BASIC ASTRO IMAGING
Session 1 : Basic Imaging Sequence
Warning: never look at or take images of the Sun without taking proper precautions to limit the amount of heat entering your imaging system. Always use professional solar filters or filter material, that are designed to reflect almost 100% of the Sun's light and heat and only allow a small fraction into your scope or DSLR.
1. Using a DSLR on a tripod:
The easiest way to take astro-images is to take widefield star trail images by simply mounting your DSLR camera on a tripod, pointing it at Polaris and leaving the shutter open for a while (minutes) on a high ISO setting. This will give you nice images of the stars trailing around Polaris as the Earth spins on its axis. This requires a reasonably dark sky or a Low Pressure Sodium filter for most urban areas to prevent street lights flooding the picture.
Check your photos with the histogram function on your camera, or simply by eyeballing the photo, to get the best setting. Once you have determined the best setting, always bracket your shots with some shorter and longer exposures, higher and lower ISO settings, and larger or smaller apertures. Don't forget that the lower the ISO setting the less noisy your picture will be and the smaller the aperture the greater the depth of focus which is especially important if you want to photograph some stars with something on the horizon. You will notice that as you use longer lenses the effects of light pollution will be reduced as you are receiving less and less pollution from a smaller and smaller section of sky as the lenses get longer.
You can get more sophisticated and buy a tracking device like an AstroTrac that will allow you to take up to several hours of exposure while accurately tracking the stars. However, you will be limited in exposure time due to thermal noise (little white specks across the image that increase with time and temperature) and the fact that DSLRs use CMOS (Complimemtary Metal Oxide Semiconductor) chips which are less sensitive than CCD (Charge Coupled Device) chips. The CMOS chips are noisier and less sensitive than the CCD chips used in most astro-cameras which are then sometimes electrically cooled to -40 degrees C below ambient temperature.
When using a DSLR you will need some form of remote shutter release to allow any vibrations from you touching the camera, to settle down before taking the image. Use a cable release or use an electronic shutter release. If you don't have a remote shutter release, set the camera on delayed shutter mode and give it say 10 seconds for any vibrations in the scope to settle out before the picture is taken. The longer the lens the longer the settling period will need to be.
2. Using a DSLR piggy-backed on a telescope:
The second way of using a DSLR camera is to buy a camera-mount that bolts to your telescope and has a camera mounting screw to attach it to the base of your camera. Attach the camera to the scope, rebalance your scope and get it polar aligned and tracking. Then use a variety of lenses on your DSLR. Point your scope at the MOON if it's up, or a bright star if not and then centre what you see in the telescope with what you see in your photograph to align the camera to the scope. Then go hunting for some goodies with the scope and click away with the camera. Don't forget to bracket your exposures. The next step is to actually use the scope as a very long telephoto lens, this is covered in the next section.
Note: For long exposure astrophotography you will need an equatorial mount or fork mount installed on an equatorial wedge. Alt-Az mounted scopes can be used for shorter duration imaging of bright targets like the Moon and planets. However Alt-Az mounts will suffer from image rotation as the object traverses the sky during long exposure shots. But even this can be adjusted for in some image capture and stacking programs to a limited extent (more on this later).
3. Choosing a camera for astro-photography:
If you don't already have a DSLR or astro-camera here are a couple of things to consider:
Colour or monochrome (black & white)
If you want to produce colour images and there are many spectacularly coloured objects out there in space, you either need a one-shot colour camera, like a DSLR, or you need to take a series of monochrome images shot through special coloured filters (usually Red, Green, Blue and Clear) and then assemble these in software to give a colour image. This is more complicated than using a one-shot colour camera but can give better results and allows imaging in other than visible wavelengths like hydrogen-alpha and Oxygen III wavelengths that can show extra detail. A one-shot colour camera has microscopic RGGB filters printed onto the chip in what is called a Bayer Matrix. Notice that they normally use two Green filters per group of four pixels. The Bayer mask is aligned with the individual pixels in the chip and then the image processor in the camera combines the information from each group of 4 pixels to give an overall colour, based on the amount of RGB light received. This is a much simpler solution for the beginner but has its limitations as above.
