Three Fathoms Observatory
Stewart Yeung

 Current Special Feature

 

 

Optical Disaster

 

I have encountered a disaster-class problem with my Meade LX200R as follows :

 

WHAT HAPPENED :

I have used the primary mirror lock to lock the primary mirror (and use a microfocuser for focusing)for over two years without any problem. The last time I unlocked and relocked it was when I adjusted the focus with the focus knob after adding a rotator to the image train, some six months ago. See an example of the normal CCD image for reference :

 

Recently on one night (2 December 2010), without my doing anything to the scope, the optical images suddenly became seriously out of collimation. The star images were much elongated along the Dec asxis. The pointing by Paramount also missed the targets due to the badly shifted focus point. I checked the other components in the image train including the Paramount and the CCD and confirmed that it was an optical problem.  See an example of the out of collimation CCD image :

 

I examined the scope on the front side by looking into the primary mirror from the corrector but could not see any mechanical fault, but I did notice from the reflection of the secondary mirror by the primary mirror that the optical alignment might be badly skewed in the Dec axis. First I tried to adjust the optical axis by adjusting the three screws of the secondary mirror. Reasonable collimation could not be recovered because the optical axis was too much out of alignment.

Then I suspected a problem with the primary mirror alignment. I tried first to tighten the primary mirror lock again: it could be turned clockwise easily, so I turned it for 180deg or so clockwise until it was tightened. Then I tried to loosen it but this time the lock knob became detached. I could not reattach it to the metal shaft again because the metal shaft only protruded out 1mm or so from the sheath :see photos of the primary mirror lock without the lock knob:

Afterward I tried to turn the focus knob: it could be adjusted very easily without forcing it. As the primary mirror had been moved by the mirror lock, I focused with the focus knob again. However the CCD images were still far out of collimation. See the following sample image and a deliberately defocused image for reference:

 

MY GUESS :

 Is the primary mirror lock damaged causing the primary mirror to shift out of alignment ?

 SOLUTION SOUGHT !

 

UPDATE on the problem on 17 December 2010:

1.Thanks to advice from fellow astrophotographers especially from PetersonEngineering.com (an article on "Resetting the Meade Mirror Lock"), the mirror lock mechanism has been reset and it's working all right. This mirror lock problem is only a secondary issue (see point 3 below).

2. The coarse focuser mechanism has also been examined. In the process I encountered a problem of re-assembling it. By patiently trying to move the mirror cell by turning it CW/CCW and pushing/pulling it along the baffle, I've accidentally found the solution to this problem.There is a protrusion at the perimeter of the mirror cell and there is a mechanical stop at the back plate. These two work together preventing the mirror cell and its attached focuser arm from rotating beyond a stop point. When the mirror cell is pushed forward along the baffle towards the front and AWAY FROM THE BACK PLATE, the protrusion becomes clear of the mechanical stop and the focuser arm can be rotated beyond the stop point.I've put back the coarse focuser assembly successfully.

3.I’ve learnt that the primary mirror cell actually fits tightly onto the baffle tube though it floats freely amid grease, thus it is not likely that the primary mirror has been tilted significantly.After examining various possible causes of the elongated star images that couldn’t be collimated, I find a malfunctioning hyperbolic secondary mirror adjustment mechanism the likely culprit.

4. I've been advised by Dr. P.Clay Sherrod not to touch the secondary mirror housing. Since I've already examined the various components in the image train including the CCD camera, filter wheel, AO-8, rotator, micofocuser, AND of course the image train perpendicularity at various connection points, the problem seems to lie in the collimation.

The problem now is in a deadlock and the observatory has been out of commission for quite a while !

 

Further Development :

On the advice of Dr. P.Clay Sherrod, the following images were taken for diagnostic purpose :

Collimated Images :

Intra-focus (note the distortion - a dark bar extending from the central shadow to the edge - at the right side of the fresnel image)

 

Infoucs

Extra-focus

 

Front View of Main Mirror

Back off Views :

 

 

 

Close Up Views:

 

 

A telling star image : the star split up into three points while being focused with FocusMax

 

Enlarged image :

 

Views from the REAR port of the back cell :

 

These diagnostic views reveal no problem at all.

After seemingly exhausted all other possibilities, I was about to embark on a major (and risky) dissembling of the scope to examine the rear primary mirror cell to decipher the collimation problem.

