One of the most important advances which has been made in astronomy in recent times has been the ability of the amateur, for a relatively modest investment, to capture images of astronomical objects of a quality which rivals those which historically only large observatories could produce. You can't completely get around the limited aperture of our smaller instruments of course, but the new, highly light-sensitive cameras which are now available to the "general public", and the relatively sophisticated mounts and guide systems make it possible to do hitherto incredible things from your own back yard, and also from fairly light polluted sites. But of course it's never quite as easy as it sounds, and unless you have very expensive equipment and are very quick to learn, our opinion is that there's a pretty steep learning curve before you will be able to produce high quality images.
These pages are aimed not at the specialist imager, but rather at the beginner - the person who has a telescope and a bit of cash, and who wants to start out on the road of taking and processing their own images. Please remember as you read this that there are numerous alternative CCD cameras, software programs and telescopes available, and only a few are mentioned here, based on our own experience. But the basics remain the same, and hopefully the information set out here will help beginners begin, and will encourage them not to throw their expensive cameras and telescopes into the nearest lake in frustration!
Taking Pictures With Film
Before we start talking about CCD cameras we thought it would be useful to say just a little bit about "ordinary" photograph taking, in the time honored way, using a real camera. I've done a bit of this myself, and have been taking pictures all my life. My father was a photographer and he taught me to estimate the correct exposures for 100 and 200 ASA black and white film, in the days when colour was "up-market". Exposures of one hundredth of a second at f16 stick in my mind, and I was usually pretty close when I guessed which speed and aperture to use in many different lighting situations. However, when I started taking photographs through telescopes I discovered that the normal rules no longer applied. It proved really tough to estimate the right exposure because it's tough to judge actual light levels when your eyes have dark-adapted, and I was only able to get photographs of the brighter planets, Jupiter and Saturn by trial and error (usually quite a lot of error!). What I discovered early on was that to take photographs using film you need:-
a mechanical camera, not an electronic one
special film which works better if it is treated in a special way
a telescope which tracks perfectly or can be guided very accurately, because the exposures you need are quite long
The reason you need a mechanical camera is because if you want to take pictures of objects other than bright planets and the moon you will need long exposures (many minutes), which means holding the shutter open for a long time. It's an unfortunate fact of life that, even when specially treated the sensitivity of film emulsion falls off rapidly after the first few seconds of exposure. That's the way photographic emulsions are designed to work. This means that after the first second or so you have to expose for a lot, lot longer to gain any additional benefit. This is what is called a "non linear response", with the sensitivity of film emulsion biased heavily towards the first few seconds. This is true even for the specially treated films which professionals use. With a battery operated camera and an electronic shutter, which is how most (or even all) cameras are made these days, exposures of more than a few seconds will mean your battery going flat pretty fast. The small batteries in most cameras are designed to briefly operate the shutter, not to hold it open for minutes at a time. Mechanical camera bodies are available through second hand camera shops, but personally we haven't yet got around to buying one, or to learning the film sensitizing techniques which you have to go through to make the film more sensitive. We graduated instead to CCD as fast as funds would allow. That's not to say that CCD is a direct replacement for film images, there are significant differences, but CCD produces almost "instant" results and quite frankly it suited our needs better. But some of the emulsion film astro-photographs which we have seen the experts take seem to have a "certain something" special about them. They have a quality and texture which is quite unlike CCD images, and the nearest analogy we could draw is between vinyl Hi-Fi recordings and CD discs. There is a warmth and body to the old vinyl which the digital medium somehow loses, no matter how expensive and sophisticated the player.
So, with your permission we will move on to discuss CCD imaging.
CCD stands for Charge Coupled Device, and put simply the working part of a CCD camera is a "chip" made up of an array of pixels (photo sensors) on a silicon substrate. While the CCD camera shutter is open, photons of light fall on the photo sites on the chip, where the photons are collected and converted to electrical current. When the shutter closes at the end of the exposure, the accumulated charge on each pixel is automatically downloaded to a computer and the whole array is stored as a digital image. The current generated by each pixel depends on the total amount of light (number of photons) which have fallen on it for the duration of the exposure, and so the generated current is directly proportional to the intensity of the light or the length of the exposure. So the response to light for a CCD camera is not non-linear as in the case of film emulsions (see above), and that is one of the key benefits of a CCD camera, the response to light intensity is linear and constant. Unlike photographic emulsions, the longer you expose, the more light you collect and the brighter the image - in a linear relationship over time. CCD cameras are also ten times more sensitive than film, and so it is possible to image very faint objects quite quickly. A lot of people actually use their camera on "focus mode" to locate, center and focus an object, allowing the camera to take a continuous series of very short exposures whilst moving the telescope around the sky to locate and center the object.
