Nautical Darkness equivalent to a moonlit sky?

[tomato]

With Astro darkness about to disappear from my location I was wondering how nautical darkness compares to a moonlit sky. I routinely image targets 40+ degrees away from an up to a 50% illuminated moon and although the results will never get an APOD, it’s definitely better than nothing. I was therefore wondering how that level of background sky brightness compares to a moonless nautical darkness summer sky? I can measure them myself but perhaps this has already been done and documented?

[WolfieGlos]

I would also be interested in this too, and to see some actual data, because I wonder if it's a bit of an urban myth that might be partially out-dated by modern sensors and filters?

Here on SGL (and elsewhere) I see that some people just shut-up-shop after losing astro-dark. I accept the further north you are makes imaging more difficult with the earlier loss of darkness, and that you'll probably not get the fainter details in targets, say tidal streams.

But last year I imaged throughout the loss of astro-dark, and captured the Eagle with the l'enhance filter with nearly 6 hours of exposure last July, and, it's in my most light-polluted portion of the sky. I've just reprocessed it with PI and XT this week - shown below - and I'm pretty happy with it compared to what I had processed previously - I might even add more exposure to it this year. I even captured M104 in 30 minutes (same area of sky) this past weekend in Nautical Darkness which really surprised me. Perhaps being fairly bright targets made this easier?

Personally, if practical this year (due to home circumstances changing), I'll be doing the same again throughout summer. It'll be interesting to see what everyones thought's are on this issue though 🙂

Edited May 8 by WolfieGlos

[skybadger]

My measurements with my Sqm meter show astro dark tapering off about 0330 UT which is 4.30 BST but getting a dark sky after sunset only arrives fully dark at 2230 UT/2330 BST. 

So 5 ! full hours, where the dark arrives at the predicted astro twilight time and sunrise doesn't impact for a full 2 hours after astro twilight ends so I adjust my imaging periods appropriately.

My latitude being 51.5N, hopefully this is representative 

[tomato]

Yes, I’m quite happy imaging broadband with up to a 50% illuminated moon, provided the target is a reasonable distance (40 deg minimum) from it, and I keep imaging through nautical darkness in the summer. I’m inclined to agree that modern sensitive sensors and LP removal software, negates the mantra that imaging outside of Astro darkness is a lost cause. I’m still curious to know if the two LP scenarios are roughly equivalent.

[woldsman]

Civil twilight is reckoned to vary from 2 to 3.5 lux over the evening but illumination depends on other factors, eg how clear the air is.

A Full Moon 's brightness depends on distance from Earth, height in the sky, and air clarity but surely is nowhere near as bright. 

Ground illumination cast from street lights or outside lights will be far stronger than either Civil twilight or the Moon. 

Assuming you image from a dark site to begin with,  given the choice between darkness and a Full Moon versus Civil Twilight and a New Moon, I'd take the 

Full Moon and pick a target as far away as possible. 

[geeklee]

I've imaged narrowband in nautical darkness before and been happy enough with the results.  Never tried broadband though - I'm interested how this compares.  I'm good with a a fairly small moon far away from an object in broadband but even then I try and choose wisely... although that sounds like there are plenty of clear nights and picking makes any difference 🤣

We'll be exiting nautical dark soon, sadly. 

[tomato]

Interesting, I think I’ll pick a couple of broadband targets over the coming months and try get some decent integration times on them to see how they come out.

[Elp]

I don't think they're equivalent. If you've been to a dark site you'll see in stark contrast what effect the moon has. A once visible milky way disappears and if the moons full nearly half the sky gets blotted out of the fainter stars (which you'd see with magnification). The moon can also affect filters and cause banding artifacts across your images. During non astro darkness you don't get this, just slight less contrast on objects. My 9 panel mosaic last summer with a 60mm refractor doing 1 hour per panel I still managed to capture over 160 galaxies or faint stuff (the majority were very very faint but if you know what to look for you can see them). If doing narrowband the non darkness affects you even less and you can image during partial sundown and sunrise (though the contrast quickly changes literally in a handful of minutes).

Edited May 8 by Elp

[Fegato]

Yes I agree - moon is miles worse at a dark site.  I live in a Bortle 3 area - and I can walk home from the pub without a torch in "complete darkness" down little country lanes when the moon is big. It's so bright it casts massive shadows!

So - I much prefer imaging in June with no moon than other times of year with any sort of bright moon. I would normally aim to use about 2.5 hours of nautical twilight - circa  midnight to 2.30. 

As an example - just using ASTAP to measure, it tells me a sub from 9 June 2023 at 01:04 in the morning, altitude 56 degrees, gives an SQM of 21.09 (Bortle 4.5 - rural/suburban).  Best I get is around 21.8.

[pipnina]

I captured this during astro twilight season in 2022 with an 200mm f4 newt and RGB filtered RisingCam571

I only got 3 nights worth, about 3 hours each night centered around true midnight. I feel like it came out quite nicely, however I am about as south as can get in the UK, so someone in scotland is likely to get far less usable time, if any!

This was before blurx/noisex so it's as true a respresentation of the data quality as can be for a processed image I suppose.

[WolfieGlos]

2 hours ago, tomato said:

Interesting, I think I’ll pick a couple of broadband targets over the coming months and try get some decent integration times on them to see how they come out.

You could always gather more data on M101 to match Onikkinen's newt 😁

In all seriousness, look forward to seeing the results.

[tomato]

49 minutes ago, WolfieGlos said:

You could always gather more data on M101 to match Onikkinen's newt 😁

In all seriousness, look forward to seeing the results.

I could indeed. I should keep better records, but I estimate around 40% of the 30 hrs I have to date was captured under a moonlit sky.