The Perseus Instrument Selector, Field Test


It wasn’t all that long ago that the back of a small telescope would have nothing more complex than a drawtube.  Today, we have an array of instruments to augment the eyepiece and many fascinating and scientifically worthwhile projects to pursue.

By April of 2008, my C-14 had an ST9 photometry camera and a DSS-7 spectrograph mounted on the back using a motorized adaption of a Meade flip mirror. Then there was a C-8 tube with an SGS spectrograph mounted on top and still nowhere to hang my photometer. 

This configuration was automated and worked reasonably well, but was awkward at best.  Moreover, the two foot long tail of the C-14 was subject to flexure because the flip mirror and other components were never designed to carry such a load.

About a year ago, at the SAS meeting in Big Bear, I approached Jeff Dickerman of Optec,Inc. with my desire for a robust, programmable 4 way manifold that could support all my instruments with full automation.  This started a chain of emails and drawings that went on for many months.

What has resulted from my problem and Optec’s engineering is a wonderful new device which I am now field testing, the Optec Perseus Instrument Selector.  (I should point out that I have no affiliation with Optec other than being a long time customer.)

Perseus is a 4.25 inch diameter, 7 inch long cylinder containing a huge rotating mirror that directs the telescope beam into one of 4 instrument mounting points.  One of these is designed to hold the newest generation of very large format CCD cameras using a 3.6 inch mounting rings.  The other ports provide two inch mounting points. 

The first impression I had when the prototype arrived was that this thing was amazingly rigid.  Plugged into the TCF-S3 focuser on the back of the C-14, there is no flexure whatsoever.  Compared to my previous arrangement, this is like night and day. Each of the ports is as solid as the back of the C-14 itself.

The circular backside of the selector consists of one serial connector, one pushbutton, a power connector, and 4 led’s that are aligned with the ports.  It takes about 1 second to move the mirror.  The worst case switch time between any two ports is 3 seconds.  The mirror actually moves in about a half second, and then takes about the same time to settle.  Given that its about a one pound mirror, this seems really good.  More important, there is little vibration.

The design is such that there are no angular adjustments.  I’ve been measuring the mirror positioning with cameras on the ports and so far have not been able to measure any deviation from one cycle to the next.  Stars fall on the same pixel rotation after rotation. 

The selector is a serial device, but I put it into an Aaxeon 502 serial to IP converter.  I run all my automation software on a Mac using Parallels.  Optec supplies a little applet to set the current selector port and this runs in the XP virtual machine and talks to the virtualized com port.  I find that the IP converted com ports are much more reliable than the physical ones.

I use independent focal reducers to bring the light to about F6.6 for the SGS, F10.8 for the DSS and F7 for the ST9.  There is a helical positioner on each port that can be used for final parfocalizing. 

More to come, when the clouds clear!

Here is the April 2009 solution.  The silver Perseus instrument selector prototype supports the SBIG SGS high resolution spectrograph on the left port, a SBIG DSS-7 spectrograph on the right, and an ST9 camera mounted below. The top port is not yet in use.

My 2008 solution to multiple instrument automation used a motorized flip mirror on the 14 and a separate C-8 OTA for the SGS.  Note that the C-8 OTA, brackets and counterweights added 65 lbs to support a 5 lb instrument, and collected 1/3 of the light.

The DSS has a pellicle beamsplitter to feed the autoguide camera.  The SGS side has a flipmirror eyepiece port, the ST9 has a Maxfilter and Optec NGW reducer.