Hidden Loft Instrument Package

Updated: 9/14/2006


I have completed, at least for the moment, the upgraded instrument package for my RCOS 14RC F/9 Aries optics system.  This note describes the components.  The imager uses a KAF6303E that is coupled to a 7-position filter wheel.  The guider camera is an SBIG ST-402ME.  A RCOS Instrument Rotator and Field Flattener/Corrector round out the package.  The components are shown in the image below and discussed in the following text.


Telescope View

Field Flattener/Corrector

A flat field corrector (FFC) by RCOS is installed in the Instrument Rotator, the red object in the above picture.  The FFC further improves the already remarkable flat field of an Ritchey-Chrétien design by correcting the curved focus field.  It is a zero power optic that simply corrects the curvature of focus field inherent in the R-C design.  In tests I have run, I have noticed less than 3% increase in FWHM to the corner of the KAF-6303 chip.  The stars are tighter and more detail is obtained in HII regions of galaxies.  A recent image of M81 cleanly split the bright double near the galaxy.  As an additional test, I found that M65 and M66 would both just barely fit in the FOV of my system, placing the galaxies almost in opposite corners of the sensor.  This image did not disappoint at all and once again FWHM measurements were easily within 3% or so tolerance.  I compared some FWHM measurements with and without the FFC.  Here were the results:

FWHM (arc-sec.) without FFC
FWHM (arc-sec.) with FFC
Center 2.69 2.39
Half-diagonal 2.78 2.42
Corner 2.90 2.40

The data was taken on two different nights with different seeing.  What is interesting is even on the night of poorer seeing, the field curvature was apparent and on the night of better seeing the correction was more dramatic.  Here is a focal plane curvature comparison.

Note there is a 2" spacer between the backplate and the instrument rotator base.  This is to locate the corrector at the proper position in the optical chain relative to the chip for the Aries optics.  While it does reduce the available back focus, I have not found this to be a problem.  The overall installation went together easily and the finished appearance it totally acceptable.

Off Axis Guider

Working with Andy Homeyer, we developed an off-axis guider (OAG) design that minimizes the optical path length and provides a very robust mechanical structure. 




The OAG attaches to the telescope via 2.7" AP threads for a solid mounting.  The pick-off prism can be seen in the OAG.  The shaft with the red handle releases a clamping mechanism that allows the prism to be rotated and translated.  The guiding camera is secured to a thick wall 1.25" holder via two polished end clamping screws.  The OAG itself is mounted to the filter wheel cover plate with 6 flat head screws to minimize overall optical path length while giving excellent mounting security.


Front View                                                                            Side View

In the front view, you can see the pick off mirror with the shutter behind it.  There is no filter in place in this view.  The pick off mirror of course is ahead of the filter wheel so there are no issues with guide star brightness loss with different filters.  The edge of the pick-off mirror is adjusted parallel to the long edge of the imaging sensor.  This is accomplished by loosening the two setscrews that mount the imager to the filter wheel and slightly rotating the camera.  The imager shutter can be manually opened by the manual shutter release, located just below the red pick-off mirror adjuster in the left picture.  The side view shows the overall assembly.  An SBIG ST-402ME is used as the guider camera and the KAF6303E camera is the main imager.

I set up the mirror position using a light box.  While taking flat field images with the imager, I dropped the pick-off mirror into the imager light cone until I just saw the shadow in the flat field then backed it out a bit.  I then took flat field images with the guider and rotated the pick-off mirror for most uniform flat field.  The imager was then focused precisely.  The two guider mounting thumbscrews were loosened to focus the guider camera.  Set up was fairly straightforward and took less than 20 minutes.

In actual use, the ST-402ME is a dream to use for guiding.  With this optical system, a magnitude 12 star provides 1500 ADU with a 4 second 2x2 binned guide exposure, which is more than enough for successful guiding.  The camera is easy to use and works exactly as advertised.  The sensitivity of the camera, the size of the guider sensor and the fact that it is ahead of the filter, makes the acquisition of a guide star very easy.  In initial tests, there was always one in the field of view with a guide exposure of 5 sec.  While not a 100% guarantee, the probability is much higher than other guiders I have used, especially when located behind the filters.

In this arrangement, there was approximately 0.6" of inward focus travel remaining.  The ST-402 clears the Instrument Rotator motor by the expected 0.3".

Initial tests look very encouraging.  Guiding performance independent of filter used is of course a wonderful experience.  With CCDAutoPilot2, I did an unattended meridian flip and rotation and the guider picked up and continued guiding without incident on the other side of the meridian.  Image registration before and after the flip was excellent.

Andy Homeyer's worksmanship and attention to detail was outstanding.  He graciously tolerated many phone calls and questions as the design developed and willingly adjusted the design to accommodate my last minute changes.  He is a very skilled machinist and a knowledgeable astronomer.  That is a combination that can't be beat.  I can enthusiastically recommend his work.


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