The overall system schematic (Click here for full sized image)
Taking great images of the night sky is difficult. There are many obstacles to overcome, any one of which can trip up the unwary and result in a poor image, or worse still, no image at all!
Alas, there is no off-the-shelf system that can be bought to allow one to start imaging.
Instead, one has to diligently research equipment and software to make sure that they can all be connected together into a symbiotic whole that will allow great pictures to be produced.
The above diagram shows a simplified schematic of my imaging system as it is/was on June 2018. Much like hiking (my other hobby), every individual will have their own unique setup, and much like hiking, its amazing how much kit is involved!
Working out what equipment and software I needed and then getting it all connected so that it all actually worked was a gratifying experience. This is your one chance to become a systems engineer!
Rather than take you through the large and rather complicated diagram above, I will, instead, take you on a tour of each sub-system…
Power distribution (Click here for full sized image)
Power is pretty important and a major consideration that one must take into account when putting one’s system together.
At the core of my power strategy are two Sky-Watcher 12v 17AH power stations. Both utilise lead-acid batteries that do need monthly topping up. If you forget, you can permanently damage them.
The big problem that I faced was that these power units only have two main power sockets each. I needed to find a way to get everything powered by just these four sockets. In addition, I would need enough power to allow imaging to be conducted all through the night.
One of the two Sky-Watcher power packs. The two main power points are visible in the centre.
It was the socket limitation that drove me toward the purchase of the Kendrick DigiFire-12 for dew control as this unit also adds a further two power distribution points. One of these points is used to distribute power directly to the electronic focuser.
The Main Signal Flow
The main camera signal flow (Click here for full sized image)
This part of the system is what it’s all about. Its primary concern is to pick up star light and then ultimately turn that light into an image!
At the heart of this system is the ZWO ASI 1600mm Pro camera. It is a CMOS based mono camera with a resolution of 4656×3520. Colour is provided by taking mono images through one of the many colour filters installed in the system. These mono images are then combined to produce a single full colour image.
The camera features a two stage cooler that allows it to be cooled down to -45 degrees below ambient. Temperature control is important and one of the biggest differences between an astronomical camera and a terrestrial camera.
The main camera, the star of the show. This is a rare view of the 17.7×13.4mm MN34230 CMOS sensor.
All electronics produce signal noise. In the case of imaging sensors, this noise tends to be proportional to the ambient temperature. For most cameras this isn’t an issue as they are only ‘on’ for a fraction of a second whilst the shutter is being engaged. This duration is short enough that sensor noise does not present a problem.
In addition, terrestrial cameras tend to take photos of well lit subjects. These subjects present a very high signal to noise ratio which is unlikely to be affected by sensor noise.
However, in Astronomy, we take pictures over many hours, at targets that are very faint. Without temperature control, the images would be swamped with sensor noise. This is why astronomical cameras are cooled down to such very low temperatures.
A big part of this sub-system is the optical train. Getting this right took a lot of research and many discussions on various forums. The two key issues that have to be addressed are:
- The camera must be exactly 55mm away from the field flattener (yes, the manual is wrong! 😛 ).
- Both the main camera and the guide camera must be able to achieve focus at the same time!
For the equipment that I use there is only one configuration that will work. Given the myriad of parts that one gets with the various items of kit, it can be exceedingly difficult to work out how it should all fit together.
To save you the time and effort in finding out, here is the configuration that I use:
The parts breakdown for the correct camera configuration to suit most standard back-focus requirements.
Sequence Generator Pro is the software that is used to command the camera – and pretty much most of the rest of the system! It determines exposure time, the selected filter and the camera temperature.
Images produced from an astronomical camera don’t really show much and must be processed. This processing extracts what little detail there is, reduces the noise and then combines and calibrates the many sub-exposures to produce a single full colour image.
Image processing is a black art that takes a fair bit of time and experience to do well. The last image that I processed (the Leo Triplet) took me an entire day – and I still don’t think I did it justice! Processing skill will make or break an image and is a very technical discipline. One that I will leave the discussion for another day 🙂
All I’ll say right now is that PixInsight is my tool of choice, with Affinity Photo being used for any final touching up.
