Antilhue - Chile
The advent of large chip CCD cameras for amateur use has put more requirements on the optical quality of the imaging systems. In the case of a type Ritchey-Chrétien Cassegrain telescope, which is inherently coma free and has "the largest aberration-free field of view of any reflecting telescope made", the ever-present field curvature and off-axis astigmatism will be noticeable in the corners of a 36mm x 24mm chip, such as the one used in the SBIG STL-11000 camera.
To counter these effects, RCOS offers a dedicated Field Flattener/Corrector (FFC), consisting of a 2 lens optical assembly in a black anodized aluminum screw-in cell. See: http://www.rcopticalsystems.com/fieldflattener.html . This FFC was designed specifically for the Star Instruments optics, used on most RCOS telescopes.
For a long time, I was unable / unwilling to install the FFC on my 14.5" RCOS because there was insufficient data on its Aries optics to ensure that adequate correction could be achieved for the f/9 configuration shown below. Note how the Precision Instrument Rotator (PIR) - that's the bright red item - is attached to the scope's back plate by means of a black anodized base plate and 6 screws.
Then, late 2005, Brad Ehrhorn and John Smith found out that the original RCOS FFC would work on John's f/9 Aries equipped RCOS, provided it is installed 2" further down the optical path. I decided to go for it and ordered the same solution from Brad: it consists of a 2" spacer and a 1/4" new adapter plate. The FFC screws onto this adapter plate and the PIR then goes over the FFC and attaches through the new adapter plate and spacer to the RC's back plate with 6 new much longer screws. The 2.7" extension rings between PIR and camera are adjusted to give the same TCC focus position reading as before the modification. This ensures optimum mirror spacing.
Results of the modification
Rather than rely only on a purely visual before-after assessment of star shapes and sizes, I used the very useful software application CCDInspector (from www.ccdware.com ). This software not only helps in precisely collimating your telescope, grading your frames according to FWHM, etc, it also has a very useful field curvature tool. Below is the result screen of a calibrated single unfiltered 15 minute exposure taken under good seeing (7/10) on Nov. 1st 2005. The scope is in perfect collimation and there is no tilt of the image plane (CCD chip surface). Note the variation of FWHM from 1.88 arc seconds in the center to 3.12 arc seconds in the corners. Curvature reads 25%.
And here is an average of 4 calibrated unfiltered 20 minute exposures taken under similar 7/10 seeing conditions on March 25th 2006. There is only 0.13 arc seconds variation in FWHM over the whole field (1.68" to 1.81") and the curvature figure has come down to an excellent 6.7% !
When looking at CCDInspector's "3-D Viewer map" the improvement becomes even clearer:
Manual FWHM measurements in MaxImDL/CCD confirm the above system evaluation. Visually, the quality of the star images in the corners is undistinguishable from that of stars in the center. Also, I could not detect any chromatic or other effect after the FFC was put in the imaging path. The RCOS FFC is there to stay ! At least until the next generation of even bigger CCD chips comes around.