NightCap Camera App in Long Exposure Mode for Phonetography
Ray Taylor
While I’ve used NightCap Camera application over the last 2.5 years with my night vision (NV) device (NVD), Mod 3C as the eyepiece in various telescopes, I have made discoveries and formed some opinions. I thought I’d share what I’ve learned, to help others who might use NightCap with their iPhone to capture astro images. Most of this narrative is a tutorial for those who are just starting to use Long Exposure Mode with NV, but all of it applies whether you use glass eyepieces or NV with a telescope, and it might be helpful even to old hands of NightCap Phonetography. There is an abbreviated checklist at the end which may be helpful, but I encourage you to read this narrative to understand the process. NightCap is specific to iOS devices; if you use Android devices, this will not apply to your equipment.
Using a phone bracket is essential to simplify this process; making these settings becomes complicated while hand holding a phone over an eyepiece in the dark. And, moving a phone, no matter how slight, during a long exposure, ruins an image. I have used a very simple phone bracket for 2.5 years that works fine for my needs, like this one:
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Any phone bracket will simplify and improve your phone imaging.
NightCap works best in Long Exposure Mode, if it is given the time needed to work its magic. A tracking mount provided the single biggest improvement to my iPhone/NightCap imaging. I started using NightCap Long Exposure Mode with my home built, completely manual alt/az mount. Any manual mount limits magnified astro imaging time to a maximum of about 3 seconds, depending where in the sky you are pointed and how much magnification is applied; at 20-30x I was pretty much limited to 1/2 second exposures.
For non-tracking use of NightCap without a telescope or other supplemental magnification, you can use the Rule of 500 to determine a maximum exposure time to avoid obvious star trails. Divide 500 by the focal length of your optics. The f:1.8 lens in my iPhone XR has a 26mm equivalent focal length; 500 divided by 26 = 19 seconds of averaging time (without adding supplemental optical magnification). Using just the iPhone to take a long exposure, you are permitted more time for a non-tracked image. For use with a telescope or other optics in front of my iPhone, the Rule of 500 does not apply.
In early 2018, I purchased an iOptron AZ Pro mount… it is an alt/az GOTO- tracking mount. Normally, these mounts are not used for AP, because tracking accuracy is limited. Traditional AP requires hours of tracking accuracy. But NV makes a subject bright enough to require only seconds to photograph… up to about 40 seconds may be needed. The AZ Pro is accurate to a few minutes, so there is plenty of time to gather the needed photons without showing star trails. By using a tracking mount, you let NightCap do what it is designed to do… take very low-light photos.
NightCap was designed around the limitations that Apple imposes on software developers. I learned that the iPhone camera can be controlled manually without NightCap… if you know how to access the controls. But even accessing those controls does not reach the level of performance that NightCap offers. I have found it to be a powerful imaging tool, even for low light terrestrial photos. This is possible because it averages many photos to eliminate noise that shows up as a grainy appearance otherwise. Averaging is different than stacking. Stacking photos involves a registration point on several photos to align them before they are stacked and averaged. Averaging does not register the photos for alignment; it only averages the images to eliminate noise from higher ISO settings, or, as in NVDs, electronic noise, also called scintillation. When noise appears in different places within several images, it can be eliminated by averaging.
Now I’ll walk through the steps I take for Long Exposure Mode in NightCap. I suggest you open NightCap to follow along. Before you begin, set the screen brightness on your iPhone to a low setting, about 20% to 25% of total available brightness. If your phone screen is set to a brighter level, it will not be showing the true brightness of what the camera is seeing and your images will come out darker than you planned, especially noticeable when you transfer them to a computer. Turning the screen brightness level down alleviates this problem and it is still plenty bright enough in the dark to see what you are doing. Screen brightness can be accessed in the control center (swipe down from the top) or by selecting iPhone Settings, Display and Brightness.
Also, under Display and Brightness, adjust your screen Auto-Lock to at least a 3 minute delay. It’s really frustrating to be taking photos, having made all the adjustments and then the screen goes dark and shuts down because you have it set for a very short Auto-Lock activation. I usually set mine to 3-4 minutes which is a LOT more time than I need to move from subject to subject, taking multiple photos.
