satellite and hot pixel track suppression in software

Started 2 months ago | Discussions thread
hha1 Junior Member • Posts: 29
Re: satellite and hot pixel track suppression in software

Jared Willson wrote:

hha wrote:

Airplane, satellite tracks and hot pixels are problem in long time exposure astrophotography. Most cameras have a dark frame subtraction option to eliminate hot pixels, but at the expense of loosing 50% of the exposure time. Most image processing software include an option to suppress artifacts in a long sequence of images. Some have the reputation of being “star eaters”. Here is how I solved this problem with an example using the “Accumulation 3.5” option in SEQUATOR.

I took a 2 hour exposure (120 shots at 60 sec each) using a f=135mm lens at f/4 of the Leo constellation. My Nikon Z6 was mounted on a SkyTracker to eliminate star trails. Rather than waiting for a satellite track, I intentionally misaligned the pole axis by 0.5 degrees. This produces 0.26 degree long star trail in 2 hours. The result of a simple star registration of the 120 images is shown on the left below. This is 1x1 degree cropped from the 10 x 15 degree field, zoomed in at 400% to make the pixels visible.

The stars are perfectly registered. The most prominent star on the lower left is +6.7 (HD97937) in Leo. The galaxy in the upper right is NGC3593 (+11.8). The faintest stars are +15. The bluish trace is produced by a hot pixel. It shows the worm period and a 3 pixel peak-to-peak worm error. Since each pixel of the camera with the 135 mm lens subtends 9”, the peak-to-peak worm error is 27 arcsec. This is a well known limitation of the SkyTracker, but it has almost no effect on the image quality with 60 second exposures. The hot pixel track is not straight and uniform, indicating that the mount flexed during the 2 hours.

On the right above is the same 120 images, but this time registered with the 3.5 sigma accumulation option in Sequator. Most of the effect of the hot pixel has disappeared, and there is no indication of the faintest star being effected. If the polar alignment had been near perfect, the hot pixel would have looked like a faint star in an unexpected location. The lesson from this (for me) is that a little polar misalignment can be helpful in eliminating hot pixels and avoiding wasted time to identify "new" supernovae.

The first option, of course, is dark subtraction.

Do you mean the internal dark subtraction, where ever 30 second exposure is followed by a 30 sec dark frame, the difference is saved as the image? It effectively decreases the SNR achievable with a 60 second exposure by more than a factor 2.

The second tool to use, as mentioned, is dithering. That's not a feature in the Startracker, of course, so there are a couple ways to accomplish it. One of them is slight misalignment of the mount. That has the advantage of not requiring any actions on your part, but it has the disadvantage of slight elongation of your stars since the tracking is off just a bit. It may not be enough to bother you, though, and that's just fine.

This is basically what I did. The 0.5 degree pole misalignment creates an 8 arcsec trail in 60 seconds. My pixels with the 135mm lens are 9 arcsec. The faintest stars with the Z6 cover 2x2 pixels. You have to look very close to see that. I have a program which does that. Indeed, for the 2000 stars identified ui=in the 10x15 degree field with the 135mm lens, the mean star diameter is close to 2 x 3 pixels.

The one other way you could dither would be to turn the mount off and on for a second between exposures. There are a couple problems wit this--primarily the risk that you will knock the mount slightly such that your frames become significantly misaligned. The second issue is that you would need to babysit the mount for the entire exposure. The third issue is that you have a slight loss of efficiency in your data collection since you would need at least a few seconds between exposures setup in your intervalometer. That's time when you could have been capturing photons. Overall, I think the slight polar misalignment is a better option just because I wouldn't want to stay right next to the camera for every second of a two hour exposure window.

You are right. That is the option have use. The camera is controlled with a intervalometer. I set it to take 120 one minute exposures, separated by 2 seconds, Then I can do other things until 2 hours are up.

By the way, walking noise may still be present even after the sigma clip stacking. Just as you found it isn't perfect at removing your hot pixel, it won't be perfect in removing walking noise--which is the exact same process, just with smaller variations in pixel brightnesses. With a Startracker, though, there simply is no dither feature, so no way to generate consistently small, random motions between frames. If you push your data hard, though, the walking noise from a polar misalignment will likely become visible.

I don't recommend an intentional 0.5 degree polar misalignment. I did this  to find out how much effort I need to put into getting a good polar alignment when Polaris is hidden by clouds, trees or mountains. The answer: With a 135mm lens you can get a usable sequence of 60 seconds each, even if you are 0.5 degree misaligned. Good thing to know. With the built in pole axis scope and Polaris visible, alignment within 0.1 degree is easy.

The walking noise is due to the fact that some of the 24 Mpixels are significantly more noisy than others, but it is still gaussian distributed. The probability of a positive 3 sigma or larger excursion is only about 0.2%. The catch is that estimating the robust standard deviation with less than 100 frames becomes increasingly unreliable,

You mentioned the cost, in imaging efficiency, of using dark frames. It doesn't have to be that bad. You could take your dark frames using just a lens cap if you can take them on a different (cloudy) night with similar temperatures. It's still a hassle, but you don't need to lose imaging time.

II like to do it the easy way:  I set the intervalometer to take 120 shots, separated by 2 seconds each, and do other things during the following 2 hours.

The real challenge is finding a night with the same temperature as the night you were imaging. Ideally, temps will match within 1*C or so. Obviously, that can be hard to arrange. Worst case, try to make sure your darks are taken on a night with lower temps than your imaging night rather than higher temps--that way your darks may undercorrect slightly, but you don't need to worry about black "pepper" spots showing up in your images from clipping.

Clipping done properly (by Sequator and others)  does not create black pepper spots, I have a program which creates a list of hot pixels. The list is not very repeatable from night to night. I also tried the "cleaning" function of the Nikon Z6.  Not sure what it does in addition to shaking the sensor, probably creates a map to interpolate across deda pixels.

There is one last tool I would recommend you take a look at, though I don't know how SEQUATOR handles this... That's building a bad pixel map. Some software can take a stack of dark frames and use those to generate a bad pixel map of all pixels brighter than a particular standard deviation from their neighbors. Then, the software will simply interpolate values for those bad pixels before you register and stack. Check to see if SEQUATOR has this feature.

Sequator does not have this feature.

Given the limitations of your Startracker, this may actually be the best of both worlds--most of the improvements from dark frames without the need to have temperature matching darks.

The SkyTracker works well enough for 60 second exposures with a 135 mm lens.  I have used it with a 200 mm lens and the results are good when reasonably pole aligned,

Thanks for your thoughts on this.

HHA

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