Transcript Astrophotography

Levels, Curves and
Layer Masks
Beginners Guide To
Image Manipulation Basics
• What is Post Image Processing?
• Image Processing Overview
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Image Collection (Exposure Times and considerations)
Image Quantity
Light, Dark, Flat and Bias Frames
Image Stacking
Level Curves and Layer Masks
• A Quick Demo Video
• Conclusion
• Closing
What is Post Image Processing?
Once you have driven for hours, set up your scopes, got all the bits and pieces working together
and spent countless hours in the cold and late of night collecting valuable photon’s you want to
be sure to make the most of the data collected and make it look as good as it can possibly be.
Fortunately for us we live in the digital age and we have a wide range of tools available to us to
help enhance the details of our images to get the maximum effect for the time we spent out in
the field.
This is where Post Image Processing comes into play. It is the use of software to remove the
details we don’t want (Noise) and enhance the details we do want (Signal).
Many people underestimate the importance of image processing and this guide will hopefully
give some pointers to get you started. It is a lot easier than most think so don’t be scared to get in
and have a play.
Image Processing Overview
From the collection of data to the saving of the final file there is a clear process you can follow
and sets of tools you can use to help you get the most out of your efforts.
This guide is based on my personal experience and is a process that works for me, you may find a
different way that works better (let me know if you do as we may learn a thing or two along the
way ). There is no right or wrong and often processing an image is a personal thing.
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Image Collection (Exposure Times and considerations)
Image Quantity
Light, Dark, Flat and Bias Frames
Image Stacking
Level Curves and Layer Masks
Image Collection
Image Collecting:
I was asked about the exposures I take to create my final images and questions like “does 10 * 1
minute images = 1 * 10 minute exposure” or How do you manage to get the faint details were
frequent ones.
The exposures you take (and settings you use) will vary depending on the target being imaged,
planning is key and understanding the range of detail and brightness will help with exposures
times.
For example the Orion Nebula (M42) has both very bright parts (The Trapezium) and faint details
in the nebula clouds. There is no way to gather the faint details in the nebula clouds without over
exposing the Trapezium part.
There is no 1 single answer but understanding the post processing options available will help you
in planning to get the data you need for the final image.
Exposures
The Trade Off
When you take a photo of a faint night sky object you need to expose the CCD for longer periods,
this results in a situation where you gather more photons or “signal” from your target but
introduce other artefacts in the image from flaws in the CCD called dark current or “noise”.
Try not to use very high ISO settings on your camera, it will introduce noise faster and reduce the
quality of your images significantly, it’s best to image for longer at a lower ISO (DSLR’s are best at
400 ISO Max)
This is why we “stack” images, When you take several long exposures you will get “noise”.
Software such as Deep Sky Stacker allows you to combine the images for comparison and can
remove the ‘noise” from them and further assist in getting a high signal to noise ratio.
In answer to the earlier question; 10 * 1 minute exposures do not equal 1 * 10 minute exposure.
This is due to the way stacking software works as the process of stacking images is not an
accumulation process but a process of comparison and removal of noise in images.
Image Quantity
It is always best to get as many exposures you can of each target you image as this will allow you
to stack and improve the Signal to Noise ratio. Deep Sky Stacker goes as far as to say you should
get at least 30 frames of your target.
I personally aim for about 5 – 10 frames of each ISO I am imaging at as I find a DSLR cannot
produce the details of dedicated Astro Imaging Cameras due to the amount of Noise they
generate, by including more frames (subs) above 5 – 10 I do not see any significant increase in
quality.
However, when gathering Dark Frames, Bias Frames and Flat Frames I suggest at least 10 of each
as these will help get rid of a lot of noise and give a nice flat image to work with. We will get to
this next.
Frame Types
Frames (or Subs) are simply another name for exposures and there are few types you can use to
assist with getting better images.
Light Frames - Light Frames are the actual images you have taken of celestial objects that contain
the valuable data that you want others to see. The key objective is to get enough data (actual
photon’s of light) to reduce the Signal to Noise Ratio. All the other frame types are used to
reduce or remove the noise in you Light Frames and flatten your image.
Dark Frames - With DSLRs and CCD Cameras, the CMOS or CCD chip is generating noise in the
frame during any exposure, the amount of noise generated depends of the exposure time,
temperature and ISO speed. To remove this noise from the light frames you use dark frames that
contain only the noise generated by the CCD/CMOS chip. The best way to get a dark frame is to
take an exposure of equal settings (duration, ISO and temperature) with the lens cap on. Since
the temperature is important try to shoot dark frames at the end of or during your imaging
session. Take a few of them (about 10 to 20 is usually enough).
Frame Types Cont’d
Bias Frames - The Bias/Offset Frames are used to remove the CCD or CMOS chip readout noise
from the light frames. Each CCD or CMOS chip generates a readout noise which is a signal created
by the electronics when reading the content of the chip. It’s very easy to create bias/offset
frames: just take the shortest possible exposure (it may be 1/4000s or 1/8000s depending on
your camera) in the dark by covering the lens. The bias frames must be created with the same ISO
speed of the light frames. The temperature however is not important.
Flat Frames - Flat Frames are used to correct the vignetting and uneven field illumination created
by dust or smudges in your optical train. To create good flat frames it is very important to not
remove your camera from your telescope before taking them (including not changing the focus).
You can use a lot of different methods including using a light box, but I found that the simplest
way is to put a white T shirt in front of your telescope and smooth out the folds. Then shoot
something luminous like a flash, a bright white light, the sky at dawn and let the camera decide of
the exposure time (Av mode), The flat frames should be created with the ISO speed of the light
frames however the temperature is not important.
Image Stacking
As mentioned earlier image stacking is the process of combining the images you have taken to
remove the noise and flatten the illumination of the CCD chip.
I am not delving into the specifics of this process in this session however, I use Deep Sky Stacker
as it is free and very effective at what it does.
You can download a copy from http://deepskystacker.free.fr/english/index.html, the manual is
also quite easy to follow so download it and have a good read.
This software will allow you to process all the frame types I listed before.
Levels, Curves and Layer Masks
Now to what I really wanted to focus on, Levels, Curves and Layer Masks. Firstly some software
you can use to manipulate your images:
Paid For Programs
Photoshop
PixInsight
complicated.
- Your one stop place to do all of the above, but does cost a lot.
- Again a one stop shop but be prepared for a ride, this baby is
Free Programs:
GIMP 2
- A free program that allows Levels and Curves and will handle most file
formats.
Perfect Layers 2
- A free program that will allow layer masking, haven’t used this but the
reviews are good and it’s free from OnOne Software.
Paint.Net
- A free software package that will allow Levels and Curves.
The best option I know of for Layer Masking is Photoshop.
Quick Video
This is a quick video I made to show you the steps you take to apply Levels, Curves and Layer
Masks.
Conclusion
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Remember to Plan your imaging sessions to get the most of your
time in the field.
Understand the nature of the object you are imaging.
Take several frames and get varied exposures.
Remember your Dark, Flat and Bias Frames.
Levels, Curves and Layer Masks are fairly easy to perform and will
greatly enhance your images.
There is free software available for you to use, I suggest you have
a look around and find a package/s that suit you.
Practice a lot, the more you try the better you will get.
Don’t over do you Levels, Curves and Layer Masks, keep your
images natural looking.
Closing
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As with all thing astronomy share your ideas and findings with
others to help them along their journey
Please support the developers of free software by making a
donation if you can afford to, even $5 could ensure they are
around tomorrow providing free software.
Pray for clear skies 
Any questions???