UPDATED 07-Apr-2020: Cropping instead of reducing image size

The Moon should be your first target when you start off in astro-photography.  It’s easy to find, does not require dark skies and you don’t need specialized gear.

So now that you’ve found yourself will a bunch of RAW photos of the Moon you’re wondering what to do next.  You took them with the RAW setting right? All astro-photo need to be taken in RAW to conserve as much information as possible because all the processing is done at the pixel-level and you want to retain as much detail as possible.

Registax is a great software for Moon and planetary stacking.  Unfortunately I find it has two drawbacks:

1. Cannot deal with .CR2 CANON RAW files
2. Crashes or gives a memory fault when dealing with large images from DSLR.

Luckily there are ways around it… You must be wondering, why use Registax if it can’t deal with large RAW CANON files?  It’s because it can align and stack images by sub-dividing your image to address atmospheric turbulence and it has one of the best wavelet analysis tool to sharpen images.

Here is what you must do: convert your RAW files to 16-bit .TIF and reduce the image size (not just the filesize, but the number of pixels in the image).  I use Digital Photo Professional 4 that came with my CANON camera, it can be downloaded. For other camera brands or photo software should allow you to also convert RAW into TIF format.

There are two possible ways to reduce the image size:

1. Resize the image – this is the fastest and simplest

Highlight the desired RAW files and select File – Batch Process

DDP4 – Main window. Selecting the desired files

In the Batch Process window select to save the files as 16-bit .TIF and ensure that you resize the images.  Normally 50% reduction will do the trick. In my case a reduction to 3000 x 2000 was sufficient.

DPP4 – Batch process window : Saving as .TIF and Resizing the images

Resizing will reduce the size of the Moon, and Registax has a better chance of dealing with alignment. It’s also a simple way to reduce noise and improve a less-than-perfect  focused image.

2. Alternatively : Cropping the image – more time consuming

If you don’t want to shrink the image, an alternative is to crop the image. With DPP4 it’s possible to apply the same crop setting to all the images, however it must be done one at a time.

First select one of the images and open the Tool palette.  Select the cropping tool and the area you wish to crop.  Once that is done, use the Copy button in the Tool palette to record your crop setting.

DDP4 – Cropping with the Tool palette

You then need to open each file individually and Paste the crop setting using the Tool palette. Once you’ve done all of that, you can select all your images and run the Batch process to save them to 16-bit .TIF as explained above.  No need to resize if you’ve cropped.

DDP4 – Image selection pane shows the crop box around each image.

Now on to alignment and stacking with Registax

Then it’s simply a matter of opening the resulting .TIF images in Registax as you would normally.

Once the alignment completed and the images stacked, your photo can be saved

But before you close the program, head over to the Wavelet panel and tweak the image to get as much detail out of the moon’s cratered surface.

If you compare both images it is clear that the 2nd one has sharper details.

As always, the best is to try different things and experiment with your setup to see what works best.

Equipment used for the above photos:
Canon 80D
Skywatch 80ED (600mm F7.5)
1/250sec ISO 200