Cooled or uncooled
For imaging of Deep Sky Objects (DSOs) it is necessary to take quite long exposures to get enough light onto the camera chip to give a good image. The more light the better the image quality which will be discussed later. Most DSLRs use CMOS chips that are inherently noisier than CCD chips and DSLRs do not contain coolers. Astro imaging cameras normally use CCD chips fitted with electrical (Peltier) coolers to reduce the chip's temperature down to circa -40 degrees C below ambient temperature to further reduce thermal noise (cameras used on the large professional telescopes are cooled with liquid nitrogen to about -180 degrees C allowing much longer exposures.
Planetary, Lunar or Solar imaging
To get the best Planetary, Lunar or Solar images you will need a high frame rate camera that can take and upload pictures very rapidly 30 - 60 times a second in a video format so that they can capture that moment of perfect seeing through the Earth's turbulent atmosphere. Typically two minutes of video will be captured and then software like Registax will be used to throw out the blurred images and then stack all the remaining good images (stacking will be explained later).
4. Mounting a DSLR or astro-camera at prime focus:
Using a camera in place of the eyepiece on a telescope is referred to as Prime Focus astrophotography. Basically it uses the telescope as a very long telephoto lens.
Set your scope up in the normal manner and align your mount so that it tracks accurately. Find a suitable object to image, centre and focus it in the eyepiece. You will need a tracking mount for anything other than short exposures and while just starting out, it helps if the target stays in the field of view while you are sorting everything else out. Put your normal eyepiece into the scope and select a nice big bright target like the Moon or a bright star or planet. Focus the scope on a star or nice sharply defined feature like a crater. Take the eyepiece out and replace with your DSLR or astro-camera.
Note: You will find that you will probably need to refocus the scope quite a way once you have installed the camera as the camera's imaging chip will be at a different distance from the scope's focal plane than your eyeball's retina was.
For prime focus astrophotography you need to take the normal eyepiece out and replace it with your camera (without its lens) as the telescope then becomes the lens. If you have a dedicated astro-camera the chances are it will come with a 1 inch or 2 inch nosepiece that slides straight in to replace the eyepiece. If not then you will need to purchase a suitable T-thread adapter.
A T-thread adapter will normally have a 1 inch or better still a 2 inch nosepiece (if your telescope can accommodate 2 inch eyepieces). The T-adapter will have the nosepiece on one end and a male T-thread on the other end. Then you need to buy a T-ring that is specially designed to fit your own specific DSLR lens mount but has a female T-thread on the other end. Install the T-adapter onto your camera and then screw the adapter's female thread onto the male T-adapter to give you a 1 inch or 2 inch nosepiece on your camera which can then be slid into the normal eyepiece holder. T-adapters can be purchased from most astronomy suppliers. See the link below for more technical guff on T-threads
5. Using a DSLR or astro-camera at prime focus:
DSLRs: Focus the image using the normal scope focussing controls. It is really helpful at this point if you have Liveview on your DSLR so that you can preview the image. Some DSLRs without Liveview can show live images through additional software run from a PC. If imaging the Full Moon start with an exposure setting of say 1/120th second and then adjust to give the faintest visible image, then refocus to sharpen the image, finally lengthen the exposure time one or two stops to give a nice bright but not over exposed image. Use the DSLR's histogram function to check that you have no pixels over say 90% of maximum exposure. Use the remote or delayed shutter release or PC software to take the picture so that you don't shake the telescope when you release the shutter. Make sure that the camera is set to save the image in RAW format NOT JPG unless you only want to work with one image. That's your first image taken, go and have a coffee!
