New Lyot coronagraph at NAO – Rozhen: presents and perspectives

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New Lyot coronagraph at NAO – Rozhen: presents and perspectives
Petrov N., Duchlev P., Kokotanekova J.
Institute of Astronomy, Bulgarian Academy of Science, Sofia, Bulgaria, nikipetrov@yahoo.com
Abstract. A new Lyot type coronagraph for prominence observations was assembled in June 2005 at the National Astronomical Observatory – Rozhen, Bulgaria. A short description and parameters of the instrument, as well as its testing and improving during the observations are presented. The results from the observations quiescent and active prominences are presented and discussed, too. The scientific objectives and future applications of the coronagraph are considered.
 
 



Introduction
The first steps to solar observations of Solar Department in the Institute of Astronomy of the Bulgarian Academy of Sciences was made in 1994 when a Solar tower with 8-m dome (Fig. 1) was built in the territory of the National Astronomical Observatory (NAO) – Rozhen. There is a parallactic mounting in the solar tower that allows assembling of at least two basic solar instruments: a coronagraph and a solar refractor. Initially 13-cm solar refractor was assembled in the solar tower for observations of the photospheric activity events in white-light. Since 2001, a 15-cm Lyot coronagraph with Hα filter was designed and built. In 2005, the coronagraph was mounted in the solar tower and from the end of 2005 regular observations of solar prominences has been carried out.
New Lyot Coronagraph at NAO – Rozhen

During a long-standing joint collaboration between the Institute of Astronomy of BAN and the Astronomical institute of the University of Wroclaw, Poland detail investigations of the construction of the polish Small Coronagraph were made. On the base of these investigations, as well as of the useful advices of Prof. B. Rompolt one of the authors of this paper (N. Petrov) in 2001 and 2002 calculated the optical system and mechanical construction of the 15-cm Lyot-coronagraph with Hα filter (1.8 Å). 

 

Fig. 1. The Solar tower in NAO - Rozhen

 

 The scheme of the coronagraph is given in Fig. 2 where are shown the essential optical details of the instrument. The objective O1 made by Carl Zeisse, Jena, the most critical component of the instrument, consists of a lens corrected for spherical aberration. Its diameter is 150 mm with 2250 mm focal length F1. The occulting cone shaped diaphragm (“artificial Moon”) D¤ is placed in the focal plane of the main objective. The shape of the occulting diaphragm, as well as its well-polished duralumin surface allows avoiding the light and heat to the sides. A hollow stem in the field lens O2 that confirms the role of a radiator supports the occulting diaphragm. In view of the different visible diameters of the Sun in different months of the year a set of 6 different occulting diaphragms was made.  

 

Fig. 2. An optical scheme of the 15-cm coronagraph at NAO - Rozhen

 

