RCOS 10RC Truss Impressions

Updated: 01/13/2003

Yesterday, December 3, 2002, Brad Ehrhorn kindly loaned me his development model of the 10RC truss for test and evaluation.  It is equipped with the Telescope Command Center (TCC) and Instrument Rotator (IR).  As I proceed through the evaluation, I'll be adding to this page.

First impressions are that this is one well executed product!  The details, fit and finish and attention to detail are very impressive.  It is easy to handle and quite a bit lighter than my previous 10" closed tube RC.  And it really looks neat!  Here are a couple of shots:

Here it is mounted on my ME along with my FSQ-106 and peeking out of my rooftop observatory.  Ready for action!

This is in the parked position, with the roof open.  The telescope is pointed due south and on the east side of the mount.  It makes the FSQ look pretty big - or the RC pretty small.

Since the scope was a newly assembled development model, I had to do a complete collimation.  This was about as easy as it gets!  I used the procedure outlined on the RCOS web site and it was a breeze after a false start on my part.  I can't tell how easy (and fun) it was to bring the scope into collimation.  And doing most of it in daylight, too.

In the evening a star test indicated a very close result - a slight tweak of the primary mirror and it was good to go.  One nice method of checking collimation is to look at a plot of the radial profile of an in-focus star.

Before star test                                            After star test

These screen captures were from Mira using the plot star profile function.  Note the scatter about the fit line in the left plot versus the right.  While there was some seeing differences no doubt, the fit is much better on the right.  I believe this is a good indicator of astigmatism and therefore collimation.  (Thanks to Adam Block for introducing me to this technique.)  The left image was taken at F/8.9 and the right was at F/7 using the AP reducer.  So, I figured collimation was close enough. 

Discounting my false start, the bench collimation took about an hour and the star test final tweak was around 10 minutes. 

Fixed Instrument Adapter (FIA)

The FIA is a wonderfully secure method of attaching the camera to the OTA.  The four sizes that came with the scope were nominally 2.55", 1.75", 1.25" and 0.75".  My ST-8E/CFW-8 was fitted with a Celestron T-thread adapter and I came very close to the specified back focus with the first three, totaling 5.55".  The FIA mounting plate is 0.30" thick.  Here is my breakdown

FIA Mounting plate:
Spacers:
T-Thread adapter and ADA-204:
ST-8E/CFW-8:

  0.30"
5.55"
2.35"
1.66"

This totaled 9.86" and the specified back focus was 9.24".  It turns out I could have been within .12" if I had used a different combination of spacers.  Well, next time...

With this arrangement, my plate scale was 0.823 arc-sec./pixel, as reported by TheSky image link.  This corresponds to F/8.9.

I then tried the AP ".75x" reducer.  After removing 2" of spacers, I was within .27" of the F/8.9 focus.  The resultant plate scale was 1.046 arc-sec./pixel, giving a focal reduction of 0.79 and a resultant focal ratio of F/7.

Before the clouds rolled in I did a test 10 minute exposure at F/8.9 using first the autoguider and then the STV/FSQ-106.  Preliminary results indicate minimal flexure, unlike that seen with other scopes.

 

Vignetting

One thing I did notice was some vignetting.  Brad had provided a number of different size light stops that thread into the extension tubes.  I put the 1.25" diameter one in.  Here is a 10 minute exposure stretched to show this vignetting. 

I made no attempt to flat field this out, although I suspect it would easily.  The average change from edge to center was around 10%.  I need to layout the light cone to see how close the stop is coming to the cone.  I have the stop located in the extension tube closest to the backplate, which is probably too far away from the image plane.

Well, that was the problem - I had positioned the light stop in the wrong place.  See the following sketch.

The above image was with the incorrect light stop.  It clearly intercepts the F/9 light cone.  I relocated the stop to the correct position.  Below is an RGB composite sky flat.  Each filter was exposed for 10 frames with an average level of 15,000 to 18,000 ADU.  The resultant images were first combined in Sigma and then color combined in Maxim.

Color Sky Flat

The resultant image was reasonably uniform.  I'll bet I could get away with using just a clear filter flat (but I won't <g>).  It also looks like I need to clean my filters.  Interestingly enough, my camera just came back from SBIG after being repaired and the upper dark dust donut looks like someone may have tried pretty hard to remove it.  All of this flats out ok, though. 

Using Maxim, I plotted the level variation both horizontally and vertically.

   

Horizontal Plot                                                               Vertical Plot

A couple of things are noteworthy.  First, the overall variation is around 2%, certainly very acceptable.  Secondly, the color variation was remarkably similar.  Not unexpected for all reflective optics but nice to see nevertheless.

