CCD Observing Manual

1.0 Introduction

1.1 Target Audience

The CCD Observing Manual is meant for anyone with an interest in using CCDs to make variable star estimates. 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 or check out our further reading and linkage section.

1.2 CCD vs. Visual Observing

Ah, the debate of champions. After years of debate on the AAVSO Discussion Group a general consensus has emerged. CCD and visual variable star astronomy are collaborative and not competing endevours. 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. Also, for cataclysmic and long period variables visual observers will observe the star while it is bright (above mag 14.5 usually) and then the CCD observers will take over while it is faint. When the star gets bright again, it is handed back off to the visual observers. In these ways, both groups can cooperate and get the most out of their observing program.

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 txt files)
• Highly recommended but not required: Have some experience doing visual variable star estimates.

The last item can save you alot 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 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 behaviour 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 mag 17 CV in outburst is awful pretty!)

Not this... 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. Below is a general estimate of the learning curve you'll go through:

Obviously it is impossible to predict your personal experience so this graph should be taken tongue-in-cheek. However, the core idea is sound. 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.

Take your time along this curve and stop at any point you want. Some people, for example, find that messing with transformation coefficients isn't worth the extra amount of effort so they settle for photometric accuracy of 0.2 mag or so. This is still accurate enough to do good work. To determine how far you want to go 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 transformation coefficients and get their error to the 0.01 mag plateau. So remember to take your time, have fun, and bite off only as much as you can chew!

1.5 Why CCD's?

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 mag 16 in 2 minutes in light-polluted, mag 4 suburban skies. With a visual telescope you would likely need 16-20 inches of aperture along with crystal clear and dark skies away from the city.

Accuracy

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 response to the amount of work you put into the observation. CCDs can easily and quickly get to an accuracy of .2-.3 magnitude. With just a little bit of work you can get that to 0.1. With a little more, you can get to .05. Finally, if you are prepared for a lot of work you may be able to get to 0.01 accuracy. With each level of accuracy is more work along with restrictions on what you can observe.

Most visual observers can get to 0.2-0.3 mag accuracy if they are very experienced. However, when the data is combined with other observers the scatter and accuracy of the light curve suffers tremendously. Scatter can be as high as 1.5-2 mags! CCD observers, however, use professionally 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 that from amateurs.

Archiving

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.

Automation

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 date (such as a cloudy night). Automation isn't easy though, and depends largely on the quality of your mount.

Fun Factor

Let's face it, when the temperature is 10 degrees outside and the wind is blowing, wouldn't you rather be inside a heated room than freezing your lips to the telescope? Also, some of us are real techheads who love the technology and the challenge of data reduction. For those brave souls, CCD observing is a real treat!