The field lens O2 with diameter of 72 mm and a focal length F2 of 220 mm is located approximately 20 mm (f @ 1/10. The field lens uniforms alight of the field of view falling on the objective O3. If the field lens were in the focal plane, any dust on the lens would be sharply focused at the eyepiece or camera. The field lens O2 images the entrance aperture S of the objective O1 on an iris diaphragm LS called Lyot Stop. The opening of the iris diaphragm is adjusted to be somewhat smaller than the image of the brilliant ring of light diffracted from the edge of the entrance objective O1 and intercepts it, avoiding another possible source of scattered light. The diffraction from the edge of the iris diaphragm LS is insignificant since the very intense light from the solar disk is almost completely retained by the occulting diaphragm D¤.
Close behind the iris diaphragm LS the objective O3 is located that forms an image of the occulting diaphragm D¤ and corona surrounding it in the camera A. The objective O3 is interchangeable and has diameter of 45 mm and focal length of 150 mm. Next to the objective O3 is located a birefringent Ha filter with 1.8 Å band-pass in the hydrogen Ha line. The filter is heated to an operating temperature of 40° C by an electronic thermostatic control with oversupply 17.2 V.
The resulting solar image can be recorded by a photographic, digital or CCD camera. The coronagraph in NAO – Rozhen is equipped with two digital cameras Canon EOS 350D (8 Mpxs) with digital matrix size 22.2х14.8 mm and Canon 1Ds Mark II with digital matrix size cameras’ resolution, obtained images have size of 3456x2304 pxs and 4992x3328 pxs. Therefore, for the two cameras one pixel corresponds to size of 6.4х6.4 μм and 6.4х6.4 μм, correspondingly. There is software that allows of the two cameras to show the observed prominences in real-time on the monitor, as well as direct registering of the prominence images on the hard disk.
Theoretical resolution of the coronagraph is 1”.1. It necessary more long observations and additional investigations to estimate the real coronagraph resolution because it, as at another telescopes, strong depends on atmospheric conditions in the region of observation. The statistical data from the observations with telescopes in NAO- Rozhen show that the mean resolution for night observations is about of 2”. The basic parameters of the coronagraph are given in Table I.
Testing and Improving
The coronagraph was built in the workshop of the Institute of Astronomy during 2003-2004. In the beginning of May 2005, it was assembled in the solar tower (Fig. 3) in NAO – Rozhen. The initial rough adjustments connected mainly with the focusing of the coronagraph were made using black-white films KODAK P3200. The first successful image of a solar prominence was obtained on July 13, 2005 using digital camera Canon EOS 300D. In August 2005 the coronagraph was equipped with digital camera Canon EOS 350D. Technical parameters of the digital camera allow obtaining long series of qualitative images with high resolution. During several months, from August to October 2005 the fine adjustments of the optical of the coronagraph system were performed. In the second half of October, the prominence images with high quality and resolution were obtained.

 

 
TABLE I
Basic parameters of the coronagraph
Parameter
Value
Diameter of the main objective O1
150 mm
Focal length of the main objective О1
2250 mm
Effective focal length of the coronagraph optical system
4500 mm
Field of view
15’
Theoretical resolution limit
1”.1
Band-pass of the Нα filter
1.8 Å
Mean diameter of the solar disk in the focal plane of the main objective O1
≈21 mm
Total weight of the coronagraph
62 kg
Altitude above sea-level
1750 m
 
 

 

Fig. 3. The new coronagraph of the Institute of astronomy assembled in Solar tower at NAO - Rozhen
 

 

On the base of joint observations with our polish colleagues and many discussions about the quality of the prominence images obtained with the coronagraph at NAO – Rozhen, were localized the problematic modules in the coronagraph optical system. As a result, in 2008 the coronagraph was improved and renovated. The main and secondary objectives were replaced with new ones possessing improved optical characteristics. The set of “artificial Moons” was rebuilt. The hollow stem in the field lens O2 that supports and cools the occulting diaphragm was rebuilt, too. By this way, the parasitic scattered light in the coronagraph tube was reduced to the highest degree.
Prominence Observations
From mid-October 2005 to the present, many observations of solar prominences were made. Long series of Hα filtergrams for selected prominences at the solar limb were obtained. The registered prominences were mainly quiescent ones but there were several cases of activated quiescent prominences.
 

 

Fig. 4.Two quiescent solar prominences observed with the coronagraph at NAO- Rozhen on 6 November 2005 at side-on position (top) and on 25 January 2008 at edge-on position (bottom).
 

 
 
Only two cases of eruptive and surge prominences was observed because the period of coronagraph observations coincides with the epoch of minimum of solar activity between solar cycle 23 and the coming cycle 24. First results from the coronagraph observations are summarized in our previous paper [1].
In Fig. 4 are represented two cases of remarkable quiescent prominences (QPs) observed with the coronagraph in NAO – Rozhen in two character extremely position at the solar limb – side-on position (Fig. 4 top) and edge-on position (Fig. 4 bottom). The side-on position of the QP on 6 November 2005 is very suitable for investigation of the internal structure of its body. The prominence shows distinctly a structure of medium-scale – several arches composing the prominence body, as well as the small-scale structure of the arches consisted of threads and knots. The edge-on position of the QPs on 25 January 2008 is the case when the long axis of the prominence body coincides with line-of-sight. The prominence shows typical for such cases helmet-like large-scale shape in which small-scale vertical threads are clearly distinguished.
Fig. 5 shows a quiescent prominence, which is undergone activation via magnetic field reconnection that leads to significant change of the prominence medium-scale structure, i.e. a significant re-organization of the magnetic field configuration of the QP occurred. This is one of rare and interesting cases when the change of the prominence magnetic configuration is significant but not drastic and the QP continues to exist. Some authors suggest the magnetic field reconnection as trigger and driver mechanism of destabilization of the pre-existing magnetic system [2, 3, 4]. Our time series of Hα filtergrams is sufficiently long (56 filtergrams) and the process of magnetic reconnection as trigger mechanism of the QP destabilization could be investigated in details.
Fig. 6 presents an eruptive prominence (EP) observed with the coronagraph at NAO – Rozhen on 22 August 2006. The analysis of the Hα filtergrams suggests that the observed EP represents a very specific case. By a kinematic pattern it is rather similar to surge prominences but by helically twisted fine structure and its evolution the prominence is rather an eruptive one.

