How to produce a defect Map in PixInsight to calibrate

[pyrasanth]

As I dig deeper into PixInsight I find that I'm beginning to use it more & more. The tools in Photoshop through Noels & Annie's actions are good but they can be aggressive in there effects on images & require very careful use.

I've begun to use long sub frame exposures particularly with HA which at slow focal ratios means that I'm often taking subs of 30 minutes. This puts the CCD under pressure and the sub frames show all sorts of hot & cold pixels which unless effectively removed make a dramatic difference to the final result.

Ideally these  pixel defects should be removed with a correct frame calibration but experience tells me that it is not always successful even with a correct calibration. I think this is the most important step towards a good image. I will show you how I produce a defect map & use it to remove the hot pixels in a long exposure sub frame. Do not run the hot pixel defect removal on a registered frame but it will work on a calibrated light frame. This procedure is very effective even after you have calibrated with flats.

The first step is to produce a dark frame of the correct binning & exposure length. This can be done off telescope for convenience. Cover the camera & take at least 15 dark frames of the length of your longest exposure. This  takes time  but is well worth the effort.

Follow the steps below in PixInsight

1. Integrate the dark frames using the settings below

Apply Global to make Dark Frame

Press the circle button to make the master dark frame. Discard the 2 high & low sigma images & stretch the newly created "integration" it should look like below

Produce Binary defect of the dark image

Open the binarize tool and click the circle on the bottom toolbar to see a real time preview as below. I suggest you experiment with the view options but maximum works well to see all pixels. Move the slider until you see as many pixel hot spots as in the image below. This is a really minute movement so be careful.

 

Invert the image

Go to the image tool bar at the top of Pixinsight & choose invert image to make the mask go white with black spots

 

Save the defect Map to a convenient location

 

Open the newly created defect map . Open the defect tool as below from PixInsight all processes if you can't immediately find it. Load the defect map into the defect tool as below.

 

Open an image file to calibrate & drag the triangle to the image- the image below shows before & after

After the calibration

 

 

Hope this is of help. If you have any questions PM me or post in this thread. Remember this procedure will not work on registered frames but will work on calibrated frames.

 

The result of my efforts on an actual image is below:

 

 

 

 

 

[ianaiken]

This looks very easy to follow. Thanks for creating it hopefully I will get to try it out on some new data this week if the weather plays fair.

[pyrasanth]

7 minutes ago, ianaiken said:

This looks very easy to follow. Thanks for creating it hopefully I will get to try it out on some new data this week if the weather plays fair.

Thanks. This is very effective so I thought its worth sharing. I suggest you run it over your calibrated frames to remove the hot pixels. However the mistake I initially made was to try & run it over calibrated & registered images and it does not work. Thinking why its because registered images have moved the pixel map so logically it's not going to work.

[Mike Hawtin]

Have you tried the Cosmetic Correction tool?  I use it instead of darks, check auto detect, check hot and cold sigma at the defaults of 3, works a treat on my Atik428 and QHY9M subs.

Mike

[Pompey Monkey]

I created a defect map exactly the same way. Works a treat for my SBIG 8300M.

[ianaiken]

1 hour ago, Mike Hawtin said:

Have you tried the Cosmetic Correction tool?  I use it instead of darks, check auto detect, check hot and cold sigma at the defaults of 3, works a treat on my Atik428 and QHY9M subs.

Mike

That tool is currently part of my workflow too. I don't use darks on my Atik 460 EXM. I've never been able to get the Cosmetic Correction tool to work with dSLR subs yet though. I think for that I probably do need darks :-)

[Freddie]

Useful stuff but I think it should be in the image processing section not here.

[pyrasanth]

1 hour ago, ianaiken said:

That tool is currently part of my workflow too. I don't use darks on my Atik 460 EXM. I've never been able to get the Cosmetic Correction tool to work with dSLR subs yet though. I think for that I probably do need darks :-)

I cant get the tool to work at all. It would be useful but I will recheck. It is just possible I was testing on an already registered frame. I will give it another go when I have time.

[pyrasanth]

4 hours ago, Freddie said:

Useful stuff but I think it should be in the image processing section not here.

Lol- I spend so much time in this section I didn't even know about the processing section but the topic is relevant to all image processing I guess- excuse my ignorance

[Freddie]

No problem. If a mod could move this it would just make it a bit easier if people were searching for help with this type of processing.

[wimvb]

Thanks for the information. I will try your method on DSLR images. The only problem is that I rely on bias and cosmetic correction  and rarely take darks anymore. I use dithering, cosmetic correction and pixel rejection during the integration process.

 

[pyrasanth]

I got the cosmetic correction tool working- me bad- I did a stupid thing when you tick the auto box nothing is selected until you also tick the hot & cold selection boxes

[wimvb]

1 minute ago, pyrasanth said:

I got the cosmetic correction tool working- me bad- I did a stupid thing when you tick the auto box nothing is selected until you also tick the hot & cold selection boxes

Yes, there are a lot of tick boxes in PI. The one I often forget is CFA image (I use a DSLR).

[pyrasanth]

I've now started to find that less is actually producing a better result at least for me. I played around with loads of options and I've come to the conclusion that unless you know exactly how they all work you can make a big mess of your images. I'm leaving the hard stuff alone & my work flow feels better for it.

[wimvb]

Agree; very often the default settings in PI work quite well. Whenever I use a (for me) new tool in PI, I start with default settings. When I start to understand the workings of the tool, I adjust settings to meet my needs. The CFA tick box in cosmetic correction must be set however if you use one shot colour (DSLR), otherwise the result doesn't make sense. In all processing I heavily rely on previews to test settings before applying to the full image.

[pyrasanth]

I'm capturing more HA data for M101 tonight. Even though the moon is nearly full the results are not too bad- I will just shoot more data to compensate for the moon. I'm using 30 minute subs with a Baader HA filter with the Atik 460- such a fantastic camera. Most of the CCD equipment I use is Atik.