Bright red artefacts

[StuartT]

Last night I took this image (16min, ISO 100, f/11) with a 17mm lens on a Canon EOS750D.

Can anyone tell me what the red dots are? They are clearly not stars.

[Cornelius Varley]

The "red dots" are mostly likely to be hot pixels on the sensor. Dark frame subtraction should deal with them.

[StuartT]

Just now, Cornelius Varley said:

The "red dots" are mostly likely to be hot pixels on the sensor. Dark frame subtraction should deal with them.

thanks. Actually, they are barely there on the JPG version I attached here, for some reason. They are really prominent on the RAW file.

[Stuart1971]

Yes deffo hot pixels, and very common on an uncooled camera, but as has been said, dark frames will remover no problem...👍🏼

[StuartT]

thanks. But just to be clear, you are suggesting I will need to stack for an image like that? How would I make several light frames given that this is a 16m exposure (so the stars would be in very different positions for each one)?

Or do you mean just take a bunch of darks and stack them with this one light? 

[Cornelius Varley]

12 minutes ago, StuartT said:

thanks. But just to be clear, you are suggesting I will need to stack for an image like that? How would I make several light frames given that this is a 16m exposure (so the stars would be in very different positions for each one)?

Or do you mean just take a bunch of darks and stack them with this one light? 

Both options will work, for a single shot you could just switch on your camera's automatic noise reduction function. If you take a series of images and process them through Startrails you can create a full image. Take a series of images and also an equal number of dark frames and stack them in the programme.

 

[Stuart1971]

46 minutes ago, StuartT said:

thanks. But just to be clear, you are suggesting I will need to stack for an image like that? How would I make several light frames given that this is a 16m exposure (so the stars would be in very different positions for each one)?

Or do you mean just take a bunch of darks and stack them with this one light? 

16m...you mean 16 minutes,...or seconds, as if 16 mins I would have thought the stars would be much more and brighter....

If you take say an exposure every minute for 1.5 hours and then say 20 dark frames of the same exposure length and then stacks the light and the darks together you would get something like this....

I did this with a canon 1000D unmodded camera on a tripod...

 

[Chefgage]

Yes. Instead of just taking one long image, take lots of shorter images such as 1 minute images. Then take your calibration frames. You can use startrails software like what what has been mentioned to stack them to create the startrails.

[StuartT]

9 minutes ago, Stuart1971 said:

16m...you mean 16 minutes,...or seconds, as if 16 mins I would have thought the stars would be much more and brighter....

If you take say an exposure every minute for 1.5 hours and then say 20 dark frames of the same exposure length and then stacks the light and the darks together you would get something like this....

 

no, I do mean 16 minutes. Otherwise, there would not be much trailing. Even 16 min is only 4 degrees of rotation.

The reason I don't have more stars is that I was at f/11 anf ISO100. (I live in town 😥

Edited April 3, 2021 by StuartT

[Stuart1971]

12 minutes ago, StuartT said:

no, I do mean 16 minutes. Otherwise, there would not be much trailing. Even 16 min is only 4 degrees of rotation.

The reason I don't have more stars is that I was at f/11 anf ISO100. (I live in town 😥

Ah, ok I used ISO 800 all the time on my DSLR imaging but with a CLS LP filter to cut out the town glow....👍🏼

[StuartT]

23 minutes ago, Stuart1971 said:

Ah, ok I used ISO 800 all the time on my DSLR imaging but with a CLS LP filter to cut out the town glow....👍🏼

ooh.. might have to buy myself one of those. I haven't bought any gear for days! 🤣

[StuartT]

ok, I think tonight's is better. 250 x 5 second exposures at 7 sec intervals combined using Startrails. 17mm lens, f/3.5 ISO 3200

 

 

[wxsatuser]

21 hours ago, StuartT said:

thanks. Actually, they are barely there on the JPG version I attached here, for some reason. They are really prominent on the RAW file.

You should be able to fix this by doing a manual sensor clean once outside and the camera has reached ambient.

The sensor clean updates the bad pixel map and should get rid of the hot. dead and stuck pixels.

[Mr Spock]

Most cameras have hot pixels. My old Nikon D300 developed a real beauty I never managed to get rid of. That was a job for the clone tool.

My D7200 has hot pixels too, but they are so small as to be of no consequence. Interestingly when I put the raw files into Photolab 4, it gets rid of them automatically.