Observing the Sun by projection

IMPORTANT WARNINGS

  • NEVER LOOK DIRECTLY AT THE SUN THROUGH AN UNFILTERED TELESCOPE
  • NEVER LEAVE A TELESCOPE POINTED TO THE SUN UNATTENDED 
  • NEVER ALLOW INEXPERIENCED PEOPLE TO LOOK AT THE SUN THROUGH A TELESCOPE WITHOUT THE SUPERVISION BY AN EXPERIENCED SOLAR OBSERVER
  • WHEN OBSERVING THE SUN DIRECTLY – I.E. NOT BY THE PROJECTION METHOD, ALWAYS USE SAFE FILTERS FROM REPUTABLE MANUFACTURERS AND FOLLOW THE INSTRUCTIONS PROVIDED
  • DO NOT USE YOUR TELESCOPE FOR OBSERVING THE SUN IF YOU DON’T FEEL WELL INFORMED ABOUT ALL THE RELATED SAFETY ISSUES

The front filters for observing the Sun in white light, that are so common today, until a few decades ago were very rare and those of decent quality, made with glass figured to strict optical tolerances, were very expensive. Most Japanese refractors were equipped with a dark glass to be screwed in the standard 24.5 mm eyepieces, despite the risks involved, or with  white screens onto which to project the Sun’s image, as one can see in the following figures taken from old catalogs of Polarex-Tasco telescopes:

It is therefore not surprising that in many old practical astronomy handbooks one can read that the most direct and safe method for observing sunspots consists in projecting the image on a screen behind the eyepiece of a refractor telescope.

Since the invention of Baader Astrosolar ™ and the renewed interest in the use of Herschel wedges – now available under several brands, the observation by projection fell almost completely into disuse and projection screens gone off the market for several years.

Recently, however, projection screens reappeared in the market of refractor accessories together with various projection devices as the Solarscope  and  Sunspotter , both specific for school teaching or group observing.

What are the advantages of observing by projection?

First, there is obviously the pleasure to replicate the historical observations of sunspots  carried out since Galileo and Scheiner’s times until those by Carrington, Schwabe and beyond. Actually the observation in white light of the Sun has always been made with this system, and we amateurs have continued to practice with this technique even when the professionals developed others and more efficient means. Notwithstanding the progresses in solar astronomy made in the last century, the projection method for drawing sunspots is still practiced among some professional observers such as the Royal Observatory of Belgium, for the daily count of sunspots (see for example page  http://sidc.oma.be/uset/ educ / en / obsen.html ).

A remarkable safety aspect of projection consists in that the image of the sun is not directly observed, and its brightness on the screen – as long as the disk is sufficiently enlarged – does not dazzle the sight. In addition, the projection allows simultaneous viewing by multiple people and therefore makes a suitable method for teaching.

There are of course some disadvantages and some precautions to be taken. The main disadvantage concerns the sharpness of the image, which is not comparable with the one that can be obtained through a Herschel wedge or a front filter of optical quality. The granulation is less visible – most often you can only guess it – and faculae can be observed only if the projected disc is carefully shielded from ambient light.

The eyepiece used for projection should not contain cemented  lenses because the heat could ruin the cement and detach the lenses. However I used Kellner and even Plössl eyepieces without any consequence, at least until the telescope used is small. Of course it is advisable to use cheap eyepieces, not high end lenses of the type used for deep sky observing.

Again, the eyepiece barrel shall not contain plastic parts  that would melt in a short time. A good choice would be a huygenian or Ramsden eyepiece from vintage Japanese or European microscopes or telescopes, that can be easily found on ebay for a few euros. The eyepiece holder and the focuser tube must also be free from plastic parts and entirely made of metal.

As the telescope is crossed by unfiltered sunlight the telescope should never be left unattended  to avoid that inexperienced people, unconscious of the risks, might look in the eyepiece. It is also recommended to remove the finder or to cover its objective: the Sun can easily be pointed at by using of the shadow method which is the safest one.

To decrease the amount of heat concentrated by the telescope it would be advisable to stop down the objective to 60 – 80 mm max: such an aperture is more than enough for observing most of the details of the photosphere, while for larger apertures a Herschel wedge or a front filter would be more appropriate.

It should have appeared obvious from the above that the projection method is reserved to refractors while it is not recommended with other optical configurations. It has been sometimes used with newtonian reflectors too, however in this case the free aperture should not be larger than 50 mm, obtained by an off-axis mask placed on the front of the tube in such a way to avoid the secondary mirror and its supports. Finally, better not to use refractors with rear correctors or field flatteners.