The variation of signal strength with light intensity across the chip creates a black and white image of whichever object you are photographing, lighter areas showing up brighter (more electrical current) than the darker areas. It is also possible to produce color images, by taking red, green and blue filtered images of the same object, and then combining them using special image processing software. This combination of red, green and blue images is much like the way a TV set produces color using three different electron guns, combining the electron beams at a point on the screen to make a color image. Luckily, these days there is sophisticated software available quite inexpensively to both operate your CCD camera, guide the telescope as it tracks the object across the sky, take the pictures and also process the images.
The quantum efficiency of a CCD chip - a measure of how much of the light falling on it is converted into a signal - varies with the camera, but for my camera it is approximately 50%. Which means as we have already said, that images can be captured in a much shorter time frame than for film emulsions. Once an exposure has been made the pixels are discharged (downloaded) and the signal passed to an on-board amplifier in the camera, from where it is converted from an analogue signal to a digital one, using a special converter. It is then sent to the computer and saved as an electronic file, but more about this step later. First, how do we go about setting up to take a CCD image, what equipment do we need, how do we connect it together, how do we take the images and save them, and more importantly how do we process those images so that they look really good?
What Equipment Do We Need?
In order to be able to take good quality images of astronomical objects the following items must be available:
A Telescope and the right connecting adapters so you can mount the CCD camera to the telescope
Either an Equatorial Mount, properly polar aligned, or an Alt-Az Mount with a Wedge (*read further for an explanation of this point)
A CCD Camera
Some form of Finder Device (I use a Flip Mirror or an accurately aligned finder scope) to locate and center objects
A Computer - preferably a Lap Top, with sufficient memory capacity to handle the files
Possibly a Focal Reducer for the telescope so you can reduce the image size to fit the size of the chip on your camera
Quite a lot of Patience
Any kind of telescope will do, but before you are able to take images you will need to be sure that the camera can be properly connected to the telescope. There are a variety of different cameras on the market, and although there are a variety of different types of connector, some attempt has been made to standardize. But beware if you purchase a Japanese telescope - such as a Takahashi - because all their fittings are different from those used in America, and if like us you own both an American and a Japanese scope, then you will end up with quite a large (and expensive!) collection of adapters and connectors. Our SBIG® ST-7E camera is fitted with a 1.25 inch diameter "nose" which can be slotted in to a receiving adapter at the end of the telescope and held in place with small retaining screws. Alternatively you can remove the nose by unscrewing it, and then screw the camera directly to other equipment in the light train, such as a flip mirror or eyepiece extension tube. Most cameras come fitted with a "T" thread for this purpose. Some people get a little nervous about holding their precious several thousand dollar camera a few feet off the ground using only a couple of tiny screws on the nose piece, which is why we usually screw ours on using the "T" thread and adapters. Others modify the assembly by fitting larger screws to give the camera a little more security. Whichever way you finally decide to connect your camera to the telescope, do please be sure to do it first in the daylight! It's really tough to fiddle around in the dark if you haven't done it before, and we can guarantee you will drop an essential screw in the grass, if not the camera itself! It's also a good idea to practice balancing the whole assembly in the daylight before you try to do it in the dark. Knowing how many counterbalance weights you need and roughly where they should be positioned can be a great help before you do it by touch and feel only. Finally, there is no worse feeling than to get to the observing field, possibly several hours drive, only to discover you are short one essential connector, rendering your imaging session useless.
To be able to take astro photographs the telescope must be equatorially mounted and accurately polar aligned. An important note is that if you have a telescope like one of the many excellent Meade® Go-To scopes, then you must buy yourself an equatorial wedge before you can attempt astrophotography. Many people have asked us why this is, because the Meade telescopes centre on an object and track it very precisely. The answer is that the mounts they use are called "Alt-Az" mounts (altitude/azimuth) which means they move up and down and left and right. Objects do not move in this way across the sky, they move in a wide arc, rotating about the celestial pole, near the star Polaris. The best illustration we can give uses Jupiter and it's moons. When Jupiter rises in the east, if you observe it through a telescope you will see that the moons and cloud belts are inclined to the horizon at an angle, rather like this:
As the planet (or any other object) rises in the sky, it will ultimately reach it's highest point for the night - called "transition" - and at that point you would see the following:
So, in this example you can see that the object appears to have tilted over, and if you were taking a picture using a camera fixed to a telescope which was only capable of moving "up and down" and "left and right" (altitude and azimuth), then the object would appear to rotate within the field of view, which would of course blur the image. At the high magnifications which we use to image distant objects, this effect becomes noticeable quite quickly, and any exposure longer than perhaps 30 seconds would make the blurring obvious. You need a wedge!!