DSLRs and Astro-Cameras: ideally your camera needs to be controlled from a PC running an astro-imaging program (of which more later). Most astro-cameras come with supplied drivers which need to be installed on your PC according to the manufacturer's instructions. Most new DSLRs and all astro-cameras tend to come with some sort of basic PC image capture program that can be used to test the setup and take a trial image. Hopefully this will include some form of looping short exposure mode that allows you to continuously take short exposure images without storing them to disk. Fine focus the setup and check the exposure and gain/ISO settings to correctly expose the image. You should be aiming for an exposure of no longer than about 4 minutes to start with. The combination of exposure time and Gain/ISO settings should be adjusted to result in a maximum pixel intensity of about - of the maximum (max. is normally 65,536) across the object that you are trying to image. Most objects will result in only a few thousand pixel intensity. Once correctly adjusted, switch to image capture mode, and take say 10 images bracketed around your nominally ideal setting and save them to disk. That's your first set of images taken, go and have a coffee and admire your work!
Explanation: Most cameras use 16 bit analogue to digital converters to convert the light falling onto the detector into electrical signals in what are called ADUs (Analogue to Digital converter Units). With 16 bit converters the signal can range from 0 ADUs to 65,535 ADUs. Some cameras go into a highly non-liner mode at the higher end of the ADU range to prevent streaking and bleeding of one very bright pixel into all the others. This typically occurs at around 50,000 ADUs so aim for a maximum pixel reading of around 45,000 - 48,000 ADUs for reasons that will be covered later. Obviously if you are imaging a very faint DSO near a relatively bright star then it is OK to let the star hit the maximum 65,535 ADUs provided it doesn't bloat out too much.
Note: When you are ready to take your first set of sub-frames (or subs for short) which area sets of images all taken at the same exposure settings, make sure that you remember to set the software to automatically store the images in a folder that you can find and that the file format is set to FITs or RAW or some other lossless format such as TIFF, do not store them in JPEG format unless you only want to work with one image.
6. Choosing the correct astro-imaging software:
You will now need some software that will capture a sequence of subs and store them for later processing. The reasons for taking a sequence of shorter images are:- The targets that you will be imaging are normally VERY dim, apart from planets and the Sun and Moon. Therefore you need to take very long exposures or a sequence of shorter exposures that can later be added together.
- Averaging a number of shorter exposures can improve the signal to noise ratio. Basically, the noise, mainly thermal noise in the CMOS or CCD chip is random but increases with chip temperature. This type of noise is reduced by the square of the number of subs averaged. Therefore averaging 4 subs should reduce the thermal noise by 2, 9 subs by a factor of 3 etc.
- Taking lots of shorter exposure subs allows any with satellite trails, clouds etc. to be rejected while still leaving a number of good exposures to average. Minimum number of exposures you should aim for is 10 but the more the better. This process is known as stacking.
- Calibrating your images is a process whereby imperfections in the imaging train, e.g. dust on filters, CCD or CMOS chips and vignetting (edges of the image receiving less light than the centre) can all be compensated for. This requires a set of Dark subs (Darks), Flat Fields (Flats) and Bias Frames (Bias) to be taken and applied to the original images (Lights). Calibration will greatly improve all images.
This all sounds pretty complicated but is handled fairly automatically by most astro-imaging software and will be covered in more detail later.
One really good astro-imaging program is AstroArt 4 or 5 and is the software that the author has been using for years which is available here. It is fairly expensive at circa £120 inc. VAT but this is cheap compared to others like Maxim DL. AstroArt will do lots of things in addition to image capture, it will also stack the sub-frames and calibrate them using darks, flats and bias subs as discussed above. AstroArt is primarily aimed at astro-cameras but does include drivers for Cannon DSLRs and can be used via ASCOM for many different cameras (more on ASCOM later).
However, there are now other software packages available like APT-Astro Photography Tool available here which if it does what it says on the tin, then it should do everything AstroArt does but only costs 12.70 Eur. It would also appear to be able to control many more DSLRs as well as astro-cameras.
Another free program for DSLRs is IRIS which appears to control many of the most common DSLRs if APT won't drive your camera.
The other option is to load the free 'ASCOM platform' onto your PC which needs the .NET framework from Microsoft already installed. The .NET comes pre-installed on most modern PCs but might have to be downloaded separately from Microsoft for older PCs. Most astro-cameras and some DSLRs plus lots of other things like mounts, observatory dome controllers etc. come supplied with ASCOM drivers. This allows many different types and models of equipment to be controlled by various different software programs that have ASCOM interfaces.
