Astronomical Society of Coonabarabran

Using “Helio” freeware

by Harry Roberts

Imagine viewing the Sun through a suitably protected telescope: the disc seems blank; then suddenly you see a small black sunspot. You check the websites, but they show no such spot. Maybe you just found a new cycle (C24) sunspot! But how do you get its position so you can make a record and tell others where to look? This is where “Helio” freeware comes into play.

“Helio” gives the heliographic coordinates of spots (or, say, H-alpha filaments) on the Sun’s disc, provided you map the spot’s position on a diagram and record the direction of drift of the Sun in the sky.

The software designer, Peter Meadows (UK) probably thought it would be used to find spot positions from projection images, but it works well if you view the Sun using cross-hair eyepieces (as I do).

Modern Maksutovs and Schmidt-Cass’s can’t be used to make projection images; never let sunlight into an unfiltered ‘scope. However, fitting a cross-hair e.p. to a suitably filtered small ‘scope converts it to a precision astronomical theodolite, assuming it’s on a stable alt-az or equatorial mount; and no drive is needed, (but can be useful).

The process of deriving spot positions is quick and easy to do, but hard to describe, so bear with me as I try.

Cross-hair alignment: Your e.p. must show the whole solar disc, i.e. magnification between 40 to 80 times, and be fitted with cross-hairs (i.e. it’s an Ortho). With drive OFF, drift your new spot along one cross hair, rotating the e.p. as needed until the spot tracks exactly along the cross-hair. (If your scope has an alt-az mount you will need to re-align the cross hair periodically; but if it’s equatorial the alignment will last all day). Note that in the field of view (f.o.v) the Sun drifts westwards, and it is the west limb that leads (E follows) when the drive is OFF.

Determine North: By gently nudging the end of your ‘scope towards north (don’t knock the solar filter off!) you can tell in the e.p. which cross-hair defines North in the field of view. Now with the cross-hair aligned, and knowing N from S and E from W, you are ready to measure the sunspot’s position. I’m assuming you have a digital stopwatch, as well as a notepad and pencil. We will make three transit timings of the Sun (Fig 1).

1. Transit T_diam. Time how long it takes for the cross hair to travel from the Sun’s west (preceding) limb to the east (following) limb where the disc is widest. Do this a few times and average the result to tenths of a second: it will be about 2min 10 sec, varying with the seasons (our orbit isn’t round!)

2. Transit T_chord. Now shift the ‘scope so the E-W cross-hair runs through your new spot, and time a transit from the west limb to the east limb, through the spot. This is a unique chord containing your spot. Repeat and average, again to tenths of a second.

3. Transit T_spot. Repeat the above but now noting the time from the west limb to the centre of your spot; repeat and average. (Ref Fig 1)

Log observation time: Note the Universal Time (UT) of your observation. It’s easiest if you use a digital watch with dual time zones, one set to UT. “Helio” wants UT, not local time. Now shut down the ‘scope, and boot up “Helio”.

Opening “Helio”: Firstly, Helio will request your local Latitude and Longitude, then the date and the observation time in UT (as above). On the dialog page a simple yellow sun map displays the current central meridian (CM) and the solar equator, as well as a black DOT that will move around the map as we enter our transit timings; when we get the dot into the position of the real sunspot “Helio” will tell us the spot’s position.

Enter Disc Diam: Convert your timing Tdiam. to seconds and tenths (e.g.131.7) and enter that number in the Diam box (top right of Helio page.)

Enter X coord (of chord): Convert timing T_chord to seconds and tenths (e.g.124.6), and divide that number by two (= 62.3). Enter this in the dialog box labelled ‘X’, lower left on the Helio page. From your earlier view of the Sun you will know if your sunspot was east of the CM (enter negative X), or west of the CM (positive X). Note that you do not have the ‘Y’ coordinate of the spot from the transits, but we can now derive it.

Deriving Y coord: Enter some trial values for ‘Y’, recalling if the spot was north or south of the E-W cross-hair in the eye-piece (In the Fig it’s south, so use negative ‘Y’ values). The DOT will move southwards toward the solar limb. If you go too far the DOT moves outside the limb and Helio gives blank values in the results box. Use tenths to get it right on the limb. You have derived the ‘Y’ coordinate for your sunspot!

Enter X coord of the spot: Simply subtract value T_spot from the value of half T_chord (it was 62.3 remember), the remainder is the X coord for your spot. When you adjust the value in the X box the dot will jump to the position of your sunspot, matching the eye-piece view. Its Heliographic Latitude and Longitude now appear in the result boxes, bottom left, of the “Helio” page.

The above steps may seem complex, but once the page is open and you are entering data, it gets much clearer. The model Sun on the page WILL match the view in your eye-piece. With some practice at aligning the cross hairs and making timings you’ll get spot positions accurate to a degree or two of the spot’s (ultimately) published position, very high accuracy for such modest equipment.

“Helio” answers tough questions like: is the new spot at the east limb the return of an earlier spot group? Does the new spot have a high latitude (i.e. is it a C24 spot)? “Helio” lets you add latitude and longitude details to sketches, like in Fig 2, and much more besides. It’s both fun and easy to use.
Enjoy Sun-watching using “Helio”.

[Ed: Note the helio web page can be found at "http://www.petermeadows.com/html/software.html".]

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