Astrophotography – ‘The Essential Guide’
I am often asked:
“How can I take a ‘Great Astrophotograph’, like the ones I see in magazines or on the internet?”
I have compiled a list of 10 Top ‘Astrophotography Tips’ for the Astrophotographer, which are useful to both the beginner and the experienced alike.
1. Be Patient
This is the most important piece of advice of all; especially for those aspiring to become an astrophotographer, and won’t go amiss with many an experienced imager as well!
Astrophotography is very different from other forms of photography. It is not like portrait or nature photography, where a ‘point and poke’ approach can often lead to a good result.
The targets to be captured are very diverse; many are extremely faint (e.g. DSO), whilst others are incredibly bright (like the sun or moon); they vary in size from the small just a few arc seconds across, to the large with angular sizes of several degrees.
In addition, the techniques used in astrophotography for image acquisition and processing are not straightforward, and require a certain amount of computer and mechanical ‘savvy’.
Anyone who expects to get successful images immediately is in for a big shock. It just does not happen (and if it does it is often more by luck than judgement).
So my first piece of advice is to take your time, be patient and expect setbacks and failures.
2. Master the Basics
As in any new venture it is important to learn the basics, don’t expect to be able to run before you can walk.
What are the basics of astrophotography?
Firstly, get to know your equipment and learn how to set it up and the fundamental aspects of its operation.
This will mean studying the accompanying manuals and documentation. You might find it useful to do some trials indoors if possible. Once you have done this then you can then begin using it in the field
Secondly, learn how to find your target and centre it in the field of view of your telescope or camera.
Begin with the moon, preferably when it is near full. It is big and bright, so you should find it easy to locate.
A full moon has the added benefit of giving off a good deal of light so you can see easily what you are doing. This is especially important when you are using new equipment for the first time and need to become familiar with its operation.
It also helps when attaching cameras or CCDs to a telescope which often involves the use of annoyingly small screws, bolts and adapters.
Once you have mastered the moon, then try centring some DSOs, ones which you know will be easily visible in your camera or CCDs FOV (see item 7).
Thirdly, learn how to focus the centred target. This is easier than it sounds.
Many a beginner will struggle with focussing, and it can take several hours or even nights to master well.
However, it is vital that an accurate focus is achieved, without it a successful image will not be possible.
It should also be made very clear that the critical focus zone (CFZ) is often very small for a CCD/telescope combination.
It is determined by the simple formula:
CFZ = 2.2*(FR)*(FR) microns
Where FR = the focal ratio of the telescope
For example with an f/10 optical system the CFZ is 220 microns or 0.22mm, and if the focal ratio is reduced to f/5 then the CFZ becomes 0.055mm, a quarter of the previous value.
The use of automatic focusers can often help achieve accurate focus (see item 6).
Fourthly, learn how to set your equipment up to accurately track the target object (see item 7).
If your telescope mount cannot keep the object centred in your FOV then it will not be possible to take long exposures, i.e. those of several minutes duration and beyond.
Long exposures are vital when taking images of faint DSOs. The longer the exposure the smaller the image noise will be, and therefore improve the quality of the final image.
The following rule should be remembered - that fewer long exposures when stacked (see items 9 & 10) produce a better image than many shorter exposures of the same total duration.
For example 10 exposures of 10 minutes are better than 100 one minute exposures.
If you are able to master these four basic operations, then you are have made an important step forward in attaining your ultimate goal. But there is still much more to learn.
3. Learn from Your Mistakes
This is a very important lesson to learn, not only in astrophotography, but in life in general.
Nothing is learnt from your successes, but learning from your mistakes will give your valuable experience in what not to do, and what works best.
I have found it useful to keep notes on every imaging session, date/time, imaging targets, seeing conditions, filters used, numbers of exposures and exposure durations, binning used, results obtained, quality of images, defects found, tracking errors and so on.
In this way I am able to determine what went right and why, and more importantly what went wrong and why?
Finally, it goes without saying try not to make the same mistake twice!
4. Learn from Others
Why reinvent the wheel, when others have done so before.
It is very important to do as much reading and research on all aspects of astrophotography as you can.
The areas which you will find most useful include: image acquisition, calibration and processing techniques; equipment and software used in astrophotography.
You should also read up on general topics such as drift alignment (if you have an equatorial mount), sky alignment (if you have a GOTO telescope), PEC methods and optical collimation.
The more you know the better prepared you will be to tackle problems you have not come across before.
It will be worth your while becoming a member of the various internet forums related to astrophotography and especially those related to the type of equipment you have.
If you are into clubs then joining a local astronomical society can often be useful, where valuable advice and help can be obtained from the ‘old hands’.
