Deep Sky Astrophotography Imaging Part I

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As many readers to my site know, I’ve been doing untracked wide-field astrophotography for a number of years now. There’s nothing quite like staying up all night capturing images of the Milky Way galaxy under the big dark skies of our natural places. As my astrophotography progressed I first decided to invest in a small portable star tracker, the iOptron SkyGuider Pro, which you can read more about here. It didn’t take long for me to find the limits of manually locating faint objects in the night sky with a ballhead and a lot of trial and error. From there I decided to dive in with both feet and the credit card. For the first year of owning the setup I didn’t use it near as much as I had planned to and there was still much I had not yet quite figured out, but the onset of the COVID-19 pandemic and restricted ability to safely travel created an opportunity to really get beyond the initial learning curve of deep sky imaging.

My goal for this article is to explain the choices I made in the main pieces of my imaging setup, why I made those choices, and a few of the lessons I’ve learned along the way – all in hopes of helping others that might be getting started in this field of photography a little extra helping hand getting beyond those early stages with a few less frustrated moments.

The First Parts of the Puzzle

Tracking Mount: After discovering the limitations of the iOptron SkyGuider Pro for imaging at long focal lengths for extended exposure times I start looking for a second tracking mount that would pick up where the iOptron left off. As I had with the iOptron, I turned to the excellent reviews over at as a starting point for my research. I wanted something with a high enough payload capacity to be useful for me as I grew into this new area of photography, with Go-To functionality, and compatibility with a wide range of telesceopes. While I chose an iOptron model for my portible tracker, as I researched it was the Sky-Watcher line of mounts that stood out as the best option. Sky-Watcher makes a wide range of Go-To from their EQ3 with a 12lb capacity to the EQ8 at a whooping 110lb capacity. The EQ3 was immediately ruled out as it didn’t handle anything heavier than the iOptron. The EQ8 was far more than I could justify in both cost and weight to move around from location to location. It really is designed for setups that don’t get moved around often if at all and I didn’t have plans to construct a permanent observatory in the yard.

The EQ6-r Pro ended up being the ‘Goldilocks’ mount in the Sky-Watcher line that was more than enough mount to start with, but I wouldn’t grow out of in a few months time either. With a published payload of 44 pounds, even reducing that by 40% for ideal astrophotography tracking, I could mount way more to this mount than the SkyGuider. The -r stands for Revised, which includes a number of upgrades over the older EQ6 model. It is a belt-driven mount, an upgrade that a large number of EQ6 owners took upon themselves to improve the tracking ability of their own mounts, a built in DSLR shutter release trigger, improved polar alignment adjustments, and a grab handle – a great addition considering the mount head weighs in at over 35 lbs on its own. Sky-Watcher just recently revised this mount again with the addition of a USB port directly on the mount itself, eliminating the need for an adapter cable to connect to a computer.

The Telescope: Already owning a wide variety of telephoto lenses that reached out as far as 600mm, it was initially tempting to purchase a big reflecting telesceope with focal lengths of double that or more. However the more I read and the more I researched one key piece of advice started to come up time and time again – start with a small refactor. For the same reason you don’t get into a front line fighter jet the first day of flight school, really long focal lengths, which usually come with heavier weight, amplify every weakness and flaw in your setup. When learning something new where precision is vital, you don’t want every flaw in that precision amplified to a point where images are ruined and frustrations turn you off the endeavor completely.

So I renewed my research in the options for small refractor telescopes. Even more than with mounts the options are extremely wide. Within refracting telescopes you have a couple sub-divisions to be aware of. Most are either of the doublet design (having two elements in the objective lens construction) or triplet design (having three elements in the objective lens construction). The Triplet design is heavier, more expensive, but does allow for better correction of optical problems such as coma and astigmatisms. This leads to telesceopes with the same focal length and aperture to have a very wide range of price tags. The Orion CT80 can be yours for the cost of a cheap 50mm DSLR lens, where the Sky-Watcher Esprit ED 80mm will run as much as many L-series professional Canon lenses. Both are 400mm focal length f/5 telescopes, but the resulting images, especially in the corners are going to be quite different – just as you’d expect with those two different quality lenses.

Also to be considered when looking at telescopes is the quality of the focuser, you want a wide diameter focuser to prevent vignetting on the largest sensor you plan to use, and that will hold the weight of your camera without any sagging. For imaging you will also need a field flattener that ensures the image coming out the back of the telescope all focuses onto a flat field, important when your imaging sensor is also flat! You’ll also need the correct adapters to fit the desired camera to the telescope, something often referred to as a T-adapter. Different manufactures of telescopes and accessories can have different diameters and threading, so ensuring that everything works together is vital.

