|With the classic Schmidt-Cassegrain telescopes, the focus is done by moving the primary mirror. In the case of the RCX400, the primary mirror is fixed and the collimation and the focus are made by moving the corrector plate with three servo motors, which facilitates the collimation.
Some other innovations of the telescope:
- correcton of coma "almost" as good as a true Ritchey-Chrétien
- corrector plate heated to prevent fogging (it works, I tested)
- carbon fiber tube to remove the thermal expansion which affects the focus
- fan to faster balance the internal temperature of the tube
- internal USB hub to reduce cable clutter
|Original declination motor with plastic gears
It is difficult to understand why Meade was "cheap" in some details that have had a significant influence on the performance of this telescope. The drive system was improved compared to those of previous LX200, but why they used ordinary quality plastic gears?
Other examples of weird cutting corners savings are shown below.
|Declination drive system disassembled
The drive systems in right ascension and declination were disassembled and modified using a kit of parts and Buck's Gears sold $130 by Peterson Engeneering.
|Declination drive system reassembled
Plastic gears have been replaced with high quality metal gears. The original spring that was too weak was replaced by a stronger one.
In right ascension, the system was modified in the same way and the tracking is now accurate as shown in photos in the Gallery.
Note also that the original electronic control circuit of the motor was removed. It was replaced by a homemade circuit located elsewhere.
|The new electronic circuits
All original circuits from Meade were removed and replaced by circuits from Jeffrey Kerr LLC. I used three types of circuits:
6 PIC-SERVO to control the servo motors of right ascension, declination, the three motors that make the collimation and focus by moving the corrector plate, and the motor of the Moonlite focuser
2 PIC-IO to read the autoguider commands, to read the temperature at the corrector and at the fork, to read the commands of the handbox, to control the heating of the corrector plate and the variable speed fan
1 SSA-485 to make the USB link with the computer and the NMC network (Network Motor Control) which connects the different circuits.
|Declination motor control circuit
Here is where the new declination PIC-SERVO motor control circuit is located: in the battery compartment of the West arm of the fork. The East arm compartment was filled with a metal weight to better balance the telescope.
Meade claimed that the telescope could be powered with eight C batteries Not only, the batteries were being emptied too quickly, but they did not provide enough current to the electronics and motors, which caused many problems. The telescope must be powered with an external source of 12 volts. I used one that can deliver 10 amps.
|Back of the optical tube
The three PIC-SERVO motor control circuits for the collimation and focus were placed in boxes added to the rear of the tube since the space was too limited inside.
The fan is still in place and is now variable speed.
There are also two connectors. The gray connector is a ST-4 type connector to connect with an autoguider camera like the Orion StarShoot Autoguider. The blue connector is an extension of the NMC network that could eventually control the motor of a MoonLite focuser.
|Inside the back cover of the optical tube
It just shows the three PIC-SERVO circuits, the fan and the two rear connectors described above.
To save a bit of money, I designed myself the control boards of the servo motors. I had the printed circuit boards made by Pad2Pad and I have designed them using their software. For the parts, I used a PIC-SERVO microcontroller ic from Jeffrey Kerr and ordered other parts from DigiKey. It was worthwhile since I made six identical copies of these boards.
|Original encoders of the collimation motors
At the rear of the RCX400 optical tube, three DC servo motors are used to move the corrector plate to make the collimation and the focus. Graphite rods connect the motors to the corrector plate which is in front of the tube. The concept is correct and facilitates collimation.
Unfortunately, Meade retained the "cheap" rotation encoding system we found on the older LX200 code wheels with flimsy plastic that caused many problems among users of RCX400. This is the first problem I encountered trying the telescope for the first time. One of the three motors did not stop and the corrector plate became quite inclined.
|Replacement of the rotation encoders
The optical encoders of the three collimation and focus motors were replaced by precision encoders from US Digital. Many amateur astronomers use encoders from that company to add a GOTO function to their Dobson telescope .
I am not sure if we could make this encoders change while retaining the original circuits from Meade. In my case, there was no problem since the motors are controlled by my new homemade PIC-SERVO boards described above.
|Inside the original East arm
The interior of the eastern arm of the fork contains the quilt of cables passing through the declination axis to reach the circuits at the rear of the optical tube. You can also see the level detection system of the telescope, the GPS circuit and other circuits.
|Inside the modified East arm
The cables were preserved but assigned to other functions. For example, the USB cable seen in white, was assigned to signals from the ST-4 connector on the back of the tube for autoguiding. The GPS is no longer used.
Meade circuits are replaced by a PIC-IO circuit, which is used to detect autoguiding signals, to read the temperature at the corrector plate and at the fork, to control the heating of the corrector plate and the variable speed fan.
|New USB Hub
The original RCX400 contains an internal USB hub with three USB connectors on the rear of the optical tube to prevent cable clutter. Many RCX400 users have reported problems with this hub.
I bought at Staples for about $55 CAD, a powered USB hub that can control up to 7 devices. Currently, it is used for three devices: the telescope itself, the main ST-8300 CCD camera and the guide camera. It is preferable that the hub be powered since some devices such as the Orion StarShoot Autoguider camera are powered via the USB port. The advantage of a USB hub is that there is a single USB cable to connect to the computer.
|The new handbox
It is obvious that the original Meade AutoStar II handbox can no longer be used with my new circuits. Anyway, I never liked navigate the sub-sub-menus of a handbox with gloves to align or control the telescope, especially in winter. Furthermore, the complicated handbox is no longer necessary since the modified telescope must be connected to a computer to function.
The new handbox is simplicity itself to use. A black rotary knob is used to select from six speeds for moving the telescope or to do the focus and collimation. The other black button is used to select the functions of the colored buttons. The red buttons are used to move the telescope or to do the collimation. Two green buttons are used for the focus. The functions of other buttons are still to be defined. A PIC-IO microcontroller ic.is inside the handbox and connects to the NMC network of the telescope.
|The Moonlite focuser
With the original RCX400, we can do the collimation and focus using the AutoStar II handbox. The same is possible in my modified RCX400 by moving the the corrector plate.
I still decided to add a motorized MoonLite Crayford type focuser to the telescope. This is in order to set the focus more accurately without risking of undoing the collimation.
The selected focuser model is the one which comes with a DC servo motor and a hand controller
|Modifications and tests of the Moonlite focuser
Some modifications were needed to make the focuser compatible with the rest of the NMC network so we can control it remotely with a computer or with the handbox shown above. For this, the manual button on the left has been replaced by a US Digital rotary encoder with 2880 steps/revolution of and a linear potentiometer was added to the right to detect the limits of race.
Here we see the focuser being tested with a PIC-SERVO board and the SSA-485 board which makes the USB connection with the computer. The results are promising. It takes a move of 4 encoder steps so that the diameter of an out of focus star reaches half the size of a 5.4 µm pixel at F/8, which is easy to reach, even with the weight of the CCD camera and filter wheel.
|Coming soon ...
The adage says that a telescope is never completed. Come again. I will add the following changes as they are done. Here is what is planned for spring:
- Installation and testing of the MoonLite focuser
- Testing the Astro-Physics CCDT67 focal reducer
- Alignment of the primary mirror that is not perfectly aligned with the optical axis