2.3 Improvements to the Observational Performance
2.3.2 Software – Telescope & Camera Controlling Systems
In the course of my diploma thesis [G¨ossl, 1998] I deployed a new camera controlling program supporting menu and simple scripting control of all camera features, log file generation, FITS
data format images9, an autofocus function, and already accessing telescope data. We (mainly
28 2. Observations
Jan Snigula and me) continued to upgrade the telescope and camera controlling software.
Telescope Control Software
In order to allow remote access to the telescope and to give means for better integration of the ob-
serving software the existing telescope software has been expanded and complemented by client/
server architecture applications. They communicate utilising an Beck IPC Microcontroller [Bec, 2006] as a relay between the observatory LAN and the telescope’s serial interface and hardware switches. The details are given in Snigula [2006, Chap. 7].
Offset guiding – Auto-dither
The offset guiding unit of MONICA is built of a commercial CCD camera (SBIG ST-7) which
can be positioned by the same motor controller as the filter wheel of MONICA. A radial trans- lation stage may be rotated by 270 degrees on an azimuth drive. In addition a motor driven adjustable diagonal mirror on the radial stage images a 1:1 subfield onto the guiding camera and allows for focusing as well as for compensation of the focal plane curvature. While the motor controls were already accessible from the MCS [G¨ossl, 1998, Sect. 2.3.2] the camera and the actual guiding still had to be controlled by a Windows PC. This “solution” was very unstable
when encountering rapidly changing observing conditions10and also involved very long parallel
interface cables which together with the high immission of a nearby radio broadcast station again lead to an unreliable behaviour. Also the field selection for “Windows-”guiding was restricted to three azimuth positions of the camera. Therefore we replaced the Windows PC with a small embedded Linux PC directly attached to MONICA. In order to relay the guiding camera control I wrote a server application for this Linux PC which makes all camera functions available via net- work and provides a flexible and robust guiding scheme. All server functions and configuration options are detailed in App. A. A graphical user interface based on the cross-platform applica- tion development library Qt [Haavard and Chambre-Eng, 2007] can be used to control the guider. This also enables remote control of the guiding unit via Internet. In addition the azimuth position of the camera is directly updated by the MONICA Controlling Software (MCS) allowing to use the complete 270 degrees for guide field selection.
When trying to minimise effects of defective detector pixels and the impact of the S/N ratio
of calibration images11on the overall S/N ratio of an image stack a technique called “dither-
ing” is the solution: Dithering slightly moves a detector relative to the observed field between
subsequent exposures to ensure that the same region of that field is probed by different detec-
tor elements. Of course dithering has to be implemented in a manner that it does not interfere with guiding. Actually guiding can even be used to perform the dithering procedure if it can be triggered by the primary camera control system which is exactly the way I went: The MCS has 10E.g. even small clouds passing by aborted guiding and the commercial guiding software did not allow any changes in the guiding scheme.
11See Sect. 4.2.6 for flat field calibration. For actual calculations quantifying differences inS/Nof dithered and undithered stacks see G¨ossl and Riffeser [2002, Sect. 2.2].
2.3 Improvements to the Observational Performance 29
Table 2.2: The combined filter throughputs+CCD sensitivity of the 7 MONICA filters relative
to the twilight sky without any filter.
filter no filter I R BV Hα V B U
throughputτ 1.0 0.2 0.4 0.4 0.05 0.2 0.2 0.05
implemented an auto-dither toggle which, if turned on, triggers dithering during readout of the MONICA CCD.
Automated twilight flat field calibration acquisition
Obtaining a “perfect” flat field calibration image for every night and every filter is mandatory (Sect. 4.2.6). The Tyson and Gal [1993] twilight formula describes the time dependency of the brightness of the twilight sky. After adjusting for the observatory’s latitude resulting in a table for the twilight length for every day of the year it can be used to adjust exposure times to achieve flat field images with a more or less constant flux. But full automation requires even more than that: To achieve an optimal flat field series the maximal number of calibration images for a given sequence of filters, which is a complex function of their consecutive throughputs and the CCD sensitivity within that passband in respect to the colour of the sky, has to be predicted beforehand and distributed among them. In addition, images within one filter band have to be dithered by an
offset big enough to avoid stars having an impact on a stacked flat field (Sect. 4.2.6).
After implementing the necessary prerequisites12 I have derived a working scheme by eval-
uating existing “optimal” flat field series performed by experienced observers. The empirically determined constraining limits for the series are:
• The minimal exposure time is 5 s to minimise systematics caused by the shutter movement.
• The maximal exposure time is 600 s to limit the impact of cosmics and stars on the stacked
flat field (Sect. 4.2.6).
• The maximal flux (i.e. the median flux of the flat) is 63488 ADU which is already enough
below the saturation level of 65535 ADU.
• The minimal flux is 16384 ADU.
• The “optimal” flux which gives both, enough flux per flat and enough total flats, is about
28672 ADU.
• Multiple filters in a series are sorted due to their throughput (Tab. 2.2), i.e. less throughput
corresponds to brighter sky.