Like we said previously, there are a lot of CCD cameras around, and we will make no attempt here to judge one type against another. We bought an SBIG (Santa Barbara Instruments Group) ST-7E camera because they have an excellent reputation for reliability, performance and after sales service, and that was good enough for us. In fact, we were so pleased with the performance of the camera that some years later we bought another SBIG camera, their larger ST-8E. One of the main variations in cameras is in the number of pixels in the "array" and also the size of the chip. The ST-7E camera which we own has a chip measuring 6.9mm by 4.6mm and a pixel array of 765 x 510, making a total of 390,150 pixels, whereas the ST-8E has chip four times larger, and with four times the number of pixels. Each pixel in both cameras has an effective size of 9 microns, and ultimately the size of the pixels determines the sharpness or "resolution" of the images it will take. With our camera, using an f/6.3 focal reducer on our C-14 telescope (operating at f/7), we achieve a frame size of 9.66 x 6.44 minutes of arc and as a guide for you this means we can just about fit M51 - the spiral galaxy in Canes Venatici - in to the frame. Using the same camera with our Takahashi FSQ-106 refractor at f/5 we get a field size of 60 minutes x 40 minutes, which hugely expands our imaging capabilities. As an approximate guide for you, this larger field size will fit all of Messier 8, the Lagoon Nebula on the chip. Multiply those dimensions by two (twice as wide, twice as deep) and you have the field sizes for our larger camera. There are more expensive cameras available, with larger arrays up to 3.2 million pixels, and in addition to getting a much larger field of view, you can also get smaller pixels (6.8 microns) which can give you better definition, depending on many factors like seeing conditions and the focal length of your telescope. We will not dwell any more on this subject in this article.
Some cameras, both of ours included, have a guide chip installed. The guide chip is a small additional CCD chip, mounted alongside the main chip and used exclusively to guide the telescope to keep it centred on the object you are imaging. How it works is that the guide chip takes a series of images of a selected guide star while your main chip is exposing the main image. If the guide star moves away from its position on the guide chip, the camera feeds commands to the telescope drive system to make corrections to bring it back to where it was. In theory that all sounds fine, but in practice it can be very difficult to do. It can be tough to encourage the camera to drive some scopes properly, and finding a guide star which happens to fall on the guide chip and which is bright enough to guide by can also be difficult. We have much better success with our short focal length Takahashi than we do with the longer focal length C-14, but all techniques require a degree of skill and practice. Whatever you do please don't despair, because there are other ways to achieve acceptable images of certain brighter objects without guiding your scope, and we will go through this in a moment. This basically involves taking a lot of shorter exposures and then electronically "stacking" them. You can also guide your scope manually using a well aligned guide scope to keep a star centred on an illuminated reticule, and this technique can be very effective, if a little tedious.
One additional comment about imaging is that, if your camera is not fitted with an anti-blooming device, you need to be a bit more careful about exposure length. Anti-blooming is a mechanism to stop individual pixels from over-filling when they become saturated with electrons. This prevents brighter stars in an image from "bleeding" into neighbouring pixels, creating an amorphous "blob" where the star should be. We chose our camera without anti-blooming, because anti-blooming reduces the sensitivity of the camera, changes the brightness balance of an image and makes true magnitude determinations difficult, if not impossible. But if you choose a non anti-blooming camera you do have to take more care when imaging, because you have to make a more careful determination of which exposure length to use. Too short and you will have too faint a signal and your sound to noise ratio will be poor. Too long and you will "burn out" the brighter stars.
What we normally do is take a guess at the correct exposure and shoot one white light image. We then measure the light intensity of the brighter parts of the image and check for burned out areas - usually stars. In this way we can establish the optimum exposure, and then set up a series for saving to the computer.