Obtaining perfect focus is a little difficult when using supplemental optics like a telescope because both the optical system and the phone camera must be focused together. When attaching your phone to the eyepiece of a telescope, you are using what is termed an “afocal” system, which simply means that one complete optical system (the iPhone) is looking into another complete optical system (the telescope), each having its own objective lens and eyepiece (ocular). I recommend focusing the telescope and the phone at the same time. But initially, set up your optical system/telescope with the NVD or glass eyepiece and focus it. Then apply your phone in its bracket to the eyepiece or NV ocular (you may need to remove the rubber eye guard on the ocular for a firm connection between you phone bracket and the NVD or eyepiece).
After opening NightCap in the main screen, I go to “Settings” (tap the gear icon at the top of the main screen) and usually make just three adjustments. I turn on the shutter release delay and set it to 3 seconds. The delay button is turned on at the timer clock icon found about half way down in the settings menu. Touch it so that it turns green. Then touch the “3s” box. This gives the telescope assembly time to settle vibrations before taking the photo and NightCap provides three beeps to alert that a photo is about to be taken; the volume of the beeping sound is controlled by the volume up/down buttons on the side of the phone. Trust me, touching the shutter release button is enough to cause vibrations, resulting in image blurring. The 3 second shutter delay is adequate for my mount/scope combination. A mount/scope combination that allows for increased vibrations might require the 10s delay. The option to this, is a blue tooth remote shutter release button available for about $8 on-line. I’ve tried this method and found it just a little cumbersome. It adds another device to carry and fumble with in the dark. The 3 second delay built into NightCap is ALWAYS there and works just fine for all of my optical+mount systems.
Then I adjust the Interval Programmer, just below the shutter release delay button. First touch the button on the left side of the settings screen so it turns green, which then allows control of the Interval Programmer. The Interval Programmer performs three distinct functions. It can control the number of images taken, from 1 to infinity (the first set of boxes under the Interval programmer button). This should be set to “1”, otherwise you will end up taking more than one photo with exactly the same settings. This is OK if you are taking several sequential shots like during a lunar eclipse. But for taking a single photo, set this to 1. The second box under Interval Programmer is the EXP (Exposure) box. The sliding button that appears under this box does not actually set the exposure for the image, it sets the number of seconds that the application will take pictures for averaging. So if you are taking a 1/5 second exposure for 10 seconds, you are setting NightCap to take 50 pictures to average… 5 pictures per second for 10 seconds. Touching the EXP box reveals a slider button beneath for adjusting the number of seconds or minutes you desire for averaging. Set the slider button to 10s to begin (more on this later). The third function that is controlled by the Interval Programmer, is the photo delay between multiple photos. NightCap can take several photos equally spaced by adjusting this setting, so that multiple images can be automatically timed, like for a lunar or solar eclipse. But for a single long exposure, this setting should be set to zero. For a single, averaged, long exposure photo in NightCap, only adjust the EXP box, and understand that it controls the length of time in seconds or minutes that photos will be taken and averaged.
I do use the ISO boost at the bottom of the settings screen to give a greater range of ISO settings. With the ISO boost setting off, NightCap Camera app is limited to the default ISO setting in the iPhone camera. My iPhone XR, has a maximum ISO of 2,500. When ISO boost is set to “Low”, the maximum ISO jumps to 5,000. On “High” boost, it increases to 10,000. The newer iPhone 11 will max out at 12,500. My older iPhone 6+ maxed out at 6,000. I do find boost useful, and it works surprising well with the Long Exposure mode, as long as sufficient time is given in the EXP setting of the Interval Programmer. But it’s a balancing act; higher ISO settings require additional averaging to smooth out the noise (graininess) introduced by a high ISO.
This photo shows what my NightCap settings look like:
NOW, you are ready to set the White Balance (WB), ISO, exposure (shutter speed) and focus… one at a time. Before adjusting these functions, it is best to temporarily set the Exposure to about 1/10 second. When the photo screen opens, NightCap will make adjustments automatically. Because the image is usually pretty dark, even with NV, NightCap will slow the exposure (shutter speed) to one of its slowest settings. This means that if the exposure has been set to 1/2 or 1 full second, it will take that long for the screen to re-fresh; it is very difficult to make the appropriate settings when there is such a delay for changes to show on the screen. By setting the exposure to a faster setting of about 1/10th second, adjustments to ISO, Focus or WB appear almost instantly as you move the sliders. I have found that a 1/10th to 1/15th second exposure setting makes the other adjustments a lot easier.
