The Deep-sky Imaging Primer

Click here to order from Amazon
(affiliate link; I earn a few extra cents if you use this)

The second edition of The Deep-sky Imaging Primer is a substantial update to the first edition, with revised and expanded text and over 325 illustrations. Printed in full color, it covers everything you need to know to capture stunning images of deep-sky objects with a DSLR or CCD camera:

  • The fundamental concepts of imaging and their impact on the final image
  • How to pick a telescope and camera
  • How to get set up and take the images
  • Where and when to find the best objects in the night sky
  • How to process images using Adobe Photoshop® and PixInsight®
  • Start-to-finish examples of image processing

Complete coverage of PixInsight® has been added, with workflows for both PixInsight and Photoshop®.  There are also two new start-to-finish processing examples.  The early chapters have been revised, with clearer explanations of important topics like noise and resolution. And a new appendix provides details of the best imaging targets for northern hemisphere imagers.  Overall, it’s a bigger, more readable book, with a lot more content.

Click here or the image below to order the book from Amazon. 

2e Cover v4 - front only (small)

Like the first edition, the book is structured in three sections:

  • Understanding Images explains how electronic sensors work and introduces the critical concepts of signal and noise.
  • Acquiring Images describes how to use telescopes, cameras, and computers to capture deep-sky images.
  • Processing Images is the longest section, covering everything you need to know to turn raw data from the camera into a deep-sky masterpiece. In addition to Adobe Photoshop, it now includes full coverage of PixInsight, which is fast becoming the main image processing tool for many imagers.

I’m happy to answer any questions at

Table of Contents

Understanding Images

1     Electronic sensors
The challenges of deep-sky imaging 8
How electronic sensors work                8
Well capacity and bit depth 9
The trade-off between dynamic range and gain           10
The importance of bit depth 11
Response curves and raw files             12
Creating color images             13
Sensor architecture 14
A CMOS revolution? 15

2     Signal and Noise
A little statistics and quantum physics             16
Signal and noise at the pixel level      17
Shot Noise   18
An example of shot noise      20
A dimmer example   22
Skyglow       23
Thermal signal, hot pixels, and dark frames   24
Read noise and quantization noise   25
Variation in two-dimensions                26
Adding signals and noise      27

Acquiring Images

3     Mounts and Alignment
Meridian flip               31
Polar alignment        32
Drift alignment         33
Tracking error            34
Balancing a mount   34

4     Cameras
DSLRs            35
Dedicated astronomical cameras       35
Sensor size and pixel size      36
Connecting a camera to a telescope  37

5     Telescopes and optics
Fundamental optical parameters       38
Telescopes for visual vs. imaging use               38
Optical aberrations  39
The Strehl Ratio        40
Refractors    40
Telephoto lenses      42
Reflectors and compound telescopes               43
Telescope quality     45

6     Image scale: matching sensor and optics
Resolution and seeing            46
Sampling     47
The three constraints on resolution  48
Field of view               49
Equipment recommendations             50
Focal reducers and field flatteners     51
The ‘f-ratio myth’ debate       52

7     Choosing objects to image
Getting a sense of scale          53
The deep-sky catalogs            53
Object types and the seasons              54
A survey of object sizes          57
Giant Objects (2° or greater) 57
Really Big Objects (1–2°)        58
Objects from 30–60' 58
Objects from 15–30' 58
Objects from 5–15'   58
Objects from 1–5'     58
Objects smaller than 1'           58

8     Focusing, guiding, and setting up
Focusing      59
Achieving critical focus          59
Autoguiding fundamentals  60
Connecting computer to mount          62
Software and settings            63
Reducing setup time               64
Dovetails, rings, and other mount accessories              65
Power supplies          65
Dew prevention        66
USB cables and hubs               66

9     Filters and narrowband imaging
Filters for imaging   67
Narrowband filters  67
Light pollution filters             69
DSLR filters and modification              70

10     Taking the exposures
Controlling the camera          71
Choosing exposure duration and gain             71
Planning for a night of imaging          73
An image capture workflow  73
Dark frames                74
Flat frames  75
Bias frames  76
Dithering light frames            76

11     Atmospheric effects
Light pollution          77
Target altitude          78
Local turbulence       79

12     Diagnosing problems and improving image quality
Wind or tracking errors          80
Diffraction patterns 81
Flat fields     81
Focus            82
Halos             82
Tips for better images             83