If all these small details are starting to make your head spin, you’re not alone. The variety of ways that you can go wrong, get the wrong accessories that might not work with your scope was overwhelming. This is where Sky-Watcher came to the rescue once again. After deciding that my budget could handle aiming for quality Triplet design I started looking at their Esprit line of telescopes. The Esprit 100, a slightly bigger and longer focal length option than the common 80mm refractors, included a host of purpose-matched accessories that I knew would work together. The package included both the flattener as well as the necessary camera adapter for the Canon DSLR cameras I already owned. It also included many accessories that would be necessary if I ever wanted to use the telescope for visual observing. The included focuser also allowed for an unobstructed view for sensors up to a Full Frame camera – meaning I could hook up either my 6D or 5DmkIV to this scope without vignetting!

The Camera: Coming from Landscape photography I already had a number of DSLR cameras in my bag. These included the all around workhorse 5DmkIV to the awesome low-light powerhouse the 6D and the crop-sensor 7DmkII as options to hook up to the telescope with the included adapter. And for the first year of imaging with the telescope (which admittedly was a small number of nights.) that’s exactly what I did. Most DSLR or mirrorless cameras are perfectly adequate for taking images of the night sky through a telescope under dark moonless skies to create amazing images. They do have a couple of significant drawbacks that you want to be aware of.

The first is noise. Oh yes, that old astrophotography nemesis of ours is back again, and while we can reduce the ISO significantly thanks to the robust tracking mount, you are now taking exposures of longer duration. As anyone who has dabbled in single-frame star trails or using extremely dark ND filters for long exposures will know the longer the sensor is on the more heat builds up on the sensor which creates noise. Heat is energy, and so is light, which is what your camera is recording. Since this heat noise is random, taking many exposures will help combat this problem, as well taking what are called Dark Frames to further mask random noise patterns to create clean clear images of the night sky.

The second limitation the simple fact that your DSLR was designed to take photos during the day, lit by daylight. This means while they take in a wide range of visible light, they also purposely block wavelengths of light on the edges of this range. These edge wave lengths cause colors to shift making it difficult to capture accurate colors as we would perceive them in daylight. These design choices become a problem when shooting near the city and the light pollution that comes with it, doesn’t matter if you’re shooting with a 24mm lens or a 600mm telescope, light pollution will wash out the details you are trying to capture. The second issue is that many nebula emit wave lengths on the very edge of what the human eye can see, exactly those that cameras come from the factory to filter out. Both of these conditions reduce the ‘signal’ that is reaching the sensor from already faint night sky targets. Each of these problems can be overcome by using non-standard-issue cameras. The newly released Canon Ra mirrorless for example omits the filter that reduces edge wavelengths, or you can have your current camera modified to remove the filter – doing this will cause color shifts when used during the day! Please read up on the specifics of these modifications prior to sending in your camera for modification!

For both of these reasons, eventually you may start to consider adding a dedicated purpose built astrophotography camera to your setup. These cameras are designed from the ground up for night sky imaging, removing features that are not needed such as SD card slots, the screen, physical shutters, even the battery; instead often have active cooling and sensors designed to capture the light coming from the night sky. Astrophotography cameras come in two flavors – Full Color or Monochrome. A Full Color astrophotography camera will effectively be the same as having a modified DSLR camera with the ability to keep the sensor at or below freezing even when the surrounding air is 80 degrees or more. They are known as One-Shot-Color cameras and the resulting images will look much like they would off any camera you are familiar with. Why then would you opt for a Monochrome camera? Two reasons, first is detail. A color camera is really just a monochrome camera with a colored filter over it. Each adjacent cell has a different colored filter, making up the three primary channels of Red, Green and Blue. The sensor’s hardware then ‘mixes’ the signal from each cell and it’s neighboring ones to decide what ‘color’ that single pixel is. By removing that filter you have just the monochrome sensor ready to accept whatever information is coming at it and does minimal processing on it. By adding a variety of different filters across the entire sensor you can take an image that is all ‘Red’, all ‘Green’, all ‘Blue’ data, and combine those in processing to create the full color image. A lot more work, but a lot more control and a lot more information that goes a long way in improving the end results. You can also use specialty Narrow Band filters that only allow extremely small segments of light through that can effectively combat even the worst light pollution and allow you to image from your backyard, even under the full moon!

