Astronomy

How to Photograph Objects Visible in a Telescope



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"How to Photograph Objects Visible in a Telescope"
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In the earlier years of astrophotography, people were restricted to the use of wet photography (35mm), but lets face it the 35mm format cameras have gone the way of the dinosaurs. In their place is what has become known as the CCD camera. These cameras have taken the world by storm. No longer are we bound by the laws of trial and error, but are instead freed up to embrace a world of creative imagery, unlike anything ever seen or experienced by 35mm cameras. No longer do we have to wait for the developers to reveal the fruit of our labors, only to find that what we worked so hard to produce, has been lost to the wrong frame setting, or the wrong exposure time, instead we are now able to view the image as it is being produced. This away if you do not like the image, you can change the varieables, and produce another.

Through the years astrophotography has come a long way. But even though this fantastic hobby has increased in technology by leaps and bounds, we are still bound by a few of the simple laws that must be adhered to, if a quility image is to be expected. These laws are simple and effect both 35mm, and CCD imaging. The first and foremost is that of Seeing Conditions. Seeing conditions revolve around those variables brought on by the atmosphere above us and the enviroment around us. What I mean by this is for example:

Atmospheric Conditions:
1. Upper atmospheric convection currents, causing waves or distortion in the images
2. Is it cloudy outside.
3. Is it raining
Enviromental Conditions:
1. Is it Humid outside
2. Is it cold
3. Is it hot
Physical Conditions:
1. Are you in the right mindset
2. Are you feeling well

All of these conditions have a bearing on how well an image will turn out. What about equipment?

The Mount

You could have a world class imaging device, but if the tracking platform is not up to par, then the camera is useless. So everything you do hinges on the mounts quality and its weight capacity - which are both contributing factors in how well your images turn out. Take for example a mount with the ability to carry large optical assemblies - but possesses poor tracking capabilities - this mount will be limited to smaller optical assemblies of shorter focal lengths.

A lot of people who get started in CCD imaging have a problem with under estimating the downfalls that one will encounter with this hobby. One of these problems is how much time do you have to devote to this mistress who will slowly suck you dry financially if you do not understand what is the right equipment to purchase, and when to purchase it.

Take for instance that you jump into this hobby with both feet - run out and buy a long focal length telescope with a large aperture. Believe me if you do - I guarantee you that you will have bitten off more than you can chew. You will be so tied up with all the different things that have to be mastered first before imaging can even take place you will become so frustrated that you will probably give up right then and there. What I am talking about are things such as tracking, guiding, balancing and rebalancing due to added equipment, calibrating the mount, periodic error correction, and so on and so on. This is not bad if you were born a rocket scientist, but for most people this is were the telescope winds up in the closet never to see the light of a star again.

On the other hand if you start with a shorter focal length, you can remove a serious number of problems that will allow you to kick back and enjoy yourself. So in essence what I am trying to tell you is - you are better off purchasing a really good mount along with a short focal length optical system.

Initial Cost

If you are like most amateur astronomers you are functioning on a champagne diet with a beer salary. So with this in mind a little forethought is needed on your part. If you are on a fixed budget, then put your money where it will do the most good, and never buy a component that falls below the set quality minimum. When buying new equipment ask around check online with the internet. Formulate your own idea of what you want. Just because one person likes this telescope or camera over another does not mean that you will like it. Also keep in mind that experience is the best policy. You do not dive off a cliff if you do not know how deep the water is.

The lesson to be learned here is start out simple and stay flexible with out going broke in the process. My suggestion is to purchase your equipment used and only buy equipment that will hold a reasonable resale value so you can up grade later. For example do not buy an expensive Cassegrain when you can really only afford a cheaper refractor. A cheaper refractor will allow you to get used to the pitfalls of imaging without the entire headache involved with having to get everything just right as in the use of a Cassegrain. Save the Cassegrain for when you have mastered these problems, and I promise you that you will sleep better at night.

Get Rid of Preconceptions

If you have been observing for a while you have accrued ideas about how to observe objects that could drive you down the wrong path when it comes time to start imaging. What I mean by this - is when it comes to simple things such as focusing and changing eyepieces the part that each of these plays in imaging is a whole new ballgame. For example the simple act of turning the focuser knob to achieve focus is replaced to having to put the scope on a star, place the camera in the focuser, and turning the knob while you watch the PC screen until the image comes into view. Some people disagree with this approach but it does work for me

In closing I would add never listen to home spun remedies on how to repair or fabricate optical or imaging components (unless you know what you are doing), and always stick to your original plan - purchase a quality mount - a quality camera - and a small refractor to start with and when you have mastered these then you can up grade to a more complicated setup.

