Cosmos Capturing: Under the Hood (and Clear Skies)

Eric Paulos
6 min readJul 13, 2021
Getting ready for a night of imaging near sunset

Glad to see so many others are filled with awe and wonder looking and learning about space and the cosmos and love that some of my images can shape your own discovery and curiosity process.

After a few recent social media image posts of some of my astrophotography sessions I had many of you ask for a bit more about how these images were created. Of course please enjoy the images but for those who are curious (warning astronomy nerd stuff below):

TL;DR — accurately capture as many photons as possible!

First I’ve been into astronomy since I was about 10 years old and taking images along the way with a variety of equipment. However, over the past 2–3 years I’ve significantly upgraded my setup. Amateur astronomy also saw an exciting upswing in interest during the pandemic and locating scopes and gear was nearly impossible. Things are a bit better now but some stuff was out of stock for almost a year. If you were discouraged by this check back now as more astro gear is back in stock and available! And please…you don’t need to start anywhere near what I describe below to have fun. I often go outside at night with just a nice set of binoculars and watch stars, planets, satellites, etc.

When I do go out to image deep sky objects, I have a more dediated astrophotography setup. My latest images are shot using a Celestron Edge HD 8" Schmidt-Cassegrain telescope with a 2032mm focal length (and 8" aperture) making it F/10 (see image at top). It’s on an equatorial CGX mount that helps cancel the rotation of the earth to enable taking long exposures without star trails. I’m also using a ZWO ASI533MC Pro dedicated color astronomy CMOS camera cooled to -20C, a ZWO ASI290MM Mini guide camera on a 60mm guide scope, an electronic auto-focuser, and an ASI Air Pro (essentially a specialized Raspberry Pi for dedicated astrophotography) to manage everything. I always select targets before I go out to image for the night so I have a plan. I select objects appropriately positioned high in the sky (to avoid atmospheric disturbances) and up for long periods of time to allow the longest possible imaging opportunities.

The process (again simplifying here) is to assemble and set up all the gear before sunset (takes about 15 min), wait until early evening when stars begin to emerge to perform a polar alignment (take 5–10 min) and is somewhat manual (i.e. computer aided but still involves manually turning knobs, etc). This aligns the axis of the telescope optics to often within a few arcseconds of pointing to the northern celestial pole. The result is your telescope aligned (see below) to cancel the rotation of the earth and keep the stars fixed for imaging.

Illustration of polar alignment concept

Next the primary imaging camera needs to be cooled to around -20C to reduce noise (this is a feature built into the camera) and all of this is controlled by the ASI Air computer that I can connect to via an iPad. Of course, all this can be run from a battery and does not require an internet connection so you can do all this in remote locations.

An autofocus sequence is run to correctly focus the image (2–3 min). Then the mount is programmed to slew to the target I want to image, a computer plate solves to make sure the image is correctly centered. I activate the guide scope which performs real-time multi-star tracking through the guide scope. This process aids the mount in accurately guiding throughout the imaging process. While a pole-aligned equatorial mount will do a great job tracking the object, there needs to be some feedback in the system to make small corrections and make sure the scope basically stays rock steady on the target. This is because I am often taking long exposures, 2, 5, 10 minutes or more requiring absolutely no movement all while perfectly and continuously driving the mount to precisely cancel out the earth’s rotation (sorry to break it to you flat-earthers but we are on a sphere!).

I then program a series of what are called light frames. These are images where you capture the light from the things you are trying to image). This could be 20, 30, 60, or more images with each each say 2–10 min long exposures (it all depends on what you are shooting and overall conditions). The Whirlpool Galaxy image below was more of a test and I only shot 20 images of 2 minutes each (so 40 minutes of overall exposure).

M51 Whirlpool Galaxy 30,000 light years away from 20 stacked 2 minute subs

In a crude sense, you can think of it as a 40-minute long exposure. Between each image, the mount also dithers (slightly moves) the position of the camera so that no two images exactly hit the same pixels on the CMOS camera (again to avoid various hardware artifacts). The clean round stars are indicative of good guiding and focus.

When you are in the middle of this image sequencing process it looks like the image below. The graph in the top left shows the guide camera tracking, the top right progress shows that 6 of 20 images have been taken, the other progress shows 105 seconds of the 120-second exposure (2 min), and you can also see what a single image looks like before all the stacking and processing as well as various camera stats at the bottom such as gain and camera temperature (-14C in this case).

Workflow on ASI Air of the image capturing process during the night

When you are done capturing your light frames you also need to take a number of calibration images to help improve your light images and remove noise. I take several different types. First I take what are called dark frames (maybe 30 or more images with the lens cap on and absolutely no light entering the system — this helps characterize the noise in the system). I also take 100 or so bias frames (a different type of calibration image), and then in the early morning another 100 or so flat frames which are done with an even neutral light. Again glossing over details here. The point is that all of these various calibration frames are used to help improve the overall image by reducing noise and other issues during post-processing.

For post-processing I use a specialized software called PixInsight to perform various operations on the images. I am always working to keep the image authentic and real so this is not to add special “cool filter effects”, rather PixInsight is a tool to help extract the most accurate signal from the noise you always have in the astrophotography imaging process. Most importantly this software allows you to easily align and stack all of the light frames of the object you are imaging. Below is a glimpse into part of the PixInsight process for my M101 light frames. The full process is not something you can easily accomplish in a traditional imaging tool such as Photoshop.

PixInsight astrophotography software capturing workflow on series of images of M101 Pinwheel Galaxy

Sorry for the long-winded explanation but happy to help others. I have so much fun doing this and can often let my equipment image overnight once I have it all set up and programmed. I still have so so much to learn along this process (but that’s what’s fun for me…along with sharing some of these truly incredible views of our cosmos)! Clear skies!

M27 Dumbbell Nebula 1360 light years away and only 9,807 years old from 30 stacked 2 minute subs
M101 Pinwheel Galaxy 85,000 light years away from 20 stacked 2 minute subs



Eric Paulos

Artist • UC Berkeley Professor of Computer Science • Roboticist • Inventor • Instigator