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Optec TCF-S Focuser

TCF-S Specification
Key points of the TCF-S specification are as follows

Problems with previous setup
I've been using a JMI NGF-S focuser (with motor) for many years, and have been very pleased with it.  It solved a major image shift problem I had with focusing the LX200 and allows good control on focus position using the hand controller, however it has certain limitations which are clearly holding back my imaging.  These are  

• collimation-limiting and flats-limiting flexure between the scope and CCD camera

• backlash/non repeatability in focus positions (unable to use @Focus, @Focus2 features of CCDSoft)

• no digital readout of focuser position

• no remote focusing (all focusing has to be done at the telescope)

• focus drift (due to temperature changes)

TCF-S Selection
I had been thinking along the lines of fixing these issues by the eventual purchase of a Meade RCX400 telescope (LX400-ACF), however I've now swung away from this scope and may eventually go for an LX200R (LX200-ACF) or indeed something different completely. 

Either way a new scope is not on the immediate shopping list. However I still would like to resolve the current issues, but also in a way that can be taken forward to a new scope in the future.  After a little research in January 2008 I decided to purchase an Optec TCF-S focuser, to solve/improve these issues.   [ Optec Website ].

Initial Impressions / benefits
The new focuser equipment arrived in mid-February and initial set-up and tests have now been conducted. Whilst there were and remain some resultant impacts of the new focuser and one or two continuing small teething issues the overall impression so far is that the TCF-S is a worthwhile investment. I now have

• firmer connection between CCD Camera and Scope.

• repeatability in focuser position, with digital readout of focus position

• remote focusing capability through network / observatory laptop 

• automated focus adjustment for changing ambient temperature  (correcting focus drift)

• realtime and recorded measurement of ambient temperature and temperature changes

The TCF-S is a far more heavy duty focuser, than my previous NGF-S focuser, which removes the previous collimation-limiting flexure between the scope and CCD camera. This should also mean that I will also get better/more reliable flat frames.  With the step motor in the TCF-S I can now move to / return to a specific focus position with confidence.  I don't have to wait at/revisit the observatory to make focusing adjustments.  I can also maintain better focus as the temperature increases/decreases during unattended or automated operation of the observatory.

Software upgrades
Whilst waiting for the new focuser to arrive, I added a number of features to my imaging/telescope control software to leverage the value from the TCF-S focuser and utilize it's remote operation capability. After fixing one or two bugs the software works well with the focuser using CCDSoft's camera object as an intermediary.

TCF-S Impacts / Issues
Impacts associated with the TCF-S that became quickly apparent after attaching the new focuser were :

• telescope balance problem

• reduced declination limit (reduced sky)

• reduced field of view

The TCF-S is a much heavier piece of equipment than my previous JMI NGF-S focuser. It is also longer in length which means that the heavy CCD camera lies further away from the fork mounting of the scope.  Together this caused the scope to become significantly out of balance.  Whilst the scope was still operatable and would still locate to star targets ok, I was concerned that the extra weight at the back of the scope would put to much strain on the declination bearings/motor. I already have a telescope balance weight pushed as far forward as possible, and therefore to rebalance the scope and strapped on some extra weighting to the beneath the front of the telescope tube. The extra weights were just miscellaneous heavy items from the garage (heavy nuts, the head of a hammer), but they've done the job.

The TCF-S is longer in length than my previous JMI NGF-S focuser, which causes the bottom of the CCD camera to hit the LX200 base / power and declination control wires at a much lower declination.  (59 degs compared to xx degs previously. This has required me to adjust the declination limit in the TheSky and more significantly has lost me several degrees of observational sky area.  Apart from looking at using the camera with a right angle mirror, there isn't much I can do with this issue in the short term.  Longer term I would hope to acquire a larger or different design of scope but clearly checking the amount of room it provides at the back will be important).

Another impact of the longer length of TCF-S focuser is that it has made a slight increase in the focal length of my imaging setup, with a consequential slight decrease in the field of view of CCD images : 15.7 x 10.5 arc mins with new TCF-S focuser, compared to 16.7 x 11.1 arc mins with previous NGF-S, equivalent to an overall 13% reduction in FOV area. Since the FOV of ST7 chip is already fairly small, this reduction is relatively significant and might reduce the overall plate solution success rates and reduce success analysis of certain Variable Star/Comparison Star pairs.

Teething problems
Teething problems with TCF-S focuser 

• shut-down issue.