Choosing a Camera

If I were to be asked what type of camera would I recommend? I would have to say that you should consider first exactly what type of imaging do you want to do. This is very important when you consider that the type of imaging that you desire has a lot to do with the type of camera that you will be using. For instance do you want to look for asteroids? Do you want to take images like you see in the magazines? Or are you interested in deep sky imaging? All of these different styles require specialized cameras. Since I do not know which of these styles you prefer. I will have to try to keep my suggestions as broad as possible. If a choice of cameras is to be made - then I would suggest that you purchase a CCD camera that has anti- blooming or ABG. Cameras equipped with this feature allow the user to take images without getting those multiple spikes on the bright areas such as stars. These aforementioned spikes are caused by an excessive build up of electrons on the CCD chip. These excessive electrons will bleed over into surrounding pixels.

If you already have a camera - and it does not have the ABG feature? Then may I suggest that you take a series of multiple images of the target - with shorter duration times? If you do this then you can enhance the finer details with out causing the stars to spike. Now remember that "with each integration image - you will have to subtract a dark frame from each of these - at the same outside temperature as the original"

What if you are one of those people who enjoy imaging nebulas? If you are, then the analog to digital conversion should come to mind. What I mean by this is you will see that there are many cameras on the market today that offer a wide variety of image bits such as 8, 12, and 16 bits. My suggestion to you would be "lean more to the higher 16 bits". The reason for this is simple the 16 bit imager will show more gradient definition between the grey levels, which will produce a better image in the long run.

What if you are interested in spectrographic or photometry work? Then the suggestion of looking for a camera that has the anti-blooming feature would be a drastically detrimental move on your part. The reason for this is the fact that as the charge builds up on the chip surface the imager will attempt to correct. This correction causes errors in the actual brightness of the target because it introduces nonlinear figures to the equation and we all know that in order to make accurate measurements of the magnitude of any bright object using known software then the input measurements should be in linear equations.

Another good feature of this type of camera is the fact that it is more sensitive to light and therefore reduces the amount of exposure time needed for integration.

What about Color Cameras?

Achieving color images is not as hard as it used to be when we only had black and white cameras (monochrome). Nowadays you have color imaging systems with color chips and monochromatic systems with add on accessories such as filter wheels which can be either automated or manual. The main differences are components and price. There are many reputable companies out there producing color CCD cameras such as Orion, Meade and Starlight Express just to name a few.

Now do not get me wrong there are still some people that prefer monochromatic cameras and use them exclusively with LGRB (luminance, green, red, and blue) filter wheels and through the years these systems have become very well automated with the computer software in use today. In fact some manufacturers such as SBIG (Santa Barbara Instruments Group) have even produced CCD cameras with built in filter wheels. The only draw back to these systems is the fact that when imaging through different colored filters you must image the target three or four times through each color in order to stack them later to produce an excellent image.

Fastar

If you have one of the larger "Celestron Schmidt Cassegrain's" like we do then you have the option to advance to a very unique imaging system known as "Fastar" This imaging system is unique in the fact that it is the only one that allows the secondary mirror to be removed and replaced with a holder that allows for a CCD imager to be installed in place of the secondary mirror. This system converts the normal f/ratio to a faster f/1.95. This new "f / ratio" is very fast when you consider that the original ratio was an f/10. This means that for the owners of these telescopes the exposure times have been greatly reduced. What used to take hours now takes minutes what used to take minutes now takes seconds. The only draw back to this system is that it was originally designed to accept the SBIG CCD camera which unfortunately is not made any more, but never fear mother is here Starlight Express does manufacture a compatible series of cameras known as the MX series that comes in varying chip sizes, with affordable prices.

Matching the Camera to the Telescope

The next problem that should be addressed is the one concerning the camera being matched with the telescopes effective focal length. The pixel size of your chosen CCD camera should be sufficiently matched to your particular telescope. Now with this problem in mind the first approach to this problem should be from the point of looking at speed versus resolution.

When you are imaging, say the Moon, or a planet you are not concerned with the focal length of the camera as much as you are worried about the resolution required to image the finer detail of the planet or moon's surface. The reason for this is simple since these objects are much brighter than deep sky objects, then the speed of the camera is not the issue, instead you are more concerned with the resolution of the telescope because the resolution of your camera should be twice that of the telescope in order to receive all the data within the image of the planet or moon. For example if you had a 5 inch (114mm) scope (it doesn't matter weather it is a reflector or refractor) it will have an approximate resolution of about 1 arc second. Now, in order to receive all the image data available, the CCD should (assuming that seeing conditions are good) be able to resolve down to 0.5 arc seconds for a target as large as a planet or moon.