If you are trying to decide which CCD camera to buy and need more detailed advice, use the SBIG link on the main page of this website and browse through the options. We can offer a couple of comments though which might help. The "E" designation in the SBIG cameras shows that they have the new Kodak "E" chip fitted. This chip is a lot more sensitive to blue light than were the older models, and although blue sensitivity is still not equivalent to the other wavelengths of light, it's pretty close. If you are offered one of the older cameras second hand, do bear this in mind. It should work quite well, but you will be required to take considerably longer blue exposures than if you had the "E" version. (But also note that you can send your camera to SBIG and have them fit an "E" chip for you!). Another question is whether it is "worth" spending the extra to buy a camera with a larger chip. Our view is that, a smaller chipped camera like the ST7 is a better choice as a first camera. In a number of ways it is easier to use, and the images easier to process than if you go for a larger chip like the ST8. Factors such as smaller file sizes and shorter download times can be significant when you are starting out in this hobby. There is no shortage of objects which will nicely fit in the ST7 field of view, especially using a scope like the Takahashi. Having said all that, if you go for an ST8 it does give the image much more perspective. Being able to "stand back" and see some of these objects, almost hanging in the blackness of space makes them look so much better.
But don't forget that the larger the chip, the larger the file size which your computer will need to be able to handle. The ST7, without binning (see later) produces a file of 765 kilobytes. Typically when you are taking a series of shorter exposures for stacking, you will shoot 120 images or more for an object on any particular night, and that's 91.8 megabytes of data! The ST8 files are more than four times bigger, so be sure that your other equipment (hard drive capacity and processing speed) is up to the job of handling these files. Also remember that when we get to the point where we want to process the images, your computer is going to have to hold several of these files in memory at the same time, and the bigger they are, the slower your work will go. You will need enough RAM memory to handle the files as well. Lots to think about hey?
A Finder Device
The first thing you will want to do when you are taking a picture of a faint fuzzy is to find it and center it in the field of view. Simple hey? You've been finding things visually for ages and you are now an expert at putting M65 in the middle of the eyepiece? Well, take that eyepiece off and poke a CCD camera up the spout and you will discover it's a mite difficult to find things - don't forget we are talking about centring the object in a field of view which could be as small as 9 x 6 minutes of arc! So most people use some form of aid to find and centre their chosen object. It is possible to use an accurately aligned finder scope, and others use an in-line device which takes a small sample of light and sends it out to the side. Another way, referred to previously is to set the camera on "focus mode" which produces a series of images very quickly. You can sometimes find and centre an object using this technique, provided you are pretty close to it to begin with. What we used on the C-14 before we bought a Go-To mount is a device called a "flip mirror". This is a box-like device which has a small mirror inside, located in the light path at a 45 degree angle, diverting the light sideways to an eyepiece, just like a star diagonal. The CCD camera fits on the end of this assembly - and stays there all the time you are working. In this way you can find your object visually and center it with the mirror diverting the light to the side mounted eyepiece. When the object is centred in the eyepiece the mirror can be flipped upwards out of the light path, allowing light from the image to pass straight through the box and fall directly on the CCD chip. One huge advantage of this system is that you can adjust everything so that when the image in the eyepiece is in focus, the image on the camera chip is also approximately in focus.
But there are disadvantages to using this technique. The first is that the flip mirror assembly adds weight to the back of the telescope. This is only important if you already have a lot of gear hung on your mount and are pushing the limits, but anything which increases the turning moment about the various axes has to be a negative. Second, all telescopes are limited by the amount of "back focus" available - the distance you can crank the focuser back to compensate for the presence of additional equipment in the light train before it reaches the camera. On our C-14 we find that, using the Meade 2" flip mirror, plus the f/6.3 focal reducer (which we use to increase the field of view and decrease sensitivity to tracking variations) and the CFW-8 colour filter wheel in front of the camera, we can still bring the system to focus. However, if we use an f/3.3 focal reducer it is no longer possible to focus, and we either have to remove the flip mirror assembly and use some other method to center the objects, or remove the colour filter wheel and image in monochrome. Something has to go.
With our Takahashi on the EM-200 mount we are only able to use a Telrad and the excellent but small finder scope which comes as standard with the main telescope. Sometimes we cheat and put the little tube on the NJP Go-To mount, but if for any reason we can't do this, it is a major source of frustration because it can prove very difficult to find the fainter and smaller objects. Even under dark skies and with much sophisticated support equipment, some of the fainter objects simply defy all attempts at finding them.
A Computer and Software
As has been said above, make sure your computer is adequate for the task. Ideally you will need a lap top which you can set up alongside the telescope and use to control the scope and take the images. Some people who are lucky enough to possess an observatory have their desk top computers set up in a next door room or in the observatory itself, but you do have to be careful about lengths of your cables and wires. Signal loss can be quite significant if you are a long way away.
We drive the telescope and collect the raw images using a lap top computer which has a 1Ghz processor, 40Gb hard drive, 128Mb of RAM and a CD burner. We would suggest that this is a minimum requirement for working with the ST-8E camera and the larger files. When we get home we transfer all the images to our desktop computer using a memory stick. We usually process the images in the field on the lap top and store stacked master files on the desk top, saving all the dark subtracted (see later) raw images on a CD disk. This means that we can always go back later on and re-process the images to use newly acquired knowledge (you will find that you learn all the time). It also means that if our hard drive crashes we don't lose everything!
