Starguide - A Practical Guide to Astro-Photography

PixInsight - Image Post Processing

Star Mask
 +

updated: 2021-05-30

Purpose

The MorphologicalTransform (MT) process is an advanced implementation of several morphological transforms. It does so by applying a function using a structuring element (a mask).

Activation

Menu: Process 🡺 All Processes 🡺 StarMask

Process Explorer: MaskGeneration 🡺 StarMask

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_SM.xisf” or similar into sub folder "aligned"

Previous Step

• Full Final Stretch or any image form chapter Improving the Image

Next Step

• Improving the Image (any step)

Tutorial

https://www.youtube.com/watch?v=MhaW1mcybzk  (Part 10  Star Mask Shrink, time=10m33s)

Reference: PixInsight v1.5 - Unofficial Reference Guide: MaskGeneration (deepskycolors.com)

Step

Description

 Action

1

Open StarMask

2

Start with the fully stretched image

from any step after Full Stretch

3

Create a Preview Box + Open the Preview

in an area with some stars. This step is required to reduce the processing amount that would be required for the entire picture

4

Main Settings

Noise Threshold: decrease for more structure, increase to prevent inclusion of noise. If you set it too high, real stars are going to get missing in the star mask, too low and it will create star masks for non-existent stars (noise).

The noise threshold level must be set above the highest RGB levels that you can see at the bottom of the PI window when you hover over the preview window with you mouse or by clicking and holding the left mouse button while hovering:

Working Mode:
 

Scale: N. of wavelet layers for structure detection. Increase to include larger structure in the generated star mask

5

Test Result

by dragging the little triangle to the preview lead to these results:

Original Preview                                  Generated Star Mask

The result looked best with the settings on the right. Try different settings until the result looks good.

6

Close the Preview

7

Generate the StarMask

Drag the little blue triangle to the main image to see if it also impacts the nebula or other other objects (this take a little while...) 

As you can see, the big nebula on the left was not impacted by the function, that's a good result!

Original Image                          Star Mark    

8

Apply the StarMask to the Original Image

Pull the tab of the B/W stretched image right below the tab of the original image to apply the star mask

The red image is the MASK which will be used further. Notice the tab on the left side of the windows turned to brown indicating a mask. 

Everything in red is protected (here this is the background). By inverting the mask you can also protect the foreground which is good if you want to reduce noise on the background.

Make sure the background (and not the stars are protected) for the next steps, the background should appear red

Hide the mask with CTRL+K

 🡺

6

Minimize the StarMask Image and the StarMask Process 

7

Open Morphological Transformation

8

Create a Preview Box + Open the Preview

on a typical star field area

 ALT+N

9

Set MF Filters and Treshold

Try following settings:

Operator  🡺 description, try Erosion first

Interlacing: 1

Iterations: 1

Amount: 100%

Threshold Low: 0.05

10

Apply the Morphological Transformation on the Preview

Test the effect by clicking on Undo / Redo

 🡺 

Original                                                  After Morph

  🡺 

11

Remove the Preview and the Mask

Menu 🡺  Mask 🡺  Remove Mask

12

Apply the Morphological Transformation on the Entire Image

13

Close Morphological Transformation

14

Save the Image

file name like  “*_i_c_DBE_ABE_BN_CC_HT_MLT_FS_SM.xisf”

or "20170921_M8_FUJXT1_i_c_DBE_ABE_BN_CC_HT_MLT_FS_SCNR_SM.xisf" in this case

using postfix _SM

file name like  “*_SM.xisf”