Beginners Guide To Levels, Curves and Layer Masks

Image Manipulation Basics What is Post Image Processing? Image Processing Overview – 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 photons 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 dont 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 dont 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. – 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, its best to image for longer at a lower ISO (DSLRs 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 photons 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 Contd 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. Its 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 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- Your one stop place to do all of the above, but does cost a lot. PixInsight- Again a one stop shop but be prepared for a ride, this baby is complicated. 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, havent used this but the reviews are good and its 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 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. Dont over do you Levels, Curves and Layer Masks, keep your images natural looking.

Closing 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???