This year I decided to add a dedicated astrophotography camera to my setup, mostly because of the COVID-19 pandemic was keeping me stuck at home rather than traveling. After more research and discussions with a number of individuals I know who have far more experience than myself, I decided on the ZWO ASI1600MM-Pro. To this I added a set of ZWO LRGB filters for color imaging, as well as the best quality Narrow Band filters I could afford from Chroma. All of these filters are housed within an electronic filter wheel that allows me to select among the filters for each sequence of images. I’ll go into much more detail on filters in another article very soon.

Basic Deep Sky Imaging Setup

With just the three main pieces outlined above, a good GoTo mount, a telesceope and a DSLR or Mirrorless camera you can start Deep Sky imaging. Polar Alignment is the same for these big mounts as they are for the smaller iOptron SkyGuider Pro or Sky-Watcher Star Adventurer, it’s just that the controls are much bigger and easier to manipulate and the polar scope is a lot easier to look through. In order for the GoTo feature to operate correctly a second alignment step is needed so the mount knows exactly where it’s pointed so it can then slew to any other point of interest in the sky. This ‘Star Alignment’ step is done after Polar Alignment is completed and you tell the mount the current date, time and location information needed for it to know where in space things are currently located. Below is a simple set of steps I go through for setting up for DSLR imaging without a computer.

