Wednesday, June 1, 2016

How to remove light pollution with "Blur and Subtract"

How to remove or at least reduce light pollution is a frequently asked question. The "Blur and Subtract" technique is a process which I use to reduce light-pollution on my astro-photography images. I live in an urban area and the skies are never black. Of course the best thing is to travel to a darker place, but most of the time that is not possible. I wanted to have a quick method to remove some of the light pollution without working too much on the computer. As it turned out this method provides very interesting results in both daylight and night photos.

The name of the technique "Blur and Subtract" speaks for itself and has three major steps:
  1. Take your desired photo (Photo 1)
  2. Take the same photo but use manual focus to blur the image as much as possible (Photo 2)
  3. Subtract (Photo 1 - Photo 2)

That is all, but if you are not familiar with any photo processing software continue to the detailed tutorial or watch this two minutes demonstration video.


Why the "Blur and Subtract" method reduces light pollution?

This method works well for light pollution since in Photo 1 the focus is usually set to infinity and in Photo 2 it will be set to just to a few cm, diffusing every star but the brightest into the background. Subtracting the background (Which is usually yellow/brown/green) will give a black background with the original stars. If you forgot to take photo 2 you can use the Blur function in a software but I think that taking a second photo is a better approach.
This method turns the following photo - Iridium Flare in Corona Borealis (Northern Crown)
Iridium Flare in Corona Borealis
Iridium Flare in Corona Borealis - With light pollution
Into this one
Iridium Flare in Corona Borealis - Light pollution removed
Iridium Flare in Corona Borealis - Light pollution removed
Which is much darker!

But you can use this method even if you include terrestrial objects and even in daylight. Strong lights will diffuse but will still be noticeable and the entire image will look surrealistic.
Here is an example. What we see here is an Iridium flare near the constellation Cygnus (Swan), and the entire summer triangle including Vega at the top and Altair at the bottom-right. The photo was taken with a wide lens (16mm). The method will work better for longer focal lengths as the change of the focus will diffuse the strong lights better.

Photo 1 - proper focus
Photo 1 - proper focus
This is the second photo. The flare is already gone which is even better for us (it improves the subtraction). The stars disappeared (except the really bright ones) and the city lights looks  bigger. 
Photo 2 - Focus to nearest point possible
Photo 2 - Focus to nearest point possible


And this is the result of the subtraction and some other processing. You might need to tilt the monitor a little bit and since it is very black a good quality monitor is required.

I applied some more fixes to the photo such as:
  1. Remove the diffused bright stars (Vega and Deneb) - Single click of the Spot Heal tool around each star.
  2. Remove the strayed light in the bottom-right caused by forgetting to cover the eyepiece during the long exposure - again one of the Heal tools did the trick.
  3. Many crops (just the flare, just the summer circle, just the flare and the buildings, etc.) but here I show the full image.
The sky is much darker and probably some faint stars are not shown (more fine tuning in software is possible if you desire), but what is more incredible is what happened to the buildings.

Detailed Tutorial

If you need an online free tool for this purpose you can use Pixlr. Here are the exact steps. The steps are identical for other software such as Photoshop or Lightroom but not everyone has them. Pixlr is a strong free alternative and has many features. However it might reduce the resolution of the image (There is a stronger desktop version but it is not free to use).

1. Browse to pixlr site, choose "Open image from computer" and select your first photo (the photo with the correct focus).
Open PXLR site
Open Pixlr site
2. The photo will appear on the screen. To open the second blur photo (Photo-2) as a new layer, go to the Layers menu and choose "Open image as layer", select the second photo.
Choose open image as layer
Choose open image as layer
 3. Check the layer window and press the "Toggle layer setting" button as displayed in the image below. This displays a dialog box with the options we need for the subtract operation.
Press Toggle layer setting button
Press Toggle layer setting button
 4. In the mode option choose "Difference".
Choose "Differnce" as mode
 5. That's it. If you need to process the image further, do it now or simply save your file with a new name as a JPEG or preferably PNG file.
Save the result file
Save the result file


More examples

Here is another example. The time is just after sunset but skies are still blue. Here is the result after I cropped most of the sky.