 

Final Development on 2 January 2011

As advised by Dr. P.Clay Sherrod, I reluctantly disassemble the image train. In the process I noted a skewed round glass plate in the AO-8. This was what I saw earlier in a quick examination a few days after the disaster, but I thought it was in its normal state. This time I questioned whether this was normal and asked for advice from fellow astrophotographers.

I was advised that it was abnormal :

 

 

View from the rear
side of the scope, with the AO-8 attached but opened up, showing the round
glass plate in the way of the light path

 

The malfunctioned AO-8 : rest position of glass plate in the Optical Path

 

The malfunctioned AO-8 : a side view showing the abnormal tilt at rest and the convoluted spring

 

The convoluted spring in the malfunctioned AO-8

 

Partially Fixed AO-8 : The Y-axis censor "at rest" position

with a resultant slight tilt upward in the Y-axis

 

The Y-axis censor position when the glass plate is pressed by hand to a Horizontal Position without any tilt

 

The condition of the Cocked Spring when the Partially Fixed (?) AO-8 is in the "at rest" position

 

Conclusive Remarks on 6 January 2011:

1. Observation was made with eyepieces only and the views were found to be normal.

2. After fixing the AO-8, the image train was put back and collimation re-attempted. The Fresnels were found to be ROUND and star images were BACK TO NORMAL STATES. At the same time the pointing was almost back to the previous accuracy, except for a shift due to resetting up the image train and relocking the primary mirror.

3. Casuse of the optical disaster confirmed : The AO-8 malfunctioned with its round glass pate at an abnormally oblique angle in the "at rest" state. the oval stars were caused by the AO-8 distortion in the way of the light path. At the same time, the REFRACTION of the glass plate shifted the focal point resulting in the widely missed pointing.

4. The convoluted spring in the malfunctioned AO-8 might have resulted (prehaps partly) from the vibration of the Paramount ME. The gears of my Paramount ME was in need of re-greasing and when the weather got colder the mount vibrated and generated a loud sound when slewing. This vibration was transmitting to the whole setup and might have caused RESONANCE in the spring which SUDDENLY became convoluted in a slewing movement of the Paramount during that cold fateful night on 2 December 2010 !!!

I have just re-greased my Paramount ME and the slewing noise and vibration have disappeared.

 

 

Lessons Leant : 

  1. Confronting a complicated system, a systematic stratagem for problem solving has to be worked out first. As Stan Moore advised early on 7 December 2010 : "Start with a simple eyepiece...the key to effective debugging is to remove or pin as many variables as possible then bring them back in one at a time." Also Danny Sperry advised later that " Step 1 when investigating potential optical issues should always be to inspect everything visually. If that doesn't turn up anything, step 2 is to reduce the variables. Remove diagonals, AO devices, filter wheels, etc." Unfortunately instead of follwing such stratagem I had my preconception about the problem and defined it as an OPTICAL disaster and I erred on probing the faults of my telescope for almost a whole month and never seriously examined the AO-8 until the very end!
  2. Knowledge of the system is crucial for problem solving. At an early stage, I actually inspected the AO-8 visually and it was already in the malfunctioned state, but I had no knowledge of its normal tilt position so I just neglected it! Here Dr. P.Clay Sherrod provided much insight and advised me that : "Distortion such as this can very well be in the camera mounting system rather than in the telescope" and " I strongly suspect the alignment of the AO8, either optically or mechanically".(But the strange thing about the A0-8 is that there was no software feedback: the exercising of the AO-8 and the guiding with it worked seamlessly and there was NO MANIFEST OF THE AO-8 PROBLEM IN THE CCDSOFT PROGRAM. )
  3. Seeking expert advice is important for problem solving. On the advice of Dr. P.Clay Sherrod in the LX200R forum, hitherto I have not tampered with the primary mirror cell or the secondary mirror assembly of the SCT. Abstinence from these risky operations might have saved the system from collateral permanent damages.
  4. Maintenance of the observatory is vital. In the final analysis, my negligence in re-greasing the gears of Paramount ME could well be the culprit of this optical disaster!

 

Acknowledgement :

Without the expert advice of fellow astrophotographers my observatory could never be rescured from this "optical" disaster. My sincere thanks to :

Dr. P.Clay Sherrod

Stan Moore

Joe Lalumia

Mark Manner

Maarten.van Leenhove

Peter Vasey

Eric w Benson

Danny Sperry

Roger Hamlett

Michael Vineyard

Pete Peterson

Rick Woods

Mike Juno

Don Whiteman

Jerry Wise

Jennifer Christine

sxinias

et al.