Instruments used for Ca K solar observation
Setup n°6 : (from 2018) :
High resolution Ca K imaging is done at full aperture (150 mm) with a 50 mm blue Astronomik filter placed about 230 mm ahead from the focus (and ahead of the Baader Planetarium FFC). This filter acts as an internal ERF.
The Ca K filter is further protected by a Baader Planetarium K-line filter.The K-line is a double stack filter. However the front element has been removed to avoid the formation of the secondary image caused by the front element.
Typical exposure time with the ASI290 is 60 frames of 4 to 6 ms for each frame.
Setup n°5 : (from 2013) :
Full disk images are made as four-frame mosaïc with the IDS 3370 camera. Typically 10 to 20 exposures of 1 ms for each frame.
Setup n°4 : (from 2011) :
Occasionaly, the SBIG STL11000 camera is replaced by a Lumenera LW 11059M camera. The Lumenera camera has the same KAI-11000 sensor as the SBIG, but the image is digitized in 12 bits (instead of 16 bits), allowing a 1 frame / rate (instead of 1 frame every 35 s). Typical runs are 1500 to 2000 frames long (instead of 150 to 300 frames).
Setup n°3 : (from 2009) :
Takahashi TOA 150 with 65 mm aperture stop.
Takahashi X1.5 ED extender.
The STL 11000 M is used without its filter wheel in order to avoid transmission loss in UV.
A Baader K-line is used as a pre-filter directly in front of the Ca K filter to reject IR light.
The Barr Associates Ca K filter was measured with a spectrograph on 21 August 2009 :
- peak transmission : ?
- CWL = 393.25 nm,
- FWHM = 0.24 nm,
- transmission at 10% : about 0.6 nm, this cold be a two-cavity filter
- temperature drift : about 0.001 nm/°C, as per available info.
Setup n°2 (from 2006 to 2008):
Takahashi FSQ 106N with 65 mm aperture stop.
This modified four lens Peztval design equipped with two fluorite lens provides a very good correction in the near-UV light (much better than the previously used Vixen fluorite 55 mm refractor which was a doublet with one fluorite element).
Takahashi NJP 160 mount with MCMT drive control. Marks on the ground allow to properly set the mount along the celestial polar axis.
The laptop computer is protected from the sun. It is linked through Ethernet network (100 Mb/s) to a desktop computer inside the house equipped with a 22" CRT screen. The outside laptop computer is only used as a relay to the inside computer. The focusing is checked on the 22" CRT screen. Pointing and acquisition are controlled by computer, but focusing is still manual.
A typical observing run consists in 150 to 300 images taken with the STL11000 M CCD camera. The run is divided in sequence of 30 to 40 images controlled by Prism software. Focusing and centering is checked at the end of each sequence. After the end of an observing run, the best image is automatically selected by a home-made algorithm based on Fourrier transform written in IDL language (selection criteria = intensity max frequency / intensity low frequency). Typically, one good image is obtained out of 200 or 300 acquired images.
The best image is then dark and flat-field corrected and processed with a soft unsharp mask.
The imaging train is composed of :
- a X1.6 extender with a KG3 anti-infrared filter (visible on the next picture). According to Takahashi, the extender provides additional blue correction to the FSQ.
- a Barr 393.3 nm Ca K filter (inside the bright aluminium ring). With the 65 mm aperture stop, the Ca K receives a F/13 incident light beam.
- a Baader FFC barlow lens. The FFC is composed of four elements with two fluorite lens. It presents a very good near-UV correction and provides a flat field.
- a SBIG STL 11000M camera with KAI-11000 interline CCD detector (4008 x 2672 pixels, 9 microns pixel size, one image per 40 s, electronic shutter down to 1 ms exposure. Typical exposure are from 10 to 15 ms according to sky conditions, which corresponds to about half the dynamic of the CCD. A shorter exposure time would be preferable to "freeze" the turbulence but ... there is not enough light with the 65 mm aperture stop. The CCD detector is cooled to -5°C.
The focusing is done to 0.05 mm accuracy with the help of a measuring gauge. Due the temperature variation along the observation session and resulting mechanical dilation, focusing is to be re-adjusted every 20 mm. The variation of focus is in the range of 1/100 mm per degree Celsius.
The FSQ rotating focuser allows to orient the CCD along the North / South celestial directions.
Heat issues :
A Schott KG3 filter anti-IR is used in front of the X1.6 extender to reduce the heat load on the Ca K filter.
With the 65 mm aperture diaphragm, the measured temperature on the KG3 is greater than 50°C (limit of my IR thermometer). Considering resolution, a larger aperture stop is not necessary because of the focus scale : 0.86 arcsec / pixel. While the diffraction limits the resolution to 1.8 arcsec at 550 nm, the resolution increases to 1.3 arcsec at 393 nm. Accordingly, the 0.86 arcsec/pixel scale is a bit short compared to the Nyquist criterion, but allows shorter exposure time, which is most important.
A larger aperture stop would provide shorter exposure time but the heat load on the Ca K filter would be too high.
An L Astronomik filter is sometimes used in front of the Ca K filter instead of the KG3. The Astronomik filter seems to provide less ghosting than the KG3 (to be confirmed).
Finally, it is unfortunately, there no full aperture Energy Rejection Filter is available for Ca K light...
The Barr Associates, Inc., Ca K filter outside its support :
- central wavelength = 393.38 nm (k2V line)
- FWHM = 0.3 nm
- outside diameter = 33 mm
The filter was manufactured using refractory ion-assisted implantation technology. This process produces filters with excellent long-term stability and very low sensitivity against temperature variation. Typical center wavelength bandpass shifts are .001 nm per degree C
If no precaution is taken, solar observation in summer may cause overheating of the step motors or the CCD camera electronics.
In order to cool down the step motors, heat sunkers are strapped to the motors.
When temperature is high, the CCD camera is also protected with an aluminium foil. Additional fan has not been necessary up to now. Water cooling would be possible but is not really not convenient to set up.
Because of atmospheric dispersion, Ca K is much more sensitive to air turbulence than visible or H alpha work (turbulence increases when the wavelength decreases). The best observing times at my site are typically from 2 hours before the meridian to1 hour after. Good conditions have never been obtained in the afternoon. In the same way, no good observation has been made when the sun is below the celestial equator.
Setup n°1 (2001 to 2005):
55 mm fluorite refractor
SBIG ST10 XME + CFW8A (with 0 - 0.3 - 0.6 - 0.9 neutral density filters)
Cak Bar filter (in the brass adapter)
Compur shutter set to 1/400 s exposure
X2 Clavé barlow lens
Van Slike focuser
Astronomik 50 mm diameter IR rejection filter
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