas Size from the menu and add as many inches as we like. For the canvas extension color, make sure to choose black.

Now we position one of the hi-res shots right over the correct place in the full image. By clicking on the little eye icon on the left side of the Layers window (make sure to leave that window open at all times) we can make the layer visible and invisible. Now we move the new layer with our move keys on the keyboard as we go back and forth between visible and invisible, until the layers clearly match. It is important to view the file at 100 or even 200%, to make sure we get the most precise alignment. If the scale is still a bit off, correct the size of the lo-res full image by clicking on that layer, and using the Ctrl + T command to resize the scale like before. Do NOT correct the size of the hi-res shot!! That would of course ruin the dimensions in relation to the other hi-res images.

From here on, it is fairly simple, but still a lot of work. One by one, we drag the hi-res close ups into the composite. Using the eraser tool, we get rid of the edges and areas that look out of focus (after making sure we have it in better focus in another image). The brush size of the eraser should be about 5 to 10% of the individual file size, so if the files are 1000 pixels wide, then a brush size of 50 to 100 pixels would be correct. This way, there will be no hard edges showing. The brush size can be easily changed with the bracket keys on your keyboard, left bracket for smaller and right bracket for larger.

If for whatever reason two adjacent images seem to have a slightly different exposure, you can correct it with the curves (Ctrl + M). When the two images are properly aligned and similar in exposure, you hit Ctrl + E to merge them. Make sure the layer order is: background (bottom), blueprint layer, composite layer, new hi-res layer. This way you will not accidentally merge the new hi-res layer with the blueprint layer.

Keep following this routine, adding one layer after the next, using the mouse to drag and the move keys for precise positioning, constantly merging and checking with the full size (lo-res) image to ensure proper overall placement. Do not forget to save every few minutes!! You don’t want to lose precious hours of work because of a power outage or a computer failure.

Often, you will need to rotate a new layer just a little bit to perfectly align it (Edit – Transform – Rotate). You may also notice after building up your mosaic in the composite layer that the scale is still slightly off – do what you can to correct it in the blueprint layer (Ctrl + T and adjust the scale using the arrows, holding down the Shift key).

If after precise positioning and scaling, there is still a discrepancy between two layers, you can use the warp technique to bring things in perfect alignment (Edit – Transform – Warp). A grid will show of the layer that you need to warp into correct placement. By clicking your mouse on the part that is slightly off, you can move a section of that layer and align it with the other layer. Note that while you are warping, you cannot check or uncheck the little eye icon in the layer window (to ensure proper placement). So it is best to change the opacity of the layer you want to warp to about 50%, that way you can still see the other layer you are trying to match it up with. I recommend viewing the file at 100 or 200% while doing this, to be as accurate as possible. Warping takes patience and practice, but it is a fantastic tool to correct minor displacements.

Part 2

Aligning the pieces of the puzzle one by one, then merging them with the layer that will eventually be our final composite is a very tedious and time-consuming process. Photoshop has a few built-in tricks that can help saving time and sometimes even improve on the accuracy of the alignment. In Photoshop CS3, I have noticed that at times it does not really work too well, the software may get confused, but I am told they improved the auto-alignment in CS4. So if you have CS4, you are lucky and should definitely try it out this way.

What auto-alignment does is a combination of positioning, rotating and scale as well as perspective adjustment, with only one or two clicks. It will perfectly align two layers that have the same information in the image, so it is important to have a good amount of overlap. First, you highlight both layers in the layer window by clicking on them while holding down the Shift key. Then choose Auto-Align Layers from the drop down menu under Edit. Now you have several options to choose from: I prefer to use Reposition Only, which is the last option, because I don’t want to affect the scale or the perspective. You can also try Auto (the first option) but be prepared for the software to do some strange and unexpected things, like moving your layers into a new position or completely changing the perspective. It takes some experimenting, but ultimately using this option can save you hours of work. After the auto alignment has been done, you still want to make sure the alignment is perfect. Click on one layer in the layer window to undo the linking, so instead of two, only one layer is highlighted. Now you click one of the little eye icons on and off, making the layer visible and then invisible. I you notice any movement (make sure you are watching at 100 or even 200%), you can use the arrow keys on your keyboard to correct. Generally, it will be right on, or sometimes just a tiny hair off. Once the alignment is perfect, you merge the two layers, so another piece has been added to the puzzle. NOTE: if after dragging in a new layer it ends up disappearing “behind” one of the other layers, you need to change the layer order. Just click on the newly added layer in the layers window and drag it to the top, so it’s visible.

Perfection is never achieved, but it is certainly worth striving for. As the composite is coming together, you may notice areas that are not as tack-sharp as you would like. It can be the result of turbulence in the atmosphere, field curvature of your optical system, or perhaps you were simply not 100% in focus while you were shooting. (Using a Bahtinov mask is a very good idea, since even with a DSLR it is sometimes hard to judge the focus.)

What I often do, is revisit all the images I have of that particular area, and many times I see another shot where it shows a little sharper. In that case, I use the lasso tool with a feathered edge (10 or 20 pixel of feathering works well, if it is a tiny area 5 pixel is enough) and put the lasso around the area I want to insert. Using the move tool, I drag that area to the composite, bring it to the right spot (either manually or with auto-align) and after making sure the exposure matches, I merge the newly added layer with the main composite. If necessary, corrections can be made to the new layer before merging, using curves (Ctrl + M), levels (Ctrl + L), color balance (Ctrl + B) or saturation (Ctrl + U).

