Uncooled astro cameras do not receive much love these days, but things may soon change, with ZWO set to release the uncooled version of the popular ASI533MC, to expand the range of planetary cameras (all ZWO uncooled cameras) that can be used also for deep sky astrophotography. The list is long and includes my ASI183MC, as well as the ASI183MM, ASI585MC, ASI1600MM, and a few other models.
Cooled Vs Uncooled
There is a reason why cooled cameras are superior to uncooled ones: they actively control the temperature of the sensor. A cooled sensor will produce cleaner images but is not this the reason why cooled cameras are said to be necessary for deep sky astrophotography.
The key words here are stability and reproducibility. With a cooled camera, you can keep the temperature of the sensor stable throughout the whole imaging session, and if you need darks, you can easily get them whenever you have time, thus spending every second of your time capturing starlight, rather than wasting a good part of it shooting darks at your location.
Uncooled Camera For Deep Sky Astrophotography: Yay or Nay?
Uncooled astro cameras can be very effective when it comes to deep sky astrophotography: low read noise, high dynamic range, high quantum efficiency, the absence of the IR/UV cut filter, high near-IR sensitivity, low weight, and full compatibility with astrophotography gear make them far superior to most classic DSLR and mirrorless cameras for everyday photography.
I have already discussed why I strongly believe uncooled astro cameras can be a terrific upgrade for astrophotographers on a budget and presented some analysis from my uncooled ZWO ASI183MC and even ASI224MC. If you are skeptical, I encourage you to have a look at my previous article entitled “Uncooled astro cameras: Should You consider them and How to use them” and to keep an open mind.
The Dark Frame Conundrum
You know the drill: after you have done with your session, while you pack away you should collect some dark frames. Darks are particularly needed if you need to remove the amp glow due to the so-called dark current.
In the words of ZWO, “[…] When it comes to CMOS cameras, “amp glow” is usually not from an amplifier. CMOS sensors are usually “fully integrated” which means that, unlike a CCD, readout electronics are included on the sensor die along with all the pixels themselves. Each sensor has at least one, often many, ADC (analog to digital conversion) and CDS (noise reduction) units on it. There are also other support circuits on the sensor die itself these days…clock generators and power supply regulators and such. These support circuits can generate heat or may even emit NIR light, both of which can cause glows. Additionally, many modern CMOS sensors include high-performance image processing as part of the sensor package, either in the form of on-die processing or a secondary processor that is directly integrated into the sensor by attaching it (often to the reverse side of the sensor.) This processing circuitry can often generate heat that may produce glows.”
Darks are needed to cancel out this amp-glow, and the amount of the amp-glow depends on the sensor temperature, gain, and exposure time. If you don’t have a cooled camera, you can see why it is said dark frames should be collected in the field so that they are taken with the sensor at the same temperature as your light frames.
Technology moves fast, and while the fairly recent ASI183MC (both uncooled and cooled) does suffer from amp glow, the newer ASI533MC PRO is amp glow free. And this technology is filtering down the ZWO product line, reaching the newest planetary cameras like the ASI585MC. If you can dither so that the stacking process will get rid of hot pixels as well as walking noise, you could spare yourself from collecting dark frames, as there are no effects from the dark current that you need to calibrate out.
We all hear people parroting over and over again that for deep sky astrophotography you MUST use (seriously, MUST?) a cooled camera, and that if you can’t afford one better to stay with your old DSLR. But they are, if not completely wrong, at least coming in too strongly.
I believe high-end uncooled cameras are far more performant than the average DSLR one may have and, on top of all the stuff I mentioned before, with an astro camera you can easily get a reading for the sensor temperature.
Ok, and why should I care about the sensor temperature if you can’t match it exactly anyway? Thanks for asking.
You see, it is said you should get your dark frames in the field while packing, to ensure you stay close to the temperature at which your light frames were collected. But if you are taking 3-, 4-, or even 5-minute exposures or longer, in the time you pack you will have collected only a handful of darks.
And if you have very few darks, don’t bother getting them at all, as they may introduce more noise than that they will remove. You need at least 20 darks to do a good calibration: if you are shooting 5-minute long exposures, be prepared to cut 2 hrs down from your time under the stars to collect dark frames. What a waste.
Building A Dark Library For Uncooled Cameras: The Spray & Pray strategy.