The ASCOM process all sounds a bit complicated but basically once you have the ASCOM platform installed and running (you can download an ASCOM diagnostic tool to check this), you need to do is:- Determine which ASCOM compatible imaging program you will use and install it.
- Download and install the ASCOM interface for the imaging program from its website. (note AstroArt and APT have ASCOM interfaces)
- Load your camera driver onto your PC.
- Find your camera's ASCOM driver (on camera manufacturer's website but also some third-party drivers on the web) then download and install that as well.
- Then from the imaging software choose ASCOM Camera and then from the ASCOM Camera Chooser dialog select your camera name. You will then be prompted to do an initial setup of the camera parameters in the ASCOM dialog box.
You are now all set to use your camera from the imaging program by setting exposure time and the number of subs that you want to capture.
7. Taking a sequence of images:
Once you have selected your astro-imaging software play around with it indoors in the warm with a cup of coffee. If you have a DSLR, mount it on a tripod and focus it on a target with its normal lens attached. If you have an astro-camera with no lens then either set you scope up indoors and point it out of the window at something about 500m or more away or just make sure that you can see a difference between images when the camera is covered and uncovered. Once you have got the hang of taking a sequence of subs you are ready to venture outdoors.
Set your scope up, align it and find a nice easy target for your first astro images such as the Moon, a bright planet or globular star cluster. Once you have the target centred and focussed take an initial test frame and use the imaging software to read the brightest pixel in the image. To get the best quality images you need to adjust the exposure time and gain/ISO to give a maximum pixel brightness of around - of the maximum possible. When imaging the Moon the exposure time will be very short (less than 1 second) but for faint DSOs an exposure time of circa 10 minutes will be required to get the brightest area of the image up to around 17,000 - 32,000 ADUs and above. The higher the better, without saturating (hitting the maximum ADU reading), for reasons that will be covered later. However, you do not want to take too long an exposure just in case a plane or satellite passes through the shot. If this happens then that particular frame can be rejected when stacking.
Once you have about 10 light subs you can then stack them using one of the above programs or the free DeepSkyStacker. This is really good software for stacking and calibrating your subs but for now just use it to stack your light subs without calibration which we will come onto later.
Stacking looks at each of the light subs and compares them to a reference image. The reference image will either be the first in the list of light subs or one that you have specifically told the software to use. As the software looks at each image it looks for recognisable features, (normally a selection of the stars in the image) and then determines how much it must shift each image to make the stars align with the reference image. The better stacking software, like DeepSkyStacker will also rotate the images to obtain the best alignment.
Note: when you save your final stacked image make sure that it is saved in a lossless format like FITS or TIFF depending on what formats your photo processing software can import. DO NOT SAVE IT IN .JPG OTHERWISE A LOT OF YOUR HARD WORK WILL BE WASTED.
Note: You may find that you need to adjust the 'visualisation' or auto intensity settings in the imaging software (F4 in AstroArt) to be able to see your target in the image as it can be quite dim compared to the bright stars.
8. Basic image processing
Once you have a final stacked image you can then start to play with it in one of the many photo processing programs that support FIT or TIFF to tease out all the details by non-linear stretching the image so that you expand the fainter parts of the image (which will have all the interesting data) while compressing the brighter parts (stars etc). This may not be so applicable when imaging the Moon.
When you first import your FIT or TIFF image into your chosen photo processing software don't be alarmed! You probably won't see very much except for a couple of stars. This is because all the interesting image data is very faint and needs teasing out of the picture by stretching the lower intensity image data while reducing the higher intensity data, a process known as non-linear stretching. This technique and others will be covered in more detail in Session 2 - Image Processing, coming soon.
The photo processing software that most pro-am imagers use is PhotoShop but the full version is now only available in a Cloud based version which has to be leased annually at vast cost. There are lots of tutorials on the use of PhotoShop for astro-imaging, special automated astronomical processes and plugins.
Although PhotoShop is very expensive, Adobe has now released a Freeware version of one of their older versions PhotoShop CS2 which can be downloaded from: here.
