Astro Session Workflow - Part 5

Part 5

Data captured over multiple nights

One thing that happens a lot in astrophotography is the collection of data over multiple nights; particularly with targets that are very faint. These targets require hours of data to achieve a suitable image.

So how do we process data that we capture over multiple nights or where multiple filters were used? The easiest way that I have found to do this is to simply use a Siril front-end called ‘Sirilic’ to all the hard work for me.

Sirilic is a Python script (with a GUI front-end) that takes all your image data and calibration frames to create a massive Siril script and then uses Siril to execute that script. The end result is a final image.

Sirilic is not the easiest software to install, and on a non-Windows system you’ll need to dive into the command line and run commands manually. This isn’t for everyone, obviously!

Multisession Method

  • Stack all sessions (separate nights, separate filters) with Sirilic to produce one image made up of all the session’s data
  • Process as per normal with Siril / Starnet++ / Affinity Photo

Sirilic does have an option that will save the final image from each session. This will allow you to tweak each sessions’s data to get the best out of it.

Affinity Photo has a similar capability to stack multiple sessions worth of data using ‘File Groups’ within the astrophotography stacking module.

Mosaics

Mosaic planning is the same as planning for imaging any other Deep Sky Object, but with a few added wrinkles.

I haven’t done that many mosaics, as I’ve usually tried to keep targets small enough to fit inside my field of view. However, I did have a go at creating a 2x2 mosaic of the (in)famous Eta Carina Nebula. Why? Because Eta Carina is large enough that it barely fits into my telescope’s FoV when imaging at the full 420mm focal length. Yes, I could drop the reducer in there and have a wider FoV due to the 357mm focal length. However, I was imaging other targets on the same night and had everything configured to use the 420mm focal length.

Telescopius has a built-in mosaic planner. Once you have entered your telescope and camera information, you can easily see the planned mosaic (see image 1). Telescopius will also provide you with the co-ordinates (RA / DEC) for the center of each panel, so you could use these to program your mount’s control to capture each panel.

Image 1

Telescopius will also provide you with the co-ordinates (RA / DEC) for the centre of each panel, so you could use these to program your mount’s control to capture each panel. There’s also the option to export the co-ordinates as a CSV file for importing into your astrophotography software.

Image 2

KStars does things in the same way. As KStars can retrieve your telescope and camera data from Ekos there’s no risk of entering the wrong information by mistake. Provided you’ve started Ekos and selected the relevant profile, that is.

Image 3

Configuring a mosaic in KStars/Ekos

Before we can capture the data for a mosaic with KStars, we need to go through a few steps first.

  1. In the Ekos ‘Image’ tab (the one with a camera for an icon), we need to setup an image sequence, Here we configure the various exposure settings - gain, offset, the number of frames, and which filter we’re going to use (if any). This has to be saved to your device with a suitable name.

  2. We setup a mosaic within the Mosaic Planner of KStars. I suggest an overlap of 10%. Check all your settings - especially your telescope and camera details before hitting the ‘Create Jobs’ button. Save the mosaic sequence file with a suitable name.

  3. In the Ekos ‘Sequence’ tab (the icon at the far-left that looks sort-of like a book) and load the mosaic sequence file (at the top-right of the window) and be sure to select the image configuration file from step 1. Check that the target is entered in the relevant text box on the left side of the screen.

  4. If required, enable the align / focus / guide check-boxes for each panel. If you’re expecting large temperature changes over the course of hte night, then select the focus check-box. (Yes, your telescope’s focus can change with the temperature.)

  5. Check the ‘Shutdown actions’ across the bottom of the window - this will park your scope and turn off the cooler in your camera when the entire sequence has completed. Or you run out of darkness.

Mosaic processing

To process the mosaic panels, I use Sirilic to stack each panel. You can either do them separately, or you can enable the ‘Intermediate stacking (multi-session)’ check-box in the Project’s properties. Doing so will cause Sirilic to save each session to a separate file. As each panel is a seperate session, this is exactly what we want.

Now we pre-process each panel with Siril. To each panel in turn:

  1. Load the stacked FIT file into Sirilic
  2. From the hamburger menu at top right, select ‘Image Information’ then ‘Image Plate-solver’. This will ensure that your image is the correct orientation and will stack properly. This is vitally important if your mount carried out a Meridian Flip during the session.
  3. Now do a ‘Photometric Colour Calibration’ to get the star colours correct. If you know your image is the wrong orientation (and the plate-solve did not fix it) you can fix this by ensuring that the ‘Flip image if required’ check-box is enabled
  4. Remove the last traces of green noise with the ‘Remove Green Noise’ function
  5. Do a ‘Histogram Transformation’ and use the Auto-stretch button. Doing so will get all the images to a close brightness / contrast
  6. Save the panel as a TIFF

This is where a dedicated panorama stitching tool is required. AutoPanoGiga does a good job with stitching the panels together, as does Affinity Photo. From here, you need to crop the resultant image to a regular square or rectangular shape, and then proceed to process it as you would a regular astro image.

Be aware that panoramas can be very large files. My Eta Carina panorama is somewhere around 70 mega-pixels in size and the Affinity Photo file was hovering around 3.5 gigabytes in size.

Part 4