  1. Unpack and setup Mount
    1. Locate approximate north and ensure that there are no obstructions that would prevent a clear view of Polaris. An inclineometer app on your phone and a compass or compass app should get you in the ballpark.
    2. Level the mount’s tripod, a ‘good’ level helps keep the balance of heavy and expensive gear well distributed, but the final polar alignment will correct for any inaccuracy in this step. I use a small 6″ level that I keep in my gear bag rather than relying on the small bubble level on the Sky-Watcher’s tripod. base.
    3. Install mount to tripod, ensure the RA axis is vertical, and attach counter balance weights. It’s important to install the weights prior to mounting the telescope to ensure becomes top heavy and tips over. If this is your first time setting up a new scope, put all weight at the very bottom, it’s far better for it to be bottom heavy than top heavy when you start to balance it!
    4. Rotate the DEC axis so that the cradle for the telesceope is parallel with the ground. This makes lifting the telescope and tightening it down much more stable and safer to do. I always do this with the moving clamp side of the outward so the scope has a solid side to rest on while being tightened down. Tigthten both RA and DEC clutches so they do not rotate freely. Open up the cradle clamp wide enough to accept the telescope’s base plate.
  2. Unpack and assemble the telescope and camera
    1. I leave the rings and base plate for the telescope assembled, I start by placing that on a flat sturdy surface (table, tailgate to the truck, etc) with the rings open and ready for the telescope to be placed within.
    2. Remove telescope from the case, extend the dew shield and carefully place within the waiting rings, making sure that the base plate and scope focuser are parallel – this helps ensure that each time you setup the camera is in as near the same position as possible. Close rings and tighten down clamp bolts.
    3. Assemble the field flattener with T-Adapter and step-up ring needed to attach it to the telescope. Remove the ‘visual back’ plate that comes installed on the scope and screw on the flattener and adapter assembly. Finally install the camera to the T-Adapter and ensure a positive ‘click’ lock in place as you would expect with a standard lens.
  3. Install Telescope to Mount and Balance
    1. Before moving scope: Ensure that the RA and DEC clutches are tight and will not rotate during installation of the telesceope, and the clamp is open for the base plate. Just in case you forgot while setting up the mount.
    2. Set one side of the base plate’s dove tail into the cradle, ensuring it is sitting inside the cradle’s clamping area, rest the base plate flat within the cradle, and then tighten the clamp down to secure the scope to the mount.
    3. Rotate the DEC axis so that the scope is in the ‘Home’ position with the scope pointing straight forward.
    4. Attach any cables for powering the mount, the camera if an external power source is to be used, anything that will be on the telescope during use.
    5. Balance the Telescope’s RA axis: Loosen the RA clutch so that the RA (and only the RA) axis can rotate. Keep both hands free to steady both the telescope and counter weight side. Depending on how freely the RA axis is without the clutch tightened, if the scope has become top heavy, you do not want the scope to fall freely into the tripod! The aim is for the counter weights to evenly balance the weight of the scope so neither is putting extra strain/work on the mount’s motors. When balanced return the RA axis to vertical and tighten.
    6. Repeat the balancing with the DEC axis, you want the forward/aft weight of the scope and camera to be even. You’ll also want to do this with the telescope already generally in focus, not with the focuser all the way in as this can significantly alter the balance. Be very cautious if loosening the clamp holding your scope to side the scope forward or aft. Return the cradle to the position it was in during installation of the scope and only loosen enough to slide the scope but not fall completely out of the clamp. Keep one hand on the scope at all times. DEC balance needs to be ‘good’ but don’t obsess over it as much as the RA balance. Without auto-guiding, the DEC motor is only in use when slewing to a new location, but having good balance here ensures that your RA balance does not greatly change when the DEC rotates.
  4. Final Setup and Imaging!
    1. Plug in power and ensure all wires/cables going to the mount have enough slack to allow for free movement of the mount around both axis.
    2. Power on mount and setup Time/Date/Location information into the hand controller. Each brand mount is different, so refer to your mount’s manual for steps. Having an app such as PolarFinder or SynscanInit on your phone makes this step easy as you just read the info off your phone and input into the hand controller. This is as far as you can really go before dark…
    3. Polar Alignment. With this bare-bones setup, polar alignment will be manual through the polar scope. Again, depending on your mount’s brand the reticle will be different, but most apps have built in examples of each brand, so take the time to configure it to match what you see. On the Sky-Watcher EQ6 the view through the polar scope is obscured unless the DEC axis is rotated, please review your manual for specific details. The more accurate your polar alignment is the longer duration shots you can take without any movement, so this step is worth getting right. Just remember the view you are looking at in the scope is likely upside down, so left is right and up is down.
    4. Star Alignment. This is easiest done with the assistance of a spotting scope attached to the main scope, but your camera in live view will be a good idea too. Using the hand controller to start the alignment, most brands offer 1, 2 or 3 star alignments. The more stars you use, the more accurate the alignment is and the closer to what you want to image in the sky the mount can get you when directed. I would recommend at minimum a 2-star alignment for this simple setup process. The alignment will ask you to choose a bright easily seen star to slew to, you then want to use the hand controller to nudge the mount’s position until the star is dead-center in the middle of your camera’s view. Having the spotting scope with a wider field of view helps find the star if it’s not visible on the camera’s field of view. Repeat these steps for as many stars as you chose to use.
    5. Focus! This is something I do during star alignment since I already have a good bright star within view. Like focusing for milky way imaging, the name of the game is to get the star as small and ‘tight’ as possible on live view. However there’s a cheap and easy piece of equipment that helps take even that guess work out of it. A Bahtinov mask is placed across the front of the telescope and produces a specific diffraction spike pattern that helps show perfect focus. The goal is for the spike middle horizontal spike to sit perfectly withing the center of the ‘X’ formed by the two diagonal spikes.
    6. Imaging! At this point you’re ready to start taking images. Using the hand controller’s catalog of points of interest choose the desired target you want to image. A little prior planning hopefully has been done and you have an idea of what you want to shoot. Larger nebula such as the Lagoon Nebula in the summer months or the Rosette Nebula or of course Orion in the winter would be logical first targets as they are bright and large enough to look good with larger sensors and refractor telescopes. Start with short high-ISO exposures to get your framing perfect. An intervalometer will be vital to controlling the camera to take a sequence of long exposures without having to jostle the camera and mount. Set the ISO to 800 or maybe 400, exposure for 2 minutes to start once you get your framing. Depending on your scope’s aperture, how good of a polar alignment and balance you have will determine how long you can shoot each frame, but this is a good starting point to work from.
The Lagoon and Trifid Nebula – 2019

Next Steps!

At this point I realize that this is going to need to be multiple part series, I’ve thrown a lot of information at you and don’t want you running for the hills – unless they’re the darkest skies you’ve got. With just the three main pieces of equipment, you can start imaging in what is a very manual process. You still need to be somewhat ‘near by’ the mount to ensure that it does not rotate too far till it comes in contact with the tripod, to tell the mount to slew to a new location if your first target starts to set in the western sky, or a different exposure time is needed. From this manual process, the name of the game is software assistance and automation. Additional tools that make things such as Polar Alignment much easier and precise, auto-guiding setups that further improve your mount’s tracking abilities and taking the need for star alignment out of the picture entirely. These are going to be topics that I will cover and discuss in Part 2 of this series. How many of these additional pieces you choose to add to your deep sky imaging setup is entirely up to you and I look forward to presenting the pros (and occasional cons) to each additional tool soon.

Dawn after a long night of imaging

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