If you can get the reading for the sensor temperature embedded in your image file, then you can build a darks library for your uncooled camera. Granted, the process is much more tedious than with a cooled camera, but still feasible.
The temperature of your darks does not need to be matched perfectly that of your lights. After all, the temperature of your light frames will not stay constant thought your imaging session, but will fluctuate following the changes in the ambient temperature.
So, what should we do? Easy: Spray & Pray !
Spray For Darks
Keep an eye on the ambient temperature at different times of the day and periods of the year. Whenever you see big changes in the ambient temperature, like 10ºC, start taking darks.
You can put your camera on your balcony and cycle through your most commonly used combinations of gain and exposure time. You can even put your camera in the fridge when you need to cool down the sensor to about 10ºC or lower. And don’t forget to change the fridge temperature (without spoiling your food). Be creative!
The idea is to cover the most common temperatures you may be exposed to when shooting at your location(s).
Just in case, though, do collect a few darks in the field while packing: better safe than sorry and you can extend your library including temperatures you may not easily reach at home.
Pray for Darks
Now that you have your darks library it is time to calibrate your most recent images.
You can use the fits headers utility to read the sensor temperature for your light frames, calculate its average and finally extract from your dark frames library those darks that match both gain and exposure time of your lights as well as the average temperature of your lights within, say +/- 5ºC.
The amp glow will be calibrated out perfectly and image noise will be greatly reduced the more darks you can use.
How To Be Prepared In The Field
Now, you would obviously like to know, while you are in the field, if you need to take darks or if you have enough of those in your library. For this, I like to plot the temperatures of your darks for each value of exposure and gain, as in the image below.
Since almost all the software I know to control an astro camera show in real time the temperature of the camera sensor, it is enough to have a look at the plots you made to see if the current temperature of the sensor can be later matched with data in your library or not.
Useful Scripts
It helps to script some of the tasks needed for this method to work. I still working on making the script as user friendly and flexible as possible, but at the moment I use three rather basic bash scripts to:
rename the dark frames with the data and time they have been collected;
get the temperature of a series of frames (either darks or lights) to have the temperature evolution and average;
fetch darks from my library that match the specified gain, exposure time, target temperature, and acceptable temperature range.
Once in a while, I use a combination of grep, sort, and AWK to quickly extract the info I need to create the plots showing the temperatures I can match with my current library.
You can download my script for free. Should you find them useful and should you improve them, I would appreciate it if you could send them to me, so that I could share them on the site (by crediting you, of course).
To use the script, you must install the fits headers utility.
ap_rendarks.sh (Download)
This script simply renames your darks with their date and time of creation.
Usage: simply type ./ap_rendarks.sh in the directory of your darks.
After you have renamed the files, move them into the directory containing your dark library.
ap_gettemp.sh (Download)
Usage: ./ap_gettemp.sh to analyze fit files in the current directory
The script also calculates the average temperature and its standard deviation. The name of each fit file, with gain, exposure time, and temperature, as well as the average temperature for the sequence and standard deviation are saved in a log file.
The script will warn you if you are mixing files with different exposure times and/or gain.
I use this script to get the temperature of my light and dark frames.
ap_darksfinder.sh (Download)
This script should stay in the directory with all your darks and it fetches those that satisfy your criteria about exposure time, gain, and temperature range. You can specify the max number of darks to fetch or ask to get all the darks that fit the criteria (up to 1000… you can change this in the script).
Usage: ./ap_darksfinder -e exposure (in seconds) -g gain -t temperature -r temperature range -n number darks or ‘all’
e.g. ./ap_darksfinder -e 300 -g 149 -t 14 -r 2 -n all
will fetch all available darks shot for 300s at gain 149 whose temperature is 14±2ºC. All selected darks will be copied into a new directory with an evocative name. Running the script multiple times with the same parameters creates multiple directories.
Finally, the script will create a log, with the name, exposure time, gain and temperature of the fetched files, average temperature, and other info.
Conclusions
I think with this second article we can consider concluded the discussion around how to squeeze the most out of an uncooled camera. Some people, I’m sure, will complain this is too much work and will be more than happy to cough out the extra cash and get a cooled camera. Others may go down the same road I’m walking right now and use happily an uncooled camera.
At the end of the day, this is for many of us an hobby: as long as we are having fun under the stars, all possible ways to play are good.