HowTo? Use Dual-ISO with star trails? Don't?

[l_d_allan]

I'm wondering if anyone has "best practices" for using Dual-ISO with star-trails?

Or is Dual-ISO + Star-Trails an inappropriate combination?

With a full-frame and 14mm or 8mm fisheye, I've used Bulb = 120 seconds, Intervalometer at ""like crazy" (shortest delay between shutter activations), ISO 100, and f5.6.

My speculation is it might vary if the image was mostly/entirely the sky, with no foreground? Then you want high contrast between the star-trails and the night sky, and perhaps Dual-ISO would be a poor choice?

Or if the foreground was important? Then you might want to get some detail in a very dark foreground during new moon?

Related question:
What about attempts to get images of the Milky Way in very dark skies (new moon and rural areas away from towns/cities)? (non star trails ... wanting little or no star movement, so not using intervalometer). Would Dual-ISO be appropriate?

[SDX]

I don't really see the purpose of dualIso in startrails.
You have to know that dualIso does have an impact on the resolution in bright and dark areas, so your stars will very likely not be as detailed as you want them to.
Furthermore, it is enough to only have one or two pictures for a good foreground. I usually start out by taking one or two pictures with properly exposed foreground, then I put the settings right in order to get detailed stars and forget everything about the foreground on those.
In the subsequent merging process, everything comes magically together to one picture, with good foreground and proper stars.

Same applies to non-tailed stars (all though it might be worth a try and see how the drop in res does work out here).

[dmilligan]

Or is Dual-ISO + Star-Trails an inappropriate combination?

Extremely inappropriate, and probably rather detrimental.

If you're stacking lots of sub frames to do your star trails, there is absolutely no reason to use dual iso, the stacking is going to remove more noise and give you more dynamic range than dual iso ever could. All you are doing is making your result worse by using dual iso.

It is a really good idea to do dark frame subtraction.

Here's a star trails shot I did recently. About 2.3 hours of exposure time means that there will be almost no noise even in the deepest of shadows:

[l_d_allan]

I don't really see the purpose of dualIso in startrails.

dmilligan: Extremely inappropriate, and probably rather detrimental.

Glad I asked, and thanks for the feedback. It did seem potentially flawed, so I'm very appreciative to get guidance.

I do tend to learn more from my (many) mistakes than my (fewer) successes.

Out of curiosity, I'd appreciate getting the settings and work-flow for the example dmilligan provided. What ISO, shutter-speed, f-stop, and number of frames over the 2.3 hours. Bulb? Did you do something different for the foreground tree and tree-line? Masking?

What software do you use for stacking? Photoshop? I use StarStaX, which is freeware, and does have provision for dark-frames.

[dmilligan]

Out of curiosity, I'd appreciate getting the settings and work-flow for the example dmilligan provided. What ISO, shutter-speed, f-stop, and number of frames over the 2.3 hours. Bulb?

1100D, 28mm lens, 28 x 300s exposure, f/1.8, ISO200
ML Bulb Timer + Intervalometer

Did you do something different for the foreground tree and tree-line? Masking?

Nope, to me masking is a bit disingenuous (i.e. cheating). The tree looks kinda weird b/c the wind is blowing it around.

What software do you use for stacking? Photoshop? I use StarStaX, which is freeware, and does have provision for dark-frames.

Calibrated and stacked in PixInsight, edited in Photoshop (HDR toning, curves, WB, saturation, etc.)

[SDX]

Even with higher settings you can still get good results. I usually shoot with f4.5 and ISO 800 at 30sec (at 10mm), and it works fine when you take an extra frame for the foreground. I usually don't mask that frame, it just goes into the stacking as any other frame. You just have to hurry and not waist time when changing settings, otherwise the stars from the foreground-frame will stand out!