We use Maxim DL to run the camera, and also to collect and initially process the images. Final processing is done in Adobe Photoshop. Some people have additional image processing software such as Mira, but we have never seen the need for them. Maxim has proven to be an excellent and very versatile program, but nothing is ever perfect for every situation. For finding things in the sky manually we use Megastar, but we use The Sky to run the Go-To mount. As a side note, Megastar runs faster and is more versatile than The Sky and we feel it is more useful as an astronomer's tool. This feeling is shared by most other astronomers with whom we work. One thing you will definitely need is a way of darkening your computer screen when you are imaging. Yes, we know that you can select "night mode" in all the programs, and that the screen will look really dark, but under a dark sky you will find that your screen is still way too bright, and you will be hounded off the observing field by others unless you do something. We have discovered that finding thick, red plastic sheeting is almost impossible, especially as you only want a small piece. Using many layers of thin red plastic laminate will cause your images to appear all crinkly and distorted. What we did was to buy and cut to size and shape two clear perspex sheets. Clear plastic is easy to find in most hardware stores. We then cut several sheets of thin red plastic and sandwiched them between the clear sheets, sealing the sandwich using adhesive tape. Our home-made screen, using this technique, has proven very durable and highly effective. If you try to test this screen out in the daylight you will find that your computer screen is probably totally invisible, but it should work just fine in the dark. You'll be amazed at how dim it needs to be to avoid losing dark adaptation.
Setting Up The Equipment
The order in which you do things is not really critical, but if you get into a routine we believe you will find that you are less likely to get to a certain point and then realize that you have missed out an essential step, or have left out an important piece of equipment. We usually set up the telescope and mount first, screw the focal reducer (where required) to the back of the scope, then add the flip mirror and roughly balance the whole assembly, remembering that when you add the weight of the camera you are going to have to re-balance. Note: If you are using one of the Takahashi "short-tube" telescopes like the FSQ-106, then the whole barrel moves in and out when you focus the scope. This can significantly alter the balance and so be sure to re-balance the assembly when you have focused the telescope. Then we plug in the computer and start it up, and once it is fully booted up we open Megastar (to find objects) and Maxim (to drive the camera). We then connect the download lead between the CCD camera and the computer, set the camera safely on the floor somewhere, and connect power to the camera. Next is to send commands to the software to electrically "connect" the camera to the computer so that the software "sees" the camera. We then start the camera cooling to the temperature which we have chosen to use. You will see later that CCD cameras need to be cooled to minimize electrical noise in the chip, and it is best to start the process early - perhaps 20 minutes or so - to allow the temperature to stabilize.
While the camera is cooling we normally use that time to polar align the telescope, making sure it is as accurately aligned as possible, and also to shoot some darks and flats (see later). Although the system will still work if your alignment is not spot on, you will find it makes life a lot easier if you are as close to polar aligned as possible. Do this and you will find you can wander off and do something much more interesting, like observing through another telescope or warming your toes indoors, instead of watching the camera all night. If you are not guiding the camera automatically, then you will need to keep the image centred in the frame as closely as possible. The reasons for this is that when you finally assemble all your images, you will only be able to use the pieces of the final image which contains all components. So, especially if you are shooting colour, if the image in your red filtered image was to the left of the field, and that for your green filtered image to the right, you will be missing pieces of the right side from one, and the left side from the other. Neither the left nor the right sectors of the final image will therefore be useable.
Finally, when you are satisfied that your polar alignment is perfect (use drift alignment if possible, to be sure), carefully mount the camera on the telescope, tighten the retaining screws, check to make sure the cables and wires are not hung up on other equipment, affecting free movement of the whole assembly, and balance your scope in both axes. As a general rule, the telescope should be perfectly balanced in the declination axis, but should be slightly heavier on the east side of the RA axis. This is to ensure that your drive motors are working against a little resistance and helps to improve tracking performance by taking out backlash. This is critical for those mounts, like the G-11, which are "excellent value for money" products, but it is less important for the more expensive mounts and drive systems, or if your telescope weighs only a fraction of the rated load bearing capability for the mount it is on. With those you shouldn't run into problems if your set-up is in balance.
