Thursday, 13 September 2018

Data Capture Workflow - Astrophotography and the Hypercam 183c

Altair Hypercam 183c Workflow

Hello!  After discussing with various people, the topic of how I go about preparing for and collecting my data for my images, I thought it would be useful to put together a workflow for others to consider using as a basis for their own imaging process.  I haven't gone into too much detail with settings in each of the steps.  These settings will be different for each person and each combination of kit, but it will be a good starting point.  So, before I proceed, this is a list of what I use for my own imaging.
  • SkyWatcher EQ6-R Pro mount.
  • Starwave 80 ED-R telescope.
  • Starwave 50mm guidescope.
  • GPCAM Mono guide camera.
  • Altair Hypercam 183c colour CMOS imaging camera.
  • Altair CCD light pollution filter.
  • LED artists tracing light board.
In terms of software, this is what I use.
  • Sharpcap Pro 3.1
  • Fits Liberator
  • Cartes du Ciel
  • PHD2

Polar Alignment

Every article I read about imaging and using an EQ mount always talks about polar alignment.  As I have found out, there are numerous ways of achieving polar alignment by using all sorts of different gadgets or software to aid the process.  The EQ6-R Pro comes with it's own polar scope, but as yet, I haven't used it.  Instead, I start off by ensuring my guide scope and main scope are aligned fairly well to true north, and the latitude is set correctly for my location (approximately 51.9 degrees north).  It doesn't have to be exact, but personally, I like to get things as accurate as I can.
My personal preference for achieving a good polar alignment is to use Sharpcap.  In Sharpcap Pro, there is an in built polar alignment tool which connects to your guide camera.  You need to have your mount aligned roughly to start off with, give or take 5 degrees or so.  Sharpcap can then plate solve the stars it sees through the guide camera and guide scope, before then giving you directions for adjusting your mount.  I took this screenshot showing what I saw during the process.

Polar alignment routine in Sharpcap Pro

3 Star Alignment

Once the mount is polar aligned, I go through the process of performing a 3 star alignment of the mount.  This process allows the mount to learn where given star are in the night sky, and can be incredibly accurate.  To carry out this alignment, I use an illuminated reticule in the telescope's eyepiece holder.  This reticule given me a crosshair target in the eyepiece meaning I can be sure that the stars used during the 3 Star Alignment routing are perfectly in the centre of the field of view.  
The alignment is carried out following prompts and instructions on the handset of the mount.  Once alignment is complete, the scope itself is ready to use, and it's time to move onto preparing the specific imaging hardware.

Fit and Focus the Hypercam

Once the scope is aligned, I remove the diagonal and reticule eyepiece, fit the 80mm extender, filter and camera and once again, return to Sharpcap.  The next part of my workflow is to get the Hypercam focussed as best I can.  I don't have a motorised auto focuser, so this is an important step that I need to get right first time.  If I need to make significant changes to the focus during the imaging run, I run the risk of making too much of a change and possibly stopping the latter process of stacking and image integration from running smoothly.
To achieve focus, in Sharpcap, I point the scope at a bright star, and then fit a bhatinov mask.  I used the zoom feature in Sharpcap and then turn the focus knob accordingly.  The idea is that the diffraction spikes caused by the mask are equally spaced when good focus is achieved.  The screenshot below shows the exposure time and gain used during the process of focusing, and the equally space diffraction spikes.

Equally spaced diffraction spikes showing good focus.

Choosing a Target

I am now ready to slew the telescope to a chosen target.  To do this, I used planetarium software called Cartes du Ciel.  I could just use the hand controller connected to the telescope, but for ease of use, I prefer the software.  For this particular night, I decided to image the Ring Nebula, or M57.  Once the mount is connected to the laptop and the software is launched, I can select a target and allow the mount to slew the telescope to the target.
M57 selected in Cartes du Ciel.

Framing the Target

Even the most accurate of go to mounts will still require the finest of adjustments if you plan on imaging a target.  This is especially important when imaging either very small, or very large targets.  You want to be sure that you fit as much of the target in the frame, and get it as central as possible.  To do this, I switch back to Sharpcap and adjust the exposure and gain settings to give me some sort of idea of where the target is.  M57 is actually quite bright, so it's fairly straight forward.  However, for dimmer targets, it can take a bit of time to get things framed accurately.  Sharpcap has a crosshair tool to help framing objects accurately.
In this screen shot, you can make out the shape of M57 near the centre of the crosshairs.  You can also see the scope controls available within Sharpcap which allow you to make fine adjustments to frame the target exactly how you want it.

Moving M57 into the centre of the frame using the crosshairs and mount controls in Sharpcap.

PHD2 - Guiding Calibration

So far, all the work has been done to get the mount slewing to targets as accurately as possible, and the target selected, framed and put into focus.  However, for prolonged exposures of more than around 60 seconds, it is necessary to run guiding software to improve the accuracy of the mount. In essence, guiding locks onto a star via the guide camera, tracks it as it moves through the sky and then issues adjustment commands to the mount to keep the guide star in the same location relative to the field of view.  
In PHD2, the guide camera is connected to the software, and as best practice, I use the Guiding Calibration feature in the software to 'teach' it how the mount is currently reacting to commands.  A guide star is selected and then commands are issued to move the star North, South, East and West during which time, PHD2 takes measurements of how much the mount has to move to move the star the required amount of steps.  The process is fully automated once started.  Once complete, guiding starts automatically.  If you want, you can leave it guiding from there.  However, I chose to perform some additional steps.
This screen shot shows the calibration steps being carried out.