Processing Images

13     Color in digital images
Raw data and image file formats         85
Visual response to color         85
Color management and color spaces 87
LAB color     88
The HSL/HSV/HSB color model            88
White balance and color temperature              88
Deep-sky color accuracy        89
Color calibration with G2V stars         89

14     Calibration and stacking
Calibration exposures            90
Throwing out problem shots                92
Stacking parameters               93
Calibration and stacking with DeepSkyStacker             94
The drizzle algorithm              97
Calibration and stacking with PixInsight        98
Creating a master bias in PixInsight  98
Creating the master dark and flat in PixInsight            99
Integrating light frames in PixInsight              101
Combining data from multiple nights               105
A PixInsight pre-processing cheat sheet          105
Using the BatchPreProcessing script 107
Diagnosing defects in calibration output        107

15     Principals and tools of post-processing
PixInsight, Photoshop, or…?               108
Types of processing tools and a basic workflow            109
Selective adjustments            110

16     Processing images in PixInsight
The PixInsight user interface               111
Working with images in PixInsight    112
Working with processes in PixInsight               115
Image and process containers             115
Saving the workspace as a project file              116
ScreenTransferFunction        116
A PixInsight post-processing workflow           117

17     Processing images in Photoshop
Layers           119
Layer blending modes            120
Creating a composite layer   120
Selections and feathering     120
A Photoshop post-processing workflow          121

18     Using masks
Masks in PixInsight 122
Creating a luminance mask with the
RangeSelection process         123
Creating a star mask with StarMask   123
PixelMath    127
Using a uniform mask to moderate an effect 128
Masks in Photoshop 129
Creating a star mask in Photoshop    129

19     Color synthesis and gradient removal
Aligning separate color channel images          131
Cropping out calibration artifacts      131
Aligning histograms with LinearFit   132
RGB color synthesis in PixInsight       132
RGB color synthesis in Photoshop     132
Narrowband mapped-color images   133
Correcting gradients               133
Gradient removal in PixInsight           134
Gradient removal in Photoshop         135
Color calibration in PixInsight            138
The PreviewAggregator Script            139
PixInsight’s PhotometricColorCalibration process      140
Combining luminance and RGB data in PixInsight       141
Combining luminance and RGB data in Photoshop     142
Creating a synthetic luminance image             142

20     Stretching: reallocating the dynamic range
What is stretching?  144
Understanding the histogram             144
How different curves affect the histogram      146
Nonlinear transformation in PixInsight           148
Nonlinear transformation in Photoshop         149
Keeping the background neutral in Photoshop            150
Posterization: the perils of over-stretching    150
The effect of stretching on color         151

21     Noise reduction
Visual noise, color, and scale                152
Noise reduction in PixInsight              153
Masking for noise reduction 153
An introduction to scale-based processing    153
MultiscaleMedianTransform 155
ACDNR          157
TGVDenoise                158
SCNR             159
Noise reduction in Photoshop             159

22     Adjusting color
Enhancing color saturation in PixInsight        160
Enhancing color saturation in Photoshop      160
Narrowband color adjustments in PixInsight 162
Narrowband color adjustments in Photoshop              165
Mapping color in Photoshop with clipping layer masks             165
Correcting star color in narrowband images  166
Blending RGB star data into a narrowband image        167
Balancing Color in Photoshop             168

23     Contrast enhancement, sharpening, and deconvolution
Creating contrast with S-curves          169
Object-background separation curves             169
Intra-object contrast curves 170
Local contrast enhancement in PixInsight      172
Dynamic range compression with HDRMultiscaleTransform    175
Local contrast enhancement in Photoshop with blend modes 175
Unsharp mask (Photoshop and PixInsight)    176
Photoshop’s HDR Toning tool             178
Photoshop’s high pass filter 179
Deconvolution vs. sharpening            180
Deconvolution in PixInsight 180

24     Star reduction and removal
Reducing background star size           183
PixInsight’s MorphologicalTransformation tool          184
The minimum filter in Photoshop      186
Reducing the brightest stars 187
Star removal and separation in PixInsight      187
Star removal in Photoshop   190