After shutting down CCDSoft and hibernating the Observatory Laptop, I find that I'm unable to turn-off the focuser at the hand set.  I have to restart the laptop and fiddle around with focuser PC connection to get to a position where I can switch off the focuser. (Simply turning off the Power to Focuser would of course mean that the current focus position/temperature are not stored on the EPROM ready for the next session)

[ More Notes : TCF-S Focuser Notes 2 - 2008-03-12 ]

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Focal Ratio / Field of View / Image Scale

Field of View Difference
My normal imaging setup uses a f/6.3 focal reducer with my 8" f/10 scope.  The observational focal length, focal ratio and field of view for my previous and new focuser setups are shown in the table below. 

The following pair of image also illustrate the slight reduction in ' Field Of View' with the new TCF-S focuser compared with previous NGF-S focuser. 

Image taken with previous focuser  FOV : 16.7 x 11.1 arc mins (Rectangle shows FOV of the new focuser)		Image taken with new focuser  FOV : 15.7 x 10.5 arc mins
Annotated CCD Image 10s Exposure, 2x2 binning, C Filter  2008-02-16  (#264548)		Annotated CCD Image 15s Exposure, 2x2 binning, C Filter  2008-02-19  (#266021)

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Critical Focus Zone

To obtain a sharp image with best possible focus, the CCD detector should sit within a zone known as the 'critical focus zone' (CFZ).  The size of the critical focus zone is based on the focal ratio of the telescope.  The faster the focal ratio the shorter the zone of critical focus is.

The theoretical size of the critical focus zone is computed using the equation

    CFZ (mm)  = (focal ratio)^2 * 2.2 / 1000

 For my imaging setup (with f/10 scope, f/6.3 focal reducer and new TCF-S focuser), the effective focal ratio is f/7.4.  Therefore

    CFZ = 7.4^2 * 2.2 /1000  = 0.12 mm 


For real world situations the CFZ value is approximately 10-30% greater than the theoretical value. Thus for my setup 

    CFZ =  0.13 to 0.16 mm

Based on the 0.000085" step size for the Optec TCF-S focuser (0.00216mm)

    CFZ = 55 steps  (theoretical)  or 61-72 steps (real-world) 

For a typical Meade SCT the focus position will move 0.19 mm for every 1 degC change in temperature (equivalent to 86 steps for TCF-S focuser).   This would suggest that for my f/7.4 telescope setup, a 0.5 deg C change in temperature would be enough to take the telescope out of critical focus (assuming scope was initially positioned at centre of its CFZ).

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Focus Profile

Focus profile was measured on a couple of occasions during Session 267 (2008-02-22).  These are shown in the graph below.

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Temperature Changes

One of the indirect benefits of the TCF-S focuser, is that it can return the temperature measured by temperture probe sensor which is stuck to the middle of the telescope tube (covered by foam insulator), allowed both realtime and recorded monitoring of the temperature changes experienced by the telescope. These telescope temperature changes should equate roughly with ambient temperature changes. 

Graph below shows temperature profile for session S266 (2008-02-19).  Two important observations from this :

•     Once the session began and the observatory roof opened, temperature fell by around 3.5 degC over a one hour period.
      (since previous practice 

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Collimation Check  (showing mis-collimation)

Previous imaging sessions had demonstrated that there were some collimation issues with my 8" LX200. Whilst the collimation was generally acceptable for regular star field imaging, given the small field of view and typical seeing conditions, lunar and planetary imaging has very probably been detrimentally impacted by imperfect collimation (e.g Images of Mars recorded during 2007 opposition ).

Previous efforts to improve collimation have been hampered by the flexure between the telescope and CCD camera, due to the heavy weight of the ST7 camera/filter wheel and inadequate bearings in the NGF-S focuser, holding the camera. Previously this has meant that  the relative position of the optical centre of the scope and the CCD chip varies depending on where the telescope is pointing in the sky. This means that collimating on a star in one particular are area of the sky, would not necessarily benefit the collimation/imaging for another part of the sky. 

In January 2008 I decided to purchase an Optec TCF-S focuser, to solve/improve a number of issues with my imaging setup including the collimation-limiting flexure between the scope and CCD camera, which should be improved as the TCF-S is a much stronger/heavy duty focuser than my previous lighter NGF-S focuser.   The new focuser arrived in mid February and upon setup it was clear that there was a very significant reduction in the amount of flexure between scope and CCD camera.  It should thus be possible to make some real improvements in improving the collimation of the scope. [ Optec Website ].

To aid collimation process, I've added software controls to allow me to quickly Focus-In / Focus / Focus Out to positions to examine the diffraction / collimation rings, controls to reenter the star after intermediate collimation adjustments, and purchased a set of Bob's Knobs  The later are not yet fitted but will eventually make the task of collimation adjustment a lot easier. [ Bob's Knobs Website ].

As an initial baseline, defocussed images of a bright star where taken to examine the star pattern and the implied mis-collimation.  Collimation adjustments will be made during a future observing session.

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