These tables have been compiled from observations made from the 23rd to the 29th of june 2000, using the MTR mode on THEMIS. Observations were performed by Nadège Meunier and Jean Arnaud with the help of Frederic Paletou (resident astronomer), Olivier Grassin and Alberto Sainz (operators). The purpose of this page is to help future observers to prepare their observation on THEMIS with informations on the temporal cadence they will be able to achieve.

Observing conditions :

        The I+stokes and I-Stokes were recorded on the same camera. Data from table 1 were derived from observations made whith 6 cameras (5 spectral domains + large slit field of view). Data from table 2 were derived from observations made with 2 cameras (1 spectral domain + large slit field of view). Observations with the different cameras are simultaneous and the processing is done almost in parallel. The columns are described below.

TABLE 1 : (With spatial scanning).
 

TABLE 2 : (with no spatial scanning).
 

NB STEP	NB STOKES	EXP. TIME (sec)	MEAN DURATION sec (min)	NB PTS	TOTAL EXP.TIME	EXP.TIME  TOTAL (%)	RANGE OF %
50 100 (b) 100	3 St. 3 St. 3 St.	1 1 1	355 (5:35) 697 (11:37) 1167 (19:27)	1 12 2	150 300 500	42 43 43	42-45
25 (c)	4 St.	0	23	1

 

COLUMN DESCRIPTION :

- NBSCAN : Number of scans in one file (for Table 1).
- NBSTEP : Number of steps performed during the observations : if there is no scan, (table2) the observation has been repeates at the same position on the solar disk ; otherwise this means that the telescope displacement is made between each sequence of n Stokes measurement. The value of the displacement is 0.8".

- NBSTOKES : shows how many time the polarizer has been moved for each position on the sun.

- EXPOSURE TIME : in second, per frame.

- MEAN DURATION : of time to make the whole observation in seconds (and in minutes:seconds). This duration includes the exposure time, the time to move the telescope (for table 1) and the polarizer, and the acquisition time. Note that during these observations, nothing was done in parallel, i.e the telescope was moved after the acquisition was finished for example.

- NB PTS : number of similar observations performed to calculate the average duration.

- TOTAL EXP.TIME : EXP.TIME X number of frames.

- EXP.TIME /TOTAL : percentage of time used for the exposure time only "efficiency".

- RANGE OF % : efficiency when several observations gave differents results.

COMMENTS :

- It takes more than 1 second to perform the acquisition from the camera.

- The efficiency is of course better when a longer exposure time is used. It is also more efficient when more Stokes parameters are measured at the same position (i.e when we limit the displacements of the telescope).

- Using the observations (a), (b) and (c), we find that for a typical scan of a region using 25 steps and 4 Stokes, for an exposure time of 0.5 seconds, the amount of time for the different operations is :

Therefore, more than half of the time is used for the acquisition, which is a limiting factor when one wants a good temporal resolution. Another result is that if the motion of the telescope and of the polariser were made at the same time than the acquisition, 20% of the time could also be saved.

- We also observed a dispersion for a similar observed sequence (see in particular lines 1, 2, 4 and 5 in table 1). We don't have enough data to tell if this is due to a different behavior of the telescope depending on the time of the day or on the position of the region on the sun, or if this is due to a variation of the acquisition time. More data will be necessary to study these effects. This problem is important for temporal sequences : one should be aware of the fact that the time interval between each sequence won't be constant !
 

Tables and comments by Nadège Meunier (meunier at bagn.obs-mip.fr). All contributions to this discussion is welcome.