The following links will be found useful:
http://www.astropix.com/HTML/I_ASTROP/TOC_AP.HTM
http://starizona.com/acb/ccd/ccd.aspx
http://www.rc-astro.com/resources/index.html
http://www.newastro.com/index.php
5. Find Your Weakest Link
The quality of any astrophotograph is dependent upon many factors, the equipment, the observing site, the acquisition and processing techniques and the imager themselves.
More importantly the astrophotograph will only be as good as the ‘weakest link’, i.e. what factor or contribution is causing the quality of the image to suffer the most.
In other words find out what is having the most adverse affect on your work, and improve on it or if at all possible eliminate it all together.
You may have to go through several cycles of soul searching and analysis to get it right, and will in all probability have to make improvements in a number of areas.
So which areas are the most common causes of poor image quality?
In order to answer this question you have to ask yourself another question, and that is what exactly is wrong with your images?
Once you have identified the weak points of your image, you can then determine what causes them and therefore take whatever steps are necessary to correct the problem areas.
To make things more difficult for any given image defect there may be several causes and several ways to make improvements.
Typically, the following are the most common areas to look out for:
· ‘elongated stars’; caused by poor tracking, poor optical collimation, low quality optics, unsuitable field flattener;
· ‘bloated stars’; poor seeing, inferior optics;
· ‘soft stars’; inferior optics, bad focus where sharp detail is lost;
· ‘graininess’; too short exposures, insufficient number of exposures, incorrect image processing;
· >‘lacking detail’; insufficient number of long exposures, incorrect image processing;
· ‘poor contrast’; incorrect image processing;
· ‘very weak colours’; insufficient number of total exposure time in RGB channels; imaging targets at low altitudes;
· ‘severe colour bias’; when the one or more colours flood the normally black background; many causes - thin clouds, light pollution, incorrect processing etc;
· ‘wrong colour balance’; when one or more colours predominate giving a ‘wrong look’; many causes; but usually incorrect total exposure times in RGB colour channels or incorrect image processing. The eye is very good at assessing colour balance –f it looks right then it usually is!
· ‘large numbers of hot and cold pixels’; defective camera (hopefully not), insufficient number of dark frames, dark frames of incorrect duration, incorrect dark frame calibration.
6. Choose the Right Equipment
Remember the saying ‘horses for courses’, the same is true of astrophotography.
There are many types of telescopes, mounts, cameras and accessories that can be used in astrophotography, but not all are suitable for every circumstance.
Some equipment is best suited to DSO astrophotography, but is equally totally unsuitable for Solar System Imaging.
For example, I would never dream of using my SBIG STL 11000 CCD to image a Lunar Crater, nor would I ever use my Lumenera webcam to photograph the ‘Horsehead’ Nebula.
It is absolutely vital that you are able to determine which equipment and equally importantly which combination of equipment is right for a particular type of astrophotography or specific target.
Here are a few simple rules to help you:
· telescopes and mounts; a good starting choice for the astrophotographer is a Schmidt Cassegrain (SCT)GOTO telescope on an equatorial mount or wedge; they are now relatively cheap, have good optics, are easy to operate and are suitable for most types of astrophotography;
· webcams are better suited to imaging the moon, sun and planets than an Astronomical CCD or DSLR camera; they are better able to capture many individual frames and therefore that elusive moment of perfect seeing;
· astronomical CCDs (ACCD) are ideally suited to DSO astrophotography and in particular fainter objects; they can be cooled to reduce system either by thermoelectric coolers and/or water cooling;
· DSLR cameras can be used very effectively for DSO astrophotography, but cheaper models can suffer from excessive system noise as they cannot be easily cooled (though Canon have recently introduced a rather expensive cooling box for their DSLRs);
· DSLR cameras have the advantage that they can be also used for terrestrial photography and are easier to master for astrophotography than a specialist ACCD.
· large format cameras DSLR or ACCDs (i.e. those with large 35mm size chips) are needed to capture large faint nebulae, e.g. ‘Horsehead’ or ‘Witches Head’ or large bright galaxies such as M31 and M33.
· wide field astrophotography; if this going to be your area then consider investing in a good apochromatic refractor with an object lens of between say 80-130 mm; this coupled with large format CCD or DSLR will allow to capture a field of the order of 2 degrees in diameter or more;
· start small; if you are a beginner don’t go out and buy an expensive SBIG or FLI ACCD and expect to get amazing results; don’t run before you can walk. Begin with a cheap webcam and image the moon and planets. When you feel more confident and want to branch out into DSO astrophotography buy a DSLR which you can use for terrestrial photography or a basic ACCD like a Meade DSI.
· Experienced astrophotographer; if you really want to capture those stunning wide field shots you see then you will have to invest more heavily in both time and money; the purchase of a high quality stable pier and mount cannot be overemphasised. High quality mounts are made by Losmandy and Software Bisque.
7. Know Your Equipment
The type of equipment you have will depend upon many factors, your pocket, your experience and your needs.