Taking The Pictures
As covered in the opening section, you have several choices to make when you are ready to take pictures. One of the more important of these is whether or not - of you camera has a guide chip fitted - you wish to auto guide the telescope while the image is being taken. Mostly this decision hangs on the brightness of the object you are trying to shoot, the ease with which you can encourage your camera to drive your mount, and the accuracy with which your mount will track without being guided. A lot of people, if they own a mount which tracks very well, and if they are shooting a bright object, will choose not to auto-guide. We only auto-guide if we are shooting a fainter object, like a dim galaxy cluster which needs perhaps ten to sixty minutes of imaging time. But you have to be careful! Remember what we said about stars "burning out"? You need to be aware that if there are any stars of other objects in the field of view which are quite bright, then a long exposure will saturate that area of your chip and can lead to disfigurement of the image. Remember the advice to take at least one exposure to assess whether you are going to experience any problems, and if the coast is clear, then auto-guide. Instructions for auto-guiding can be found in the imaging software which you are using.
But what if you cannot or do not wish to auto-guide? Well, there are well established procedures you can use to create excellent images, and the following procedure describes how to do this. Basically, the procedure involves taking a series of shorter images and then combining the best ones electronically, and everything which follows concentrates on how to do this.
This is perhaps the place to explain in more detail why we cool the camera. A CCD imaging chip is simply a complicated little piece of electronic circuitry. It is therefore subject to electronic "noise" which is nothing more nor less than random rumbles. This electronic noise can be reduced significantly by cooling the chip below ambient temperature, and our camera is capable of reducing the temperature by up to 30 degrees C, using an on-board thermoelectric cooler. Although it is possible to achieve up to 30 degrees reduction in temperature from ambient, we normally work between minus 5 and minus 15 degrees. This is because at those temperatures we find the noise to be not noticeable, and also because if you try to cool the camera down too far, the operation of the cooler itself has been found to introduce noise to the image. Normal background electronic noise is visible as "sparklies" all over the image. When we first saw them we were all excited because we thought they were stars, but no such luck - they're noise. The advantage of a CCD imager is that you can take a "blank" exposure - called a Dark Frame - and subtract it from the main image to produce what we call a Dark Subtracted Image. Here are some examples of what we mean. Please note that we have degraded the file quality for this web page so that download time is reasonable:
This is a ten minute raw image, just as it comes from the camera, showing "sparkles" caused by electrical noise. Note how some of the stars are "bloomed" by over-exposure
|This is a "Dark Frame" made up of three exposures with the shutter closed and combined to ensure that the dark is truly representative of the noise|
Another thing we have to do is to take an image called a "Flat Field" which captures imperfections arising from dust on the window covering the CCD chip, dust on the telescope optics, fingerprints and so on. We can then electronically subtract both the dark frame and flat field from the raw image to create an improved image.
|This is a flat field, taken through the Takahashi telescope system used to capture the above image of Messier 33. It was a 2 second exposure in an almost dark sky.||This is the original image with the dark frame and flat field subtracted from it. We have also electronically subtracted cold and hot pixels which is a function of the individual camera|
So, what we are about to do under our dark sky is to take a series of images, just like the image of Messier 33 which we have used above to demonstrate dark subtraction. But first we must be sure the image is properly focused.
Focusing is absolutely the most critical thing you will ever do when you are imaging. We have wasted more hours than we care to think about, taking out of focus images which looked fine at the time, but were fuzzy when we got them back home. Let's assume that you have found your image and centred it in the camera using the flip mirror, and have remembered to flip the mirror up out of the way (it's amazing how many times we forget to do this). As stated previously, Maxim is equipped with an excellent function, called "Focus Mode" and how this works is that you select a short exposure time - say 3 seconds - and bin the image 3x3 (more on this in a minute) so that you get small files and rapid download. If you then select "continuous" and start the camera clicking away in focus mode it will take a continuous series of images for you so that you can fine tune your focus until the image is nice and sharp. Do be sure to focus through the filter which you are going to use to take the first series of images, and you may have to re-focus every time you change filters. Although filter sets are supposed to be par focal (they all focus at the same point), they often aren't. You should also be aware that focus will change through the night as your telescope and equipment cool down. Check it frequently. The software programs also provide you with little electronic histograms and gizmos to help you get the final image pin sharp. Use them, and then you will know that you are as close to focus as the seeing is going to let you get that night.
On the subject of "seeing" you need at this point to make an assessment of how good the seeing is that night. If seeing is poor, then taking un-binned images can be a pointless waste of time and computer space (un-binned images take longer to download and occupy more space). Under poor sky conditions it is not possible to achieve image definition above a certain limit, and the increased resolution of the un-binned mode does not improve what can be achieved using 2x2 binning. If the seeing is average to good, then taking un-binned images will give you much more flexibility and sharpness in your images and although download time and file size are longer and larger, the added time is well worth while.
