A Sensitive Visual Photometer for Solar Research

Yngve Ohman

The paper describes one of the visual photometers which have been developed at the Stockholm Observa-tory and its solar station in Anacapri, in connection with the solar patrol which has been operating inthese observatories for several years. The instrument, which uses a Savart polariscope as detector, canbe used for measuring disk phenomena as well as limb phenomena, and is particularly well adapted foruse as an auxiliary instrument when working with birefringent filters. It has proved useful in spectro-photometric measurements as well.

In connection with the solar flare patrol operatingsince 1952 at the Swedish Astrophysical Station inAnacapri and at the Stockholm Observatory in Salts-jbbaden since about 1947, considerable efforts havebeen made to develop simple auxiliary instrumentsfor measuring areas and intensities of various solarphenomena, such as flares, filaments, and prominences.The first instrument (1) developed for this purposeconsists of an orthoscopic eyepiece, where three holesin a transparent polaroid film are visible togetherwith the solar image. The three holes correspond tothe limiting sizes of flares of importance 1, 2, and 3.By turning the eyepiece with respect to a birefringentfilter the solar image viewed in the eyepiece willchange its intensity except for the small areas visiblein the holes. In fact, the holes appear in this wayas artificial flares which may be compared with realflares seen near the holes through the polaroid film.

A more refined instrument (number 2), based ona similar principle, was devised by the author in1956' and has been used with great success at theSwedish Astrophysical Station in Anacapri.2, 4 Inthis photometer the polaroid film is cemented to aplate of calcite, the principal section of which is per-pendicular to the vibrational direction of the polaroid.In this way two images are seen of the hole in thefilm, the intensities of which vary with differentorientations of the eyepiece with respect to the bi-refringent filter. A setting 0 which gives the ex-traordinary image of the hole (undisturbed part ofthe solar disk) the same intensity as that of the singleimage (ordinary) of a flare observed at the side of

The author is at the Stockholm Observatory, Saltsjbbaden,Sweden.

Received 14 August 1962.

the hole allows the intensity of the flare if to be cal-culated according to the formula:

= ( + -tIa1 ). (1)

Here Io is the intensity of the undisturbed solar diskand a a transmission factor which can easily be deter-mined by comparing the two images of the hole.This method of solar flare photometry is particularlysensitive for objects of low contrast, as a consequenceof the fact that the artificial bright object is addedto the undisturbed solar surface as expressed byformula (1). Very bright objects and notably thosewith filamentary structure may be somewhat moredifficult to measure, but a trained observer arrivesat good accuracy also for such objects. Even darkfilaments (prominences on the disk) have been satis-factorily measured with this device' by locating thefilament on the hole.

In order to find an instrument which could allowaccurate determinations even of flares with a fila-mentary structure and of flares and prominencessituated outside the limb, experiments were startedin 1960 with a modified photometer using a Savartpolariscope as detecting device.6 The experimentswere very successful, and a new photometer wasdevised which has been found to give the same highaccuracy as photometer number 2 described abovebut with still more possibilities. In fact from theextremely good experience we have had with thisphotometer (number 3) it might be worth trying itas a standard photometer in solar research. Itsprinciple is as follows:

When the light has passed the birefringent filterM (Fig. 1) as plane polarized light, it is split into twobeams polarized perpendicularly to each other in

January 1963 / Vol. 2, No. 1 / APPLIED OPTICS 89

M

[

Fig. 1. Schematic plan of the pholpolariscope as detecting device. M isordinary polarizer, I a calcite plate, ESavart polariscope.

the calcite plate I connected w:at the end of which is mounted tlS. With this arrangement thesomewhat displaced images of theis rotated with respect to the birelative intensity of the two imathe superimposed light of thethe central region of the undisturlno trace of polarization, that is,fringes, the two images have elsame intensity.

Suppose now that we have a of view. With the same settingfringes are now well visible inflare. In order to make the frimages disappear (Fig. 2) we iangle 0 and obtain the intensitypressed in the intensity of the nEIo, as unit, by means of the formula

if = o tan2 (450 + 0).

S mospheric and instrumental). The photometer isvery well adapted for such a procedure.7 In factstray light outside the limb can be observed in thesame way that we observe a prominence. Supposethat K, is the apparent intensity ratio found for thestray light and that Kp is the same for the prominence.The true intensity ratio for the prominence is then

tometer using a Savart obtained as:a birefringent filter orthe eyepiece, and S a i = - P (4)

1o I -K.

We have arrived at very good accuracy by this method,and have been able to establish small intensity varia-

Lith the eyepece E, tions in prominences which seem to be related some-he Savart polariscope times to long distance disturbances from flares.8

observer sees two The photometer has been found to be very con-

sun. If the eyepilece venient when used to measure sunspots as well.7

refringent filter, the Moreover it has been found useful for measuringtwiages , chae- f equivalent widths of Fraunhofer lines.7

twvo images, say of In the first instrument which has been in use now

bed solar disk, shows in Anacapri for about two years the calcite plate Ino trace of Savart has a thickness of 10 mm. In the same instrument

iidently exactly the the Savart polariscope has been so made that the

fringes appear with a spacing of about 5 min of arc.

olar flare the field This has been found very satisfactory when workingas before the Savart with a solar disk formed by a lens with f = 150 cm

both images ofr the and together with an eyepiece with f = 25 mm. Ininges in one ot the-irn the eyepiece anof the flare, if, ex-

eighboring solar disk,

(2)

In order for one to determine 0 more accurately,the other image of the flare is now weakened by turningthe eyepiece in a symmetrical position, so that theSavart fringes disappear in this image instead. Thedifference in the two settings gives the quantity 20.

