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PESKYWAABBIT

Newbie imager here. Hot pixels?

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Hi guys, so I finally got my HEQ5 PRO mount to track correctly and I decided to take a bunch of photos of the Orion Nebula(or at least part of it).

I took as many photo's as I could with my Canon 1000D DSLR connected up at prime focus. I used 1600 ISO (max), 400 ISO and 100 ISO all with a shutter speed of 30"

 

I have found these red, green and blue squiggles I think are called 'hot pixels' in all of my photo's after I've used DeepSkyStacker to merge them. They may be hard to see on here but in Photoshop they are everywhere :(

Could someone please explain to me what is causing this and what can I do to avoid it in the future please. I feel like I've ruined a perfect night out! :help: 

Or maybe there is a way to fix this?

 

I was using a Skywatcher 200p for these.

ISO 1600 30"

Iso1600.png

ISO 400 30"

Iso400.png

ISO 100 30"

Iso100.png

 

Thanks, guys!

 

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Yes indeed those little trails (it's good the they are trails, this means not perfect guiding / tracking but in this case it's a good thing) are consequence of hot pixels.

Hot pixel simply means that chip at that pixel is not behaving as it should - could be some sort of small manufacturing defect - amplifier associated with that pixel is really sensitive to temperature - so on long exposure it saturates that particular pixel more quickly then others. You don't really have to worry about it, it is quite common and there are couple of ways to solve it.

First thing to do is learn to take dark frames - that means same iso, same exposure length - and later subtract from light frames (there is option in DSS to do that) - but dark frames don't get rid of all hot pixels, and sometimes turn hot pixels into "cold" ones :D (if pixel is both saturated in light frame and in dark frame - difference will be zero - same as cold pixel). Second thing you can do is turn on cosmetic cleanup in DSS - to remove hot and cold pixels. Third option that helps both with hot / cold pixels and strange things that might happen to your light frames - such as passing satellite or cosmic rays hitting sensor - is use sigma clip stacking (also option in DSS). For it to work, ideally, no light frame should be perfectly aligned with any other - and you have that - hence little trails of hot pixels - this means you can use sigma clip stacking as is.

Also - work on your focusing, you are a bit off. And check focuser / camera - there might be a slight tilt - try to fasten everything properly so you don't have loose bits.

 

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There are two ways to get rid of hot pixels from your images.

1. As @vlaiv has pointed out, you can use dark frames.

2. Use a technique called Dithering.

 

The idea of dithering, is that between each sub-frame, you move the telescope slightly.  This make the image that you want to capture fall on different pixels.  Do this for every frame in a your imaging sequence, and you'll end up with the image jumping all over the place, which is a good thing.  Then when you stack the images, you run an alignment process.  What will then happen is that those hot pixels will not fall on top of each other and will be treated just like any other noise in the image.

You can combine dithering with dark frames too, the process means that the dark frame needs to be applied to each sub frame before the alignment process takes place.

 

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