25     Cosmetic repairs
Cosmetic repairs in Photoshop           193
Correcting elongated stars   193
Cosmetic adjustments in PixInsight  194

26     Image composition
Framing the scene    195
Orientation 196
Color             198
Step away from the computer             198
Images and reality   198

27     Plate solving and mosaics
Plate solving and image annotation 199
Mosaics        200

28     A DSLR processing example    203

29     A CCD processing example      210

30     A Photoshop processing example      216

Exercise Answers      221

Moonlight Tables     224

Deep-sky Highlights           228

Index         233

50 thoughts on “The Deep-sky Imaging Primer

  1. How to buy this book in India. From Amazon, shipping charges are more than the cost of the book.

  2. Hi Charles, I live in Canada and discovered that doesn’t carry your book, which I thought was odd… anyway, is there anywhere in Canada that can source it for me (bricks-and-mortar or online)? – Rod

    1. Hi Rod –

      Sorry for the book not being listed on I can have a copy sent to you (media mail) for US$32. That takes 1-3 weeks. (Priority Mail is ridiculously expensive, but available for $47.) If you are interested, email me at deepskyprimer*AT* You may also check’s rates to ship a copy to you in Canada, as they may actually be able to ship it cheaper than I can.

      Thanks for your interest!

  3. Hi Charles,

    I’d like to thank you for your book! I’am french, I was trying to find a book about deep sky imaging but in french there are only 2 or 3 books that are dealing with astrophotography in general. Then I find your book on Amazon by chance. I looked inside and found it very good, with simple explanations… I bought it and I’m not disappointed! I had read the first part of your book and I find the answers to many questions… Thanks again! I’am going to read the next parts 🙂

    1. Thank you for your kind words about the book! I am happy to hear that the explanations are clear to you. I hope you enjoy reading the rest of it, and please send me a note if you have any thoughts or suggestions.

      Best wishes,

  4. Charles,
    You produced a wonderfully readable and information-rich book! I bought a copy about 4 months ago, and find I’m constantly referencing it for one thing or another. Yours is the first imaging book I’ve purchased, and I would eagerly recommend it to others. Excellent work!

  5. I think the book is great, however the font used is way too small. It is nice looking, but I have to make a real effort to read it. I would have much preferred a thicker book, but easy to read, after all books are meant to be read, not to be looked at. That is too bad, as the book deserves all the praises, and I do hope that a future edition considers this point of view. Thank you.

    1. Thank you for your helpful comment, Angelo. I am working on the second edition, and I have indeed increased the font size by one point. I’d rather have a thicker book too, but the book’s cost is based on the number of pages, so increasing the font size can increase the final price of the book. To balance the increased font size, I’m planning to increase the size of the book itself from 8×10 inches to 8.5×11 inches.

      Best wishes,

      1. Hello Charlie,

        Thank you for your reply. I understand your concerns and am glad to read there will be a second edition with an increased font size. Do you know when it will be out? Thank you.

        Best regards,

  6. Pingback: Academics - Page 2
  7. Have you considered releasing the book as an e-book? It’s great having a paper copy to read from cover to cover, but I find e-books much better for a quick reference. An e-book can go with me wherever I have my iPad. Eithe e-pub or pdf would be great.

      1. Hi Charlie, would like to buy both your Primer and The Astrophotography Sky Atlas. But I’m just interested in ebooks, for several reasons. I travel a lot, like to look back on stuff I have read, and then save shipping duration, cost – and environment. Unless truly forced – a principle. Many great hi-res tablets these days. No idea why you would choose iBook platform lock-in. Kindle is at least everywhere. ePub is great, can even accept (Adobe) DRM. If you do, please let me know, and don’t forget adaption to be within the readable! (this guy, like plenty of others, forgot to check:

      2. Thank you for the feedback. Now that the ePub and Kindle formats seem to be maturing, I will consider offering the second edition as an eBook. I’ve had several similar requests, and I completely understand the convenience of eBooks. I read most of my magazines now on the iPad. There are some challenges with the eBook formats that still make me hesitant due to the nature of my books: they are full-color with high resolution images. Neither book would be appropriate for monochromatic eReaders, as a lot of information is communicated with color in the figures, and one of the chapters is specifically about color. I fear the atlas is possibly too high resolution to work on an eReader. If I had a way to prevent those with monochrome devices from purchasing the books, I would be more comfortable. I don’t want anyone to be disappointed after downloading an expensive eBook!