A USER-FRIENDLY SCREEN

To illustrate the projection method I decided to make use of a commercial device of this kind marketed by Tecnosky of Felizzano (AL, Italy). The  Tecnosky Sunscreen – very similar to a couple of Japanese products branded Vixen – is formed by two aluminum discs 205 mm in diameter, the black one to be connected to the telescope which also acts as a shield from direct sunlight, and a blank disc which is the true projection screen. The latter slides on two tubular guides attached to the black disc so that by varying the eyepiece-to-screen distance one can vary the solar image size.

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The black disc that acts as a support for the screen and which is attached to the eyepiece holder of the telescope by means of a T2 thread
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The projection screen is a white metal disc that slides on two guides in order to have the desired distance from the eyepiece. The position can be fixed by means of screws

At the center of the black disk is a hole with a T2 female thread (telescope side) for connection to a 1.25″ or 2″ nose which in turn shall be inserted in the telescope focuser. On the other side of the disc there is a 1.25″ eyepiece holder for the projection eyepiece or a diagonal, as one can see in the below figure. In the latter case the projection device will make a 90 ° angle with the axis of the optical tube, which may be useful on several occasions.

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Example of connection to the focuser of the telescope. Since the refractor has plenty of backfocus I have been forced to add a couple of extensions both before and after the black disk.

The white screen has an excellent finish, very uniform, but with the use inevitably gets dirty and scratched, this is why I recommend using as a projection surface a sheet of white paper attached to the disk. The maximum diameter of the Sun disc that can be projected is 170 mm because the guides reduce the useful diameter of the white disc.

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The projection screen applied to my refractor stopped from 150 mm to 75 mm. Here the screen is used in direct projection, but it is also possible to arrange it at 90 ° with the axis of the tube using a common diagonal.

HOW TO USE THE SCREEN

The black disc shall be attached to the eyepiece holder of the telescope and the white projection screen initially placed at about 20 cm from the eyepiece (a 25 – 30 mm f.l. eyepiece would be a good starting point). The objective cap – which until now has been kept covered, shall then be removed and the scope pointed to the Sun by using of the shadow method. A large circular spot of light can now be seen on the white screen. However its circular edge is not the true Sun’s limb, it’s the edge of the eyepiece field stop. The true limb of the Sun shall be searched by moving the telescope tube until it appears as a curved fuzzy edge of the  light spot. By moving in and out the focuser tube – without changing the distance between the two disks – we will look for the position where the edge of the Sun appears to be perfectly clear. If sunspots are present focusing can be refined on these details. Now by changing the distance between the discs, the size of the projected Sun can be lead to the desired diameter.

The solar disc should not be vignetted by the field stop of the eyepiece, that is some space should be left between the true limb and the stop. Detailed observations of groups may require to enlarge the disc by changing eyepiece with one of shorter focal length and/or increasing the distance between the eyepiece and the screen.

The  projected image diameter  – supposed perfectly in focus – is linked to the screen-eyepiece distance and the focal lengths of the objective and eyepiece by the formula

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where  Dp  is the diameter (in millimeters) of the projected image,  Ds  is the apparent diameter of the Sun in radians – variable between 0.00917 and 0.00948 at aphelion and perihelion respectively –  d  is the distance in mm between the eyepiece and the white screen,  F  the objective focal length and  f  the eyepiece f.l. An approximate formula of immediate use that assumes an average value for the apparent diameter of the sun is this

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which provides directly the eyepiece-screen distance required to obtain a solar image of diameter  Dp  in mm.

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Fixing the blank to the white disc by a clip allows to draw comfortably. By varying the distance between the screen and the eyepiece the size of the projected image can be lead to coincide with the edge of the blank disc. In this case one can see the form adopted by the Royal Observatory of Belgium (USET data / image, Royal Observatory of Belgium, Brussels). The sheet should be rotated on the screen to align the EW line with the direction of the Sun’s drift in the sky due to the Earth motion.

The diameter of the blank should be chosen taking into account that  a small image is brighter but less resolved, it will show faculae better but doesn’t allow to distinguish small spots and pores. On the other hand a  larger image will be resolved but less bright and less defined. It is clear from the above formulas that the size of the projected image is proportional to the distance between the screen and eyepiece, while the magnification is, as usual, depending on the eyepiece focal length. One shall therefore find the most suitable combination between disk size, brightness and detail. This is done by trial and error, in general a projected disc diameter between 100 and 150 mm can be a good compromise.