The auto-guiding system
We live on a planet that spins rather fast. The end result is that the stars appear to whizz by when viewed at any kind of magnification. To get around this the whole system has to be polar aligned with the Earth’s rotational axis.
The problem is that polar alignments are never exact, nor are the engineering tolerances of a mount good enough to allow for long trail-free images.
To get around this issue, a second camera is deployed whose sole job is to keep an eye on just one star and its relative position.
If the star moves, the system will command the mount to recenter the star back to its original position – in this way the system is always precisely centred on the targeted object.
In my system, the star light comes in through the telescope, and is then split off to go to two separate cameras. The light splitting is achieved by a prism in a device called the Off-Axis-Guider – or OAG for short.
Most of the light goes through to the main imaging camera, with the rest going to the guide camera – in my case a ZWO ASI 290 mini.
The image from the guide camera is then analysed by a piece of software called PHD2. This software is setup to be locked onto a star – the guide-star. It is looking for any relative movement of that star. If it detects any, it sends a correction signal to the mount to recenter the star.
PHD2 in action! The top pane shows the locked on star, the lower pane shows the guiding corrections sent to the mount.
Autoguiding is a complex subject, and like image processing it’s a bit of a black art. However, to complicate things further, stars can appear to move around due to atmospheric turbulence. The trick is to make the autoguider sensitive enough that it detects a drifting star, but not so sensitive that it ends up chasing atmospheric seeing.
The pointing system
You are not going to be able to image anything if you cannot point the telescope at the object being imaged!
My pointing system has three modes of operation:
- Manual control with an X-Box joystick controller
- Slew-to-object-on-a-map commands sent by Cartes Du Ciel – a planetarium program
- Closed loop slewing via Sequence Generator Pro.
Of the three pointing modes, closed loop slewing is the primary one. The other two tend to be used for system calibration purposes that will be discussed in a future post.
Closed loop slewing is a marvel of modern engineering.
The way it works, is that Sequence Generator Pro (SGP) takes a picture of the night sky with the ZWO ASI1600 main camera.
SGP then sends that picture to Planewave PlateSolve 2. This piece of software identifies every star and object in the image. Once it has worked its magic, it knows precisely where the telescope is pointing. This pointing data is then sent back to SGP.
SGP then commands the mount to point at the object of interest, as per the loaded imaging sequence. Once the mount has completed its slew, another picture is taken and once again, it is analysed. Any deviations from the original commanded location are then sent back as corrections to the mount.
This process keeps repeating until the object in question is perfectly centred.
All mount commands are routed via EQ-Mod which is the software interface to the mount.
The focusing system
Focusing is extremely difficult to do manually. In fact the tolerances are so tight (we are talking microns here) that a human couldn’t possibly make the small adjustments required.
Unlike terrestrial images, stars really highlight poor focusing. That’s because they are points of light and it’s very easy for our eyes to detect if these points aren’t razor sharp.
As with other aspects of my system, focusing is controlled by Sequence Generator Pro (SGP). I’ve programmed it to perform an auto focus at the start of a session, after a filter change and after any large ambient temperature variations.
The way it works is that SGP will take a series of images with the main camera and then make a number of adjustments to the focuser. These adjustments will bring the image into focus and then back out of focus.
Once SGP has done this, it has enough data to calculate the precise focus point. It then commands the focuser to move to that point via a connection through the Lakeside hand controller (The hand controller can also be used to manually command the focuser too).
The downside to this, is that one must already be reasonably well focused prior to a SGP focusing run. Luckily for me, I have stored the rough in-focus position in the Lakeside focuser’s unpark command. This is initiated from its hand controller.
Setting the initial focus is normally one of the first things that I do after setting up the telescope as it also allows me to get it balanced properly for the night ahead.
The filter control system
My system takes its images through a black and white camera as these are the most sensitive and they provide the greatest flexibility for gathering data.
To synthesize a colour image I need to take images through various colour filters and then combine those images.
After a lot of research I chose the Baader 36mm filter set. These are of a reasonable quality, not too expensive and large enough not to cut off the light cone through the telescope. In addition, all the filters are parfocal – that is they all reach focus at the same point.
The system currently supports both narrow and wide band imaging.