 

Fig. 6. An eruptive prominence observed with the coronagraph at NAO – Rozhen on 22 August 2006
 

 

 

Fig. 5. An activated QP observed with the coronagraph at NAO – Rozhen on 15 November 2005
 

 

 
 
 
The analysis of the quality of the prominences observed between 2005 and 2008 suggests that the optical system of the new coronagraph, especially after its improvement, and the altitude of the instrument above the sea level give good perspectives for high quality observations of solar prominences from NAO – Rozhen.
 
Scientific Objectives and Future Applications
The basic science objectives of the coronagraph are:
· fine structure and dynamics of quiescent prominences;
· dynamics and internal structure evolution of eruptive prominences;
· dynamics of active prominences (surge and spray);
· monitoring of solar prominences.
The equipment of the coronagraph and the solar tower is not fully completed. In future, the coronagraph will be equipped with spectrograph that will allow registration of the line-of-sight velocity from fine internal structure of quiescent and eruptive prominences. On the base of such observations, we could investigate the dynamics of the fine-scale internal structure of the prominences and especially prominence oscillations.
In the near future, a detail investigation of the influence of the atmospheric conditions at NAO-Rozhen on the quality of coronagraph observations, as well as an exact determination of the real resolution of the coronagraph will make.
One of the important coming tasks is the creation of an archive of the observational data and its availability. Using digital camera for image registration we obtain images directly in file format that makes easy the creation of the archive of our observational data. The preparation of the catalogue of observed prominences is coming. In the frame of the Bulgarian Virtual Observatory, the Solar department of the Institute of Astronomy of BAS, started web site www.astro.bas.bg/sun/SVO [5] where the data set and catalogue of prominences will be available on-line.
 
Acknowledgements
The authors are grateful to CNSys Computers & Networking, Sofia and OPTICOELECTRON GROUP, Panagyurishtefor financial support of the coronagraph building. The coronagraph realization was supported by National Scientific Foundation under Grants F-1510/2005 and D0-406/2005.
 
References
 [1] Petrov, N., Duchlev, P., Kokotanekova, J., and Rompolt, B.: 2007, First Results from Coronagraph Observations at the NAO – Rozhen, Suppl. Bulg. J. Phys. 34 part 2, ASTRONOMY AND SPACE SCIENCE, M. K. Tsvetkov, L. G. Filipov, M. S. Dimitrijević, L. Č. Popović (eds.), 86.
[2]   Antiochos, S. K., DeVore, C. R., and Klimchuk, J. A.: 1999, Astrophys. J. 510, 485.
[3]   Chen, P. F. and Shibata, K.: 2000, Astrophys. J. 545, 524.
[4]   Moore, R. L., Sterling, A. C., Hudson, H. S., and Lemen, J. R.: 2001, Astrophys. J. 552, 833.
[5]   Petrov, G., Dechev, M., Slavcheva, L., Duchlev, P., Mihov, B., Kopchev, V., and Bachev, R.: 2008, Astronomical Virtual Observatory. Bulgarian Virtual Obcervatory – place and role, VI Serbian-Bulgarian Astr. Conf., 7-11 May 2008, Belgrade, Serbia, Publ. Astr. Soc. "Rudjer Boskovic" vol. 9, (in press).