Reconstruction of the Lhires III at higher intensity (focal ratio of the collimator of f / 8 to f / 5)


The Lhires was built for the standard C11 telescope. This has a focal ratio of f/10. My C14 has f/11, but at a focal length of 3910 mm. Because of the long focal length the stellar disks in the focus have diameters of 60 to 80 um dependent of the seeing (2" to 4" in germany). Thus I have a relatively large slit width of 40 um used to fall even with less than 50% of starlight through the slit, the rest is lost (and is used for autoguiding). This unsatisfactory situation has led me to rebuild my Lhires III to faster focal ratio. Instead of a 30 mm collimator was a 40 mm diameter lens fitted (achromatic doublet). At 200 mm focal length, the formally focal ratio is f / 5. The conversion makes sense only if the seeing star image can be reduced to 30 or 40 um. This is achieved by using a focal length reducer (Shapley lense) in front of the slit.

The first serious field test of this arrangement managed successfully during the measurement campaign on WR 140 at MONS telescope on Oberservatorium Teide in Tenerife (December 2008-March 2009). Here the focal reducer Celestron 0.63 is used. It is important to a fixed screw connection between the spectrograph and the reducer.

Left side picture
My friend Berthold builds the Lhires at the Cassegrain focus of the 50cm-MONS telescope.

 

Our spectroscopic equipment at MONS telescope:
Mirror box for finding and centering of the observation target.
Between the mirror box and the Lhires see the reducer (with red printing).

Solution in my own spectrograph, prepared by my friend Dr. Berthold Stober.
In a brass bush the the collimator (40mm diameter, 200 mm focal length) is caught.
The brass tube has an external thread which is screwed into the support of white plastic and can be moved for the purpose of focusing.
The larger collimator compared with the rightmost part of the original Lhires III (lens diameter 30 mm).
A neon spectrum, recorded with a 40 um slit. The drawn numbers are the FWHM of neon (in units of 9um pixels). The FWHM corresponds roughly to the slit width.
The same neon lines on a larger scale. The profiles are satisfactory symmetrical.
From the end of May 2009 I have my private Lhires III equipped in this way with a larger collimator. This allowed me to use a reducer (Celestron 6.3) in front of the slit and get such a small disk of star on the slit that I could set this to 22 um slit width. This increases the resolution of the spectra significantly. With a blazed 1800 g / mm reflection grating (dispersion 0.20 Angstr. / Pix), I get the neon's FWHM = 3.5 pixels. That is 0.7 angstrom resolution, or R = 9300 at 6563 angstroms with good overall efficiency.  

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