Astro Session Workflow - Part 1

Why I’m writing this

About a year ago I gave a presentation on astrophotography to my team at work. It was a “Get to know your co-workers better” session where we explained one of our hobbies over the course of lunch. After working on it for a couple of weeks, I ended up with a presentation of about 35 slides and talked for just under an hour.

When I have a new image (or two) ready, I share them with the team. I don’t think they quite understand just how much work goes on ‘behind the scenes’ to arrive at the final image, and that I merely point the camera at the sky and out comes an image.

Now, I’m getting the (very faint) itch to do another presentation showing what’s involved with capturing an image I’m happy with; and to provide them an idea of just how many things can go wrong.

I’ve also talked to quite a few people just getting started in astrophotography. This is a way for me to straighten out all the ideas in my head and to make sure I’m giving good advice. I won’t be covering topics I have no experience with, such as using a monochrome camera, imaging the planets or the sun, and certainly nothing about running an automated observatory.

So let’s get started.

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Part 1 - First Steps

Target Selection

Before we can start capturing any images we need to carry out a reasonable amount of preparation. The first step of which is to decide exactly what object we are going to image. There are plenty of books and catalogues available to peruse, such as the Messier Catalogue compiled by French comet-hunter Charles Messier in the 18th century.

I tend to flip through my copy of ‘Imaging the Southern Sky’ by Stephen Chadwich and Ian Cooper, looking for something that is visually appealling and visible with the equipment I have.

There is also the website ’telescopius.com’ with allows you to browse through multiple lists of targets, all searchable by size, magnitude, rise time, and more. Once you have selected a potential target, Telescopius will display further information and images of it, a small chart with the direction of the target (from your configured location), it’s maximum altitude and when it passes the meridian. You can also enter your telescope and camera details to see an approximation of how large the target will be in your field of view.

When I’m imaging from home, my view to the North is restricted by buildings and aiming West I start to hit the light pollution dome from Melbourne. So I try to find a target that will be reasonably high in the sky and to the South as much as possible.

Of course, there is also the chance that an interesting looking target will not be visible from your location at the specified time. A prime example of this is the Orion Nebula. For most of the year it’s below the horizon for me, only appearing in late November (shortly before sunrise) and disappearing again in March / April (shortly after sunset). This chart from Telescopius demonstrated this clearly.

I also use a free planetarium program called ‘Stellarium’ (and it’s online equivalent at ‘stellarium-web.org’) to give me an idea of how high in the sky the target will be. It also gives me an additional idea of how large the target will by in my field of view. Apparently you can also use Stellarium to control a telescope if you have the right hardware configured. I’ve never tried this, so cannot comment on how well it works.

Environmental factors

This is where knowledge of your imaging location comes into effect - knowing where the nearest towns or cities are so that you can avoid the glowing domes of light pollution, knowing whether there are trees or buildings that will block your view in particular directions.

Harking back to my imaging location, I am in suburbia so I have light pollution all around (but worse to the West) so I try to image targets that will be above 45 degrees elevation. (There are other reasons for this but I’m going to skip over them for the moment.) For what it’s worth, my location is about a Bortle 6 rating.

In comparison, the ASV’s dark sky site is about a Bortle 2 rating (much better than home!) although we still have to contend with the light pollution domes of Melbourne and Bendigo.

Weather

Of course, it makes no sense to get all ready for a night of imaging and be stopped by clouds or rain. I use 4 source of information for determining whether it will be a good night for imaging.

The three online tools are ‘yr.no’, ‘clearoutside.com’, and ‘cloudfreenight.com’. None of these are perfect and I have had several cases where they all disagree with each other as to whether I have clouds or clear skies.

The final source of weather forecasting are my eyes. I have had nights were I do not bother setting up because all 3 online resources say it’s cloudy (or will be cloudy later in the night) and when I look outside it’s perfectly clear. I’ve also had the opposite happen!

Nothing is more frustrating than driving for 2 hours to a darker location on the premise that it’s going to be clear (according to all the forecasts) and to have clouds roll in. Or worse yet, rain.

Filters

I was a bit conflicted as to where to put this section. It’s part of the hardware setup (next section down), but the filter selection is determined by the target you are imaging.

For targets that are emitting light across the visible entire spectrum, such as stars, galaxies or reflection nebulae, then it makes sense to use either no filter, or a filter that will cut out spurious IR (Infrared) or UV (Ultraviolet) light as much as possible. Doing so will help to reduce halos or ‘blooming’ around brighter stars. Halos and blooming can be reduced or removed in post-processing but it’s best to reduce them as much as possible when capturing the images.

Emission nebulae, on the other hand, benefit from the user of filters to allow only specific wavelengths of light through to the camera sensor. The three most common filters are Hydrogen-alpha (656 nanometres), Sii (Sulphur-2, at 672 nanometres), and Oiii (Oxygen-3, at 496 nanometres). There are also red, green and blue filters that will only allow through light of the particular colour.

If you are using a monochrome camera, then it’s necessay to use filters to capture light of different wavelengths. Then you are able to assemble your final full-colour image from the Red, Green and Blue images.

On the other hand, if you have a colour camera (also called an OSC - One Shot Colour - camera) then you are automatically capturing the entire visible spectrum of light. Adding filters such as the Ha filter will reduce the amount of light pollution captured but at the cost of only capturing red-coloured light.

I have two filters, the Optolong L-Enhance narrowband filter for emission nebulae and a UV/IR cut filter for pretty much everything else. To help clarify what frequencies of light the L-Enhance passes through to the camera, Optolong provide the following chart. You can clearly see the two spikes at the wavelengths of Ha and Oiii. If I’m imaging an emission nebula then I’ll capture one set of data with the L-Enhance and a second set with the UV/IR cut to capture the colour of the stars.

A full discussion of filters, their types, and applications is a bit beyond the scope of this post, so we’ll leave it here for now. If I ever get the chance to upgrade to a bigger telescope with a monochrome camera I will try to write an article about it.

Gear Setup

Now we’re finally getting to the action.

The weather is looking good, our target has been selected, and now it’s just a matter of setting up the hardware. This is all very straight forward - the tripod has to be close to level, the mount roughly aligned with the Celestial Pole (in my case, the South Celestial Pole), and your telescope assembled with all your equipment.

This assumes you’re using a tripod. If you have a permanent pier, then it should already be level and aligned with your respective Celestial Pole. (And why are you reading this? It’s aimed at lesser-experienced astrophotographers!)

When aligning your mount, don’t forget to allow for the magnetic offset - the south magnetic pole (where your magnetic compass points) is not the same as the South Celestial Pole. Most compass applications on smartphones should automatically allow for this, but check anyway. (Oh, and don’t trust those smartphone apps too much, either. I’ve seen them be up tp 45 degrees off.)

This (old) image shows my setup assembled, roughly aligned and ready for action. As I image from the same location in my back yard each time, I have marks on the concrete where my tripod feet go. I also know how much to adjust the tripod legs to get it almost level straight away.

Part 2

• Astro