|
|
|
Astrophotography Guide
Introduction
Capturing the light photons from a deep space object, (DSO) is a difficult task that requires careful planning and a lot of patience. Nothing can teach you better than experience. There are many resources available in books and on the internet to point you in the right direction and arm you with knowledge and equipment, but understanding is by doing. There are many different techniques of capture and processing as there are endless types of equipment. This tutorial lists just one, fairly basic method, assuming you already have knowledge of the skies, a properly aligned telescope and a digital camera.
Basic concept
Light behaves in a fairly predictable way. The further away an object is, the more light and colour it loses. Look out your window. Notice the trees are green up close? The ones further away in the hills are a purple or blue sort of colour. The ones furthest away can appear almost black!
So it is with DSO’s. On the side of a lot of department store telescope packaging you will notice pretty pictures of galaxies and nebulae. In reality, the naked-eye view is far less appealing. Our eye captures photons of light and registers them in the brain instant by instant. The colour and light is there, but just too weak for our eyes to detect. That’s where a camera comes in. When you open the shutter of your camera, the light photons strike and accumulate over as much time as is needed to capture the detail. You can do this as many times as you like and later stack the images to produce a clear and detailed color photograph. Care must be taken to capture the right amount of light (timed) so that the image is not over or under exposed. Too much time with the shutter open can burn out the image. You must also use the tracking functions on your telescope mount to accurately follow the movement of the stars so from the camera’s point of view, nothing is moving. I use a combination of the telescopes built-in sidereal slewing speed, and manual guiding using a joy-pad.
Considerations.
Seeing. This is how clear the sky is. Many things can affect seeing.
1. Humidity. Moisture in the air scatters light photons. It blocks out the light from space and also bounces light around in it’s midst.
2. Light pollution. Light from nearby sources such as streetlights or distant sources such as a city or even the moon, can fill the sky with light and leave little contrast with DSO’s, to the point where they can overpower them altogether.
3. Altitude. Your height above sea level or above the surrounding terrain can impact your seeing conditions. Notice when a fog develops, it usually starts in the valleys. Dew builds up quickly on your optics as a result. Most large observatories are perched on top of mountains where the air is thinner, so they have less atmospherics to look through.
4. Position in the sky. It often is not good to photograph an object that is low in the sky, as you are looking through a lot more atmospheric anomalies that can ruin your shot. The sky is most stable straight up or somewhere high. Try to plan your imaging so that your target coincides in time and position to best optimize this.
Temperature. This plays a major role in a couple of ways.
1. Optically. Temperature in the atmosphere can affect image stability, like watching the shimmering oasis effect above a hot road. Heat from the roof of a nearby house can be a problem, as can heat from chimneys. Even weather patterns and high altitude jet-streams can have an effect.
2. Electronically. The photosensitive chip in your camera registers light photons on each pixel sensor. Heat can also leave a light signature called “noise” on these sensors. This can be caused by the ambient temperature of the environment it is placed in, or through the electrical charge in the camera causing the chip to heat up. Many people combat this at first by allowing their equipment to cool down outside for an hour or so before use. Others use modifications such as cooling fans on the equipment.
So you see, there is a lot to get right, even before you get near the telescope isn’t there? This can make the hobby very frustrating. However, the harder it is to achieve a goal, the more rewarding it is when you get it right! You may be starting to appreciate that a simple pretty picture is not as it seems. A lot of patience, skill, sleepless nights and hard work go into that one perfect shot!
Setting up.
This section assumes that you have selected a target, know where to find it and are imaging at the appropriate time of night/year.
Equipment.
Camera: Canon EOS 400D Digital SLR with shutter release cable.
Lens: Skywatcher Pro Series ED80 Apochromatic Refractor.
Tracking Platform: Celestron Nexstar 11GPS SCT on a heavy duty wedge.
Laptop Computer.
Accessories.
USB cables to connect camera to laptop.
T-ring adaptor to connect camera to telescope.
Optional Canon AC power supply. (Batteries tend to die before the night is out.)
Illuminated reticule 12 mm eyepiece
Countdown timer. I use the one on my mobile phone with an alarm.
Software required.
For capture and organising: Standard software included with camera.
For stacking and processing: DeepSkyStacker or Registax IRIS
For post-processing: Corel Paintshop pro or Adobe Photoshop.