Morale of the story:  Do a layout before inserting light stops!

 

Precision Instrument Rotator

I had been dying to try this out and finally did last night.  First off, it takes up a lot of the backplate of the 10" scope but does fit!  And, it works like a champ.  For my setup, I used 2" of FIA spacers since the IR adds 2.95" to the optical path.  I slewed to some targets via TheSky, picked a guide star to use, noted the required rotation in TheSky and commanded the rotator to go there.  The guide star was on the autoguider chip!  This was all done remotely from my office.  It was repeatable and accurate.

A couple of caveats.  Before going completely remote, determine your cable routing and rotation limits!

I just had to take an image with this sweet setup!  Here is M77 with a short LRGB of 5x5m:3x7m:3x7m:3x10m.  RGB was binned 2x2.  18 darks, 36 bias and 10 flats were used for calibration.

M77 - First Light with 10RC Truss

 

Collimation

Here, I mean alignment between where the 10RC is pointing and where the FSQ points.  With my other RC, I had to do some shimming and build some adjustments into the top dovetail plate to get the two optical axes even close.  With the 10RC, I just clamped the FSQ on.  With the 10RC centered on an object, the resultant image with the STV/FSQ was around 5 arc-minute off.  It was still comfortably in the STV FOV and very usable.  This was a pleasant surprise and is a tribute to the mechanical alignment of the RCOS product.

FocusMax

FocusMax (FM) worked very well with the TCC focuser.  I ran FM as a separate application and selected "RCOS TCC as the focuser", as opposed to running it through the Maxim interface.  I generated a number of V-curves automatically using the V-curve wizard.  The resultant automated focus was as good as I could do manually, indicating a good algorithm.  I did notice that the V-curve data was better than I had seen with my other RC, which uses a stepper for the secondary focusing.  By better, I mean a closer fit to the straight line and less slope intercept difference.  I believe this is due in no small measure to the very high precision servo controller for the secondary focusing.

In short, FM worked very well and focus was very repeatable.  FM does not seem to log temperature from the TCC application.  I will get this info to the FM guys and hopefully it can be addressed.  Also, with FM talking to the TCC focuser and changing the TCC application from the mini-window to the standard size one, I received an error message: "Port already open".  I clicked OK and continued without any problems.  This was repeatable.

12/8/2002: I did some critical evaluation of focus as recommended by FocusMax and what could be done manually.  FM 2.x recommended a setting of 13631.  A careful manual adjustment, while watching both the image of the star and the FWHM in Maxim, indicated a best focus at 13691.  This is around 1.5 CFZ (critical focus zone) away and is clearly not optimal focus.  I believe that any slight collimation error can make the spot astigmatic slightly out of focus and it rotates by 90 on either side of focus.  That, coupled with the difference in unfocused images depending on which side of focus one is on, can lead to FM giving an erroneous prediction of focus.

Larry Weber, FM author, told me about the v 3.x beta which, among other things uses a different curve fitting algorithm.  V2 used straight lines on either side of focus; v3 uses a hyperbolic fit to the V-curve.  I loaded v3, ran 7 v-curves and now FM recommended focus was 13672, about 1/2 CFZ difference.  I suspect this was as close as possible.  Resultant FWHM's were a lot better, diffraction spikes were sharper and more detail was clearly visible.

Here is a link to M77 taken with improved focusing and using the color data from the above image.  I also took a shot of V838 and saw FWHM's as low as 2.2 arc-sec.

While I suspect that collimation can be slightly improved and may try that, I believe the new version of FocusMax is a significant improvement.  For those using FM with an RC, I highly recommend the version 3 beta, which can be downloaded here.

By the way, did I mention the Precision Instrument Rotator is awesome? <g>

Telescope Command Center

I have been working with Dan to test some control parameters for the secondary heater.  As part of that effort, I took a baseline temperature curve last night from opening the roof of the observatory to a steady state condition.  The secondary heater was not on but the primary fans were under automatic control to keep the primary mirror within 2 F of the ambient.  Here is a plot of that data that comes from the logging feature of the TCC application:

    

                     Primary cooling                                  Primary, secondary and ambient

Both curves begin with the roof opening.  The slower thermal response of the primary delays cooldown as expected.  It seems to take around 75 minutes to get within a couple of degrees of ambient with the fans running at 100%.  After that, the fans come on as needed to maintain the difference.  The right curve shows the primary, secondary and ambient more clearly.  The cooling dip at 300 minutes was around 3 AM.  It actually agreed with recorded weather here locally!  Note that the fan came on to bring the mirror closer to ambient as needed.

The next test will be with the secondary heater being controlled at a temperature slightly above ambient.

Usage Notes continued here.

 

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