UPDATED: Altair 26C issue resolved!

[Stratis]

UPDATE: All issues fixed!

Rather than necropost and drag this back to the top of the forum, I'm editing this top post. Previously, I had concerns over a strange magenta gradient appearing in the darks and bias frames of an Altair 26C.

Initially the other forum members reassured me (see below for very helpful replies) that all is well, and indeed I was able to resolve the issue.

I was able to trace the problem to the use of a Raspberry Pi 4 as the imaging computer. There is something strange in the way the RPi's internal USB hub interfaces with the 26C, as by switching to a Windows PC and NINA the gradient (however mild) disappeared completely leaving beautiful even darks.

 

Edited January 23, 2023 by Stratis
Updated Issue

[ONIKKINEN]

Here is one of my darks taken with a RisingCam IMX571 (same manufacturer as yours), 60x 60s debayered stretched and screenshot in Siril:

Looks not too different to yours. The important thing to note here is that it is VERY stretched and the actual differences in intensity are not that big. Sensors are not perfect and you will inevitably have some kind of difference in intensity of different coloured pixels (and possibly a gradient like these) so i wouldn't worry about it. Of course if you have light leaks then retake the darks. You should have the camera off the scope and completely plugged from light in a dark room when taking darks. If you have the camera on the scope the darks will be ruined in most cases unless you have basically perfect darkness at your site. I believe you might have some light leaks as you have a full 1 ADU difference in median pixel values in the green channel between the left and right edges (mine has 0.3ADU). It may also be just that i am comparing a stack to a single frame so might be nothing after all.

But, an issue you should fix and it may actually have something to do with this is your offset which is way too low which results in 0 value pixels that carry no value and cannot be used for proper calibration. You should increase offset until no pixel values are at 0 in a bias frame. My camera had a default offset off 768 which is just fine and there was never a reason to touch it. I recommend increasing your offset to at least 500.

[Gary Clayton]

I concur, they look the same as my darks, apart from your offset. 

[tooth_dr]

Mine are the same 2600MC

[Stratis]

Thank you guys!

The only experience I have with OSC imaging has been a 585-based sensor (which produces profoundly different output) and my 6D which is hardly comparable. 

Really good to know it's normal behaviour. I shall have to learn how to process it out  

[Stratis]

15 hours ago, ONIKKINEN said:

But, an issue you should fix and it may actually have something to do with this is your offset which is way too low which results in 0 value pixels that carry no value and cannot be used for proper calibration. You should increase offset until no pixel values are at 0 in a bias frame. My camera had a default offset off 768 which is just fine and there was never a reason to touch it. I recommend increasing your offset to at least 500.

Offset was the setting I had the least idea about, thank you for clarifying.

Looking at the histograms you're clearly right, I was wondering why the values chopped off so abruptly like that.

If I may ask another newbie question, should I be using Bias frames or Dark frames with this camera? From what I've read a Dark frame includes the data the Bias frame would?

[ONIKKINEN]

8 minutes ago, Stratis said:

Offset was the setting I had the least idea about, thank you for clarifying.

Looking at the histograms you're clearly right, I was wondering why the values chopped off so abruptly like that.

If I may ask another newbie question, should I be using Bias frames or Dark frames with this camera? From what I've read a Dark frame includes the data the Bias frame would?

Bias frames can be used as darkflats to calibrate your flats, provided they are roughly in the same temperature and exactly the same offset. I would take a set of darkflats of roughly the same exposure as the flats instead of bias.

For lights you will need to take actual darks to properly calibrate out your offset and the (extremely) little dark signal the camera produces. If you are very lazy you could also use bias/darkflat frames as dark frames for short exposures, have done that and not sure there were any issues. But hardly any reason to skip darks as you take them once and forget about it.

So im short, darks to calibrate lights and darkflats(or bias) to calibrate flats.

[Adam J]

On 21/12/2022 at 00:29, Stratis said:

UPDATE: All issues fixed!

Rather than necropost and drag this back to the top of the forum, I'm editing this top post. Previously, I had concerns over a strange magenta gradient appearing in the darks and bias frames of an Altair 26C.

Initially the other forum members reassured me (see below for very helpful replies) that all is well, and indeed I was able to resolve the issue.

I was able to trace the problem to the use of a Raspberry Pi 4 as the imaging computer. There is something strange in the way the RPi's internal USB hub interfaces with the 26C, as by switching to a Windows PC and NINA the gradient (however mild) disappeared completely leaving beautiful even darks.

 

Makes me wonder if its not using the internal memory and only working on USB 2

[Stratis]

16 hours ago, Adam J said:

Makes me wonder if its not using the internal memory and only working on USB 2

There was definitely a bandwidth issue. This is known with the RPi in that it doesn't really keep up with USB3 specs, especially not when all its ports are loaded as it's basically just a soldered-on £5 USB hub, but I had hoped that the internal memory buffer could compensate.

It seems it does compensate in that the data is delivered and somewhat faster than USB2 speeds, but the signal is somehow biased in the process. I used a USB-C hub with a Surface Go 2 tablet (I normally use it for focusing and polar alignment at the mount) to take frames with NINA and the output was way better.

Damn shame as the RPi is perfect in every other respect, sips power and super lightweight as remote control box.