PHD2 Calibration steps.

PHD2 - Guiding Assist

There are many settings in PHD2 which can be changed to try and improve performance.  To be perfectly honest, I don't understand most of them, but they all seem to be important.  So, to help me put the correct values in all these places, I use the Guiding Assist tool in PHD2.  Guiding Assist stops any guiding that is taking place and just measures the movement of the selected star.  From these measurements, the software can measure the increments in which guide commands need to be issued, and also if selected, the backlash of the mount.
Guiding Assist is left to run for around 2 minutes so it can make all the measurements it needs to.  Once completed, it then allows you to adopt the suggested settings and automatically enters them into the appropriate fields.
This screenshot shows the measurements that are taken and the different information it provides, and the second screenshot shows the results at the end of the process.  Remember that this changes from night to night and needs to be done every time everything is set up.

PHD2 Guiding Assist in progress.
PHD2 Guiding Assist results.
Once the settings have been adopted, PHD2 then starts guiding.  Before I carry on, I prefer to leave the guiding process run for a few minutes and watch the graph in the software,  Theoretically, the flatter the lines in the graph, the better guiding is working, and the better the mount is set up.

The difference in the graph shows the point when PHD2 Guide Assist stops taking measurements, and then starts guiding.

Starting the Imaging Run

At last, I'm ready to start turning my attention to capturing my images.  There are a final few calibration steps I carry out before starting the image capturing.  Back in the Sharpcap software, I have recently started making use of the Smart Histogram.  The feature analyses the sky conditions, and with some input from me about such things as how long I want the total exposure to come to, and the shortest and longest individual exposures I am willing to use.  For example, if I know I only have 3 hours total to spend, I can tell Smart Histogram that I want to collect 90 minutes worth of data in total.  Smart Histogram is then able to give me the optimum gain and exposure length to use for the parameters I set.  Again, this process can take a bit of time because Sharpcap actually takes a range of individual frames of differing lengths and gains to build up it's graph.  The remaining time left out of the 3 hours I can use to collect calibration frames for use during image processing.
This screenshot shows the histogram analysis in action.  Once the results are displayed, you can input them into Sharpcap ready to start the imaging run.

Smart Histogram calibration steps.

The Capture

Now it's time to start the data capture process.  The mount is set up, the software is configured, guiding is running and I now know the optimum settings to apply to the Hypercam in Sharpcap.  All I need to do is set the imaging run off within Sharcap.  I can set the imaging run to either take a fixed number of exposures, to run for a certain total amount of time, or take pre-configured numbers of exposures.  Using the information from the Smart Histogram tool, I usually know by this point how long or how many frames I need to take.  It's just a question of applying the settings and setting the process off.
Each frame is displayed in Sharpcap so you can see what's going on.  You can also see how much time is left in the run and how many frames have been taken.

Hypercam settings used for imaging M57
It's important to keep an eye on the PHD2 graph throughout the imaging run.  Normally, just keeping an eye on the graph is sufficient.  However, PHD2 does give an audio and visual alarm if it loses contact with the selected guide start because of cloud or dew.  To monitor this, I tend to have both the PHD2 window and the Sharpcap window open at the same time.

Monitoring the data capture of M52, and PHD2's guiding performance.

Capturing Dark Calibration Frames.

Once the main imaging run is complete, and all the light data is collected, my last job of the night is to collect the Dark calibration frames.  These are simply additional images taken using exactly the same parameters as the lights at the same focus and camera orientation.  The only difference is having the cap on the end of the telescope.
This screenshot is of Sharpcap taking the Dark calibration frames.

Collecting Dark Calibration frames.

Flats and Fits Liberator

This is usually the last task of the night.  The following morning, I will then take my flat images, but they can be done at anytime providing the focus and camera orientation are maintained throughout.  This is really important, so usually, I take the scope off the mount at night, put it onto a flat surface and don't touch anything until the flat frames have been acquired.
From reading up on various forums, and asking questions about obtaining flat frames, I have settled on the routine of:
  • Stretching a white T shirt over the end of the telescope.
  • Use an LED tracing/drawing board to give an even light covering over the end of the scope.
When I first got my Hypercam 183c, I joined the Altair Astro Google Group.  On there, I made contact with RobinG (who I think is the writer of the Sharpcap software), and he offered the following piece of information on taking flat files with the Hypercam 183c, and how to work out the correct exposure etc.

Mean flat values to aim for in Fits Liberator
Taking this on board, I then set about experimenting taking single flat files and opening them in Fits Liberator until I achieved the desired values.  It's important to note that I leave the gain settings the same, and purely work with exposure length.  It's also important to note that the optical train remains exactly the same.  In other words, if all the images are taken using a certain filter, then that filter remains in place for the flats.
This is a screen shot of Fits Liberator showing where to find the mean values to aim for.

Fits liberator is free software available to download and use.  The image statistics highlighted in yellow is where to look for your flat files values.

So that's my workflow for a typical night's imaging.  It seemed quite daunting at first, but once I got used to it, I found that I can get through everything in about 30 minutes.  Many of these steps will be redundant in a permanent set up in an observatory.  That would be ideal.
Anyway, thanks for reading, and I hope you find it useful.

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