by Don Selle
Exposure in daylight photography is a pretty cut and dried subject. You need to set the exposure time required to fully illuminate the scene you are shooting while achieving a specific effect or look in your image.
With your DSLR or smartphone, the exposure is pretty much automatic. The algorithm is built into the camera. Truth be told, the vast majority of picture takers never get outside the boundaries of the camera’s automatic settings.
Not so with astro-imaging. Dedicated astro-cameras have very simple, open-ended controls, there really are no boundaries or standards when it comes to exposure time. Even with a DSLR there is no guide to tell you what settings to use for exposing your individual subframes, nor for how many subframes you should take (total exposure time) to have a well exposed final image.
While astronomy image capture software will allow you to specify all of the parameters for exposing your subframes, and some even allow you to specify fairly complex image capture sequences, to the best of my knowledge, there is no built-in exposure algorithm here either. There are a few tools out there that will help you to calculate things like optimum subframe exposure times, and SharpCap’s Smart Histogram function which can help you to visualize your exposure sequence, but that’s as far as things go today.
All of this adds up making astro-imaging a bit more challenging than your vacation photos, but don’t let that discourage you. Many have gone before you and have left a clear trail to follow. You can become a successful astro-imager!
So how do you come up with a reasonable exposure sequence and total exposure time that will work with your imaging rig and for your target of choice? This article will help you answer this question.
The subject of exposure for astro-imagers can be divided into three main topics and we will address each in a series of three separate article. These are:
- Total Exposure Time required to complete a quality image of various celestial objects
- Determining the optimal sub-exposure time to minimize noise and ensure an efficient cadence
- Exposure of sub-frames with high dynamic range (ie bright stars and dim nebulae) targets
In astro-imaging, we are constantly fighting a battle against noise, both in our targets and in the background sky around them. The primary weapon we have in this battle is exposure time, both optimum subframe exposure time as well as the total number of stacked and combined subframes to increase the total exposure time (sometimes referred to as total integration) of your final image.
In this first article on exposure, we will focus on estimating the total exposure time needed for assembly of a reasonable image where the noise is not too evident, even when the image is viewed at full resolution.
Total Exposure Time - It’s all about SNR!
The image at the top of this article illustrates what we are trying to achieve when choosing the total exposure of our target. On the left is a crop from a single 2-minute subframe shot at f/4. It is obviously very noisy, which makes it look grainy and the noise degrades the sharpness of the image. On the right is a similar crop taken from a stack of 85 2 min subframes or almost 3 hours of total exposure time. The reduction in the noise and increase in sharpness is obvious and dramatic.
The images demonstrate why we stack and combine multiple subframes. It is to increase the SNR of the image by effectively increasing the total exposure time of the finished image. The right-hand image has a higher SNR than the left. In fact, because SNR is proportional to the square root of the number of subframes, combining 85 x 2 min subframes we have increased the SNR of the finished image by slightly more than 9 times.
SNR is the acronym for signal to noise ratio. The signal in the image is the light coming from our target. It’s what we are interested in capturing and turning into an image. Everything else added to your subframes which is not signal is noise. We can remove most of this noise, but not all of it.
Those viewing your astro-image are used to seeing well exposed, low noise daylight images. For your image to be pleasing to look at, it needs to have at minimum a fairly high SNR. Let’s call this the Minimum Required SNR or MRSNR.
When we brighten (contrast stretch) an image, the battle for this MRSNR takes place mostly in the dark and mid tone areas of the image, the parts of the image that are only slightly brighter than the sky background. When we brighten these areas, the noise becomes more obvious, and unless the SNR here is higher than the MRSNR, our viewers will notice the noise.
The sum of the duration of your stacked subframes included in the final image is your total exposure time (or total integration time). Total exposure time is one of the key elements of a good astro-image. In general, the longer your total exposure time, the higher your final SNR will be. You will see an improvement in the darker parts of the image.
If a longer integration time will improve SNR then we should just plan for 5 or 6 hours of exposures, right? Not really, the effect of extending exposure time has diminishing returns. Remember that SNR is proportional to the square root of the number of subframes you stack. So, to double the SNR of a single subframe, you would need to stack 4 subframes, triple the SNR would take 9 subframes etc.
In addition, not all targets are equal. Some are much brighter than others, so the signal coming from them is very strong. It is therefore easy to achieve the RMSNR with an hour or two of total exposure. Why waste more imaging time if the difference in the results are not that great?
Note – those wanting to get into the full details should view this presentation by Dr. Robin Glover, the author of SharpCap. The information in it is the basis for much of these articles on astro-image exposure.
Estimating Total Exposure Time
This means that we need to develop a method for estimating how much exposure time will be required for different targets. Over several years I have developed a workflow to do this. The steps I follow when we are planning an imaging sessions are:
- Start with a few total exposure rules of thumb based on experience. I will provide you with the ones I use as well as specify the details on the imaging set up so that using the concept of exposure value, you can convert the times to match your system.