When measuring objects which are fainter thanthe solar disk, say sunspots, filaments, or prominencesoutside the limb, the formula (2) will be replaced by:

ip = I tan2 (450 - 0). (3)

Figure 3 shows a photograph taken through thephotometer when a setting has been made on a prom-inence outside the limb. As can be seen from thisimage the Savart fringes have completely disappearedin the prominence visible against the displaced solardisk of the fainter image. When such measurementsare reduced, the apparent intensity ratio is generallytransformed to one with the intensity in the centerof the solar disk as unit. It is necessary thereforeto make a determination of the limb-darkening functionfor the birefringent filter.

In the case of prominences and sunspots it is oftennecessary to apply a correction for stray light (at-

Fig. 2. The importance 3 flare of July 15, 1961, observedthrough the photometer using a Savart polariscope. The Savartfringes are absent in the weaker (lower) image of the flare whichis superimposed on the brighter and somewhat displaced imageof the solar disk. This corresponds to a correct setting.

90 APPLIED OPTICS / Vol. 2, No. 1 / January 1963

Fig. 3. Limb prominence of July 9, 1962, observed throughthe photometer using a Savart polariscope. The Savart fringesare absent in the stronger image of the prominence which issuperimposed on the weaker and somewhat displaced image ofthe solar disk. This corresponds to a correct setting.

the Stockholm Observatory solar patrol instrument,which has a focal length of 189 cm, a polariscopeis used which gives a separation of the fringes ofabout 8 min of arc. As the fringes are visible atinfinite distance, a myopic observer must use hiscorrecting glasses when working with the photometer.

A small improvement of the instrument may be

made by placing a half-wave plate in front of thecalcite plate. The measurements can then be madeby turning this plate instead of the eyepiece. Thismethod would particularly facilitate measurementsof prominences because of the fact that a more orless radial displacement of the two images is preferablefor such a purpose.

Though our photometer number 3 is similar inprinciple to the classical Wild photometer, it doesnot seem to have been used by other observatories.In our opinion it facilitates to a considerable degreethe classification of solar flares, and could make pos-sible a refined flare classification using two symbols,one expressing the area and the other one the intensity.It is worth noting in this connection that we have foundno trace of systematic difference' between measure-ments made with this photometer and the photometernumber 2 working according to formula (1). Ifthe birefringent filters are of similar quality, we believethat different observatories would obtain very similarresults. In this connection it should be mentionedthat the accuracy found by us compares well with thatof high quality photographic determinations.

References1. Y. 6hman, Stockholms Observatoriums Ann. 19, 4 (1956).2, Kerstin Fredga, Stockholms Observatorium, Medd. 100

(1958).3. F. Eleman, Stockholms Observatorium, Medd. 135 (1962).4. Y. hman, Stockholms Observatoriums Ann. 22, 4 (1962).5. Kerstin Fredga, Stockholms Observatorium, Medd. 126

(1960).6. Y. hman and Britt Ahnstrbm-Sandgren, Stockholms

Observatorium, Medd. 125 (1960).7. N. J. Bergsj6, Stockholms Observatorium, Medd., in press.S. Y. hman, A. Lindgren, and Ulla Lindgren, Stockholms

Observatorium, Medd., in press.9. Kerstin Fredga, Stockholms Observatorium, Medd., in press.

Letters to the Editor continued from page 88

An Unusual Property of MgF,Films in the Infrared

Milton Laikin

Pacific Optical Corporation, Inglewood, Calif.Received 10 September 1962.

Evaporated MgF2 films have long been used as antireflectioncoatings for glass parts,' and have also been used as a protectivecoating for aluminum mirrors.' Silicon monoxide is sometimesused as a protective coating for aluminum mirrors.' However,if the magnesium fluoride is baked at 250'C for 10 to 15 min,mirrors so protected are highly resistant to abrasion.' Mag-nesium fluoride is also used because of its ultraviolet transmit-tance as a protective coating on aluminum where high ultravioletreflectance is desired. 5

Referring to Fig. 1, curve 1 shows the reflectance of unpro-tected aluminum as compared to aluminum overcoated with a250 m optical thickness (index of refraction times thickness asmonitored with green light) of magnesium fluoride, curve 3. Thereflectance is reduced at 16.5 by 15% due to the magnesiumfluoride. Aluminum is first evaporated onto the cold glassblock. The heaters are then turned on to raise the temperatureof the work to 250'C and MgF2 is then evaporated. All this isdone at pressures of about 5 X 10-5 mm Hg.

Essentially the same curve is obtained if the MgF2 is evaporatedonto work which is at room temperatures and the heaters turnedon immediately after the evaporation (while still under a vacuum).

However, a different reflectance is obtained, curve 2, if theheaters are not turned on at all while under a vacuum. The filmis, of course, quite soft. Its reflectance has dropped by about 6%at 17.5 as compared to the plain aluminum. This reflectance

January 1963 / Vol. 2, No. 1 / APPLIED OPTICS 91