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How to Observe Variable Stars

by Bob Nelson, Ph.D. (Prince George Centre)

The Equipment

It is important to get good quality equipment that does the job for you -- that much is obvious. However, it is also important to get equipment that makes observing easy and pleasant to do. If your equipment is awkward and difficult, you will most likely give up in frustration or lose interest. Variable star observing should be fun!!

I favour the Dobsonian telescope for observing variables because of its simplicity and because you can get large apertures relatively inexpensively (enabling you to see fainter stars); therefore my comments will be confined to this instrument. You do have the disadvantage of having to move the telescope every so often as the Earth rotates, but I find this not too serious (except at higher powers). Later, I'll make specific remarks on using equatorially mounted telescopes (with setting circles) and binoculars. (Yes, Martha, binoculars are good here too.)

Whatever telescope you chose, you'll need the following:

  1. Coarse finder -- this is to get the telescope pointing in the right part of the sky. This can be the "gunsight" type or the more sophisticated Telrad projection system. In any case, you need a better system than sighting along the side of the Dobsonian's box (if it has one).
  2. Telescopic finder -- this is to "zoom in" to your pointer star. The straight-though finder (telescope) can be awkward; on the other hand, the usual diagonal (prism or mirror) attachment reverses the image (not good). The best is probably the Amici prism which does not reverse (but is expensive). In my system, the finder with an Amici prism is mounted on top of the telescope tube and the view through the finder is exactly the same orientation as when I lift my head and look up at the sky. (You have to try the usual single-reflection diagonals to really appreciate this!!) Also, the flat part of the telescope tube allows me to lay a chart flat and move my eye rapidly from chart to eyepiece while illuminating the chart with a suitably dim red flashlight.
  3. A telescope (Newtonian or Schmidt Cassegrain) with well collimated optics -- to pin-point images. A fast focal ratio is preferred because it gives a wide angle of view, making "star hopping" easier. (However, proper collimation is then critical.) As mentioned before, an equatorial mount and setting circles are not essential.
  4. Eyepieces -- you'll need a good selection to go from 40 x to 400 x or more. Barlows are good. You don't need the premium eyepieces costing $300 or more each. Orthos are good but do not give the wide apparent angle of view. Erfles and Plossls give a wider field but are more expensive.

There are two methods for finding the field of view of your telescope:

  1. Timing the passage of a star on the celestial equator across the field of view. (Use: 15 deg/hour -- 15 arc min / min)
  2. By calculation -- diameter of field stop x plate scale ("/mm) (plate scale = 206,265 / f where f = focal length in mm)

Additional Items

  1. Red flashlight -- any reliable flashlight will do -- just tape the clear part with red translucent electrician's tape.
  2. Observing chair -- adjustable in height.
  3. Warm clothing -- 'nuff said.

Charts

First, you'll need unaided eye finder charts to get you started in the right direction. Skalnate Pleso and Atlas 2000.0 are good for this. For each variable, photocopy the appropriate area of the sky onto stiff paper, add a title and identify north.

Next, you'll need AAVSO charts to give you the fine-scale (telescopic) field of the variable, with the comparison stars and their magnitudes. They come in various scales: 'b' -- 60"/mm, 'd' -- 20"/mm, etc. I can supply you with copies to get you started; later, as you get into it, you can order your own at $0.25 each or download them from the AAVSO web site). Locate the brightest star in each 'b' chart and mark it in pencil -- this will be your "AAVSO pointer star". (I use different shapes for each star I want to mark like circles, squares, triangles. Corresponding stars on different charts can then be quickly identified.)

Now go back and mark the boundaries of the AAVSO 'b' chart on your naked eye chart. Mark your pointer star. If this is too dim to see with your naked eye, mark a nearby bright star -- this will be your "naked-eye pointer star".

Before You Go Out

I highly recommended that you carefully (with a good quality drafting compass) draw appropriately sized circles on white paper and photocopy them onto sheets of mylar. (Each size will correspond to the angle of view through the finder or telescope eyepiece on the appropriate chart). Then, you'll be able to place the transparency over the chart and know exactly what you're looking at.

At the Telescope

Check the alignment of the optics first to ensure pin-point images. (See the articles in Sky and Telescope, 1988 March-April for how to do this.) Then aim the telescope at a bright star (so that you can't mistake where you're looking), centre it in the eyepiece, and adjust both finders carefully. Check your work.

Now to find your variable! Swing your telescope around, and with the coarse pointer, aim the telescope at the naked-eye pointer star in the naked-eye finder chart.

Now look for the same star in the telescopic finder -- it should stand out (since you chose the brightest one around). Move the telescope gently to put the star on the cross-hairs. Remembering to orient it correctly, take the chart (or the next one at a slightly larger scale) and compare it with what you see in the eyepiece of the finder. The various star patterns in the eyepiece (I see a lot of triangles, hockey sticks, 4-sided figures and the like) must correspond to what you see in the chart. This is where the transparencies come in -- it is essential that you know what your field of view is. Take care with this step. When appropriate, I like to mark the patterns (called asterisms) in pencil for quick reference.

If things don't seem to fit, go back, checking carefully. You may be able to wander around (lost in space?) and get the right star. However, finding the pointer star should not be a problem. (If you insist on using a diagonal which reverses from left to right, you'll either have to do mental gymnastics -- not recommended -- or turn your chart face down and shine your flashlight through it. A portable light table is easier to use but still not good.)

