Chapter 7 — Sample Observation

In this chapter, we will review the step-by-step instructions that were presented in Chapter 2 by making a simulated estimate of the variable star Z Ursa Majoris, or "Z UMa."

1. Find the field - Figures 7.1 and 7.2 show you the vicinity of this variable. Beginners should find the field of Z UMa easily, because it is located within the "pan" of the Big Dipper. Figure 7.3 shows that Z UMa lies fairly close to d (delta) Ursa Majoris.

2. Find the variable - There are several strategies you can use to find the variable. Because it lies relatively close to delta UMa, you might contemplate a star hop from there. However, there is also a 5.9 magnitude star just south of the variable as shown on the "b" scale chart. Both of these are good starting points if you are planning to star hop. Alternatively, you might wish to skip the hop altogether and attempt to zoom directly to the variable. Here are some tips depending on the finding method you are employing.

From delta UMa - Pointing at the third magnitude delta UMa is easy. Figure 7.3 shows the area from delta to the variable on the AAVSO Variable Star Atlas.

You now have a choice to star hop by either using the finder (if you have one) or by using a low power eyepiece in the main scope. A good finder scope (8x50 and larger) will show many of the stars on the AAVSO atlas. One advantage of using the main scope is that you can get the orientation correct right away.

From the 5.9 comparison star - Almost any finder scope will reveal the 5.9 star near the variable. Only under the darkest skies will this be visible for use with a 1x finder. However, this star is about equidistant from delta and gamma (see Figure 7.4) so it is easy to point at its location. Because of its brightness, it should be fairly conspicuous in the main scope. From there, you can use the "b" scale chart to do a short star hop to the variable (Figure 7.5).

Directly to the variable - This means using your chosen finding method to point as close to the variable as possible before ever looking through the main scope. An observer who only uses setting circles will almost always use this technique. It is probably the most popular method among variable star observers.

With a 1x finder, you will use delta and gamma as guide stars. With a finder scope, you can also use fainter stars (like the 5.9) not visible to the naked eye.

Figure 7.6 below shows a small reflector telescopic view near Z UMa. Just as you would with the real telescopic view, your task is to match this with the estimate chart shown in Figure 7.7.

Figure 7.7 - AAVSO "b"-scale chart of Z UMa with a 2.3 degree Field of View circle drawn in.

Figure 7.6 - Z UMa field

The beginner will generally find this challenging for the following reasons:

1. The orientations probably don't match.
2. The magnification will almost certainly render an image that's a different scale.
3. The limiting magnitudes will not match.

All three of these fall in the category of "telescope familiarity" and should become easier for you as you gain experience with your instrument. Here are some tips:

(1) Orientation. Failure to get this right means frustration. You may find it next to impossible to match up star images if the orientation is wrong. A big advantage of star hopping from a bright star or asterism is that the orientation problem is taken care of before you zoom in on the variable. The orientation diagrams presented earlier can be of great help. However, when in doubt you can always let the field drift. The direction of the drift will always be WEST. In Figure 7.6, south is tilted approximately 45 degrees to the right.

Caution: If you are using a telescope with an odd number of reflections (refractor, Schmidt-Cassegrain, etc.), ideally you want to use an AAVSO reversed chart.

(2) Magnification. The "b" scale chart shows a relatively large area of the sky. Therefore, you probably want to use your lowest power eyepiece. You'll also want to know the actual field of view. The field of view shown in Figure 7.6 is 2.3 degrees. This 2.3 degree circle has been drawn onto the "b" scale chart shown in Figure 7.7.

(3) Limiting Magnitude. In general, you will find the "stars" on the charts much more visible than those in the eyepiece! This mismatch can also make field identification difficult. Because it is more difficult to see stars in the scope, it is usually better to look for bright stars or star patterns (asterisms) in the eyepiece first, and then attempt to locate them on the chart.

A technique that many observers utilize who choose the "directly to the variable" approach is the reverse star hop. If the variable field is not readily apparent at first glance, scan around the field looking for asterisms in the field of view (FOV). Once one is spotted, then go to the chart and find it there. You now have a known place from which you can then do the star hop (presumably back) to the variable. Because of the small scale, the "b" charts work particularly well for this method.

In the Z UMa field, there is a trio of 8.6-8.8 magnitude stars just north of the variable. Once you have found these stars in your FOV, the variable is as good as located.

Tip: If you spot what appears to be a very noticeable asterism, draw it on your chart. This will help the next time you are finding this field.

