Galaxies can be difficult for a lot of people to shoot simply because they are what’s called a broadband target. What this means is that they emit a wide range of wavelengths of light, including much of the visible part of the spectrum. The difficult part comes if you have any amount of light pollution because most light pollution interferes heavily with this galactic light, causing huge reductions in contrast and detail.
This doesn’t have to be a huge problem if you know how to work around it, but there are a few things you should know before you start trying to capture a galaxy.
Light pollution at its core is just any artificial light that drowns out the night sky by interfering with the light coming from space. It is very frustrating for every amateur astronomer and can make imaging broadband targets much more difficult. If you want to know what your light pollution is or the area you’re going, you can look at the link below and type in your location. Change the data to Viirs 2020 or whatever the most up to date information is at the time.
For anyone in a suburban or urban area, galaxy imaging can be frustrating. You’d get all your frames into your computer and realize you don’t have much detail or contrast and the image has a lot of weird multi-color banding happening all over. There is a solution to this, flat frames.
Flat frames are really like a uniformity frame and are very easy to take. They will add contrast back into your image and also remove much of that nasty banding. All you have to do is tie a white t shirt over your telescope and point it into a clear sky shortly after sunrise or before sunset. Temperature doesn’t matter and you’ll be looking to take as fast an exposure time as possible while keeping your histogram between 33% and 50% to the left. If you use the software NINA, there is an automated utility built into it already so just use that. If not, take your frames and add them to your stack in deep sky stacker or pixinsight. There will be an option to include them in both programs, but don’t add them as light frames. These will also remove dust spots on your sensor.
The difference can be staggering if you live in an area with a lot of light pollution and I think flats are something that everyone should do regardless of where you shoot.
My biggest recommendation to newcomers to the galaxy scene is to not use filters to capture them. Filters inherently block wavelengths of light, but because galaxies emit a broad part of the spectrum, your filter is blocking part of the galaxy’s light. There are a lot of people who try to use a light pollution filter to shoot galaxies and while you will get a picture of a galaxy, the colors will be wrong since the filter is blocking detail and certain colors.
However, if you have some more experience in astrophotography, try using an Ha filter on top of your color image. Ha filters will capture the nebulosity in the galaxy, making nebulae and narrowband targets within the galaxy stand out. This can be a great way to get a picture that looks more interesting and different than the usual galaxy images.
This goes along with the filter section, but in the exact opposite way. LRGB is a method of capturing a color image of a broadband target with a monochrome camera. You essentially get a red, green and blue filter and you take images with each of them, then align and stack them on top of each other after to get your color image. These filters are generally inexpensive compared to narrowband filters as well. What this means is the full resolution of your camera is being used for each color band, meaning the detail you can capture in a galaxy is multiple times greater than in a color camera. Most of the galaxy pictures you see in my gallery were taken in this way.
You may be wondering what the “L” stands for. That’s luminance, which is blocks the UV and IR wavelengths of light to leave you with the visible and Ha spectrums almost exclusively. This means it acts as a sort of detail layer, creating very sharp and well resolved images. You can then stack this on top of your color layers to add detail to your galaxy. Luminance is something that really doesn’t work on a color camera due to the bayer matrix so it is reserved for mono cameras only.
Galaxies can have massively varying sizes and shapes, so choosing one that fits with your gear well will be key. Unless you’re trying to shoot just the core, trying to image Andromeda with a high focal length scope is probably a bad idea since you won’t capture much of it or you’ll need a mosaic. Galaxies like M81 and M82 are good for medium focal length and wide angle scopes, with some very dim nebulosity around them. M51 is a good one for medium focal lengths as well, in the 600-1000mm range. Higher magnification improves the look of M51 since it appears bigger, but I liked the results I got from a 600mm reflector.
They also vary in brightness hugely. M51 is very bright and easy to capture contrast and detail even from light polluted areas, whereas M101 is one I always have some issues with in my suburban/urban transition area. You can check surface and apparent brightness differences if you want to know how easy it will be to shoot your target.
I’ve been shooting galaxies in a pretty light polluted area for years now with good results, so it isn’t impossible to do. You simply need to take advantage of the software and techniques that are out there. Dynamic Background Extraction in Pixinsight is fantastic and Light Pollution Removal in the astornomy tools action set is also great. Try all of these things out and I guarantee you’ll at least have an image that is better than doing none of this and in most cases you’ll have something you’re actually happy with.