For wideband imaging the system uses Luminance, Red, Green and Blue filters. Wideband imaging tends to get used for galaxies and star clusters and produces your standard colour images.
For narrowband, the system uses a Hydrogen Alpha filter (Ha), an Oxygen III filter (OIII) and a Sulfer II (SII) filter. Each of these filters are tuned to the common wavelengths that are emitted by nebula. Pictures taken through them are ‘false colour’ pictures and tend to use the common ‘Hubble Pallette’ to represent these wavelengths in a pleasing way.
As with other parts of the system, Sequence Generator Pro (SGP) also controls the ZWO EFW filter wheel. In this case it will follow an imaging plan within a pre-made sequence:
A typical imaging sequence
In the above sequence, I have taken 30 + 10 x 120 sec luminance photos and 10 x 120 sec photos through each of the colour filters. Each object will have it’s own plan written for it. I’ll cover detailed planning in another post.
The dew control system
Dew control is pretty important and something that some imagers neglect. I know I used to with my previous system 🙂
We need dew control as there is a high likelihood that dew will form on the main telescope lens (known as the objective) during an extended imaging session. If left unchecked, it can affect the quality of the images and even damage the main lens.
To alleviate this, a Velcro heater band is placed around the telescope, very near to the objective lens.
The Velcro dew heater band can be seen around the objective.
My system has two temperature sensors, one that just hangs down for ambient readings and another that’s under the Velcro strip to measure the lens temperature.
Prior to system switch on, I program the Kendrick DigiFire-12 to keep the lens a fixed number of degrees above the ambient temperature. The actual value varies from night to night and is dependent on dewing conditions.
The polar alignment system
Achieving a good polar alignment is critical to allow the system to take images without trailed stars.
Trailed stars occur because of the Earth’s rotation. We don’t normally think of the Earth as rotating particularly fast, but if you were to view the stars under any kind of magnification, you would be very surprised at how quickly they zip by!
On my previous imaging runs I used to use the polar scope running through the mount, but for whatever reason, I have never managed to get a satisfactory alignment with it.
The EQ6-R mount’s polar scope
To resolve this I have now purchased the QHY Pole-Master which is a camera that fits to the front of the mount:
Farewell Polar Scope!
This camera replaces the polar scope and with the aid of the Pole-Master software, it will allow me to obtain very quick and accurate alignments. Once alignment is achieved the camera is disconnected and removed.
The Pole-Master hasn’t been used in anger yet, but I’m expecting great things of it!
The planning work flow
Planning what to image can take time – especially if you do not have the right software.
I normally start my planning sessions by browsing the internet looking for interesting astronomical objects. Once I find one, I use SkyTools 3 to tell me if it is observable from my location and to tell me the optimum time of year to image that object:
Checking out M101’s visibility over the year.
In general, you want to image an object when it is at its highest elevation for the year as this can vastly decrease the amount of atmosphere that the telescope has to peer through. Less atmosphere means higher contrast and less image distortion due to turbulence.
For the M101 galaxy example above, SkyTools 3 is telling me that the optimum time to image M101 is between May and June, but anything over the green line is good.
Once I have determined that an object will be visible from my location, the next thing I do is check out its size in relation to the camera sensor and telescope combination:
I use The Sky-X for this. It allows me to take a simulated look at how the object will appear in the system. From this I can determine which of my two telescopes is best suited to fully frame the object.
Once I have identified the equipment that I will use, I then write an imaging sequence plan in Sequence Generator Pro:
The M101 imaging sequence plan.
The plan includes absolutely everything, ranging from filters used, the number of exposures and their durations, when to focus and also where to look. These plans have to take a lot of factors into consideration and will be discussed in a future blog post.
Once programed, one simply hits the play button and in theory, the system will complete an entire imaging run fully autonomously with no human interaction required.
That’s it for the grand tour of my system.
Hopefully it will help guide others with regard to some of the considerations that they must take into account when building their own systems.
It also highlights how complex these systems are. In fact, it’s a minor miracle that these systems work at all given the number of disparate hardware and software components that go into their making!
Choosing and getting all of these parts to work well together can prove to be challenging. But hey, that’s most of the fun! 😛