        I still have a long way to go with the second edition though, so perhaps things will be different by the time I’m ready to publish it. Or perhaps I need to create a specific version for tablets that is designed for the format. I’ll investigate my options, and appreciate any additional thoughts you have.


  8. Hi,
    I am hesitant to buy this first edition if the second edition is being released soon. What are the plans for the second edition, still looking for ab April release date?

    1. Sorry, work commitments have delayed my ability to finish the second edition. I do not expect to have it ready until later in the year now.


    1. I’ve re-written sections 1 and 2 already, and I’m re-writing section 3 with a much greater focus on PixInsight this time. I have another book coming out next month (an astrophotography atlas), so work on that has delayed the second edition of this book. Thanks for your interest! When the second edition comes out will mostly depend on my work schedule, but I don’t think it will be ready before the end of this year.


  9. Hi,
    I bought this about 18 months ago as a newbie. It’s one of the best you can buy on the subject. Covers most of the significant concepts in a relatively simple langauage that one can understand. Especially stuff like full well capacity, the coverage of the field of view and then processing tips as well. Well written with clean colour images. I still use it whenever I need to brush up. I bought several books before but this is the stand out for DSLR especially.


  10. Charles:
    I have the First Edition. I was working thru the exercises and came to Exercise 1.3 on page 33. The answer on p.185-186 confused me. How did you get 18.3 e- per second? From Ex 1.2, the thermal noise was 1.68e- per second at 20 deg C.

    1. You are correct, this is an error in the book that carries over an old version of the question. The answer to 1.2 should read:
      “An exposure of 20 minutes at 40 degrees Celsius would produce 20 min x 60 sec x 1.68 e- per second = ~2014 electrons. Dark current would fill 2014 / 25500 = ~8% of the full well capacity. The brightest capturable level would be 25500 – 2014 = 23486. The noise from the dark current is SQRT(2014) = ~45 electrons. The read noise is given in the problem as 10 electrons, thus maximum theoretical dynamic range is 23486 / (45 + 10) = 428, or 53 dB.

      For the same exposure at -15C, following a similar set of calculations, the dynamic range would be 25468 / 15.6 = 1629, or 64 dB.

      (Note that we are simplifying our calculation of total noise here by not using the proper summing in quadrature method, as that is not covered until the following chapter, but this simplification does not have a meaningful impact on the results.)”

  11. Hi! I see there’s mention of a second edition of this book being prepared last year. Will that be released soon? I’d like to buy it.

    1. I took a year off from writing the second edition to create The Astrophotography Sky Atlas. Now I’m working on the second edition again, but it’s still a long way from done. Sorry for the delay. It can be hard to squeeze in the time to write with a full-time job and family!

  12. Hi from New Zealand. I wonder if your Astrophotography Star Atlas includes the Southern Hemisphere for all of us below the equator? I assume that you have probably spent most of your focus on only the Northern Hemisphere. thanks.

    1. Hi Larry – The atlas covers the entire Southern Hemisphere in the same detail as the Northern Hemisphere. With so many great objects visible from “down” there, I couldn’t leave out the southern skies!


  13. Hi, I´m here to bug you, good Sir, for a digital version of your work. In my search for just that I see illegal downloadable copies in pdf-format out there. I simply refuse to go that way, it would not be fair to all that work you´ve put into your books.

    SO many people have a full colour tablet these days, I really think your work would reach more people that way. Pictures could be enlarged for potentially better viewing than on paper.

    E-books tend to come cheaper as well, no postage, no paper, instant delivery, has a lot of benefits. Please concider some sort of comercially available digital version of your work soon.

    For my personal interest, the Deep-Sky Imaging Primer is my first wish. I´m rather impatient when I see something I need for my hobbies. I´ll probably acquire the printed version none the less, while I wait for my wish to come true (around Christmas? 😉 ).

    Kind regards and hopes for clear skies
    Bjorn, Denmark

    1. Hi Bjorn – I appreciate your thoughtful note. As the e-reader market evolves (and I get feedback from potential readers like you), I am reconsidering my publication options. I still need to do some research on pricing structure, file formats (Kindle vs. ePub vs. iBooks), and resolution. All of my efforts right now are still on finishing the upcoming book, but once I’m done with that, I can see about releasing a digital version of The Deep Sky Imaging Primer.


