Of all the new stuff in the Seestar app update this month, the one I keep coming back to isn't the flashy one. RGB curves are great. Aircraft-trail removal is great (Atlanta skies, I'm looking at you). Comet nucleus stacking has people in the comments going nuts. But the feature that's going to quietly make every single image a little cleaner from here on out is the new Dark Library.
ZWO posted an infographic about it this week and it's been one of the most-shared Seestar posts on Instagram since. I figured it deserved more than one graphic. So here's the plain-English version: what dark frames are, what changed, and what I'd do right now as the nights get colder.
First, What Is a Dark Frame? 🔭
Every camera sensor makes a little noise of its own, even with no light hitting it. You get hot pixels (those single bright red, green or blue dots), a faint thermal glow, and general grain. And here's the part that matters: it gets worse as the sensor warms up and the longer each exposure runs.
A dark frame is just a photo taken with the light blocked off. It captures that sensor noise and nothing else. Subtract it from your real images (your "light frames") and a big chunk of the junk goes with it. ZWO's graphic shows it as simple math: light frame minus dark frame equals a cleaner picture.
The catch is that a dark only works well when it matches the light frame: same exposure length, same gain, and as close to the same sensor temperature as you can get. A dark shot on a warm summer night doesn't line up with a frosty November session, and the subtraction gets sloppy.
What Actually Changed in V3.4
Seestars have always had a built-in dark filter, which is why your scope does its little calibration routine before imaging. What's new is that it now keeps those darks and organizes them into a library instead of just using them for one night.
According to ZWO's breakdown, three things are happening behind the scenes:
It matches on every single frame. While you're imaging, the Seestar checks the library at each captured frame and picks the master dark that best fits that frame's exposure and temperature. That matters because a sensor doesn't sit at one temperature all night. It drifts as the air cools down.
The library grows on its own. Darks get sorted by exposure setting and build up over time. The more nights you shoot, and the more different temperatures you shoot in, the better the matches get. ZWO literally calls it "the more you observe, the better it gets."
You can get at the master darks. For the first time, you can inspect them, save them, and pull them into your own editing software.
And it's not just the new Pro models. One Seestar S50 owner on GitHub reported that after updating to firmware 9.75 and app V3.4.1 (released October 19), his original S50 started creating a "darklibrary" folder, adding a dark .fit file on each startup. So if you have the original S50 like I do, update the app and the firmware.
How to Find Your Dark Library
In the Seestar app, go to:
That's where your master darks live, sorted by exposure settings. Poke around after a couple of sessions and you'll see it filling in.
The one big rule: don't delete them. ZWO's graphic has a giant yellow warning box about it. Those master darks are your scope's history of temperatures. Wipe them to free up space and you're back to square one, with poorer matching until it rebuilds. If storage is tight, copy them off the scope (more on that below). Just don't delete them from the scope itself.
"The Dark Library doesn't get better because you pressed a button. It gets better because you went out and shot on a lot of different nights."
Why "Before Winter" Is the Whole Point
This is the part I think most people will miss. The library is built from the temperatures you've actually shot in. If you've been imaging through a Georgia summer and early fall, your library is full of warm-night darks. The first truly cold night is when it has the least to work with.
So the move is simple: get out on as many of these transition nights as you can, even short sessions, as temperatures slide from the 60s down into the 30s. Every new temperature you shoot in gives the scope another close match to pull from when it's 28 degrees in January.
Two honest caveats. ZWO hasn't published exactly how finely it splits temperatures, or how many darks go into each master. So I can't tell you "you need X nights." And with my S50 Pro hosted remotely at Astro Peak in Utah, I'll likely see a very different temperature spread out there than in my Atlanta backyard.
A cheap way to see what you're dealing with is to log the air temperature at the start and end of each session. A little digital thermometer/hygrometer does the job, and it also tells you when you're getting close to the dew point.
Using the Master Darks in Your Own Software
If you process outside the app, this is the big win. For years, the standard advice for stacking Seestar .fit files in Siril or anywhere else has been "there are no matching darks, so skip calibration." Now there are.
The basic approach in Siril-style workflows is to copy the master dark into a darks folder next to your lights folder and let your script calibrate as usual. The maintainer of the popular naztronaut Siril scripts confirmed on GitHub this week that the scripts just use whatever darks you give them, so a single master dark in that folder works fine. Make sure the master's exposure length matches the lights you're stacking.
Personally, most of my processing runs through StackingWizard and AstroWizard, with the RC Astro tools on top.
One heads-up from general astrophotography practice: the classic advice for building a dark library by hand is at least 15-20 darks per set, and many imagers reuse a library for up to a year. The Seestar handles the count for you, but the "up to a year" part is a good reminder that this library is a long-term thing. That makes it even more worth backing up.
Gear That Helps
Nothing here is required. The Dark Library is free and automatic. These are just the small things that make it work better.
ThermoPro TP50 Digital Thermometer/Hygrometer. The cheapest way to know what temperature range you're actually building your library in, plus humidity so you see dew coming. 4.6 stars from over 155,000 ratings.
Samsung T7 Portable SSD, 1TB. For backing up your master darks (and the rest of your .fit files) off the scope without deleting anything on it. Fast, tough, and it's what I'd trust for a library I want to keep for a year.
The Bottom Line 🌌
The Dark Library isn't the update feature that'll get the most screenshots, but I think it'll have the longest-lasting effect on image quality. Update the app and firmware, find the library under Me › Device Management › Sensor Calibration, leave those master darks alone, and get out on as many fall nights as you can before the real cold hits. Your January images will thank you.
Sources: Seestar official: "Meet the New Dark Library" infographic · Seestar App V3.4.0 announcement · ZWO Seestar downloads (V3.4.1) · naztronaut/siril-scripts issue #122 · Galactic Hunter: Building a Dark Library (checked Oct 23, 2026)