Category: Astrophotography method
Posted by: Tom How
In an ideal world, we'd all have our telescope observatories at the top of mountains with totally uninterrupted views in any directions. For those of us who do not have a handy mountain in the back garden, we have to contend with trees and houses getting in the way. This creates a hard limit on the objects we can use for Astrophotography. It would be nice to have a perfect horizon, but most of us cope by patiently waiting for the required target to arrive in a region of clear sky. British astronomers have lots of patience!
Most of us modern astronomers use computerised star chart programs. Such software allows you to enter the horizon limits in each direction and create a local horizon line (yellow on the chart) above the zero degree horizon (grey on the chart).
With the installation of my new homemade telescope mount combined with the passage of time, my map of the local sky has changed. The annoying huge tree to the south east has grown, whilst pruning has created some gaps in other areas. As it was cloudy today, I decided to recreate my horizon map.
This is a simple yet time-consuming task. You have to slowly move the telescope around the sky, using the scope to locate the line between the sky and the trees/houses. The telescope mount tells us at which altitude the telescope is pointing. This information is recorded around various points of the sky - the job only takes a hour or two.
Then is is a case of entering this data into your planetarium software (Skymap wins an award here for dreadful such a dreadful interface) and use the resulting plot when planning your observations. When it is dark, it is very easy to suddenly find yourself shooting images through a tree and wondering why the stars like so odd. With a decent chart setup it is easy to avoid silly mistakes.
Here we can see my plots in each direction. As it happens, most of my astrophotography is done in the north east, as this does the darkest sky. Many interesting objects appear in the south as well, but I can usually only track these for a couple of hours, so it is harder to get a decent image.
Most of us modern astronomers use computerised star chart programs. Such software allows you to enter the horizon limits in each direction and create a local horizon line (yellow on the chart) above the zero degree horizon (grey on the chart).
With the installation of my new homemade telescope mount combined with the passage of time, my map of the local sky has changed. The annoying huge tree to the south east has grown, whilst pruning has created some gaps in other areas. As it was cloudy today, I decided to recreate my horizon map.
This is a simple yet time-consuming task. You have to slowly move the telescope around the sky, using the scope to locate the line between the sky and the trees/houses. The telescope mount tells us at which altitude the telescope is pointing. This information is recorded around various points of the sky - the job only takes a hour or two.
Then is is a case of entering this data into your planetarium software (Skymap wins an award here for dreadful such a dreadful interface) and use the resulting plot when planning your observations. When it is dark, it is very easy to suddenly find yourself shooting images through a tree and wondering why the stars like so odd. With a decent chart setup it is easy to avoid silly mistakes.
Here we can see my plots in each direction. As it happens, most of my astrophotography is done in the north east, as this does the darkest sky. Many interesting objects appear in the south as well, but I can usually only track these for a couple of hours, so it is harder to get a decent image.
Category: Astrophotography method
Posted by: Tom How
The British weather affords us few opportunities to image the night sky, so we don’t want to waste precious astrophotography time hunting and the sky for our targets. Once you have a permanent observatory, you need to make sure your telescope pointing is as accurate as possible.
Category: Astrophotography method
Posted by: Tom How
Really good astrophotography requires long integration times. Many hours of data go into a good narrowband image. In the UK we do not get many clear nights, and when it is clear, the astro-imager still needs to get some sleep. As the astronomer is the telescope’s least reliable component, a lot of these problems are solved by removing him from the equation as much as possible....
Category: Astrophotography method
Posted by: Tom How
I has discussed before the importance of using field fields to calibrate your images in astrophotography. In this article I've presented some sample images to demonstrate why a flat field is so useful in astrophotography, especially when dealing with images of nebula.
Category: Astrophotography method
Posted by: Tom How
How long are you supposed to expose to take a decent flat field? Most folks are quick to tell you that you need to expose long enough to get the signal about halfway across the histrogram, but this isn't the whole story. Does the same theory work for DSLRs?
Category: Astrophotography method
Posted by: Tom How
A really deep, high contrast and low noise astrophotograph takes a large amount of exposure, even on a fast telescope. All beginners make the mistake of not concentrating full on a single target