[RCC] Streaky artifacts in M42 and request for critque - Forums

Tim Ulama

Topic starter

Feb 20, 2022 · Edited: Feb 20, 2022

Hi everyone and greetings from Germany!

I hope the awesome astrobin community can give me some help

I recently processed data from M42 and i noticed two things:

The stars at the edge of the picture are not round
There are streaky artifacts all over the picture:

My setup is fairly simple, a stock EOS 700D, SW Star Adventurer and a Canon Ef 70-200 F4 is usm L lens.
I captured data over two nights, total integration time is around 2 hrs. During each image session I took 70x60" lights, 20 darks and added 20 flats and 20 biase-frames from my library. The data was stacked and processed using Affinity Photo (i also tried DSS with pretty much the same result).
You can see the final image here:

M42 - New Version with more Data

Could 1. be a result of lens distortion? If tracking isn't good, shouldn't all the stars show elongation?

Are the streaky artifacts in 2. a result of improper calibration? Should I add more calibration frames? I think I can also see colour splotches, which I think are typical for Canon sensors?

I am grateful for any advise! Also feel free to critique any other things you might notice.

CS
Tim

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Yuxuan

Feb 20, 2022 · Edited: Feb 20, 2022

For 2, did you not dither?

Tim Ulama

Topic starter

Feb 20, 2022

Yuxuan:
For 2, did you not dither?

No. I am not controling the mount via a pc or anything. How can I dither manually with the Star Adventurer?

andrea tasselli

Frequent contributor

Feb 20, 2022

The answer to the above question has been already answered fairly recently by someone who had the same issue. Do a search in AB. And yes, you'd need to dither, especially with some cameras (Nikons are largely unaffected). As for the other question (i.e., not round stars) it is because your lens has significant optical "distortions", so to say. What you see is field coma and can be minimized by further stepping down the lens but cannot entirely removed, am afraid. You would need a way better lens than that for it to give nice round stars edge to edge.

Haakon Rasmussen

Feb 20, 2022

Hallo Tim,

I have the had the same streaks, and it was due to Darks not having exactly the same temperature as the Lights. You could read out a couple of Darks and lights with Raw-Therapee to see if there are temperature differences. If there are, try to retake your Darks. If there are no difference, i am afraid you need to dither.

CS
Haakon

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Tim Ulama

Topic starter

Feb 20, 2022

@Yuxuan Thanks for the clue about dithering!
@andrea tasselli Thanks for pointing me to the other post!
@Haakon Rasmussen Temperature should be the same. I took the darks frames immediately after the light frames and there wasn't a notable change in temperature.

Haakon Rasmussen

Feb 20, 2022

Hallo Tim,

I too took the Darks direct after the Lights, temps were different nevertheless.

CS
Haakon

Rob Foster

Feb 23, 2022

Hi Tim, I’ve had issues with banding with my modified Canon 80D, and have learned that banding can be an issue with Canon sensors In general even with well matched calibration frames. More signal helps a bit, but I have had to rely on the Canon Debanding script in Pixinsight for very early correction in the linear stage, then the band reduction tools within the Astrophotography Tools plugins for Photoshop which do a great job late in development. Not sure this is your issue or what you might have available within Affinity. Good luck! Rob

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Tim Ellison

Feb 23, 2022

Hi Tim,

I concur with what has been already said.

I had issues with streaky effects which looked rather similar to yours. I'm using a Canon 600D. After a deal of head scratching and experimentation I worked out that it was caused by the temperatures of my darks being too high. It's always possible that your streaks are caused by something different, but I reckon it would be good to eliminate the hot-darks possibility before looking elsewhere. Have you looked at the temperatures stored in the image tag data? They don't tell you the whole story, as I believe they show the temperature at the start of the exposure and don't tell you anything about how that changes during the exposure - but they're the best available information. The sensor will usually warm up during the course of an exposure, and then cool during any dead time between exposures. So if there are differences in that dead time it could possibly result in darks that are a bit too warm. Was the camera in the same environment for the lights and the darks? Or was it taken into a warm environment to shoot the darks? The tag data should give you a good steer on whether or not there is a temperature issue.

On the question of the non-round stars, that may be down to imperfections in the lens. But there is something else that might possibly be going on as well. At shorter focal lengths there is a projection effect which can elongate stars towards the edges of the image. This is caused by the fact that the "celestial sphere" is being projected onto a flat camera sensor. I have seen it in some of my wide angle images - at maybe 15mm focal length. I'm not sure whether your setting of 200mm is short enough for the effect to become apparent. In theory there are ways to correct the effect, and I'm working on that, but haven't got it fully figured out yet. There's always more to do in this game!!

Your shot of M42 looks very nice by the way!

Hope the above helps.

Tim

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Tim Ulama

Topic starter

Mar 8, 2022

Hey everyone!

Thanks for all the helpful comments!
I've checked the temperatures from the images. During the first session Lights-temperature is around 15-17°C and Darks are pretty much the same temperature.
Lights-frames of the second session, however, start at 16°C and end up at 10°C. The Dark-frames from this session start at 6°C and end up at 10°C. So there is a larger difference. I read somewhere that a few degrees difference between Lights and Darks are not a big deal, but more than 5 degrees could cause problems. I shoot with a 2 second interval.

Next time I am out there, I'll keep an eye on the temperature. Since I use magic lantern, the live view displays the temperature.

CS
Tim

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Tim Ellison

Mar 8, 2022

Hi Tim,

Those temperature differences look pretty good to me. I can't imagine getting much better than that without a temperature-controlled camera. I generally operate with differences of a few degrees between lights and darks and don't have any serious problems. The times in the past when it did all go wrong for me was when the darks were at temperatures perhaps 10-20 deg C warmer than the lights. I cringe when I think back to the times when I was doing that!

There may be differences in the processing perhaps? I'm using PixInsight. I did discover (the hard way) that there is a right and a wrong way to perform calibration with that package. Some quite well respected sources of knowledge actually have aspects of this a bit wrong, and can result in problems. I'm not sure if there is any possibility of similar issues with Affinity Photo or Deep Sky Stacker, but it might be something to ask the community about. When I was doing it "the wrong way" with PixInsight, I found I was getting single-pixel black holes in the calibrated frames, where values had gone negative during the calibration arithmetic and been clipped to zero values. Dithering then made these holes move around in the image producing "streaks" or "worms". In the absence of dithering, the imperfections in tracking, even if small, could produce a similar effect I think.

I hope you manage to solve the problem. Good luck with it all!

Tim

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