Don't forget to get the serial number for this freeware version which is:
Windows Version CS2: Serial Number: 1045-1412-5685-1654-6343-1431
Please note that MKAS accepts no responsibility for the trustworthiness of this third party website and it is your responsibility to check for any additional downloads that may come bundled with CS2 from their website. Always read the messages given by the installer software and disable anything other than the software you intend to download.
Another simpler and cheaper photo processing software is PhotoPlus X7 from Serif. There is a free starter version here with the full version only costing £79.99 to download. The author has used a previous version of PhotoPlus and it does contain most of the functionality of PhotoShop including, Levels, Curves, Layers and various Filters. These are all the tools that you need to get really great results and there are even Tutorials on You Tube.
Almost all the PhotoShop astro-imaging tutorials can be followed using the much cheaper PhotoPlus but the tools used will be called something slightly different and be located under different menus, but not much different, it's easy to guess!
Very Basic Image Processing
Image processing will be covered in more detail in the second session on Astro Imaging. However, to get you started, here is a very basic tutorial. This has been written based on PhotoShop CS2.
After loading in your stacked image don't panic! The chances are you won't see much apart from a few stars as most of the interesting image data will be too dim to see on the screen.
M57 Stacked and calibrated in AstroArt 4
First find the find the Levels tool (Image:Adjustments:Levels) which should bring up a box something like the following:
Typical Levels graph on first importing your image
This histogram shows the relative percentage of pixels(vertical) that have the intensity 0-255 (horizontal). This shows that the vast majority of the image is compressed into the lower 10% of the available intensity in your image. But that there are the odd one or two pixels with intensity right the way up to 255. Exit the Levels tool without changing anything
Now find the Curves Tool (Image:Adjustments:Curves) which will bring up a graph looking like the following:
Typical Curves graph on first importing your image
This will initially show a straight line graph running from lower left to top right. Notice the light grey area to the left which is where all the image data is. Click on the line and stretch it upwards fairly steeply on the left side so that the new line is above the original straight line. Then click on the new line at around 20% across to the right side of the graph to set a break point on the new line. Then click on the new line again further to the right to force the new line over towards the top right and then maybe add a fourth point to give a linear section up to the top right corner. The resultant graph after stretching should look something like this:
Curves graph after first Curves stretch
What you are aiming for is to produce a straight line section across the image data followed by a gradual tailing off of the stretching to give a straight line section running up to the top right. Do not let the line hit the top of the graph, only in the top right corner.
OK the stretch and you might start to see some of you image coming through as shown below:
Image after first Curves stretch
Now go back to the Levels (Image:Adjustments:Levels) and look at what has happened:
Levels after first Curves stretch
You will notice that the peak of the histogram has moved to the right because now the low level data has been increased in amplitude while the high level data has not.
Now we can get rid of some of the area to the left of the heal of the graph by moving the black point arrow across the graph BUT make sure that you leave some space between the arrow and the heal of the data, never remove it all, like so:
First Levels adjustment
Just before making this adjustment notice that the black area of space in you image has taken on a slightly grey appearance. OK the adjustment and you will notice that the background has returned to virtually black.
Now make another Curves adjustment to result in a similar shape of graph as previously. Depending on how bright your image data is you may need to be less aggressive with this second stretch as shown below:
Second Curves stretch
Now when you click on OK you will almost certainly see the background start to lighten and turn grey. You may also notice straight areas at the very edges of the image that show different levels of grey. This is caused by the stacking software having to move each sub-frame to match the position of the stars on each sub-frame. This will be dealt with as the last step of the process.
Image after two stretches and one Levels adjustment
Now go back to the Levels and remove most of the black level to the left of the heal of the image data.
Second Levels adjustment
The background will have reverted to almost black again. The amount of black level you remove using the Levels dialog and the resulting black level in your image is a matter of taste. Some imagers prefer to have a totally jet black background while others prefer a slightly off-black background as they say this is more natural.
Now all that remains if to get rid of any areas around the outside of the image that have been caused by stacking alignment.