As I continue adding new layers to the composite, I check from time to time with my “blueprint layer” (the lo-res full image of the moon) to see if I am still putting every piece in the right spot. If I notice any discrepancies, I can use the warp tool (Edit – Transform – Warp) to bring the composite back in alignment with the blueprint.

Part 3

Once the composite is together, the main problem with a subject like the moon is the huge contrast range between the brightest areas and the terminator. The bright areas are typically overexposed, showing a lack of detail, and the areas around the terminator are usually on the dark side. We want to bring out the detail in the highlights by toning them down and increase the contrast and brightness of the darker areas. The tools to use for this process are the lasso (with a feathered edge) in combination with level adjustment (Ctrl + L) and afterwards, the burn tool.

First we delete the lo-res blueprint layer. It has served its purpose: everything is in the right place. So we are left with only two layers: the final composite as well as our black background. We can possibly rotate the composite layer a bit (Ctrl + T, bring the mouse outside the outlines of the layer, close to a corner until the rotate arrows appear) and move it to where we like its position the most. We can even crop the image more if necessary. Now we set the lasso tool to a feathered edge of 250, which is the maximum. Put the lasso around the brighter parts of the moon, which usually means the whole image except for the terminator. Hit Ctrl + L to show our exposure levels. There are three cursors at the bottom: by moving the middle cursor to the right, we bring down the midtones. The image goes a bit darker and shadow areas start to appear, especially close to the darker side. Don’t push it too far, just add a bit of shadow. Now move the whole outline of the lasso towards the brighter side, using the move keys on the keyboard. Again, look at the levels and bring down the midtones. Move it over even further and again, bring down the midtones. This way, you gradually increase the detail in the highlights, until the lasso is only around the brightest parts at the edge of the moon. Once those have been brought down, the whole image should look more even.

Now we put the lasso around the darker areas near the terminator. We may need to bring the feathered edge down to 150 or so, depending on how large our image is. Hit Ctrl + L for levels and move the right cursor towards the left side. See how far we need to move it to get sufficient brightness in the darker areas. It makes so much difference when the contrast has been adjusted! By now, we should have a fairly decent and evenly exposed photograph. To increase the contrast further, we can go to Image – Adjustments – Brightness/Contrast. Taking the contrast up a notch (maybe up to 12 or so) will help bringing out the Mares, I always like them to be fairly dark. You can use the burn tool with a large and soft brush size to deepen the shadows in the bright areas even more. Set the range to shadows and the exposure to 5%, and then go over the bright areas and the Mares several times until you like how the shadow detail is emerging. Using the lasso tool, you can also make local changes to brightness or contrast. Always use a feathered edge: the smaller the area, the less the amount of feathering. Then hit Ctrl + L and use the cursors to bring the values up or down.

Part 4- Last Chapter: Sharpening

It is amazing how much proper sharpening can add to the image. Many techniques can be used, but let’s keep it simple here. I like using Smart Sharpen as well as the Unsharp Mask. If your file is really large you will find that at 100% it shows kind of fuzzy. From the menu, choose Filters – Sharpen – Unsharp Mask. My advice is to play around with all the cursors (Amount, Radius, Threshold) to see its effect. A lot happens to the image and there are no set standards to give. I often use about 50% for the amount, a radius of 2 to 5 pixels and a threshold around 15, but every image is different and you need to use your own judgment. Go as far as you think you can push it without looking unnatural. Right after you sharpened, you can always go to Edit and Fade Sharpening to reduce its effect.

Now you have your final large composite; save it as a TIFF or as a PSD file. It will be great for large prints, but way too large for the Internet. To share the image with other photographers, reduce the size considerably, to maybe 1600 x 2000 pixels for a large JPEG, save it at 9 or 10. After reducing, use Smart Sharpen (Filter – Sharpen – Smart Sharpen) to keep all the detail; again, play with the cursors to get the best effect. The radius will be less now it is a smaller file (between 0.7 and 1.5 px) and the amount should not exceed 70% to keep a natural feel. In the areas that still need a little extra punch you can locally sharpen with the Sharpen tool, set at 10% or so for that special final touch.

Wishing you good luck in creating spectacular composites!

Other mosaics by Martin Cohen:


Full size version of Milky Way with Messier Map here!

Martin Cohen is a professional photographer and amateur astronomer in Santa Monica, California. (photos of the moon) (photos of Havana, Cuba)

This article: "Seamless Mosaic in Photoshop for Astrophotography" was

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    1. AstroPhotography Tonight

      Jeff- thanks for the feedback! Martin did an awesome job with this tutorial! Ray Shore

  1. great tutorial Martin/Ray, well done! I love the results it gets. It’s about time I get myself a copy of Photoshop πŸ˜‰

    Looking forward to more of this!


    1. AstroPhotography Tonight

      Hi Robert!

      The credit all goes to Martin on this one! He does great work with Photoshop. He is working on a much larger mosaic right now that will be posted in the article soon. It will be a nice piece of work!

      Thanks for the feedback Robert…it’s always well appreciated!

      Ray Shore

  2. Thank you so much for this article! I used it today and with a little bit of figuring what I was even doing (first mosaic) the article was a HUGE guide. I was using CS6 and it was exactly the same. Thank you again! πŸ™‚

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