«The AAVSO CCD Observing Manual AAVSO 49 Bay State Road Cambridge, MA 02138 phone: +1 617 354-0484 email: aavso Copyright 2011 AAVSO Preface ...»
CCD Observing Manual
49 Bay State Road
Cambridge, MA 02138
phone: +1 617 354-0484
Copyright 2011 AAVSO
This manual is a basic introduction and guide to using CCDs to make variable star
observations. The target audience is beginner to intermediate level CCD observers,
although advanced CCD users who have not done any photometry will also find this
The AAVSO CCD Observing Manual had its origins with the AAVSO's CCD observing program itself, which began in the 1990s. Much has changed since the start of the CCD era of variable star astronomy, and you may find things in this document that are out of date. This document is a work in progress and always under revision. Let us know where you find it helpful, and where it needs work!
Please send any feedback or suggestions to firstname.lastname@example.org.
A number of people have contributed their time, knowledge, and expertise to this manual over the past decade, and we are grateful for the contributions of everyone involved.
Particular thanks go to the following individuals for their contributions: Aaron Price was responsible for creating the original manual and writing the bulk of its content, much of which remains in this edition. Chapter 4 on photometry was a joint effort by Bruce Gary, Arne Henden, Aaron Price, Gary Walker, Doug West, and Ronald Zissell. Thanks also to the many observers who have commented on and corrected this manual since its first publication.
Clear skies, Arne Henden, Director Aaron Price Matthew Templeton American Association of Variable Star Observers Cambridge, Massachusetts December 2, 2009 Table of Contents
1.1. Target Audience
1.2. CCD vs. Visual Observing
1.3. Are You Ready? (Prerequisites)
1.5. Why CCDs?
2.2. CCD camera
2.4. Computers and Software
3.0 CCD Camera Skills
3.1. Calibration Overview
3.2. Calibration: Bias Frames
3.3. Calibration: Dark Frames
3.4. Calibration: Flat Frames
3.5. Finding the Field
3.6. Image Integration Style
3.7. Tricks of the Trade
4.1. Introduction to Photometry
4.2. Differential Photometry
4.3. Calculating and Submitting Uncertainties
4.4. Special Topic: Bright Star Photometry
5.0 Observing Programs
5.1. Cataclysmic Variables
5.2. Long Period Variables
5.3. High Energy Network
5.4. Eclipsing Binaries
5.5. Low Amplitude Stars
5.6. Supernovae/Novae Patrols
5.7. Designing Your Own: Using AAVSO VSX
6.0 Observing Techniques
6.1. Using the AAVSO Variable Star Plotter
6.2. Reporting Observations to the AAVSO
6.3. Using the MyNewsFlash System
6.4. Using AAVSO Special and Alert Notices
6.5. Using CCD Views
7.0 More Information and Administrivia
7.1. Discussion Groups
7.4. Additional References
1.1 Target Audience The AAVSO CCD Observing Manual is meant for anyone with an interest in using CCDs to make variable star observations. Most of the information is written with the beginner and intermediate CCD observer in mind. However, even the most advanced CCD observer will probably find some information useful. CCD photometry has been called an art by some because of all the intricate details involved in getting a good result. Art and science both have one thing in common — neither has experts.
If you have any questions we encourage you to contact us at headquarters, or browse our further reading and external links section.
1.2 CCD vs. Visual Observing After years of debate on the AAVSO Discussion Group a general consensus has emerged. CCD and visual variable star astronomy are complementary rather than competing endeavors. Each brings their own
strengths and weaknesses to the table:
The most successful observing programs in the AAVSO combine the capabilities of both programs. For example, the eclipsing binary committee routinely uses visual observers to follow an EB to get a general idea for the time of minima. Then the CCD observers schedule a night where they can refine that time.
Visual observers are also able to monitor many variables in a given night, and can in principle provide faster notification of important events like outbursts; CCD observers can then concentrate on active objects to obtain high precision time-series or calibrated photometry.
To the right is a light curve of SS Del. You can see how the visual observers took over near maxima (brighter than mag 14) and the CCD observers took over near minima (below mag 14). This is a great example of cooperation to build a terrific light curve.
If you are currently a visual observer and are happy with your current program then there is no need to invest the time and money for CCD work.
Only consider CCD work if there are specific programs that you would like to participate in that require the unique benefits of the CCD, or if you are a lover of technology and would just enjoy the challenge it presents.
Remember the goal here — to have fun and collect real data for the scientific community. Both can be achieved by CCD and visual observing programs.
1.3. Are You Ready? (Prerequisites)
Before getting started you should have some experience with your CCD camera. You should:
Know how CCD cameras operate (well depth, shutter types, linearity, etc.) • Have basic experience using your CCD camera.
• Have good working knowledge of computers (especially regarding processing data and text files).
• Highly recommended but not required: have some experience doing visual variable star estimates.
• The last item can save you a lot of time. "An ounce of prevention is worth a pound of cure." In general, for every visual variable star estimate you will probably end up saving yourself ten times that amount of time in your CCD learning curve. Try to make at least one hundred visual variable star estimates. Pull out some binocular variables and just follow them once a week for a month. If you haven't already, get a copy and become familiar with the AAVSO's excellent Manual For Visual Observing of Variable Stars.
This experience will be key to teaching you how to identify fields, how color affects estimates (important later when we get into filters), the behavior of a star's light curve, how to submit data, and perhaps most importantly – patience! Also, visual observing is usually quite fun and addictive, so practice will help make sure you enjoy variable star observing. After all, for most of us this is a hobby right? CCD Observing has many facets to it. Each one of these areas you gain experience with is one less area you have to be concerned with while learning CCD observing. Get the basics out of the way now so you can focus on the hard stuff later.
