Gradient in dark frames - Forums

Topic starter

1 hour ago

Hi everyone!

I recently bought the Player One Poseidon C Pro camera and after a very long period of cloudy nights it finally cleared and I got to try it. When taking the lights everything was normal. When the sun rose again the pictures got very saturated (half the pixels were just violet and a the other half were just green). I stopped capturing and went on to take the flats and flat darks. I have always used the sky to take my flat frames and everything went smoothly (with my old DSLR). The flats are also great to shoot with the new Camera.
But when it got to taking the flat darks things got strange. I put on the telescope cover and covered the whole telescope with the camera and everything with a blanket. That way I should eliminate any possible light rays getting into the image train. But the darks don't look like the usual darks. Instead, there is a very noticable gradient, as you can see in the attached picture.

📷 Dark_Poseidon.jpg

Afterwards I also tried taking bias frames like usual (same setup with blanket). As you can see in the second picture the bias frame also doesn't look like a bias frame should look like.

📷 Bias_Poseidon.jpg

For both the dark and bias frames I reduced the gain and offset of the camera to 0 (when shooting lights I use gain: 125 and offset: 25).

Does anyone have a idea what could cause the very strang looking dark frames and bias frames? Has anyone else had this issue before and how did you fix it?

Thank you very much in advance!

Lukas

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Mikołaj Wadowski

1 hour ago

Both darks and bias need to be taken using the same offset and gain as your lights. And you don’t need darks anyway with your camera. Try taking new bias frames at gain 125 and offset 25 and see if they calibrate well.

As to the gradients, did you try taking dark frames in an unlit room? My guess would be a light leak. If you took both darks and bias frames outside, a blanket probably wasn’t enough to block all of the light from reaching the sensor.

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Dunk

1 hour ago

I have this same camera and have never seen anything like this… except when I had a light leak in my OAG.

Maybe try taking darks..in the dark and see if this still happens?

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Joseph Biscoe IV

1 hour ago

The gradient above is a light leak. Best to track down the source of the leak in a dark environment with a flashlight. Avoid daytime dark capture and build a dark library for your common exposure times on some cloudy nights. Mikolaj already mentioned this, but make sure to take the darks and biases at the same gain and offset.

I would caution you to always use dark frames. Relatively short Bias frames are unstable as each exposure will heat the chip up. Some testing I did with runs of 100 bias frames at different exposure lengths (0”, .25”, and 1”) all show from .5c-1c of temperature variation. This probably means very little with capable image processing software these days. But it means a lot for accurate calibration. If you want that sort of thing. Dark frames are much more stable for cmos chips.

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Joseph Biscoe IV

1 hour ago

This second post was sent in error. On my phone, and I can’t delete it.