- Select targets for the imaging session based on their placement in the sky and the rule of thumb total exposure times.
- Find quality images of your selected targets online where details of the equipment used, and exposure times are sufficiently defined. Translate that information to your imaging system using exposure value.
- Modify your imaging plan as necessary with this new information
This approach should get you close to the total exposure time needed to achieve a reasonable image where noise is not obvious. It may turn out that you are off a bit for various reasons, with the worst case scenario being that during processing, you may realize you need to add more exposure time due to actual conditions encountered (seeing, transparency etc.).
But first, a brief backgrounder.
What is Exposure Value (EV)?
Simply put, EV is a measure of how much light hits the imaging sensor of your camera, hence how bright your image will be. What follows is a brief (non-technical) summary of EV. We will use EV to help us estimate the total exposure time we will need for a given target.
Besides exposure time, EV is made up of three additional factors, focal ratio or f number of your optics, sensitivity of your camera, and the bandwidth of any filter used in front of the camera. When we find good images of our intended objects online for which the imager provides equipment and exposure details, we can use these factors to adjust the exposure times to fit our own imaging system. In this way, we can get a good idea of the total exposure time that we will need when using our own equipment.
Without getting into the detailed math, here is how changing a single factor changes the EV:
- Exposure time - EV is proportional to exposure time. Double the exposure time, you double the EV this. In photography is also known as a 1 f-stop change in exposure time (shutter speed).
- Focal ratio – EV is inversely proportional to the log(base 2) of the f number. The lower the f/ratio, the brighter the image will be if exposure time is held constant. The log2 means that to double the EV by changing the f number, you would divide the f number by 1.4 which is the square root of 2.
To convert a 2 hour exposure time of an online image taken with an f/8 telescope to one using your f/5.6 telescope, keeping all else the same, we use the following equation where fa and ta are the f ratio and exposure time of the online image and fb and tb are the focal ratio and exposure time for the same image with your telescope :
tb= ta 〖fb〗^2/〖fa〗^2
tb = 2hrs *((5.6 *5.6)/(8*8))
tb ~= 1hr
Sensitivity – This has everything to do with the specific “quantum efficiency” of the sensor in your camera. For our purposes where we are developing an estimated total exposure time, we will simplify this by using a rule of thumb.
- CCD cameras will have a value of 1
- Most CMOS cameras using older sensor technology will have a value of 1
- CMOS cameras with back illuminated Sony sensors will have a value of ½
- Most DSLRs will also have a value of 1
- DSLRs with back illuminated Sony sensors will have a value of ½
- Gain – is accessible on CMOS cameras and is equivalent to ISO for a DSLR. Technically gain is a factor in calculating EV, however we will ignore it here. For CMOS cameras, you should use the gain setting that results in the lowest read noise for your camera. For DSLRs – most have an ISO sweet spot where low light noise is minimized in the raw image file (do not confuse with low light in camera noise reduction which you should avoid). See note below for more information.
- Filter bandwidth – I have included filter bandwidth for completeness, however, I personally do not use it for estimating total exposure time, due to the fact that I normally would not use exposure time for a narrow band image unless I was planning to do a narrow band image myself. While there is a difference in EV (assuming a constant exposure time) between a 6nm and a 3nm filter.
Be advised however that when adding a dual or tri-band filter to a OSC camera, exposure times will need to be increased significantly. An 8x or 10x increase in exposure time is fairly normal, since the bandwidth of the NB filter is 10% or less of the unfiltered bandwidth of the camera.
- Site Conditions – Most of the imaging I have done has been from the HAS dark site or from much darker sites such as the Texas Star Party in Fort Davis. Sky darkness does make a difference in that, at darker sites, the lower level of light pollution results in subframes with slightly better SNR in the darker areas than would be expected at a more light polluted site. I have typically not corrected my exposure times for this though on the basis that I would end up with a better image this way. If you are Urban imaging, however (which I have no experience doing), you should probably add 50% more total exposure time in addition to correcting for any filters you use. Remember while light pollution can be subtracted from an image, all light has a random component so you will always leave some noise behind when you do the subtraction.
Note – DSLR ISO Invariance – see this video for details: https://www.youtube.com/watch?v=d8QV00mkJW4
Starting Point – Rule of Thumb Exposure Times
Here are a few starting points of total exposure times for several DSOs. They are based on the following conditions:
- f/7 130mm refractor
- Canon DSLR /ASI071 – OSC CMOS camera
- Assumes imaging done at a medium dark site (Bortle 4 to 5) such as at the HAS dark site in Columbus. Due to the effects of “skyfog” noise you should probably double the times for imaging in town (this is just my guess so experiment)
- Cut times in half if your imaging system is f/5, divide by 5 if your imaging system is f/2.8 or faster.
