Starguide - A Practical Guide to Astro-Photography

 updated: 2021-05-31

Purpose

Manually increase the contrast.

Implements pixel intensity transformations by interpolation of functions defined through arbitrary user-definable points. 

Activation

Menu: Process 🡺 All Processes 🡺  Curves Transformation

Process Explorer: IntensityTransformations 🡺 Curves Transformation

Input

Stretched image, file name “*_i_c_DBE_ABE_BN_CC_HT_MLT_FS.xisf” or similar into sub folder "aligned"

Output

Stretched image, file name “*_i_c_DBE_ABE_BN_CC_HT_MLT_FS_CT.xisf” or similar into sub folder "aligned"

Previous Step

Background Improvement

Next Step

Any topic under Color Correction

Tutorial

https://youtu.be/FxM5WGrvhoI?t=937  (Part 9  HDR starts around timestamp = 15 m and 35s)

Step

Description

 Action

1

Open Curves Transformation

2

Create a Luminance Mask

this will create a B/W luminance image.

Then hide the mask (CTRL+K)

3

Open a Real-Time Preview Window

4

Enable Track View

this will show the histograms of the different colors

5

Click on an Important Section on the Real-Time Preview

(selecting a typical area on the image) 

and the Curves Transformation will show the discrete color values of this point in the histogram windows as vertical lines.

Then click on the white line where the vertical color lines meet the diagonal white line.

6

Repeat this Step on a Darker Section of the Image

Again click on the white line where the vertical color lines meet the diagonal white line.

7

Repeat this Step on a Background Section

to protect the background in the image

8

Repeat this Step on a Bright Section of the Object

to protect the bright sections of the image

Now there are 4 points on the diagonal white histogram line.

9

Try moving the 2 Midpoints to Different Positions

and watch the result - do not exaggerate as this can destroy the entire picture.

The example at the right is a fairly good result.

 You can set additional points on the line and check the results.

10

Close the Real-Time Preview

11

Apply the Settings on the Entire Image

by dragging the little blue triangle an on your image.

12

Remove the Mask

Menu 🡺  Mask 🡺  Remove Mask

unless you want to keep it for the next step!

13

 Close Curves Transformation

unless you want to continue with the next step!

14

Save the Image

or "20170921_M8_FUJXT1_i_c_DBE_ABE_BN_CC_ HT_MLT_FS_SCNR_SM_HDR_MLT_CT.xisf" in this case

using postfix _CT

file name like  “*_CT.xisf”