Zigzag Hot Pixels

[brenski]

Hi I'm hopeful someone will have come across this phenomenon before and can tell me how to fix it.

♦ Zwo ASI 183MC Pro (cooled) ♦ Skywatcher Evostar 72ED DS-Pro ♦ SW Star Adv Pro 2i ♦ Optolong UHC LPF 2" ♦ Stellamira Field Flattener / 0.8 Reducer

♦ Lights are 30s 150gain (x60)
♦ Darks are 30s 150gain (x15)

Some information:
♦ "zigzags" are appearing on my first stacked images after Sharpcap capture - immediately after preliminary Black-Level adjustment in PShop (ie before any real processing)
♦ Zigzags are all over the image (crop below is from lower right centre of Orion nebula)
♦ No matter which method I use to apply darks: as Master Dark subtractions in Sharpcap or as Darks in DSS or "noise" files in Sequator - these zigzags still appear.

What am I doing wrong? Any help appreciated, thanks

[admin]

Hi,

The zigzags are following the movement of the image due to the mount not tracking perfectly - probably some declination drift added to some periodic error in RA.

Two approaches to consider :

1) Check your dark frames : They should have exactly the same settings as your light frames, and be taken at the same temperature. If there are differences then they will be less effective and may leave behind hot pixels after subtraction (or, equivalently, very dark pixels)

2) Add dithering to your workflow - this makes small, random movements of the mount at intervals between frames. The trails are a lot less noticeable when there is no real pattern to them.

Note that no matter how good your dark frames are, you will always get some warm/cool pixels left over after subtraction, and if you stretch the image hard enough then these will become visible.

cheers,

Robin

[brenski]

cheers Robin.

Darks are already been taken at same exposure/gain/temperature as the Lights. They're done during the same session.
will definitely look at dithering, thanks

[RonAM]

I always understood the colored squiggles to be hot pixels not corrected by dark frames but the last couple weeks of imaging have me scratching my head a bit. Sharing in this thread for thoughts.

1. My ASI294MC pro camera has very low dark current.
2. Imaging Comet ZTF the past few weeks using SharpCap livestack with 2 different mounts yielded one image with the hot pixel trails, the other without.

Temperature, Exposure per frame, Total exposure and Gain were all the same between the 2 sessions.

The Feb 7 session was on a tripod, no mount tracking, letting the livestack adjust for the moving star field. So the frame edges get lost as stacking proceeds, but at 4-5 minutes it’s not too bad. This session produced all images with no sign of the hot pixel squiggles. For sure the star field moved across about 20% of the camera frame during the 4 minutes total time so I should expect hot pixel trails, yet there are none.

The Feb 13 session was on an excellent tracking mount where there was no loss of frame edges during the session & yet this is the session where every image has the colored hot pixel squiggles. I would expect short hot pixel trails but not this large as the tracking was very good.

Cropped versions of the respective typical images are attached. They may end up in reverse order vs my comments but you can tell which one shows the hot pixel trail.

I guess my question is “Why don’t they appear for the no-tracking tripod session?”

[admin]

Hi,

essentially the difference is that in the non tracking case, the image is moving by several pixels with each frame (most likely), so that any trail from a hot pixel is broken up, and much less obvious. In the tracking case, the movement is gradual, so the pixel trail is continuous, or nearly so.

Another thing to be careful about is the amount of stretch that is applied in post-processing. No matter how good your dark subtraction, each dark frame has random fluctuations in it, so the master dark also has some random fluctuations (although smaller, by a factor of square root of number of dark frames). If one pixel in the master dark is a little dimmer as a result of these random fluctuations then the result on dark subtraction will make that pixel a little brighter than it should be in each frame. If you stretch the image far enough, you will see that discrepency, no matter how small it is initially.

cheers,

Robin

[RonAM]

Thanks Robin - so for the No tracking case the more separated hot pixels blend in with the background stars. It would take quite a “Where’s Waldo” search to find them in the midst of all the stars.

[admin]

Hi,

yes, I think that's the case. It's also the premise on which dithering works - by breaking up the easy to spot trail, the hot pixels are still there but less detectable by the human eye.

cheers,

Robin