  14. Thanks, Charlie. And best of luck with your current work.
    I will order the hard copy asap. Can’t wait…

    To promote the digital version you could stress the practicality of having it along in dark conditions (if the book should find it’s use in real-time-next-to-scope situations, idk.), some ebook-apps have night lighting, so one doesn’t spoil night vision.

    I use both Blue Fire and iBooks, the former comes in both iOS and Android. Some inspiration:

    Clear skies (and clear types)

  15. Hi Charlie, minor copy error you’re probably already aware of. On page 171, figure 230 covers up the text. It’s not a critical error as the intent is clear in the image, but I wanted to make you aware of it. Stu

  16. I was wondering if your book shows information on filter wheels? I have two stellarvue refractors (80 & 105 triplets). What are the advantages of electronics over manual? Or is there any? I haven’t found anything that talks about filter wheels. Or what size filters are best 1-1/4” or 2”).
    Thanks in advance.

    1. Hi Stephen – Electronic filter wheels have enormous benefits. Mainly, it’s because you can set them to be controlled from your computer, allowing you to automate your imaging session with software like AstroPhotographyTool or Sequence Generator Pro. Otherwise, you’d have to sit by your scope all night and turn the filter wheel manually. Also, manual filter wheels won’t have the same accuracy in turning to the exact same spot, which is required for flat fielding.

      As for filter sizes, there isn’t a “best”–it depends on the sensor size, how close the filters are to it, and the focal ratio. For most APS-sized sensors, 1.25″ are okay for focal ratios slower than f/4. SBIG cameras of this size don’t put the filter as close the sensor, they usually use 36 mm filters. Larger sensors require 2″ filters.

      Hope that helps!

  17. Hello Charles, would it be possible to promote the information and image of the book in the next issue of the amateur astrophotography magazine please?

    Looking forward to your reply

  18. Charles, I am enjoying your book. However, to get the most out of it you kind of need to be in front of the computer working through the examples you give. It would really be helpful if you could provide the images you use in your book to demonstrate PixInsight for download so that we could replicate your results as we work through the examples



    1. Hi Stuart – I’m so glad to hear you are enjoying the book! The files used in the examples were several gigabytes in size, and I’m not sure I have them all at this point. Is there is a specific example you’d like to try? I can see if I can find the original data and send your way.


  19. Hi Charles,

    Thanks for this very interesting book. I’m trying to work out the solution for ex 2.2 in the second edition. If my reasoning is correct then reworking the formula would yield a = 1.22 times wavelength devided by sin(1/206265). I do not get 173mm. Even using 173 mm in the formula itself and calculating the angular resolution does not yield 1/206265. What is wrong with my reasoning?

    Regards, Bart

    1. Hi Bart –
      I should have better explained in the text that the wavelength and aperture values are both in the same unit (meters). So the wavelength becomes 700 nm = 700*10^-9 meters. Your equation when you solve it should look something like:
      sin(1/206265) = 1.22 * 700*10^-9 / a
      with the value on the left in radians, it becomes
      4.848*10^-6 = 854*10^-9 / a
      a = 854*10^-9 / 4.848*10^-6
      a = ~0.176 meters = ~176mm

      Not only will I update the completed answer to show these steps better in the third edition, but I’ll also change Rayleigh’s criterion to drop the sin-1 function. I was being technically correct by including it, but for small angles, it doesn’t any material impact to the accuracy, so Rayleigh’s criterion just becomes resolution = 1.22 * wavelength/aperture. I think you’ll agree that equation is much easier to use! Reworking the problem with this approximation, it becomes:
      1/206265 = 1.22 * 700*10^-9 / a
      4.848*10^-6 = 854*10^-9 / a
      a = 854*10^-9 / 4.848*10^-6
      a = ~0.176 meters = ~176mm

      Thanks for bringing this to my attention.

      Clear skies,

  20. Hello Charlie;

    What is the chance that you will be issuing a revised edition again? PI has advanced quite a bit in the 4 years since your last update.

    Thank you!

    1. Yes! I’m working on it now. Not only has PI advanced, APP and Affinity Photo have become a part of many people’s toolbox. I’m hoping to release the 3rd edition this fall to cover all of this.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s