Taking a Series of Images
So, the first thing you need to decide is whether or not to "bin" your camera, and now is a good time to explain just what binning is. Imagine your camera chip as an array of pixels, all working independently so that each pixel sends it's own signal to the amplifier when you download the image. That is called working in un-binned mode. But these cameras are capable of connecting together groups of pixels, either in groups of four or groups of nine. The net effect of this is to create clusters of pixels which work together as one pixel. The first case (clusters of four pixels working together), is called 2x2 binning, and the second case (clusters of nine pixels working together), is called 3x3 binning. It's hard for us to tell you all the circumstances under which you would choose to use one or the other forms of binning, beyond what has been said above. In our case we only ever really use un-binned and 2x2 modes. What binning does is it increases the speed and sensitivity of the camera, but it also reduces the resolution. For galaxies and globular clusters you are really looking for as much resolution as you can get, and so binning goes against that goal. However, if you want to work faster, perhaps if clouds are threatening or time is short, or if the seeing is particularly poor and you are wasting your time trying to get very high resolution images, then a 2x2 binned image will be adequate, is much faster to acquire and the file sizes are considerably smaller than for an un-binned image (195 Kb). Hope this helps.
The next thing you need to decide is the exposure, and a lot has already been said on this. Exposure will to some extent be dictated by which type of object you are shooting. For galaxies and faint nebulae you need long exposures, but for the brighter star clusters - especially globulars - we have found that shorter exposures avoid "burning out" the core of the cluster. The exposure times will also vary depending on the aperture of your telescope. The C-14 collects vastly more light than the FSQ-106 (14 inches versus 4 inches) and as a very rough guide we use about 5 to 10 seconds in the C-14 for bright globulars, but 30 seconds to a minute in the FSQ for the same object. In the C-14 we use 1 to 5 minutes for galaxies, but with the FSQ we sometimes expose, guided, for 30 minutes or more. We usually shoot 20 or 30 white light images for an object and perhaps only 10 colours through each filter, re-focusing each time we change filters to start a new color sequence. The white light image is known as a "luminance" and gives sharpness and definition to the object. Another tip is that if you use a dew shield (we use a heated Tuthill dew shield on the C-14), this can act as a very effective "sail" if there is any kind of breeze. This can, and will move the scope just enough that your images will be fuzzed. Take it off if you can, and remember to re-balance the scope! If you can't take it off because the humidity is too high, then pray for calm conditions. It took us quite a while to realize that this was what was causing some of our "tracking" problems.
When your fingers have finally gone completely numb, and when you've decided you've had enough for the night, you are going to want to close down without damage to the equipment. STOP!! You're about to forget one of the most important steps in the whole process - you need to take some Dark Frames. Remember that these frames will be necessary for you to be able to subtract the electronic noise from your images. Before you turn things off, select "Dark Exposure" on your camera control software, and take three or more exposures at the same temperature and for the same exposure time as you used for the images. If you have taken exposures at more than one temperature, or have exposed for more than one exposure time, do make sure that you take darks for each exposure time and temperature. You take three or more darks because you are going to combine them to average out random effects like cosmic ray hits, which can spoil your dark frame. You are going to take them at the same time as your main exposures, not because it is impossible to do them afterwards, but because you want as many of the conditions to be the same as they were when you actually took the pictures. Some people even make sure that there is absolutely no random light around when they shoot their darks. The cameras are so sensitive that bright lights can be detected and can spoil a dark frame, even with the shutter closed.
Once you are sure you have got yourself some good darks, you can continue with the close down procedure. It is good practice to go to your camera control software and select "Go to Ambient" to allow the camera's temperature to gradually return to normal, rather than just turning it off. This helps to avoid condensation inside the instrument and like we have said, is in any case good practice. Although the cameras are fitted with a moisture absorbing attachment - or desiccant - it is not unknown for droplets of moisture to appear, spoiling the images. Following this warm-up procedure helps, and coupled with regular regeneration of the desiccant as recommended by the manufacturer, we have been able to avoid any problems with condensation. You can be taking the equipment apart while the camera is warming up, and it can be laid carefully out of the way on or under the observing table while you work. Last step is to power down the software and turn off the computer.
So, you now have a lap top computer stuffed full with all sorts of images, and you're dying to see what they're like. Let's go indoors and see what comes next.