If you are using a white phosphor NV tube, changing the WB has a dramatic effect on the color of the photo. I have found, for my taste, that I prefer a slightly blue tint, which means that I set NightCap WB between 4000 and 4200. Setting it higher will begin to change the tint to green; lower and it turns to a much darker blue. You can experiment with WB to find a tint you prefer. If you do not set the WB, NightCap will set it automatically using default settings from the phone, which sometimes works, and sometimes doesn’t. Often, if left to the automatic setting, it will go to a greenish tint when using NV. The choice is yours though. With glass eyepieces, I do allow NightCap to select the WB setting which comes out much better. WB is set manually by moving a slider button that will appear temporarily at the top of the screen, when you touch the main (photo-taking) screen. Once you have found a WB setting you like for NV photos, remember the number for future manual settings.
Now move to the left side of the screen to adjust the ISO. With a long pass filter on your NVD, a higher ISO is required than going filterless. With a narrow H-a bandpass filter, even more light is suppressed, thus requiring an even higher ISO setting. For now, find a setting that is bright enough so you can focus stars. You will come back to ISO settings while adjusting the Exposure.
Manual focus adjustment is the slider button at the bottom of the main screen in NightCap. This button focuses the phone camera… not the telescope. Precise focus of stars in an astro image is critically important. For a long time, I used 3x reader glasses from the drug store to focus stars on the phone’s screen. But there is an easier way to use manual focus in NightCap. On the very bottom of the main screen, on the right side of the shutter release button, is a separate slider that can be moved to the right or left to zoom. Make sure you’ve got at least one star visible on the screen when you zoom in. Next, use your telescope focuser to focus stars to their smallest diameter while watching the screen. Now, slowly move the focus slider button at the bottom of the phone screen and watch the focus on the screen; stop when it looks perfect with the smallest points of light. You can return to the scope focuser to re-focus it… when it is perfect, lock the focuser draw tube so it will not move accidentally. If you refocus the telescope, be sure to finish by moving the focus slider on the phone screen as a final adjustment. When focus is reached, remember the number which can be found at the top of the screen… it will be needed again if you leave this main screen, for instance, to review an image that has been taken. When perfect manual focus has been achieved, you can zoom back out.
Alternately, you can move the small floating box in the main screen to allow NightCap to automatically adjust focus; just touch it with your finger and slide it over some stars . The red light in the box will turn green when focus has been achieved and is locked. I have found that placing the box on medium sized stars about half way between the center and edge of the image is the best position for the autofocus box. Sometimes, especially if the image is very dim, autofocus will not function properly. I always adjust focus manually to avoid this issue.
Next, you are going to set both the ISO and the exposure. Keep in mind that a lower ISO is always better; a lower ISO produces less noise than a higher ISO. So you might start by setting the longest possible Exposure at 1/3s, 1/2s or 1 second (currently the maximum exposure/shutter speed for different models of iPhones) and then adjust the ISO, so that the image is not too dark or too bright. These two settings work together in a uniform way. Once you have found a suitable brightness for the image, further adjustments can be made by moving both settings in opposite directions to achieve similar results. These are your final settings.
This photo shows the four slider buttons that control WB, ISO, Focus and Exposure at the main screen perimeter. The small floating box near the bottom is the auto-focus box. A readout of your settings is found at the top, under the icons (too small to read in this low res image). The slider adjustments are shown briefly when you touch the screen. Be careful not to move your finger too close to the center of the screen, lest you change a setting that you've already made. In other words, keep your finger close to the edge of the screen where the slider lines appear.
At the bottom of the main screen, just left of the shutter release button, are three red lights followed by FOC, EXP and WB. As you make manual adjustments to these three settings on the main screen, the red lights will change to green. The green color means that those settings are locked and will not change unless you make an additional manual change to them. If you touch the FOC, EXP and WB, a menu box will open showing these same elements but larger. You can lock (green light) or unlock (red light) any of them by touching each element. Close this menu by touching anywhere on the iPhone screen.
