A NEW H-ALPHA SOLAR LIMB-PATROL SYSTEM* George Carroll. Lockheed Solar Observatory, Lockheed-California Co. Burbank, California

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1 A NEW H-ALPHA SLAR LIMB-PATRL SYSTEM* George Carroll Lockheed Solar bservatory, Lockheed-California Co. Burbank, California This paper describes in detail a new hydrogen-alpha solar limb patrol now in successful operation at the Lockheed Solar bservatory. The system is designed to add a properly exposed photographic record of all observable prominence actions extending above the sun s limb to the existing standard disk record. For various reasons, photographic records of flare-associated prominence actions have been difficult to obtain on a patrol basis. Visually a 0.5 Â bandpass biréfringent filter centered on Ha will show excellent solar disk detail and acceptable limb detail at lower intensity. However, photographically, because of this much lower intensity, limb prominence activity will not show unless the exposure is increased to the extent that the disk image is then greatly overexposed (by about a factor of ten). It has been clearly demonstrated in time-lapse prominence studies at various solar observatories (particularly the Sacramento Peak bservatory and the High Altitude bservatory) that a 4 to 8 Â bandpass filter used with a coronagraph will show the largest percentage of active prominence motions. A filter of less than 4 Â bandpass will frequently fail to pass the strongly Doppler-shifted components of prominence action. Certainly a 0.5 Â filter is too narrow for recording the dynamic flareassociated prominence actions. Solar astronomers, realizing the importance of recording solar prominence activity simultaneously with disk activity, have devised several systems to attain this end. The most successful of these systems (in use by the High Altitude bservatory and the CSIR, Sydney, Australia) is one in which the two outside polaroids of a 0.5 Â Ha filter are mechanically removed and replaced by clear glass to maintain focus, and a metal occulting * Presented at the Sail Diego meeting of the Astronomical Society of the Pacific, June 12-13,

2 432 GERGE CARRLL disk is then inserted at the first image plane. The film, after previous exposures to the disk with the 0.5 Â bandpass, is then re-exposed to record prominences with the 2 Â bandpass (resulting from removal of the outside polaroids). The occulting disk permits use of the broader bandpass as well as preventing any further disk exposure. At the Lockheed bservatory it was felt that such a complicated electro-mechanical device would be impractical for a solar patrol that must operate almost entirely unattended for long periods of time. The writer therefore began a study of several possible methods which would accomplish the desired results and still remain simple and reliable. The combination solar-prominence and flare telescope described here is the result. It should be remembered, however, that this system was designed to fit into a flare-patrol system already in use and cannot be considered the best possible mechanical design for a combination flare and prominence patrol. The system is, in effect, two complete systems which are almost identical optically. System A is a conventional coronagraph without an occulting disk and is used to produce the disk image through a 0.5 Â biréfringent filter. System B is a conventional coronagraph with an occulting disk and is utilized to form an annular image of the chromosphere and limb through a 4 Â interference filter. There are no moving parts, and the two images are optically combined to be recorded simultaneously with a single exposure on the same film plane as shown in Plate I. A study of the schematic diagram in Plate II will assist the reader in following the detailed description of the system. The three unique features of the system are readily apparent : ( 1 ) The use of a 4 Â interference filter in system B passes ten times as much Ha light as the 0.5 Â biréfringent filter of system A (permitting a single photographic exposure of shortest possible duration for both images). (2) Two 45 mirrors are used to bring the limb image of system B to the final image plane. Note that the 45 mirror located back of the filter in system A has a hole cut through it at 45. This hole is just large enough to clear the light beam that forms the disk image. (3) System B

3 PLATE I A typical picture from the Lockheed solar patrol telescope, showing disk and limb through a 0.5 Â Halle filter, and second larger limb through a 4Â interference filter.