12Telescope offsets controlled by the camera software, date dependent twilight flux prediction for Wendelstein, maximum number of flat fields per filter prediction, automatically constructing the filenames fulfilling the naming convention sky filter yymmdd ###.fits, etc.
30 2. Observations
• A flat field series starts centred on a blank field. Offset the pointing by 2000 in declination
and right ascension between every image (flat field dither offset). Change direction of the
offset after the every “odd” filter change (first, third, fifth, etc.).
These constraints are applied to the following procedure:
1. Calculate the numbernof achievable images per filter:
n= 15 N 1+ N X i=2 kTi −Ti−1k ,with (2.1)
N =number of filters in the series,
Ti =relative throughput of a filter band compared with no filter, and
i={U,B,V,Hα,BV,R,I,nofilter}(Tab. 2.2).
2. Capture 2 s pre-flat series images with a window readout of the centre 100× 100 pixel
region13 until the sky is bright or dim enough to start the series: For dusk the predicted
flux for the minimal exposure time must be less than the maximal allowed flux; for dawn14
the predicted flux for the maximal exposure time must be more than the minimal required flux.
3. Calculate the next exposure time ∆ti with a modified Tyson and Gal [1993] twilight for-
mula:
∆ti =
lnati−ti−1 +Alna∆ti−1 −1
lna −(ti−ti−1) ,where (2.2)
∆ti−1 =the previous exposure time,
ti−ti−1 =the time span elapsed since the previous exposure start,
A = fluxoptimal/fluxi−1, the amplification factor needed to derive an optimal flux level con-
sidering the flux level of the previous exposure, and
a=10κ/τ, with
κ= 0.094 min−1 the empirical normalisation factor for the exponential twilight brightness
law derived by Tyson and Gal, and
τ = actual length of twilight in minutes which has to be determined for the latitude of
Wendelstein and every day of the year. If the derived exposure time is beyond the limits reset it to the nearest limit. Start an exposure.
4. During CCD readout move the telescope by the flat field dither offset. If there is an “odd”
filter change coming up, reverse offset direction for the next offsets.
13To reduce the overhead from 90 s for a full image to less than 20 s for the centre 1% window.
14“DAWN, n. The time when men of reason go to bed. Certain old men prefer to rise at about that time, taking a cold bath and a long walk with an empty stomach, and otherwise mortifying the flesh. They then point with pride to these practices as the cause of their sturdy health and ripe years; the truth being that they are hearty and old, not because of their habits, but in spite of them. The reason we find only robust persons doing this thing is that it has killed all the others who have tried it.” [Bierce, 1906] Quot erat demonstrandum: Observing astronomers are men of reason.
2.3 Improvements to the Observational Performance 31
Figure 2.4: The number of flat field calibration images achieved per night has continually in- creased. First by experience (red to green), then by introducing the auto-flat procedure (green to blue). The cumulative histogram counts the number of twilight flats normalised to the average of 280 clear nights per biennium. The later the histogram saturates or the slower it grows the more
flat field images were recorded. Example 1 (blue 250/29, green 280/29): In ’04-’05 for 250 (of
280) nights no more than 29 twilight flat field images could be obtained which implicitly means that for 30 nights more than 29 “flats” were recorded; in ’02-’03 no night had more than 29 flats.
Example 2 (blue 150 /16, red 225 / 16): In ’04-’05 for 130 (normalised) nights more than 16
“flats” were obtained whereas in ’00-’01 only 55 nights surpass 16 “flats”.
5. If the current filter is done and there are still filters on the list, select the next filter.
6. Whenever the flux actually was too high before the end of the series or the predicted flux after a filter change would be too high (using the exposure time derived for the previous filter) repeat a pre-flat to readjust the exposure time.
7. Go back to 3, but repeat the last filter in the series just unless the abort conditions for dusk or dawn respectively are met. For dusk: The previous exposure was with maximum exposure time and is below the minimum flux threshold. For dawn: The previous exposure was saturated and the following pre-flat is also saturated.
Fig. 2.4 displays the gain in flat field images per night. (See caption and compare the blue bars to the green and red ones.) The average number of “flats” per twilight increased from 14.34
32 2. Observations
Figure 2.5: The MONICA imaging overhead (excluding pure camera overhead as CCD wipe or CCD readout). The overhead is the percentage of “idle” time of the camera in the span between the first and the last science night sky image of each night. Observations performance
has been greatly improved until 2001 and remains on that high level since. In ’97/’98 about
50% of the observations had less than 40% overhead (i.e. summing up the four “0, 10, 20, and 30% bins”) whereas more than 90% of the observations did so since 2000. The data of
1997/1998 and partially from 2000 had to be put together manually, because no automatic logs
were available. Nights with less than 30 minutes “shutter-open” time and guest observers who produced in average more than 50% overhead were ignored. Observation breaks longer than an
hour were also put offthe record, because they are presumably due to clouds passing through.