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  • Ed Dozier

Stack Star Shots with CombineZP

How can you make one of those cool star field shots, without making the stars turn into streaks? Is there a way to take these pictures without having to buy special hardware? Yes.

Star shot made from multiple photos, using CombineZP.

There are few things you will need to make good star field pictures. Not surprisingly, the better (and larger) your camera sensor is, the better chance you’ll have to produce quality results. A stable tripod is a must. A lens with a wide aperture will really help. Get a remote release (or a cell phone app) to trigger your shutter. Finally, you’ll need software to align and combine multiple exposures.

What you won’t need is a motorized mount that rotates your camera to track the stars; that’s what the software is for. There are many programs that “align” multiple exposures via a simple shift, but the list gets pretty short when you add the constraint to fix rotation. The Earth rotates, causing the stars to appear to move in an arc. I have been using a (free) program called CombineZP that can fix rotation, scale, and shift changes when combining pictures.

The CombineZP program was written by Alan Hadley; he’s a really smart guy, but is a little bit challenged by grammar and spelling (to say the least). In case you’re interested, the program name refers to “stacking/combining photos in the ‘Z’ direction” and the “P” is short for “pyramid”. He uses a “pyramid” algorithm for some of his photo stacking operations, which is really great for solving many issues involving overlapping hairs on bug close-ups when doing focus-stacking. Intense stuff. His program’s Help system explains this and many other things. Alan’s program can do much more than bug shot stacks, as I’ll show you. Here's a link to his free program.

The CombineZP program works with Windows10 and many earlier versions of Windows; I use it in Windows 10 x64, although it’s a 32-bit program.

I almost forgot to mention that you also need a really dark sky. City lights and the moon will generally ruin your results. The higher altitude and less humidity you can get, the better.

The kind of photography I’m talking about here doesn’t work for night landscapes (with a horizon), because you can’t mix a fixed horizon with moving stars. This article is about pure star shots.

When I photograph stars, I will typically use my Nikon D610, which has a really, really good full-frame (FX) sensor. My go-to lens is my Tokina 11-16mm f/2.8 (DX), even though it’s not supposed to work on a full-frame camera. It works just fine at 16mm, although I typically crop the edges a bit to rid some vignetting and frame-edge astigmatism/coma. If I owned something as snazzy as the Nikkor 14-24 f/2.8, then I’d definitely use that instead.

To get my star photographs, I will typically set my camera on manual exposure, ISO 3200 (or less), f/2.8, and a shutter speed of 10 seconds for 16mm shots. Shutter speeds longer than 10 seconds at 16mm will result in star streaks. This kind of photography requires manual focus on infinity (it’s smart to pre-focus while it’s still daylight). These shots will be under-exposed, but the CombineZP (and some other post-processing software) will brighten things up in the final picture.

If you choose a longer focal length lens, they you’ll need to use shorter shutter speeds to avoid getting streaks instead of points of star light. Take a test shot and zoom in on it to view how much streaking you see.

I’d recommend you take a minimum of 4 shots to combine. The more shots you have, the better results you can get. Don’t wait too long between shots.

Manual method using CombineZP for stacking shots:

Convert your Raw star shots into 8-bit Tiff, LZW compression, with an image editor of your choice. CombineZP won’t accept Raw format or 16-bit.

Start CombineZP.exe

Click “Enable Menu” icon to see the menu system.

Click File | New

Select the TIFF photos (as-shot order), then wait until each shot is loaded into the stack.

Select Stack | Size and Alignment | Auto (Shift + Rotate + Scale), OK.

(This will align and replace each shot in the stack with the aligned shots.)

Your screen will probably look black after the alignment is done, but that’s normal.

Select Stack | Enhanced Average to Out

Lowlight Gain (0=none) enter a value between 0 and 50. Press OK.

Highlight Attenuation (0-1000, 0=none) enter 0. Press OK.

Brighten (1000=stay same) enter 2000 (for 1 stop brighten, 3000 for 2 stops, etc.), Press OK.

The “Enhanced Average” step lets you tune the exposure adjustment of the photos, and then combines them (and reduces noise via averaging the shots).

When processing is done, mouse-drag a rectangle around what you want saved. In this case, it’s as if you’re using a crop tool.

Click File | Save Rectangle As | myStarShot.png

(You can choose an output format from jpg, tif, bmp (24 or 32 bit), gif, png.)

Now, your “stacked” shot is ready for final adjustment in your favorite photo editor. You will probably want to do additional noise-reduction, Levels and Curves adjustment, white balance adjustment, and apply an un-sharp mask.

Create a Macro to Automate Star Stacks

If you’re a little more ambitious, you can create a macro to do your star stacks, once you settle in on a recipe you like. Alan explains how to make macros for his program, but here’s a Cliff’s Notes version if you want to try it out.

The CombineZP program has several collections of macros, saved in files that have the .CZM extension. Inside these collections, you can have up to 10 macros. Macro names that look like “_Macro4”, “_Macro5” etc. are place-holder (inactive) macros without commands in them (unless you put some there).

Since the default macro set has 10 active entries, you’ll need to either make your own macro set or alter an existing macro set.

Find an appropriate “macro set” (.czm files) that has an available macro via

Macro | Load Macro Set

(I will choose “Enhancer.czm”)

You’ll want to replace a place-holder name in the set with your new macro:

Macro | Edit | Macros.

Click on “_Macro 3” to alter it (if you used Enhancer.czm). Note that your new macro name cannot begin with an underscore character, or it won’t be runnable via a user click.

The Macro Editor, before any changes.

Rename an unused macro (starts with “_Macro”) to a name without an underscore. Here, we’ll call the new macro “Star Stack”.

Add steps, along with any parameters it needs, by selecting a “Command” in the drop-drown list.

For the first command, we want to align the already-loaded stack of photos:

Align the stack

Click “Update/Paste” to add the Align command to the macro. This command will replace each original star shot in the stack of loaded images with aligned ones (not touching your original .tif files). You now have a new “stack” of images to perform further operations upon.

Next, we want to get the “average” of each shot in the stack, to rid noise and atmospheric interference effects. We also want to enhance the light in each shot while combining it with the others.

“Enhanced Average to Out” command with (3) parameters

Click the “Update/Paste” button to save the averaging command.

The “Enhanced Average to Out” command expects to operate on a stack of images (with any number of images in the stack). It will then place the results into the “Out” location, which is visible on the screen.

Click the “Save Macro” button, once all of the steps are added.

Click the “Ok/Update” to exit the Macro Editor.

The finished Star Stack macro

The new macro stack

Click on the “X” to close the “Edit Macro” dialog.

For use in the future, you will want to save this macro set into a new file.

Click Macro | Edit | Save Macro Set As | StarStacker.czm

Try out your new macro:

File | Empty Stack (to clear out everything)

File | *New (select the original .TIF files of the star shots)

Macro | Star Stack

(It should now run and do both the alignment and averaging)

Do the usual “save rectangle as” to save your results.

After you’re done running the new macro, you may want to restore the system with the default macro set (for focus-stacking).

Click “Macro | Restore Standard Macros

The program now looks like it did when you first started running it. You can now do regular focus-stacking operations.

To get back to your new star macro, do this:

Macro | Load Macro Set | StarStacker.czm

This particular example isn’t very sophisticated, but it shows you the way into the world of CombineZP automation. There are a great many more macro sets to explore that are provided with the program. You can use the help system to research the commands in the sample macros to learn more.

Now, get out there and shoot the stars.

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