Plan to Pre-Process

Part I – The “three astrophotography conditions”

01.04.2023

1) Introduction

Hi there and welcome to a series of posts that shall introduce/guide you through the most important steps in astrophotography. The idea behind this series is to show you all aspects, starting from preparation over planning until the final image. I won’t get too much into details as this series is made for interested readers and those who would like to start with astrophotography.

This post will introduce you into astrophotography basics and the main astrophotography conditions.

You can see all posts of the series “Plan to Pre-Process” here.

Sidenote; I’m a hobby/amateur astrophotographer. It’s my intention to highlight the basic aspects of astrophotography and explain them to beginners. In other words it’s my hobby and I’m maintaining this website in my free time. So bare with me if some content is outdated, at the time of writing I’m showing what I’m doing to achieve results that I’m happy with. Those blog entries are written in English and are automatically translated into German with DeepL.

2) The “three astrophotography conditions”

I see three major “conditions” that are relevant for planning an astrophotography imaging session;

  • Weather conditions (clouds and wind)
  • Seeing conditions (light pollution, air turbulences, fog and moon phase)
  • DSO (deep sky object) conditions (visibility and altitude, object type, rise and set time, size and transit time)

.. let’s have a look into the details!

2.1) Weather Conditions

Weather conditions can be easily looked up by checking a (trusted) local weather service. In general you want to have none to low wind (gusts) so that your guiding (or tracking) is not affected by it. As my observatory is located in Austria, I’m using ZAMGs‘ “tonight” weather service to get a good idea what’s going to happen tonight. Of course you also don’t want to have any precipitation during your session.

As a summary; you want to run your session in a cloudless night (/w no precipitation at all – that’s what “CS“, clear skies, stands for) with zero to low wind speeds.

2.2) Seeing conditions

2.2.1) Light Pollution

As initially stated, light pollution is an issue in astrophotography – the darker the night-skies are, the easier it will be to get some nice frames. My observatory is located in a so called “Bortle 4” region. The Bortle scale reaches from 1 to 9, where 1 stands for “excellent dark sky” and 9 for “inner city sky“.

  • If you’re interested, check handprint.com for more info on the Bortle categories
  • To get an approximate estimation / Bortle categorisation of your location, you can check lightpollutionmap.info

There are different filters available that can help in light-polluted areas.

2.2.2) Air Turbulences

Especially during summer, it might happen that – when you’re collecting data for an object that is sitting “over the roof” of a house. Due to the heat of the roof (the sun was burning down on it the whole day) there might be air turbulences in the (cold) air of the night. There’s nothing really you can do about it, except to look for objects that wouldn’t be affected by such structures.

2.2.3) Fog

Especially during winter, it will happen that fog is preventing you from making use of the clear, dark and long nights. Sadly there’s nothing you can do about it.

2.2.4) The moon and its lunar phases

Last, but not least, there’s our old friend, the moon. While being a very interesting object to photograph, it may throw a wrench into your works, simply because of brightening up the night sky. During first quarter to third quarter, I always try to collect data for objects that are “away” from the moon, but more on that a little later.

Two great resources to check the current lunar phase;

The following image shows the different lunar phases;

The different lunar phases from new moon to full moon and back to new moon.
Lunar phases by Orion 8, source: Wikipedia

As a summary; the darker the night-sky, the better in terms for collecting your data. While being in a lower Bortle region is for sure beneficial, it’s not impossible at all to also take nice frames in urban-/city-near locations. You also want to avoid air turbulences (like those caused by roofs/buildings) and foggy nights that are part of the colder months and taking frames of objects that are close to the moon when it’s between the first and third quarter.

2.3) DSO (Deep Sky Object) conditions

2.3.1) Deep Sky Object – Altitude

I’ve set up two thresholds for target selection in terms of altitude (based on my observatories’ location and its sorroundings);

  • > 33° above horizon for “standard” targets
  • > 27° above horizon for “special” targets that are “rare” (like Thors’ Helmet)

.. but in any case never below 25° over the horizon, because you might get into several issues like light pollution (over cities and urban areas), as well as haze. Also don’t forget that the lower the angle you’re looking at an object (in space), the “thicker” Earths’ atmosphere layers will become – so lots of potential seeing drawbacks bound to it.

To determine the altitude of an object during your imaging session, you could use different software like Stellarium, Sequence Generator (Pro), or even a web-based tool like Telescopius (which is really easy to use).

2.3.2) Deep Sky Object – Type

The object type determines which filters you’ll need to capture the right signal based on its wave-length(s). As I’m working with a so called “OSC” (One Shot Color) camera, I don’t have to bother with too many filters like;

  • L(uminance)
  • R(ed)
  • G(reen)
  • B(lue)
  • SII (ionized sulphur)
  • OIII (double-ionized oxygen)
  • Halpha (hydrogen)

..that you need to use in combination with a mono camera.

While it may be more “convenient” and beginner-friendly to work with OSC, the downside is that OSC sensors don’t see the full spectrum of light it reaches, because the sensor is coated with a “Bayer field pattern” that enables you that every frame you take is actually an RGB image.

So, in my case, the only thing to worry about is whether it’s sufficient to take frames “only in RGB” (because an OSC camera already produces RGB images), or if the object type would require a (dual) narrowband filter (OIII and Halpha, like the Optolong L-eXtreme 2″).

Of course you can filter and edit it to your needs – there’s a section on this in the next part of this series.

2.3.3) Deep Sky Object – Notable Times

For rise- and set time, size and transit time – as I mentioned Thors’ Helmet earlier, you can see what I meant with “rare” target if you have a look into its object details on Telescopius and an eye on the objects’ altitude chart. At the time of writing this post, there’s only a tiny timeframe available for taking frames of that object. So, once again, your planning for such “rare” events is crucial.

In addition to that, the size of an object determines whether you can “just” shoot one panel or if you need to work with a mosaic (based on parameters like sensor-size, focal length) – Telescopius has a built-in simulation, so you can easily see how different parameters affect your framing.

The transit time is basically the time where an object is at its “highest” point during an observation time frame. And remember, the “higher” the object is, the easier it may become to frame it :)!

Closing Words

In the next part of this series, we will have a look into how your preparation could be incorporated into a session planning. I know there’s much to take into consideration, but no worries – we will have a look into all of that, step by step and one at a time. CS!