Now push the shutter release button at the bottom/center of the main screen. The phone will beep during the 3 second delay and then NightCap will take your photo. During this initial photo, you have 10 seconds (the time you previously set for averaging) to look at the screen, see the settings at the top of the screen and try to remember the ISO and exposure time, the white balance setting and the focus number (between 1-100). Why? Because if you leave this main screen to review the image (by clicking the stacked boxes icon in the upper left corner of the main screen), the four settings (WB, ISO, Focus and Exposure) will be deleted because Long Exposure mode will revert to automatic adjustments. Chris, NightCap’s developer, can’t do anything about that; it’s an Apple issue over which he has no control. So when you return to the main screen after reviewing an image you took, you will need to re-set those four settings. Knowing what they should be makes re-setting them a lot faster.
When the image is being taken, it is important to keep your eye on it as it is being built in NightCap during the averaging process, especially just before it is complete. If you see too much grainy appearance, if the brightness of the object or the background sky is too light, or if you want to change something else in the photo, make the adjustments to WB, ISO, Focus or Exposure, or return to the settings menu to increase averaging time (EXP box) and immediately try again. If the image shows a lot of noise, lowering the ISO can help, but only if you can also slow the exposure to achieve the same image brightness. If you cannot change the ISO/Exp, then return to the settings screen (gear icon) and increase the averaging time in 5 second increments. Toggling between the main screen and the settings screen does not delete your locked settings, so click on the EXP box again and slide the button for another 5 seconds (now 15s total). You might have to reach 20 or 25 seconds before the grainy appearance of high ISOs is managed to your satisfaction. The longest I have needed at ISO 10,000 is 35 seconds (using 1s exposures) with my NVD used as the eyepiece.
Every time you make an adjustment to the optical system, like changing filters or eyepieces in afocal or adding a reducer, flattener or barlow, you must re-focus the telescope and then check or re-adjust the four primary settings of WB, ISO, FOC and EXP. I am used to this process and it goes very quickly for me.
As long as you remain in the main screen or the settings screen of NightCap, and your phone does not shut down in Auto-Lock, your settings will remain locked. If you are using a GOTO mount, you can leave the WB and Focus settings alone and just make minor adjustments to ISO/Exposure for really bright or dim subjects. I have taken up to 34 images of different subjects this way in less than an hour, and not felt rushed. By making a list of what I want to photograph in advance of the session, it goes very fast.
You can mix visual NV and phonetography, but it is a lot slower if you are constantly removing the phone/adapter from the NVD each time you want a visual comparison; and, you will loose your settings when the phone is turned off. Splitting your time between NV visual and photographic use seems much more productive; when taking photos, I take them consecutively until I’m done.
If you want to use your iPhone with NightCap, without night vision… follow the same instructions above in Long Exposure mode, using your glass eyepieces with your phone. Point light sources from clusters, bright double stars and colorful asterisms are usually the best subjects when taken from a dark site. Luna and the sun (with proper filtration) are very bright subjects that mostly do not need the Long Exposure mode, but images of these two subjects do improve with judicious use of manual settings in NightCap. Filters can be used, but any filter reduces light throughput, requiring a longer exposure, higher ISO, or longer averaging time. Making appropriate settings really amounts to a balancing act; the higher the ISO, the more averaging time will be needed to smooth out noise in the image; the longer the exposure time, the longer the averaging time needs to be for an appropriate number of photos to average… to average 40 one-second photos takes 40 seconds, whereas to average 40 one-quarter second photos takes only 10 seconds. Balancing the ISO, Exposure and Averaging time is the key to the Long Exposure Mode in NightCap.
One of the great benefits of using NighttCap’s Long Exposure Mode, is being able to see how the ISO and exposure adjustments you make effect the image you intend to take. After you’ve taken a few images, you will watch the screen as those two adjustments are made. Generally, for the lowest ISO setting, you are going to use the slowest exposure setting. Remember that once you have slowed the exposure setting to 1/2 or 1 second, making any other adjustments will take longer to show up on the phone screen. AND, the slower the exposure is set, the longer the averaging time will need to be for greatest effect. Generally, I want at least 35 exposures to be averaged, which means that the EXP box in the settings menu needs to be set accordingly; 1s exposures will require 35+s of averaging, 1/2s exposure will require 15-20s of averaging; 1/3s exposure will require 12-15s of averaging, and so on. Very bright subjects requiring a faster exposure of 1/10th second at the lowest ISO setting do not require the Long Exposure mode.