As we said a while ago, we normally immediately download our raw images to the desk top computer, just in case of hard drive problems. How you handle this is up to you. The first thing you will want to do is to prepare your master dark frame, which involves median combining the three (or more) darks which you shot at the end of the imaging session. The image processing software will lead you through this procedure, and at the end of it you will have what we call a "Master Dark" which should look somewhat similar to the one we showed previously. Now open all the raw images which you want to dark subtract. Please note that the number of images you can open at once, and the speed at which your computer will work will depend heavily on the number of images you try to open and the memory capability of the computer. With 64 Mb of RAM we are able to open twenty unbinned images (765 Kb each) without problems, although it wheezes a bit. Now click on Process/Set Calibration, and select the master dark you made just now, using the selection button for dark frame on the menu panel. You will find if you click on Calibrate All, you should find that your computer will automatically subtract the dark frame from all of your images.
Note: You will notice that there is another little box in this menu called "Flat Frames". For the first years while we were learning how to take images we did not take flat frames, and really did not notice any deleterious effects as a result. Most experts will tell you not to bother with flat frames for about the first year, or until you have thoroughly mastered the rest of the techniques. We agree with this statement, and so will not be dealing with flat frames in this presentation. Now you have to save all the dark subtracted images, and just in case we ever want to go back and re-do the dark subtraction, we save each dark subtracted file to a different directory on our hard drive, at least until we are satisfied that the final images are OK. While we are doing this we also do one more thing. Not all of the pixels in your expensive camera operate properly. We're not talking this time about electrical noise which can be subtracted out of the image, we are talking about "Hot Pixels" and "Dead Pixels". These are pixels which, for some reason at this point in time, on this occasion, chose either to work overtime and show up as a bright pinpoint, or not work at all and show up as a dead spot. Either way, these pixels need to be removed from the image, and you do this by selecting "Kernel Filters" in MaxIm and then clicking on "Dead Pixel" - OK, and then "Hot Pixel" - OK. Use the default settings for how much severity to use in these filters to avoid degrading the image. When you've done this you can re-save each image. What usually happens in our case is that we will have some images in which the telescope has clearly moved, or for which the tracking was poor. We normally discard those images at this stage. They are useless, nothing can be done with them, and the temptation to use them will ultimately degrade your final image. But be sure as you do this that you are deleting the right ones - once they're gone, they're gone!
Now we have come to the point where we want to stack our individual 10 second, 30 second or one minute or longer images to create a master image which is roughly equivalent to a continuous exposure of several minutes. We do this in Maxim by selecting "File" - "Combine Files" from the drop down menus, and then highlighting all the files we want to stack together. Click on OK and then what we normally do is manually align two stars in each image to ensure that any effects of field rotation due to less than perfect polar alignment are removed. We normally stack all the images from one set of filtered images, working our way through Blue, Green, Red and White Light, ending up with a set of four stacked masters. We try to make sure that the total number of images we stack for each colour is about the same, to avoid mismatches when it comes to combining them to form the final colour image. Using the same image series we used to demonstrate dark subtraction, here is a complete set of stacked masters for M33, the large spiral galaxy in Triangulum. You will see that the sizes of the stars in each image are approximately the same. This is important. What you will find, especially if your focus has changed slightly between filtered image series, producing slightly larger stars in one image as compared to another, is that when you combine these images to make a coloured image, the stars will have little coloured rings around them, with the colour of whichever image had the larger stars!
|The 30 minute Green master image, made by stacking three 10 minute images||The Luminance master image, made by stacking eight 10 minute images|
|The 30 minute Red master image, made by stacking three 10 minute images||The 30 minute Blue master image, made by stacking three 10 minute images|
The final step is to combine these images together, using the "Colour Combine" option in Maxim to create a colour image. During the combining process you are asked to align the images once more, and we usually again use manual alignment using the two star option, choosing stars from opposite sides of the image. The final click on OK requires you to select the LRGB colour balance. This parameter will be affected in part by the camera and filter set you are using. It is a good idea, when you have a few hours to spare to take an image of something which has a known colour, like a person's face or a colour photograph to which you can refer afterwards, and then decide on which balance your system needs to be true. Our system seems to require R:G:B in the ratio of 1:1:1.6. Yours may be different, so experiment. And also be careful if you have been taking images of an object which is particularly low in the sky. Blue light is absorbed by the atmosphere much more strongly than red, and so the same exposure time may not give you a proportional image in the different colours. Combining these filtered images together in the same ratio as you would have done for an object which was overhead could give you an image which is much redder than should be the case.
And that's it - click on your final OK and you should see this:-
We hope your first experiences with CCD are more successful as a result of the information contained on these pages. Feedback and comment are welcomed - critical or otherwise.
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