First Attempt At Astro Time-Lapse

March 31, 2024

First Attempt At Astro Time-Lapse

Steve here. This is my first attempt at doing an astro time-lapse. The photo above is one of the frames I captured. The subject is a random patch of Southern sky over our backyard last night, about 30 degrees behind Orion. This was all a learning experience, so I was tickled pink to get anything better than a smear of fuzzy light.

(Click on any of the photos here to see them as a gallery.)

The video below is the resulting 24-fps with no image pre- or post-processing other than converting from RAW to JPEG, using a zoom lens at 55mm: 81 frames exposed at 120 seconds each, for a total imaging session of 162 minutes, 2:42:00. The camera battery ran out and the sky started to cloud over at about the same time.

Here's the full video (watch it 0.25x or keep tapping the spacebar to play/pause repeatedly):

Firsts

There are some firsts to celebrate in this!

First real astrophotography images.
First astro time-lapse.
First time polar-aligning a mount.
First use of this camera (manually focus through the viewfinder, no Bahtinov mask).
First use of this mount.
First work with RAW format images.

There are many opportunities for things to go wrong in setting up and running this, with lots of details despite all the modern technology automating things.

The one I was most worried about was polar alignment. I knew this could be finicky, and a make-or-break issue for tracking. It turned out to go pretty easily, and the results show that I did a good enough job. Combined with the delicacy of focusing on point-sources of light, this could ruin the shots.

The only mistake I made was that after I set the camera dial to M for Manual, I forgot to the set the shutter speed to Bulb (where the mount opens and closes the shutter remotely via electronic cable to make long timed exposures). I had gone back inside to let it run and was sitting there running through the setup in my head when I realized that, and ran back out to set it. Fortunately I only missed a few frames (or I can use them as my calibration dark frames).

F-stop and ISO? Uh, yeah, the camera was apparently set to f/5.6, ISO 400, left from the last time it was on auto.

Conditions

We live in East Boothbay, ME, USA. The natural beauty of coastal Maine is simply magnificent, with gorgeous skies. That beauty is the reason we moved here from the Boston area as a pre-retirement step.

The sky started out absolutely clear, eventually clouding over in the early morning, with temperature around 37 F.

According to https://www.lightpollutionmap.info/, our location is class 4 on the Bortle scale, which is "rural/suburban transition". Especially when facing south, over open ocean. Here's the map centered on our location, where you can see Boston is bright 160 miles to our south:

According to The Deep-sky Imaging Primer, it's possible to use light pollution filters to improve the camera view if you live in a light-polluted area. They filter out the common wavelengths of light used in streetlamps, such as sodium and mercury vapor. Unfortunately, they don't help with broad-spectrum LED lighting.

Another option is to use narrowband filters to collect only light from specific emission spectra produced by various celestial objects. Full-spectrum white light stars won't benefit from them.

Astro Time-Lapse

What exactly is astro time-lapse? It combines regular daylight time-lapse with astrophotography. Daylight time-lapse means taking a photo once every N seconds, then stringing them together to form a video sequence.

Astrophotography means taking long exposures while tracking a target continuously in the sky. That allows time for the image sensor to accumulate photons from the dim target, moving through the sky with it. Multiple images can be "stacked" to accumulate the final image.

Astro time-lapse uses long tracking exposures as well, but instead of continuing to track the target between exposures, it resets to the original position to wait for the next exposure to start. It essentially "ratchets" the target through the sky and across the sensor, one image at a time.

The resulting astro time-lapse video shows the star pattern moving through the sky while foreground objects remain fixed in place. So like a time-lapse of people waking through an area, it's a time-lapse of stars walking through the area. Because each of the frames is a long exposure, they contain many bright stars (and the motion blur of the tracking over the foreground is small enough to be beneath our notice, giving the illusion of fixed objects relative to the frame).

This is a much brighter and more numerous star field than is visible with the human eye. When I pointed my camera and set it, I could see a few dozen stars in that area. The video captured hundreds. That's part of the magic. What's fascinating is that you can see color variation in them, a hint of yellow or orange versus pure white or slightly bluish.

It'll be interesting when I do some image processing to see what shows up, since the human eye isn't sensitive to the wavelengths occurring in many the celestial objects. But the camera has captured them. The other part of the magic is teasing that data out and making it visible.

