Blobulous Revisited – Part 1

It’s pretty amazing what you can accomplish with just a little bit of effort.  Below you’ll see a star trail formed using the Blobulous technique that I described in an earlier column on Advanced Star Trail Tricks.  There are many ways to achieve this effect. The method I described in the prior article – adding the last frame using Screen or Add blending – works especially well if the skies are dark. But here, I used high ISO shots about an hour before moonrise. Because of the high ISO exposures with a soon to rise moon the sky was blue not completely dark.  Adding the last frame to the stack as described earlier will work but it makes the Milky Way blow out and the sky overbright. I wanted the Milky Way to remain noticeable.


Selecting Images to Work With

There are few important things to note here. The radio telescope tracks the sky and slews from location to location causing blur. Each 20 second exposure was shot at ISO 3200, f/2.8 using a 15mm fish-eye lens on a Canon 5D Mark II. I carefully picked a range of images where the start of the sequence through to the end of the sequence included only images where the dish did not move “too far”. The next four frames show how dramatically the dish moved in subsequent shots. Had I included them it would have made quite a mess.


The dish moves far in 80 seconds…

To say it clearly: looking at the individual frames is what allowed me to zero in on the specific images I planned to stack. After selecting the range of images (C_072887 to C_072935),  I stacked them using good ‘ol brightness mode stacking. You can stack using a ton of methods but our favorite, of course, is to use the Advanced Stacker+ from though the free TEST Stacker can do the job, as can a variety of free tools that we describe in this article and in our Star Trails Webinar.

To create the shot below on the RIGHT you’ll need Photoshop or some equally featured program that allows layering and masking like GIMP. Even the lowly, stunted Photoshop Elements will do the trick. After doing a normal brighten mode stack, the output looks like what you see on the left.  After some image manipulation magic, we achieve the result on the right.


Before (left) and After (right)


It would be tedious to show step by step what I did, so I will start with a “one shot” view. You’ll notice there are 9 layers that contribute to the final image.  The top – SLC-SCA – is obvious, it’s my watermark.

Results with all layers shown.

Results with all layers shown.

However you can also see the brighten mode stack (at the bottom), and a copy of that same stack with a layer mask.  If you look carefully it’s pretty obvious that the black on the mask corresponds to the upper part of the moving dish.  Sharp eyes will notice two additional blacked out parts corresponding to the other radio dishes.  Notice that above the stacked layer is a single frame: C_072934 – the next to last image from the set used in the stack. All the layers above the single frame are adjustments to correct color, white balance, hot pixels (the Heal layer), and to do some sharpening.


The single frame (934) is blended in Lighten mode with the stack. The stack is at 36% opacity.

The essential bit of magic here is that the Brighten layer is only used at 36% opacity. 36% was chosen by eye. I slowly reduced the opacity until the Milky Way became noticeable rather than a blur and yet the star trails remained noticeable.

The image was nearly complete by painting out (excluding) the blurred stack areas using a black brush at 100% opacity with a small amount of feathering – i.e. a brush that was not 100% hard.  It’s easy to paint on the mask and watch how the blurry dish from the stack is replaced by the non-blurry single image from C_072934.  If you’re wondering why I didn’t use the final image, C_072935, it’s because it was a tiny bit blurrier than the preceding image.

The rest of the adjustments are straight forward – they were all meant to fix contrast, darken the overbright areas and then correct for the color saturation increase that occurs when you darken an area with color.

We cover complete details in the PhotoManipulation webinar series.

Here is the image obtained from the next set of exposures after the dramatic dish move. This image used the “Streaks” stacking method available in the Advanced Stacker, but is otherwise identical in creation.


For yet one more take – using yet another Advanced Stacker Mode (long streaks), there is this.

Long Streak Runnin'

In the next column I’ll show all the steps I took to produce the first image on this page.


What do you think about this?