And another interesting application on this royal ponciana blooming tree
 royal ponciana blooming tree
 royal ponciana blooming tree
 Into this strange colors.
 royal ponciana blooming tree
 royal ponciana blooming tree

And a photo of the sun (Warning: photographing the sun requires special filters. Never look directly at the sun!). The sun is very bright so blurring it will create a larger and dimmer image of the sun (a red giant lookalike)) and create an interesting image.
Double sun
Double sun


Tuesday, May 17, 2016

Carnival of Space #457

Hello all and welcome to a new CoS. Something always happen in space and here you will find what happened in the last week.
We will start with Mercury Transit from last week. What I love about such events is that they are so global. For several hours many people all over Earth watched this event from different countries, Universe Today collected some of the best photos and videos and present them in a single article:
Mercury Transit
Mercury Transit


One of my own videos is in UT's article and that is always an honor, but If you want more information and details including my photography setup please read my own summary of the transit as well.

Two more articles from UT:
From Chandra:
From BrownSpaceman:

Take in mind that next week Mars will be in opposition and as close to earth for the next two years, so grab your chances and aim a telescope to it and enjoy. It will not be as big as the moon, but even if you don't have a telescope you will notice its red bright color easily.

That's all for today. Have clear nights and long days!






Tuesday, May 10, 2016

Mercury Transit summary

Mercury Transit is a rare occasion on which Mercury pass in front of the sun, a tiny tiny "eclipse".
My setup was like this. I used a Bresser 130N and Canon DSLR to photograph through it, however there was a slight focus problem and there are no good results from it. Another Camera, Nikon P900 was mounted as piggyback on the scope and worked properly and Another Camera Canon SX50 for handheld photos and for the Sunset video below.
My setup for Mercury transit
My setup for Mercury transit
The beginning was not very promising the entire sky were cloudy and the sun could hardly be seen. However from time to time there was some less clouds and it was possible to see the transit.
Here is my first photo. Notice sunspot 2542 in the middle and tiny Mercury (imagine the Sun as a clock and Mercury is at 11).
Mercury Transit
Mercury Transit
 And here is another photo in which you can see how Mercury moves.
Mercury Transit
Mercury Transit

The sky cleared a bit so I put all cameras on auto mode using the telescope and a mount, and left to find a place where I can see the sunset

Here are two videos from the transit. The first one is about 2 hours of the Transit until the sun was too down.


In the sunset videos don't miss 00:40 where two airplanes transit the sun as well!
Here is a general view
Photographing the sunset
Photographing the sunset

And the video.

And just the highlights of the sunset in regular speed (The airplane is in the beginning)

Mercury Transit sunset with airplane
Mercury Transit sunset with airplane

It was a very interesting and unique event!




Monday, April 18, 2016

Use Orion to see Mercury

Mercury is an elusive planet. It is small, dim and most of the time to close to the sun to be seen at all. Yet , during its greatest elongation it is far enough from the Sun. In this period Mercury can is easy to spot after sunset or before sunrise.. Today 18-Apr-2016 Mercury is in greatest western elongation meaning it is easy to watch.
It is even easier as you can use the famous constellation Orion to help you find Mercury. Just continue the straight line from Orion's belt and go almost to the horizon and you will see Mercury. Actually Mercury is brighter than any of Orion's belt stars so you might be able to see it even before. Start watching about 45 minutes after sunset and look about 15 degrees up from the place the sun set. There is no other bright star in this area so when you finally see the white dot, it is mercury. And if you own a telescope try to observe Mercury and see its tiny but noticeable phase.
Here are some photos I took tonight to guide you
The first one is without annotations:

Orion and Mercury
Orion and Mercury
And here are annotations. Do not confuse the airplane with Mercury.
Orion and Mercury

 And if we mentioned an airplane here are some more airplanes (Looks like the IAF is practicing for Independence day)
And a bird on the background of the moon (From 17-Apr-2016)