Astro Photo Question (trying this forum)...

John Retsal wrote:

[ I asked this over on the Astrophotography forum and got zero replies ]

I'm going to try my hand at some wide-field Milky Way photography.

Camera is Nikon Z6II with the Z 14-30 f/4 lens.

Location has Bortle 2 skies (for those who know what that means), so pretty dark.

I'd be shooting at 14mm and maybe 20mm, wide open at f/4 tripod mounted, no tracking and VR off.

I'm pretty sure I need to keep my exposure times (per the NPF rule) to around 11 sec for 14mm and 8 sec for 20mm in order to have no trailing visible.

Personally, I use the "500 rule" as a guideline. Divide 500 by the focal length you're using to get a maximum exposure time that won't produce objectionable star-trailing. That term, objectionable, is key. It's a subjective threshold that might be too short or too long according to your aesthetic. A 30-40 second exposure at 14mm should produce a pleasing night sky result. A 25-ish second exposure should deliver the goods at 20mm.

For reference, my favorite photo of comet NEOWISE from the summer of 2020 is a 4-second, untracked exposure made at 200mm. Not to get too deep into the weeds on that one, but it's a stacked image made from eight exposures of the comet at ISO 12800. I did the stacking in Photoshop and it really does a nice job of mitigating noise visibility.

If the subject of the photo is a constellation or a bright star that you want to capture with diffraction spikes, then I would recommend observing the 500 rule a bit more closely. However, the more the stars function as framing for another subject, the less important it is to avoid any visible trailing at all costs. If your subject is the summer Milky Way, choose settings that capture the breadth, depth and color of that display. If there's a bit of star-trailing, who cares. The stars are supportive elements in the scene. If your subject is a bright comet, ignore the 500 rule and choose settings that put the comet front & center. Nobody's going to be looking at the stars. They're all going to be wow'd by the comet

What I'm unsure about are other settings such as ISO (taking into consideration the camera's ISO Invariance) and white balance.

I'll often shoot at ISO 6400 with a full-frame body. As mentioned, I used ISO 12800 for comet NEOWISE.

And a bonus question, is there any advantage in shooting in b/w in terms of the quality of the images?

Thanks all!