Equipment

The equipment is based on this video by AstroBackyard, Trevor Jones: My CHEAP Astrophotography Kit for DEEP SPACE! (I upgraded from the Canon Rebel T7 entry-level DLSR kit to a Canon EOS R100 entry-level DSLM kit), all purchased off-the-shelf as specified in that video, nothing designed, built, or modified by me:

Other than saving a few hundred by using an entry-level DSLR with a single lens, this is pretty much the lowest range you can go with new equipment for a removable lens camera. Remember that anything to do with optics is like a BOAT: you can always Break Out Another Thousand to get higher-end equipment with more features, better quality, etc.

Two things to watch for when buying camera items are the lens series (this camera uses the Canon RF series), and the type of remote cable jack (this camera uses the C1 style).

If you've ever held a higher-end Canon DSLM like an R5 with an RF IS STM lens, this R100 with plain RF STM lens (not an IS lens) feels like a toy, but it's still quite capable. In his video, Jones says he wasn't sure how the entry-level T7 kit would perform, but was very impressed with it and found it quite suitable for beginners.

It was based on that judgement that I got the R100. I've ended up completely skipping over the DSLR generation of cameras, going from pocket digital cameras to removable-lens DSLM.

Here's what that looks like in daylight (I used the Sky Adventurer to control the camera shutter, so this doesn't show the Neewer Timer Shutter Release, i.e. intervalometer):

Software

All the software I used is either provided as part of MacOS on my Mac, or downloaded free online. I didn't do any custom algorithms or software.

Control App

I downloaded the Sky-Watcher Star Adventurer Console app for my Android phone (Moon Phase Calendar is another good one to have):

Here's the app main menu (in night mode):
I used the Polar Clock Utility to show me the correct position of Polaris in the mount's integral polar scope. I used that to polar align the mount, adjusting the RA and declination knobs. Note that the app clock shows 0 at the top, but the scope reticle actually has it at the right. Position Polaris at the numerical position on the clock.
I switched the mount on in App mode. That advertised a WiFi access point that I connected the phone to. Then I connected to the mount in the app.
I selected Astro Time-Lapse:
I left Exposure (Sec) at its default 120 seconds.
I left Frame Rate (fps) at 30 (NTSC).
I set Video Time Span (Hr) to 5 hours. At 2 minutes per frame, 30 frames per second, each hour of elapsed time produces 1 second of video, so 5 hours is just under 5 seconds worth, 150 frames. The mount uses some additional time per frame, so ends up calculating 142 frames for 5 hours.
I enabled WiFi Off on Run and hit the Run button.

The rig started its magic just after 9 PM. I brought it back inside at 5:30 AM when I got up to let the dog out. The camera battery was dead (it had been below 40 F overnight, cold enough to shorten the battery life a bit).

Image Conversion

I pulled the SD card and plugged it into a card reader plugged into my Mac. I copied all the photo files over.

I tried to open them with Preview and Photos, intending to combine the images into a video. Neither would open them, because they don't support RAW images. I remembered I had set the storage format to RAW so that I wouldn't lose any image data. Always shoot in RAW and discard data later during processing once you know you don't need it.

I dowloaded GIMP, because it can handle any kind of image, and it can export JPEG. When I tried to open a photo, it reported that it didn't have a RAW loader installed.

I found this tutorial on RAW loading in GIMP: How to Open a Gimp Raw File (RawTherapee Plugin & Darktable). It turns out the RAW image formats are vendor-specific, so require vendor-specific loaders.

I downloaded RawTherapee. I tried to set it to connect it to GIMP, but so far that hasn't worked. It also works as a standalone program, with batch processing.

In RawTherapee:

After some experimentation, I went into File Browser on the upper left and navigated to the RAW file directory:
I hit Ctrl-A to select all the images.
I hit Ctrl-B to add the selected images to the batch queue (see Keyboard Shortcuts).
I went into Queue on the upper left.
I clicked on the start button. It exported all the images to JPEG.

In QuickTime Player:

I selected File > Open Image Sequence... and navigated to the RawTherapee output directory.
I hit Cmd-A to select all the JPEG files and clicked on Choose Media.
I set Frame Rate: to 24 Frames Per Second. Even though I had shot at 30 fps, they can be retimed to stretch out the number of seconds a given number of frames takes to play, producing a slower-motion output.
I clicked on Open. That generated the video.
I clicked on File > Save... to save the video to a .mov file that I then uploaded to YouTube.