However no matter what equipment you have it is vital to understand how it works, what its strengths and weaknesses are, as well as some important specific characteristics, i.e.:
· Know your FOV; it is important that you know the Field of View (FOV) for your telescope/camera/focal reducer combination. This will help you know whether your equipment is suitable for imaging a particular object, i.e. will it fit in your FOV.
· Know your Image Scale; this is for a given telescope/camera/focal reducer combination tells you the resolution, i.e. how many arc seconds there are per pixel.
· Effect of Seeing; it is important that you know this. Typical seeing will give an error of between 1-2 arc seconds per pixel error, so if your resolution for your equipment is 0.3” per pixel, you are going to be wasting your time except on those very rare nights of perfect seeing!
· Mount Tracking; The errors in your mount’s tracking will add further problems to the equation, given that an tracking error of less than 1” per pixel is considered acceptable for most telescope focal ratios.
· The Right Equipment Combination; in order to be able to image every type of target well, you are likely to need more than one telescope and more than one camera or CCD. Each must be chosen carefully to enable you to successfully take images for your chosen target lists. If money is tight then choose a combination which best suits your needs.
8. Get a Workflow
In order to master the art of astrophotography you must have a workflow(s). What is a Workflow?
A workflow is a set of steps which are carried out in a defined sequence beginning from the moment you choose your imaging target and ending with the finished masterpiece on your computer screen or printed out and hung on your living room wall.
Here are a few guidelines which should help you in defining a Workflow:
· It should be suited to your needs, the equipment you have, the software you use and the conditions of your observing site;
· It should make use of the astrophotography guidelines given here, what you have learnt from experience and what you have learnt from others;
· You will probably need more than one workflow, a single workflow will not be suited to all types of astrophotography or indeed all types of object; e.g. you may well need a workflow for DSOs, one of lunar astrophotography and others for imaging the sun or planets. I also have a separate workflow for processing M42, which is a large and very bright object, requiring a different set of steps;
· In the case of DSOs it should be designed so as to minimize noise in the image as much as possible;
· It must also produce images which do not look false or are over processed, i.e. they should look as natural as possible.
A sample workflow suitable for DSOs is given below, it only contains the outline of the steps required, and you will need to work on it to customize it to your needs:
1) Choose your imaging target and the equipment you will use; making sure it is suitable for the object you plan to image (try using Ron Wodaski’s CCDCalc software, it will help you with Image Scale & FOV for your equipment combination, see link: http://www.newastro.com/book_new/camera_app.php);
2) Plan your exposure sequence, i.e. how many exposures, their length and the filters you will use;
3) Choose a suitable Date and Time to image your chosen object, i.e. is it visible from your observing site, what time, which direction, do you have an unobstructed view, is it too low down etc.
4) Acquire Images according to your defined exposure sequence;
5) Calibrate, align and combine raw images for each channel (L, R, G, and B) using appropriate software (I use CCD Stack or Maxim DL).
6) Perform the colour balancing on each colour channel image based on filters used allowing for camera gain and atmospheric extinction (see camera manual and say Handbook of Astronomical Imaging)
7) Combine the RGB channel images into an RGB colour image using appropriate software (I use Maxim DL)
8) Perform image correction and enhancement on L and RGB images separately (here consult references in section 4 for guidance and help).I use Adobe Photoshop, together with a combination of Eddie Trimarchi’s Fitsplug, Russell Croman’s GradientXTerminator, and Noel Carboni’s Astronomy Tool plugins for this step
The above is just one example, the one you finally adopt will be different but one which works for you.
9. Master Image Acquisition, Calibration, Correction and Enhancement Techniques
This is just a reiteration of probably what you have guessed already – you need to master the techniques associated with the three basic steps of astrophotography, i.e.
· acquisition of the raw images;
· the calibration of the images to produce images free of system noise, hot & cold pixels, blooming which have been aligned and then stacked; and finally
· image correction and enhancement.
You will find it beneficial if you familiarise yourself with the various astrophotography software programs on the market, see the link:
www.artdeciel.com/software.aspx
This will give you an insight as to what software to use and what is best for you, your computers and your pocket.
10. Finally – Become Computer and Mechanically Literate
It has almost certainly become clear to you that astrophotography is very different from terrestrial photography, and a lot more difficult.
It involves considerable use of computers to control the telescope, the camera and more than likely other devices such as a focuser and a filter wheel.
In addition you will find yourself having to maintain, assemble, attach, troubleshoot and setup a variety of mechanical components including the mount, pier, power supplies, maybe even the observatory and not mention a ‘spaghetti’ like array of cables of various sorts – power, communication and network.
You will find it very useful if you acquire a moderate level of both computer and mechanical ‘savvy’.
Good Luck! Clear Skies!
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