With More Experience - Another advantage you'll gain over time is a feel for the brightness of stars in your telescope. For example, once you've seen a variety of 9th magnitude stars on the charts, you'll inherently know how bright such a star "should" look. With additional experience, you'll also get a feel for how bright it should look under moonlight or other adverse conditions. This helps immeasurably when finding variable fields.

3. Find the comparison stars - Here your task seems straightforward: find at least one star brighter and at least one star dimmer than the variable. The difficulty will vary in direct proportion to how far away the comparison lies. A technique that often works well is to locate "probable" comp stars in the FOV. That is, locate a star you think is slightly brighter or dimmer than the variable. Then, locate that star on the chart. Chances are, it will indeed be a comp star. If not, try another. When you run out of probable comp stars, then you should consult the chart.

Caution: In your zeal to find the variable, your mind can play tricks. You may be unfortunate enough to find a pattern of stars that "looks" like the chart and believe that you've found the variable! In this step, you are not only finding comparison stars, but you are also helping to prove your identification. Heed simple warning signs. If the chart shows a comp star that is not visible in the scope or very different than the magnitude indicates, chances are greater that you have an identification problem rather than a new variable star!

Though all you need are two stars to bracket the variable, you are strongly urged to locate additional comp stars. Are the magnitudes consistent? If they are not, why? Does just one of the comp stars look suspicious? Be sure to recheck the positions. You will find that stars are plotted on the AAVSO chart with extremely high accuracy. If there is only one comp star that seems off, it is best to disregard it and use the remaining comp stars.

4. Estimate brightness - Once you've located suitable comparison stars, you can finally perform the estimate step. Figure 7.8 (below) shows our field with Z UMa centered and with South up. From this view, it appears that the variable is between the 80 and 83 stars in magnitude, and you will interpolate your estimate from these.

Caution: Most new observers will find the estimating of real variables more challenging than in this demonstration. Does the interval between the 80 and 83 seem small? It is! Consequently, you should not be surprised if your estimates differ a bit from those of other observers.

Figure 7.8 - Z UMa field with comp stars

For the purposes of the demonstration, let's assume an estimate of 81.

5. Record your observation - The following information should be recorded:

Name of the variable: Z UMa.

Designation of the variable: Though this is not mandatory because you can theoretically look it up later, writing this at the time of observation will help catch many potential errors. For example, in the cold of an observing session your U's might end up looking like V's and vice-versa. The designation will immediately fix such problems!

Date of your estimate: You can write this for each estimate, but since it is common for observers to start a new page for every night of observing, the date is normally placed at the top of the page. You should always use the double date format to avoid any confusion between before or after midnight.

Time of your estimate: Observers use both local time and Universal Time (UT). You should be consistent, whichever time you use. The precision of your recorded time depends on the type of star. See Table 6.1, in Chapter 6 for guidelines. When in doubt, it never hurts to be more precise. Many observers record all their observations to the minute regardless of the variable type.

Magnitude of your estimate: In this case, it was 8.1.

Magnitude of the comp stars used for the estimate: We used the 80 and 83 comp stars.

Chart used for making the estimate: Since we used the standard "b" scale chart dated 12/97 shown in Figure 7.7, we could code this "S/B/1297". The S stands for Standard. The other kind of chart issued is a Preliminary, and this would be noted on the chart itself. New observers should avoid using the preliminary charts. The B stands for the "b" scale of the chart. Had Z UMa turned out to be very bright, you might have used the "a" scale chart. The exact format of this field can vary (For example: S-B-1297, s/b/1297, SB1297 etc.), but all three pieces of information should be present: type of chart, scale, and date. On many of the charts, only the year is given, so that is all you can specify.

Notes on any observing conditions which might effect seeing: Many of the usual conditions like moonlight, haze, clouds, etc., should be coded with the standard abbreviation letter. You will find a list of these on this, "Abbreviations for Comments" page. Other comments should be written out. Figure 7.9 shows what a sample notebook entry might look like for our sample observation.

Figure 7.9 - Excerpt from observer's notebook

Though the "W" code (indicating weather) is specified because of the windy conditions, we did not show the estimate as approximate, as would be indicated as "8.1:". As an observer, this decision is yours. By specifying the code, without the magnitude as approximate, you are indicating that the condition existed, but you didn't feel it impacted the accuracy of the estimate. The opposite cannot occur. If you specify the estimate as approximate, you must specify a reason for the uncertainty.

Gene Hanson with his 18-inch f/4.5 Obsession reflector and 6-inch f/5 telescope