1.4 Expectations In general, this manual will focus on aspects of CCD observing specific to variable star photometry. With a few exceptions, we won't go into the details about how a CCD works, how to cool it, etc. Also, variable stars are usually just dots in the image so you won't find any help here in taking pretty pictures. (Although to some of us an image of a 17th magnitude cataclysmic variable in outburst is awful pretty!) This!
Images courtesy A. Henden (USNO) You are starting out on a very long journey, be prepared and patient! The good news is that you can start taking useful data almost immediately! However, you'll find the learning curve very long and at some times steep. In the beginning you'll find that choosing equipment is a pretty daunting task. But once that is done you will settle down into a steady routine of making observations and slowly improving your accuracy.
After you have mastered the basics such as dark, bias, and flat fielding then it gets pretty tricky in the world of transformation coefficients and all-sky photometry. We'll keep the material in this manual limited to what you'll need to go from having a CCD-equipped telescope to submitting variable star observations to the AAVSO.
Take your time in mastering CCD observing, and enjoy the learning experience. To determine how far you want to push your equipment and abilities you should compare the goals (and requirements) of your observing program against the amount of work you want to put into it. In some cases it can take years of experience before a new observer can master CCD photometry and get their error to the 0.01 magnitude level. So remember to take your time, have fun, and bite off only as much as you can chew!
1.5 Why CCDs?
The two main reasons why CCDs are great for variable star observing is because they are more accurate and more sensitive than the human eye. Additionally, you can archive and automate your observations.
Below is a summary of a few of these advantages:
Sensitivity With a CCD you can see objects much fainter than is possible visually. For example, a 10" telescope with a Starlight Xpress MX916 CCD can get to 16th magnitude in two minutes in light-polluted, suburban skies with a visual limit of 4th magnitude. With a visual telescope you would likely need 16–20 inches of aperture along with crystal clear and dark skies away from the city.
The linear nature of a CCD means that when proper procedures are carefully followed the photometry can be very accurate. In general, the accuracy of your result is in direct proportion to the amount of work you put into the observation. CCDs can easily and quickly get to an accuracy of 0.2–0.3 magnitude. With just a little bit of work you can get that to 0.1 or 0.05 magnitudes. Finally, if you are prepared for a lot of work you may be able to get to 0.01 accuracy or better. Each level of accuracy will require more work and restrictions on what you can observe.
Most visual observers can get to 0.2–0.3 mag accuracy. However, when visual data are combined with those other observers the scatter of the light curve increases due to differences in observer physiology, observing conditions, and skill. The magnitude scatter of some red variables can be as high as 1.5–2.0 magnitudes. CCD observations can be more standardized than visual observations. CCD observers often use standardized filters to make their observations. This makes the bandpass for each CCD observation very similar to each other, meaning that the combined accuracy can be up to.01mag. This is one reason why the scientific community prefers CCD over visual data. The professionals use the same type of filters so they can easily combine their data with those from amateurs.
CCD data is recorded digitally by a computer so it can be easily archived. These archives can be sent to others for further review, the data can be recalibrated at a later date (Ex: if the sequence is changed), and any questions that arise over the data can be answered. You can also reanalyze your images at a later time, and even use your images to discover new variable stars.
Advanced CCD observers can automate their observations to make better use of their observing time. For example, one observer may set an observing plan to image 5 objects all night long while they are asleep.
Then all the observer needs to do is wake up a little early to turn off the system and can reduce the data at a later time. Automation isn't easy though, and depends largely on the quality of your mount.
2.0 Equipment An unknown speaker at the first AAVSO CCD workshop said, "Calculate the expense of all the required equipment, multiply that by three, and that is what the final cost will be."
2.1 Telescope Purchasing a telescope is a big step. A telescope should be chosen with your entire observing program in mind. Don't just purchase a telescope based solely on CCD requirements unless you plan to never use it visually. Also remember to take into account things like portability and size.
When it comes to CCDs, the most important features in a telescope you want to consider are:
Aperture: This one is easy. The larger the aperture – the fainter you can go. However, remember • that with CCDs modest sized telescopes can do a lot of work. The vast majority of stars in the AAVSO program are 16th magnitude or brighter, so purchasing a large telescope isn't required.
However, we do have some programs which need large aperture like the High Energy Network's Gamma Ray Burst program. Note that bigger aperture has the downside of smaller FOV (for same f-ratio).
Field of View: CCDs are notorious about having small fields of view (although this is changing as • technology progresses). Small fields of view make it hard to get the comp stars in the same field as the variable. It also can make it frustratingly difficult to find the object. The smaller the f-ratio the better.
The Mount: This is crucial. The mount's ability to find the object ("goto") and the tracking will • profoundly affect your observing experience. Also consider the setup time. A mount that isn't very accurate, takes forever to align, and tracks poorly will leave you so frustrated that you may lose interest in the hobby. If possible, do not skimp on the mount. Equatorials are a must because alt-az mounts cause field rotation during medium and long exposures, which is impossible to compensate for without an image rotator.
The simpler the optical system the better. Stay away from focal reducers and anything else that • can cause vignetting unless you really know what you are doing. Also stay away from color corrected systems and in general anything that adds a nonuniform effect to the field or changes the natural color of the incoming starlight. Celestron's Fastar system cannot be used for photometry.
Also watch out for poor baffling because it can create ghost images that look like stars in your final image.