If the unaided-eye pointer star is not on your AAVSO 'b' chart, you'll have to "star-hop" to the pointer star which is. Carefully and slowly (in case you go the wrong way) move the telescope and, at the same time, move the transparency on the chart. With practice, you'll know which direction to move, and how much. When the AAVSO pointer star is centred on the cross-hairs, it is time to look in the main telescope. Insert the longest focal length eyepiece (for the widest view). Now take the AAVSO 'b' chart. If, in your finder utilizing the Amici prism, north was, say, at the upper right, then, in your main telescope, SOUTH will be at the upper right. AAVSO charts at a scale of 60"/mm or finer are always printed with south at the top.) Orient the chart correctly and check the star patterns around the AAVSO pointer star -- again these should correspond. (Use the mylar transparencies here too.) If things don't agree, go back and recheck your steps.

It things agree, you are most of the way there! Now star-hop across the chart slowly until you approach your variable. (Again, go slowly because you may inadvertently pull the telescope in the wrong direction or misidentify a star.) You should now be able to see your variable or at least the field around it (if the variable is too faint). Are the stars in the vicinity too faint? Switch to a higher magnification and, if necessary, switch to a finer-scaled chart. very sure in your identification -- remember, everything must correspond EXACTLY. When you are sure you are looking at the variable, it is time to start the process of estimating the magnitude.

By the way, lest you think that finding your variable is a long and complicated process, please be advised that experienced observers can usually find the variable in less time than it took you to read the previous section. It will take you a while the first few times, though.

Estimating the Magnitude

Your first task is get the approximate magnitude of the variable. Find another star which is about as bright. (On AAVSO charts, the decimal points in the magnitudes are omitted to avoid confusion with a star.) Next, try to "box in" the variable -- try to find a star on the chart which is a little bit brighter than the variable, and one which is a bit fainter. At first, they'll all look the same; however, with practice, you'll be able to pick out the differences.

Now there are various methods you can use to estimate the brightness of the variable. One method I don't recommend is to stare at each star directly. You can't do this for the fainter ones anyway and for the brighter ones, you run into problems with colour differences. Your fovea (that part of the retina used when you look directly at something) sees reds more brightly than does the rest of the retina. (This is called the Purkinje phenomenon.) Unfortunately, most variables are red in colour, and many comparison stars, white or nearly so. If you stare directly at the stars, you'll get readings for the variable which are 0.2 to 0.4 mags too high.

Instead, I use one or all of the following methods. Stare midway between the stars you wish to compare and you'll see which is glowing more brightly. For some reason, this is important: twist your head so that the line joining the stars is parallel to the line joining your eyes. Also, it is important to place the midway point at the centre of the field. Another method is to compare the brightness of the de-focussed images. Yet another method (use this when all else fails) is to glance really quickly from one to the other and back again.

To "nail down" the brightness, use the step method. If the brighter comparison is, say, 9.2, and the dimmer is 9.9, estimate the steps: brighter -- var -- dimmer. If the steps are about equal, then the magnitude is 9.5 or 9.6. If the variable is closer to the brighter, the magnitude is 9.3 or 9.4, and so on. Probably the error will be around 0.2-0.3 magnitudes, although experienced observers should be able to get 0.1 if the stars are not too dim.

If the variable is very dim, use lots of magnification and, of course, averted vision and proceed as above. Luckily, there are probably good comparisons nearby. For stars that are really dim, things get quite hard; you should probably avoid faint variables at the start.

If the variable is very bright, you have the opposite problem -- there is probably no comparison bright enough in the same field. There are two solutions -- hop around (with the telescope, of course), using your memory or the apparent brightness, or use the finder for some other telescope).

When you have the brightness estimate, record it, the time, the comparisons used, the variable name, and any other pertinent information in your notebook. You're done!!

Using Equatorially Mounted Telescopes

These are more convenient because they track, and south is always at the top (unless you use the cursed single-reflection diagonal). You also have the advantage of using the slow motion RA or DEC controls. If the axis is properly aligned, you can use the setting circles and skip the first step with the coarse pointer when searching for the field. You will still have to do the final star-hopping, though. (The digital setting circles on CAT telescopes may enable you to move directly to your star. If you think this is worth several thousand dollars, go for it.)

Using Binoculars

I have no experience doing binocular variables, but you probably need a holder of some sort to keep the binoculars steady. You'll be restricted to the brighter variables, but this makes the job of finding your star easier. With smaller magnifications, the rotation of the earth will not be a problem.

Suggested Magnitudes

Instrument

Size

Stellar Magnitude Range

Binocular

7 x 35 mm 

2 - 8

 

7 x 50 mm

3 - 9

Telescope

6" (150 mm)

6 - 12

 

8" (200 mm)

7 - 13

 

10" (250 mm)

7.5 - 13.5

 

13" (330 mm)

8 - 14

Experienced observers can go about one magnitude fainter in each case. Note: I have dropped the limits for both binoculars one magnitude from what calculation predicts because you cannot increase the magnification for these devices.

Conclusion

While other observers may do things differently, I have tried to present my step-by-step set of instructions on how to get started in variable star observing. Years ago, I started observing variable stars for what I could learn from it. I have indeed learned much; but more than that, I have gained an enduring fascination with this subject. At the very least, it gives you something to do after you have looked at the usual show objects in the sky (and possibly dabbled in astrophotography). And, unlike with astrophotography, you will also have the satisfaction of knowing that you have made your contribution to progress in astronomy.

By the way, did you know that the bulk of the data on long period variable stars comes from amateur astronomers? It all began in 1855 when Argelander, realizing that there were far more variable stars than professional astronomers could cope with, called upon amateur astronomers to take up the task. Since then, the AAVSO (and others) have accumulated over 5.5 million observations, providing a wealth of data for professional astronomers to analyze. The need continues; there is lots of observing for everyone to do.