4 PLATE II ptical diagram of the combination solar-prominence and flare telescope.

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7 SLAR LIMB-PATRL SYSTEM 433 forms an annulus of the solar limb approximately 10% larger than the disk at the final image plane. In this way the beam is large enough to strike mirror No. 1 outside of the hole area. This results in a double presentation of limb phenomena, making flare recognition easier for integration with prominence activity in the same region. Because system B is the new system and the point of this paper, the following description and comments will be largely confined to it. The ray paths shown by the diagram are those that form the solar limb region. The occulting disk is placed at the first image plane, for most efficient eclipsing of the disk, and also to prevent excessive heat from getting beyond this point. (System A requires a heat-reflecting element just back of the stop to remove heat from its beam.) In both systems, the stop is placed at a point where the objective lens is imaged by the field lens. This effectively blocks stray light and maintains best contrast and image quality. The optical train is designed and adjusted so that the solar annulus and occulting disk will both be in focus at the final image plane. The second objective or imaging lens directs the rays through the filter at the smallest possible angle since this is the condition for best filter performance. Back of the second objective lens Y a 45 mirror (No. 2) is placed to direct the light through the 8 Â multi-layer interference filter. The beam then passes through a converging lens to the No mirror and then to position on the film plane. The interference filter, mirrors Nos. 1 and 2, and the converging lens should be as close to the final image plane as mechanically possible, since the success of the system is strongly dependent upon the images being very nearly in focus as they pass mirror No. 1. Final exposure balance of the two systems is best achieved by adjusting the size of the stop openings. Since optical alignment is critical, it is desirable to build the system so that each element can be adjusted and aligned individually. Both mirrors must be fully adjustable and aligned to produce a perfectly circular annulus. Experience indicates that it is advantageous to mount as many components as possible in units, minimizing the alignment problems of the two systems. An easily

8 434 GERGE CARRLL removable cover for the system provides protection and easy access for frequent cleaning of the optics. A number of occulting disks are required to compensate for the annual variation in size of the solar image. The choice of occulting disk size is strongly dependent upon three criteria: (1) performance of the solar guider, (2) average seeing conditions, and (3) tendency of the filter to produce reflection and diffraction rings. Certainly the smallest occulting disk permissible is one that passes as much chromosphere as possible but effectively blocks all photospheric light; consequently a good solar guiding system is a primary requisite. The parameters of the lenses used for the Lockheed prominence system are given here for reference purposes only, and were chosen to be compatible with the existing system. bjective: 4-inch aperture, 78-inch focal length; field and objective lens: 2-inch aperture, inch focal length; converging lens: l^-inch aperture, 9-inch focal length; hole in mirror No. 2: 0.78-inch diameter. Two operational disadvantages of this combined prominenceand flare-patrol system should be discussed. First, the low contrast of solar disk phenomena in Ha necessitates using a rather high-contrast film (Eastman IV-E), and the shorter dynamic range of this film makes it less desirable for photographing prominences than the Eastman 103-Ha film. f course, the unique double presentation of the limb in this system partially compensates for this and allows easy separation of limb flares from fainter prominence actions (Plates III and IV). The second disadvantage of the combined system is the much higher dependence of the limb system (owing to the wider bandpass filter) upon clear sky conditions. In observing periods when the sky is very bright or clouds very frequent, the limb system should be blocked out to avoid the possibility of adding scattered light to the solar disk image. In practice, this has not been a serious disadvantage, and it has been necessary to block out the limb system only during rare cloud conditions that produce very strong red scatter (thin cirrus clouds).

9 SLAR LIMB-PATRL SYSTEM 435 The advantages such a system offers are : 1. No moving parts, minimizing the need for frequent adjustment of critical components such as occulting-disk position. 2. Single photographic exposure of shortest possible duration for both activities, with adequate exposure of low-intensity limb events. 3. Double presentation of limb phenomena if the limb-patrol image is purposely larger than the disk image. This makes limb flare recognition easier for integration with prominence activity in the same region. 4. The broader bandpass of the separate limb-patrol filter enhances the possibility of recording strongly Dopplershifted components of prominence activity.