This is simply done by selecting the rectangular selection tool (dashed rectangle in Photoshop) and drawing a rectangle around the inside of all the edges of the image and then selecting Crop (Image:Crop) which will leave you with the finished image to save. Make sure you save it in a lossless format like TIFF and then also save it as a Jpeg to email to all your friends. At this point you may want to congratulate yourself and have a drink of something stronger than coffee!
Final cropped image
9. Common problems encountered during image processing:
The above is a very basic image processing guide. A more detailed one will be available shortly.
However, to keep you going, the following is a list of the most likely problems that you will come across:
1. The image has no colour - this is either because it was taken with a black and white camera or you forgot to tell the stacking software that it was dealing with a colour image. OR you may have imaged using a binning technique that groups four or more camera pixels into a single super-sized pixel. This reduces the amount of time required to acquire a reasonable signal level from a faint object. Binning normally produces a black and white image only.
2. The colours look wrong - this can either be because you selected the wrong Bayer matrix pattern in the stacking software or you need to adjust the colour balance of your image. If the majority of the stars do not look white then this is probably the cause. Refer to the Help File for the photo processing software to find out how to balance the R, G and B colours.
3. Stars look oval or worse still short streaks - this is caused by your telescope's mount not tracking absolutely accurately. To get the best (small round) stars most mounts need auto-guiding which is another level of complexity for astro-imagers and will be covered elsewhere.
4. Round doughnut shapes in your image - these are called dust doughnuts, motes or bunnies. They are caused by small grains of dust either on you camera chip, on any filters in front of the chip or possibly a star diagonal if you used one. Try to clean your filters and the front face of the camera chip BUT please read up on how to do this first and DEFINITELY DO NOT USE A HANDKERCHIEF TO RUB THE DUST AWAY. No matter how tiny the dust it can permanently scratch glass surfaces so always use a puffer to blow as much dust away as possible, then use a special CLEAN lens brush and then finally maybe a professional Lens Pen. You will need a magnifier and LED light to see the smallest dust grains. To determine where the dust is, look at the size of the doughnuts. If they are totally black small specks then they are on the front face of the CCD chip, take great care cleaning this. If they a small but with a definite clear centre then they will be on a filter (maybe IR blocking) which is close to the chip. If they are larger, they are still further away from the chip. This can be corrected by taking Flat frames and calibrating your image during the capturing process.
5. Vignetting - this is where the centre of the image looks brighter than the outside. This can be corrected by taking Flat frames and calibrating your image during the capturing process.
6. Image lacks punch - this is normally associated with hardly any difference between the target brightness and the background sky brightness. This is caused by not having captured sufficient light from the object being imaged. This means you need to either increase the gain of the camera (results in higher noise levels), increase the exposure time (puts more demand on the tracking of the mount) or choose a brighter target to get started with (best solution for beginners).
10. Improving the image quality - autoguiding and image calibration
Once you have taken you first image and processed it you will want to make a few improvements. The simplest and cheapest way of improving your images is by a process known as calibration. This is actually a cunning sequence of digital manipulations, using some special images taken at the same time as your image sub-frames. These typically consist of Flats (as discussed above), Darks and Bias frames which are all required to compensate for things such as dust doughnuts, vignetting, thermal noise and bias in the raw data obtained from the camera. Once the raw data has been calibrated it will allow you to tease much finer details from your images. This will all be covered in the next Astro-Imaging Session.
Auto-guiding is the process of using a second camera (guide camera) to monitor a star somewhere close to the object that you are imaging. This camera is controlled by another piece of software (or could be the same image capture software) which looks at a star image and monitors its position on the guide camera. Any change of position is then computed and used to issue commands to the mount to move the mount to reposition the star image on the guide camera chip. Guide cameras are either mounted on a separate smaller refracting telescope that is bolted to the main scope or they can actually monitor a part of the light coming through the main scope but off-axis from the imaging camera. This is achieved using a device called an off-axis guider (OAG) which inserts a small prism into the light path just outside the imaging camera's field of view. Both techniques have their benefits. Which is best for you all depends on what type of scope and camera setup you are using to image with.
These techniques will be covered in more detail in Session 2 [TO FOLLOW].
Article by Ian Hargraves
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