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WilliamOptics ZS73 and RedCat51 side by side.



I recently bought a WO RedCat51 (the WO FanBoi I am) deciding to mount it side by side with the WO ZS73 to maximize imaging in the fickle UK environment.

The 73 has the x0.8 reducer flattener to give an equivalent focal length of 344mm.




The RedCat on the other hand has a focal length of 250mm.



With some magic from PixInsight or APP I should be able to merge the images acquired by these two into a reasonable sized image. Maybe use the Redcat as the full size image with some detail in the centre acquired by the 73.


Anyway to achieve this I have bought a Primaluce Lab side by side Losmandy plate for dual platforms complete with two single clamps.




I added the Deep Sky Dad AF3 autofocuser to the RedCat. This was very easy to fit and was completed in 10 minutes.







The DSD hardware is a combination of aluminium and 3D printed hardware. I am impressed with the build quality and it can fit on the new RedCat (mine) and the older RedCats with the shorter dovetail.


Focus using the ASI183MC is at the 5mm mark on the draw tube. 


The WOZS73 has the Lakeside focuser fitted and this has been in use for a couple of years without issue. Below you can see my custom bracket which I spray painted in Ferrari red to match the rest of the hardware.





The mount had to have a minor modification to the DEC cable management so I could turn the DEC head through 90 degrees so the custom mounting plate carrying the dewheater PSU then sits at the back end of the scopes. This picture is upside down but you can see the mounting plate secured to the underside of the Primaluce replacement puck for the HEQ5Pro.




The puck head is also turned through 90 degrees so the side by side Losmandy bar can be mounted.

The telescopes then simply sit in each respective clamp on the side by side bar.


Guiding Hardware

The RedCat doesn't come with a carrying handle bar as a default and one has to be ordered separately. As this doubles up as a mount for a mini Vixen bar to hold a guide scope I bought one.

The ZS73 on the other hand nowadays comes complete with a a carrying handle bar cum mini-Vixen mount. However as I bought mine back in 2019 it didn't come with one. It did, however, come with a pair of WO red guidescope mounting rings that fix to the M6 tapped holes in the tube ring clamps.






I decided to purchase some Svbony gear as an experiment and ordered a mini-Vixen dovetail complete with guide telescope rings. I discarded the tube rings, tapped out the 5mm holes to M6 and attached the WO red tube rings to it.



A pair of Svbony finderscope mounts with small dovetails were also purchased and the 4mm holes in them were opened up to 6mm so it could attach them both to the ZS73 tube rings. Then the guide scope could be transferred between to two scopes dependant upon which one was being used as the main scope.



I have a stash of unused Svbony bits now but no doubt they will come into use in the future. I am impressed with these parts as they seem to be very well made. On a side note I have one of their illuminated Red LED crosshair reticules for the Dobsonian and that is well made too. So Sybony sell some decent kit.


If it is desired to use a single scope on the mount then that can still be easily done by rotating the DEC puck through 90 degrees again and mounting the scope in the conventional way. The cable management can cope with both scenarios as there is sufficient loop in the loom. 


This picture shows the scopes in side by side configuration prior to load testing. I was also missing a short power lead which has now been installed. The secondary computer and power source are also seen.



This next picture shows the underside of the puck with the dew controller/power distribution unit mounted in a custom plate secured to the puck clamps.



Computer or Computers

The thing about EKOS is you can multiple image with a single computer by firing up two instances of KStars/EKOS to do it. However this puts the Raspberry PI computer under a bit more load than necessary. As I have a number of PIs I decided to use the existing "PI in a box" via INDI to control the main scope, mount, guiding, image acquisition, focusing, plate solving etc. I have a remote computer (actually a Linux virtual machine) in the house that runs KStars/EKOS and that just connects to the INDI Server on the PI to do its stuff. Images are stored on the PI and transferred to the computer in the house simultaneously as they are captured. Belt and braces.


For reference the "PI in a box" is actually the 30A 13.8v power supply unit, an ethernet switch, the raspberry PI with associated mains power supplies contained in a plastic storage container with holes for cable ingress and air vent. This is normally underneath the scope tripod and it helps keeps any moisture off in the dead of night.



PSU with ethernet switch and Raspberry Pi 4 on top. The switch is connected to the workshop LAN and then to the house using an gigabit ethernet cable.




Storage box - "Really Useful Storage Box 42 Litre"


The secondary scope is controlled by a second PI mounted on the DEC plate and will just take pictures and adjust focus. That PI runs Astroberry with KStars EKOS started locally. It stores its images on the PI itself. There is a flat CAT6 ethernet network cable running from the ethernet switch in the "box". That is again controlled from a second instance of EKOS running on a virtual machine inside the house. This PI however is powered by a power bank. I decided a long time ago to have the imaging computer a distance away from the scope, I was running Windows in those days on a laptop and I purchased long cables (EQMOD USB3 etc ) so have continued to use those ever since.



Both scopes are using ZWO ASI 183s, either the mono with a filter wheel or the OSC version which is used solely for broadband. These cameras can be swapped easily between the scopes and the wiring I have done on the harness can cope with any changes. According to AstronomyTools CCD Suitability calculator this sensor size is ideal for all of my scopes including the bigger sister scope ZS103.


Mass and balancing

I took great care to measure the mass of each component to ensure it was still within the capacity of the mount and it is well in. The total mass including side by side rail (1.4kg) , computer with powerpack and both scopes complete with wiring is 8.4kg.

To balance the scopes I first found the balance point of each scope by placing their respective dovetails onto a handy cylindrical object. In my case it was a round 12mm diameter metal bar. I marked this in pencil on each dovetail.

Then I mounted each scope onto the side by side rail and did the same on that. Again marking the balance point.


To fit the rig to the mount I found the best way was to fit counter weights, mount the side by side bar, then mount each scope in turn. I balanced the DEC first of all and the actual measured balance point only had to be moved about 5mm one way and it was then perfect. Then RA was adjusted as normal by adjusting the counter weight positions.   


Power Draw

Soak testing the entire rig came after measuring the current drawn from each item. It added up to 5.7 amps at 13.8 volts. This was with both coolers running at 100%, the mount slewing at full speed and all dew straps at 100%.

I did notice that the red power light on the HEQ5Pro started flashing. This represents a low voltage to the mount. I measured the actual voltage in the system and it had dropped to below 11v. The mount really likes to run at 12v and the voltage drop had been caused by the length of power cables, which cannot be shortened BTW and the combined load of everything running at 100%.

I have ordered a 12v to 12v step down/step up voltage regulator which is usually used in caravans, motor home, narrowboats when similar conditions arise. This is simply fitted locally to the cable that feeds the mount. No matter if the voltage drops to 8 volts it will produce a regulated output of 12volts up 4 amps. The mount itself only draws a maximum of just over 1 Amp when both axis are slewing and the mount is loaded up with both scopes.


That module still has to arrive but will be fitted soon.



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