This enables you to view a much fainter object lying very close to the bright one.
Using this device will help you to see very close double stars, especially where the primary star is extremely bright, or faint moons around the planets, such as Phobos and Deimos orbiting Mars.
These are not available commercially, but using very simple materials, such as black card and insulation tape, a very effective Occulting Bar can be constructed in less than half an hour.
Having long suffered from astigmatism, after being driven absolutely bonkers by changing glasses a multitude of times and a lot of soul searching, I finally went ahead and had laser surgery over two and a half years ago.
Have you ever wanted to know what effect laser surgery might have on your observing?
Here’s my account of what my experience was like and how my observing and astronomy will never look back. (Pun entirely intended).
My Observers guide to Comet ISON for later this year and into 2014 has just been published.
It is currently available on Amazon Kindle download:
Containing maps and charts to help you find the comet while observing and costing less than a pint of beer, how could you not?
Taking images of the night sky doesn’t have to be complicated and you don’t need any specialist equipment to do so.
Following this guide will allow you to produce images for yourself without having to take out a second mortgage just to take many fabulous and rewarding images. It can be done using the simplest of photographic equipment.
1. A camera.
Any camera that is capable of taking an exposure of more than a few seconds exposure is capable of taking images of the night sky. Small compact cameras sometimes have a Program or Night Landscape mode. This automatically sets a long exposure of a few seconds and focuses the camera at infinity.
Some nice constellation images can be taken this way. The image below was taken using such a camera using Night Landscape Mode and shows Taurus quite nicely and the star colours of Aldebaran, Betelgeuse and the Pleiades Star cluster.
A DSLR has far more flexibility in what you can achieve, so is the preferred option if you can afford one. There is a lot of dispute on the Internet regarding the different makes of DSLR that are ideal for taking astronomical images. Most people seem to regard the Canon range as being the best, with lots of talk of the Nikon stripping out data when it takes an image. If you have a Nikon or other make of camera, excellent images can still be taken regardless of the make. Most of my images on this Web Site have been taken with Nikon cameras, so there is no necessity to change camera if you already have one to hand.
However there is a lot of third party software available for the Canon range, so if you are considering buying your first camera, I would recommend a Canon for this reason only.
Make sure that you set your camera so that it does two things:
1. Doesn’t use long exposure noise reduction.
2. Saves the resulting image as a RAW file.
(I save my images as both large JPG and RAW, you will see why later).
Also set the ISO setting to at least 400 or preferably more (800-1000).
The higher the ISO setting the brighter the resulting image and the fainter the object you record in less time, but you also get more noise on your image.
The lower the ISO setting the darker your image, and the longer the exposure needed to see faint objects, but you do get less noise on your image.
2. Lenses. The lens you use will determine what you capture. A wide angle lens ~20mm will show a wide expanse of sky (i.e. a wide field of view). A 100mm lens will show a much smaller field of view. A variable focal length lens will enable you to frame different constellations better, but the quality and f-ratio is normally less. A bigger F-number will produce a fainter image, and will need a longer exposure.
3. A sturdy tripod. Holding the camera still is essential. You need something that will hold the camera steady for the extent of the exposure. You don’t necessarily need a full sized tripod. A Gorilla Pod, or something like it, can also be used. A mount like this is light and portable and will enable you to mount the camera on any convenient sturdy support. I have managed to take Milky Way shots with my DSLR on holiday this way, with the camera supported on the back of a Sun lounger.
4. A means of making long and multiple exposures. Many cameras are only able to make exposures of up to 30 seconds on their own. To take images of faint objects, you need to make exposures of a minute or more. To achieve this, you can buy accessories which can be used to make longer exposures. These can be as simple as an infra-red release which will open the shutter when first pressed, and close the shutter when subsequently pressed. An attachment for the camera can be more complicated and will enable you to program the camera to take a number of images one after another. This method of taking images is almost essential unless you want to stand there and keep releasing the shutter manually. As I know from experience, this does become somewhat tedious after a few hours.
Taking Your images.
Now we have really come onto the business end of things. Let’s assume you’ve got your camera mounted on a tripod and are out under a clear sky. Pick an area of sky you are interested in and point your camera towards a familiar constellation. If you have a variable focal length lens frame your chosen area of sky accordingly.
At this stage you need to know two things:
The shorter the focal length of lens used, the brighter the image and the less star trailing is visible.
The longer the focal length, the fainter the image and the more star trailing is visible.
Take an exposure of about 20 – 30 seconds. You may find when you look at your resulting image, that the stars are more than pinpricks. Even if you set your lens to infinity (which is where the stars are) your lens may still not be properly in focus. You may need to experiment a few times and adjust the focus to get really pin sharp stars. Once you have achieved proper focus, you can now take your images.
Take a number of exposures of the same object and vary your exposure in each one. Take images using 10 seconds up to 1 minute exposures, increasing each exposure by 10 seconds. Inspect each image and note at which exposure you can start to see star trailing.
You will have then found the maximum exposure you can take before trailing is visible with that focal length lens. (As a rough guide if you use a 20mm lens you can get away with about 45 seconds exposure before the star trailing as a result of the Earth’s rotation starts to show).
Once you have found this, take a number of exposures at that same setting, pointing at the same chosen point in the sky.
Without touching the lens, move the camera to a different point in the sky and take a number of images in the same way. If you do change the focal length of the lens, you will need to check your focus once more before continuing.
Processing Your Hard earned images.
After a few hours you should have a few sets of “almost” identical images on your camera.
Now comes the fun part in getting the data you have captured so it gives you a pretty picture.
I use two pieces of free software to help me process images:
This is a nice piece of software which allows you to add image together. As its name suggests, it enables you to add images together to produce star trail pictures. If your camera is static on a tripod (as yours currently is) the stars are moving across the sky due to the Earths rotation. This means that between each of your exposures, the sky has moved. If you take enough images and put them into Startrails it will show the difference in the stars position and you will see the movement quite easily. The use of JPG’s that I suggested earlier would be better for this as they are smaller images and more easily handled by the Startrails software. To show you how it works, below is a 20 second image of Aquila taken with my DSLR from a dark sky site in Wales. It doesn’t look very exciting, does it? I took 17 images exactly the same and dragged them into Startrails. The resulting image is shown second . That does look a little more exciting now, doesn’t it?
Startrails enables you to see the movement of the stars and produce star trails just using the images you have already taken. You can also see the colours of the stars in an image like this too.
Startrails will also produce an animation of the stars movement as an AIV file as well if you want it.
Deep Sky Stacker (DSS): http://deepskystacker.free.fr/english/index.html
Now things get really exciting. Using the very same 17 images RAW images of Aquila shown above I pulled them into DSS and processed them. Unlike Startrails, DSS looks at the images and determines where the stars are. So when it processes the images, despite the fact that the stars have moved between exposures, it will register the stars on top of one another and add the images together. As a result of this faint objects you can hardly see on the original images are made brighter as the data is added together. the image below shows the result of stacking those same 17 RAW images of Aquila together:
Now you can see the bright band of the Milky Way and the dark rift at the side of Aquila.
If you look carefully you can also see the Coat Hanger in Vulpecula.
I’m sure you will agree that this is not a bad result for a camera standing static on a tripod.
Go on, give it a go.
You won’t be disappointed.