Starguide - A Practical Guide to Astro-Photography

PixInsight - Master Calibration Files

Master Flat Calibration Frames Part II

updated: updated: 2021-08-20

Purpose

Once we have calibrated the individual flat frames, we'll integrate them.

After bias and dark subtraction, the flat frames are strictly composed of illumination data. Therefore, we must match the illumination levels of all flat frames. This is the key to do a good flat frame integration.

Illumination fitting is done by multiplying each flat frame to match the average pixel value of all flat frames. This is done by selecting multiplicative normalization in the Image Integration section of ImageIntegration.

Average pixel value fitting is also important to achieve a good rejection of outliers. In the Pixel Rejection (I) section, we'll select the flux equalization option to normalize the frames.

As in the bias and dark frame integration, we'll disable image weighting.

The pixel rejection algorithm to use will depend on the number and type of flats that we have. In the case of sky flats (where we have stars on a highly illuminated sky), or in case of using a small number of flat frames, the best rejection algorithm is percentile clipping. This is the best option to reject the stars.

Activation

Process 🡺 All Processes 🡺 Image Integration

Input

Calibrated flat files (folder: e.g.: "\_Astro\ASI294 Calibration\Config 3\20191022_Flat ASI294 -10C\Calibrated Flats"

calibrated flat files have a "*_c" postfix

Output

Calibrated Master Flat File  -  folder e.g.
"\_Astro\ASI294 Calibration\MasterFrames\ASI294\ASIAIR Masters\Config 04 Flats"

Previous Step

Master Flat Generation I

Next Step

Master Flat Generation II

Tutorial

PixInsight — Master Calibration Frames: Acquisition and Processing

Step

Description

Action

1

Open the Image Integration module

2

Add Flat frames

Take care to use the same exposure, temperature and optical configuration as used for the light frames.

3

Image Integration Section

• use Multiplicative Normalization

4

Pixel Rejections (1) settings

• use Percentile Clipping or Winzorized Clipping
• Normalization: Equalize Fluxes

5

Pixel Rejection (2) settings

• The percentile rejection limits must be very restrictive. Usually we'll set the limits (especially the high limit) below 0.02. The figure on the right shows the rejected high pixel values of a three sky flats frame set. Although a restrictive clipping limit will affect the signal-to-noise ratio of the resulting master flat, this is the only solution to get rid of the stars in all the frames.
• If we acquire a large number of dome or box flats (flats without stars), outlier rejection is rather simple. Winsorized sigma clipping with permissive rejection limits will do a fine job.

7

Run the Calibration Step I

Click on the (Apply Global) or hit F6

8

Save the Calibrated Master Flat File

File name e.g.: "ASI294_Master_Flat_180sec-10C-20x.xisf"

into folder 
"\_Astro\ASI294 Calibration\MasterFrames\ASI294\ASIAIR Masters\Config 04 Flats"

9

Next Step: Light Frame Calibration