- If you are shooting with a mono camera you should spend at least 50% of your exposure time on the luminance channel with the other 50% split between R, G and B channels. So for a 3 hour total exposure 1 ½ hours should be Luminance with 30 mins each for the color channels. You might also consider increasing the suggested exposure times by about 50%, and spend it on the color channels. This will give you better color detail with a higher SNR.
So here are a few “rule of thumb” exposure times.
1.Globular Clusters – 30m to 1 hr total exposure. Globular clusters range in brightness from 4th to 17thmagnitude, though the majority are around 9th to 11th magnitude. I would plan 30 minutes for the brighter globs and an hour for the dimmer ones. The trick for imaging globs is to keep your subframe exposures short enough so that the brighter stars are not saturated and blown out. Saturated stars do not show their colors. (more on this in part 3 of this article).
2.Open Clusters – 30m to 1 hr total exposure. OCs like globs range in brightness from 4th to 17th magnitude, though the majority are around 9th to 11th so the recommended exposure times are the same. Like globs, the trick for imaging OCs is to keep your subframe exposures short enough so that the brighter stars are not saturated and blown out. Saturated stars do not show their colors. Another big exception comes if the OC is part of a surrounding nebulosity. You may well want to expose for the nebulosity with a separate shorter exposure for
3.Galaxies – 3 hours – less for the few really bright ones like the Andromeda galaxy and M33. The rest of the Messier galaxies are all in the 8th to 10th magnitudes so this total exposure should work well for them as well as NGC galaxies up to 11th or 12th magnitudes. Lots of targets in this range!
4.Bright Nebulae – 1hr to 3 hrs. In my list of nebular targets there are about 25 that have listed magnitudes between 4th and 10th. For the brighter nebular targets like M42, 1 hour exposure time will probably suffice. In fact M42 is a special case, since it’s dynamic range makes it necessary to composite images of varying duration in order to capture the full range of brightness (this is called HDR imaging). For NGC and IC bright nebulae, you should plan on at least 2-3 hours of total exposure. Here you will benefit greatly from your online research. If you (like me) prefer combining OSC and duoband filter images, at least 50% if your total exposure should be with your duoband filter.
5.Reflection Nebulae – 2-3hrs. Most blue reflection nebulae are fairly bright because they are reflecting the light of a nearby star or stars. In fact, capturing the Pleiades will take you less than an hour of total exposure. What generally makes these objects more interesting are the bright nebulae and dust clouds that surround them, which need to be exposed sufficiently in themselves. Avoid over exposing the reflection areas though, or you will have difficulty pulling out the lovely blue color.
6.Dark Nebulae – 1 hr to 3hrs or more depending how extensive you wish to show them. The time required to capture dark nebulae is really based not only on the DN itself, but also what is around and behind it. If it is in front of a bright patch of the Milky Way or a bright emission nebula, expose for these features and you should be able to get the dark dust clouds to show up well with the right processing.
7.Galactic Dust and the Integrated Flux nebulae – Well here I’m going to punt, since I have not done much imaging in this area. I have done a lot of looking online however, and total exposure times here are commonly 8hrs to 11hrs, so patience is a virtue!
A Few Additional Rules of Thumb
When selecting targets to image you should keep a few rules of thumb in mind.
- Select targets based on your experience level. If you are just starting out, you should select targets that are bright and are therefore fairly easy to capture. These for the most part will also be simpler to process and give you some good experience fairly quickly. These targets would include bright open and globular star clusters. Since stars are essentially point sources (spread somewhat by unstable atmosphere) the brighter a star is, the higher its SNR.
Once you have some success here, then try imaging the larger and brighter nebulae and galaxies. Many (me included) started imaging the brighter Messier objects.
2.Plan to image your targets when they are best positioned. Here are a couple rules of thumb that will help you here. Your target should be at least 30 degrees above the horizon (unless your target is very low in the south and there is no other way to image it). You should also your imaging session so that when you start a target, it is rising in the east and is nearing the meridian.
If possible, and your mount does not force a flip, you should attempt to image through the meridian. You need to be aware that depending on the particulars of your imaging rig you’re your target selection, there is a risk that you could run your camera into your tripod or pier si be careful. Imaging through the meridian means that you are exposing your subframes where the light’s path is through the least amount of distorting atmosphere.
3.Find images of your intended targets online. Its always a good idea to have the end in mind when you start a project. Astro-imaging is no different. Once you have your target selected, search online for images others have posted online. Generally you will be able to good examples that include information about the equipment and exposure times used.
4.Experiment with your exposure settings. The timings given here are based on rules of thumb which came from my experience and mistakes. They also do not consider some of the newer AI driven noise reduction programs available which are making their way into the processing workflows of many astro-imagers. As good as these are, still nothing beats good SNR in your images.
In part 2 we will discuss finding your optimum subframe exposure time including how it can improve the overall SNR of your final image.
As always, feel free to contact me with any questions or comments!