On the other hand if you are imaging deep sky objects you are looking at just the opposite situation, you are more concerned with the speed of the camera rather than the resolution of the telescope. A good CCD camera will probably have a pixel image span of around 2 arc seconds, when mated to say a 14 (356mm) to 24 inch (600mm) scope. So in other words the focal ratio should be as fast as possible, when considering the limitations of the required focal length of the telescope, so that the required exposure times can be reduced. To make a long story short - it would seem that fast optical systems such as Newtonians with a range of f/4 or f/5 mated to heavy duty tracking platforms - or mounts are needed for superb CCD imaging. But really this is not necessarily the case. If you use a CCD camera with a small pixel array like Orion's Star Shoot DSI, and you use it in single binned mode, you can produce good images with little signal to noise ratio - in images that took only a few minutes of exposure time - when attached to small relatively inexpensive refractors such as Orion's ED80.

Now that we have discussed the right camera's - the right telescope's - we need to elaborate a little on the right mount. We talked about this a little earlier on in this article - but we did not go into any great depth. So in view of this let us talk a little about the right mount and also we will touch on guided and unguided images.

Guided vs. Unguided Imaging

One of the hardest things - and certainly one of the most strenuous things - that a telescope mount can perform is imaging with a CCD camera. This is a task that most telescope mounts on the market today are certainly not up to with out some type of additional help. This is due in part to the fact that most mounts manufactured today lack the precision needed to accomplish unguided tracking for anything longer than a few minutes. That is the reason why many people who choose to get into CCD imaging prefer to enhance their mounts with some type of guiding assistance such as an additional guide scope or off axis guider.

Off Axis Guiders

Even though off axis guiding is considered by most to be the preferred method, it does have its drawbacks. The most apparent one is that although you can guide on a star that is in the same field of view as the object being imaged, the problem stems from the fact that a suitable bright guide star cannot always be found within that field of view. That is why the good off axis guiders allow you to tilt the internal mirror, and allow for you to rotate the guider housing (such as the Celestron radial guider), but even this in most cases is not enough. In view of this problem some top of the line CCD cameras are fitted with a separate guiding chip within the camera itself, but believe me you pay for this luxury.

The Guide Scope

In another method called the guide scope, a separate CCD camera is attached to another small refractor scope that rides piggy back on the larger imaging scope. Cameras such as SBIG, Starlight Express, and Orion, make for good guide cameras when coupled with a guide scope. The most frequent problems associated with this type of set up are the ones that manifest themselves in the physical mechanical misalignment of the guide scope and the main optical tube. This is termed as differential flexure, and is caused by the mounts that the guide scope is mounted to, and the warping of the optical tube itself caused by the weight of the optical tube on the mount itself.

Mirror Flop

Another problem that is encountered more times than not, is the one that occurs when the telescope is tracking; the primary mirror will shift as the scope moves from one angle to another. This image shift can be eliminated somewhat by using the mirror locks (the screws that protrude from the rear of the scope, and are used to secure the primary mirror when the scope is transported from one place to another) by securing these screws the mirror is not allowed to move, thereby eliminating image shift while the scope is tracking. This flexure and mirror flop is very common in SCT's.

Matching the Guide Scope to the Telescope

The next problem to be addressed is the one that concerns matching the guide scopes focal length with the main optical tubes resolution. Too long of a focal length in the guide scope can cause the guide scope to track on what is termed as atmospheric scintillation which can cause the guide scope to send the wrong tracking corrections to the mounts drive motors. On the other hand - if you do not have enough focal length in the guide scope - your guide scope will not track with enough precision to match the resolution of your guide camera. In other words the guide scopes focal length should only be long enough to allow the guide scope to about double that of the imaging CCD camera.

The Importance of Balance

Well - now that you have all this cool looking equipment hanging off the telescope - you have just added another problem that can affect the image in your CCD - and that is balance. When adding extra equipment such as cameras and guide scopes - your scope will have to be balanced in both axes. This is crucial to the overall performance of the imaging system - because it can throw off the anti backlash - and the periodic error correction causing the normal smooth movements of the mount to become erratic and shaky. So remember that when setting up an optical imaging system - always check and recheck the polar alignment - the backlash in the drive gears - the tracking rate on your hand controller. In addition to this, be sure the mount is secure and stable. Then check for any flexure between the guide scope - the main scope - and also check to make sure that the primary mirror locks are engaged before you begin tracking.

Now that you have taken into consideration all of the varieables needed to aquire a good image, consider this, how do I process the images?

Image Processing:

A good place to start in image processing is to begin with the basic software systems that are out there. Systems such as Software Bisque, Maxim DSLR, Photoshop
are some of the best just to name a few. By following the instructions provided, you to can have images that will rival those found in magazines.

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