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DRAWING

With compass and ruler or any graphics software once can draw on a blank white disc to be attached to the screen where to report the observed details by using of a delicate pencil. The disc, of diameter between 10 and 15 cm, should be as this one:

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The projected image on the screen will have the orientation above, with north up and west to the left. If the screen is placed after a diagonal the east and west direciton will be exchanged.

Since no astronomical drawing makes sense if it is not oriented with respect to the celestial directions , the cross in the circle shall be used for this purpose. The horizontal branch of the cross must be placed in the E–W direction along which the Sun moves in its diurnal motion (which can be determined by turning off the tracking, the movement of the Sun will indicate the west). Once the blank has been properly oriented one can proceed to draw the visible details with light pencil marks trying as much as possible to prevent the screen to vibrate. The observer shall report the outline of umbras and penumbras and, at least roughly, the facular regions.

At the end of our efforts we will get something like this

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Of course detailed drawings of limited regions can be done simply by increasing the diameter of the sun and/or the magnification:

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Richard Carrington’s drawing documenting the solar flare occurred on September  1st, 1859 which led to exceptional auroral events worldwide even at low latitudes. Carrington observed with the projection method, the solar disc had a diameter of 11 inches and was projected on a glass plate painted in yellow. Note the orientation of the drawing with the north at the top and the west (P) to the left.

Although not strictly necessary an  equatorial mount with sidereal tracking will make things much easier and will also keep the Sun with the same orientation with respect to the cross. An altazimuth mount with autotracking may also be fine provided the drawing doesn’t take too much time.

ONE STEP FURTHER

Those who want to give a little more value to their drawings can determine the location of groups and spots with respect to the Sun coordinate system.

The orientation of the sun changes throughout the year because in the course of Earth orbit we observe it from different views. This animated GIF, from the excellent web site of Peter Meadows , shows how the appearance of the Sun varies during the year:

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In particular, the celestial N-S direction coincides with that of the true Sun only on certain dates of the year in January and July, otherwise there is alway a discrepancy between these two directions, i.e. the position angle of the Sun’s north pole. The angle is positive to the east and negative to the west , it is indicated by the letter P in solar ephemeris and should be reported on the edge of the drawing disc using of a goniometer:

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In correspondence with the value of the angle a line is drawn through the center of the drawing which represents the true meridian of the Sun as one can see in the below example:

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where the position of the true N pole is indicated with a P.

But we have not finished yet because not only the Sun appears to “dance” like a pendulum swinging to the east and west through the year, but even the apparent inclination of the solar poles toward Earth varies by a few degrees which is indicated by the symbol B0 in solar ephemeris. If we want to determine with enough precision the position of groups and spots, we must also take account of this angle. For this purpose we can make use of the so called Stonyhurst discs, grids of the solar coordinates system to be superimposed to the drawings made by projection or by direct vision:

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Each disc corresponds to a certain value of B ranging from -7 ° to + 7 °. In selecting the disc for a certain value of B0, one shall take the one corresponding to the nearest integer: for example, if on a given date B0  is 1.8° we will use the disc corresponding to 2 °, if B0 is -4.3 °, we will use -4° and so on. The disc shall be superimposed on the drawing so that the central meridian of the Sun traced on the Stonyhurst disc coincides with the one that we have drawn based on the value of P, that in this way:

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Each Stonyhurst disc corresponds to two values of B0, one positive and one negative – i.e. + 1 / -1, + 2 / -2, etc. – the “side” to be used should be made coincident to the north on the drawing.

As one can see from the figure above there can be a considerable discrepancy between the N-S celestial directions and the solar meridian. Based on celestial directions, those corresponding to the cross on the blank, we would have assigned the group marked with the number “14”, the northern hemisphere of the Sun, while the Stonyhurts disc, once properly oriented, shows that it actually belongs to the southern hemisphere.

Stonyhurst discs can be freely downloaded from  this site  and printed to the desired diameter on fine paper, glossy or transparent. Alternatively in the used market one can still find the so-called “Vixen acetates”

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SOFTWARE

In order to know the orientation of the solar disc at any date I recommend two free softwares,  TiltingSun  and  Helio Viewer . Both are simple and intuitive, and allow to import scanned drawings or CCD images to which directly superimpose the grid of the solar coordinates.

RECOMMENDED READINGS

There are a few readings which I highly recommend. How to observe the Sun safely, by Lee Macdonald (Springer) is a very good handbook for the solar observer which covers well the projection method. I also recommend Solar Sketching , which contains an entire chapter dedicated to the projection and is also an must have reading for those who want to devote to the drawing in white light and monochromatic light (H-alpha, Ca-H, Ca-K). Both books are also available in Amazon Kindle format.

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