Optical Train
Connect your equipment and power on. With the main guiding telescope powered on and aligned, my equipment is connected as follows: My unmodified Canon camera has its standard lens removed and replaced with a T-ring adaptor. This then fits directly into the eyepiece holder at the rear of my 80mm refracting telescope. (ED80)
The ED80 is mounted piggy-back style to the top of my Celestron. The Celestron has electronic motor drives to track the stars. (More on that later) Both telescopes are aligned with each other so that they their centre of view is exactly the same. The ED80 is less powerful in light gathering ability than the Celestron, but has a much wider field of view. The advantage of using it like this is because I can image a whole nebula instead of just part of it. Also, because the Celestron offers a much higher level of magnification compared to the ED80, my guiding on a star can be super accurate. A tiny wrong movement in the guiding Celestron, can go relatively unnoticed in the ED80. Were I to do it the other way around, a tiny wrong movement in the ED80, would show as badly elongated stars on the image at the end.
I have a shutter release cable hanging from the camera, and a USB cable carefully threaded through convenient points on the telescope, down to a laptop mounted next to the pier. I also have an AC power supply instead of a battery for the camera. Its cable is also carefully routed out of the way. Be careful that the cables do not catch on anything as they can destroy your equipment in an instant if they do. To keep a guide star centered in the Celestron, I use a 12 mm Illuminated reticule eyepiece. This has glowing crosshairs so you can see them against the black of space.
Preliminaries.
Remove the memory card from the camera so that the image files go straight to the camera’s software on the computer.
Slew your telescope to a bright star or easy target close to where you intend to image. If you focus on something and then move your scope to a completely different area, then mirror flop etcetera can knock it out again.
With the camera hooked up to the PC, when you turn on the camera a screen will come up called EOS utility. Select remote shooting and another screen will come up titled EOS 400 Digital. Leave that there. Look through the back of the camera and focus the telescope as best you can. Set the camera on bulb and take a five to ten second exposure. When you take the first photograph, Zoombrowser EX will come up with a scroll bar and a preview of your photo. Select the JPG image and use the magnifying tools. Preview the photograph on the computer and magnify it to look for stars that occupy very few pixels. Move the focus very slightly on your telescope and take another photograph. Repeat the procedure until the stars on your magnified preview occupy as few pixels as you can. This produces a very good focus. There are focus software packages available, but I think this is a very good way to do it manually and gain an understanding of what you are trying to achieve.
Now that the camera is focused, delete the images you just took. You don’t want them getting mixed up with the rest. The idea now is to dedicate yourself to only one object for the night. Don’t get greedy and try to image everything in one night. Take lots of photos of the same thing to give you as much as possible to work with. From now on, start taking notes. Note the temperature. Note the exposure length. Note the details of your set-up. Everything. This will all come in useful when you make mistakes and have failures. Count on failures, they will happen! These failures are a good thing though. Each one will teach you something. Believe it or not, the failures teach you more than the successes!
First photos. (Lights)
Photographs are also called frames or subs. The frame or sub with the subject in it is called a “light frame.” Centre your target in the camera finder and in the crosshairs of your guide scope, making sure you have a good guide star in the middle of the crosshairs. This star needs to be clear and sharp as you will be watching it for the duration of the exposure.
Get yourself comfortable in front of the eyepiece. If you have a chair that seats you at a comfortable height, even better. You are going to be there for a long time. There are a few automatic guiding programs available that can be used with a webcam instead of your eye, but I think it is better to learn the hard way, and appreciate it better by doing it manually first.
Set the camera on bulb (For the Canon 400D it is “M” on the top dial). Set the ISO at 800. Set your timer for, let’s say, two minutes and 52 seconds. (I used it on the Trifid and Lagoon nebulae near Scorpio the first time.) Press and lock the shutter open and then press your timers start button.
Using the joy-pad on the guiding telescope, make sure the guide star stays in the centre of the crosshairs for the entire three minutes. Do not let it drift off. If you bump the telescope even once during the photo, release the shutter, delete the photo and start again.
At two minutes 52 seconds, the alarm will go off. This last 8 seconds is to give you time to let go of the joy-pad, find the shutter release in the dark and activate it at three minutes exactly. (The display on the back of the camera will show the time in seconds.) That’s pretty much it for capturing light frames. Do as many of these as you can. Fainter objects will need longer exposures and brighter objects will need shorter ones. Each object will have an optimal number of exposures and frames, depending on the object, night and your equipment. This is why you write everything down!
Dark frames (Darks)
|