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5: Off-axis guide cameraIn the beginning I plan to use the ASI120mm as guide camera for this system. Many details are 3D-printed and made off plastic. It flex more than metal brackets. Then it's good to have a off-axis guider that look through the same lens as the photos is taken from, no internal flex. Guide camera:
I bought this AIS120mm camera used. It's the older model with USB 2 ports. It caused some trouble and I had to flash the camera with an upgrade software, it's only in Linux system you have to do this: Upgrade ASI120mm camera. 
The camera has a back focus of 12.5 mm. It's a monochrome 1.2 M pixel sensor, the pixel size is only 3.75 my and the sensor size is 4.8x3.6 mm (diagonal 6.5 mm). The off axis adapter I use now in the beginning has a hole diameter of 9x5.5 mm which work with this small sensor. In the future I plan for a bigger sensor to get a wider fov and then I must built my own off-axis adapter with a bigger hole, something like 10x8 mm. Off-axis adapter:
This is the part I plan to use, a prism from my T48 off-axis guider in the beginning. It has a tiny hole of 9x5.5 mm that connect to the guide camera. Later I will design a new one with much bigger opening. 
Something like this it will look, from the camera side. 
From the lens side it look like this. On top of the tube is the prism placed, not drown here. Behind the off-axis tube is space to install a 2 inch IR/UV block filter. Future design:
The thin unmounted filter give more free space and I can use a bigger off-axis adapter. It's designed to take a rectangular hole up to 10x8 mm to the guide camera. I can then have a bigger sensor to get a bigger fov. The off-axis adapter point in direction to the cameras bottom side. The only side where there is space for this guide camera. The mount for the guide camera is the T2 (42 mm) standard. Must find where I have to focus point and fit the dimension to that. Still there must be a focuser for fine tuning, the normal drawtube isn't very appealing, maybe a helical focuser if it's enough space for it. In this drawing I have a 45 degree tilted first surface mirror, but could be a prism also. Back focus:
After some calculations I found where I have the focus point for the guide camera, 20.5 mm below the yellow device. The ASI120 camera has a back focus of 12.5 mm. Now I need some focus mechanism between the yellow and green device that doesn't rotate during focusing, 8 mm left to built this focus device on. Ray trace:
The blue line represent the light ray coming in from the lens to the main camera and the guide camera. A bigger opening to the guide camera had let in more photons. With a medium format lens this is much easier to solve with its bigger image circle. Guide camera attached on the main lens:
If my calculations is correct the guide camera sit +/- 2 mm from correct focus. No other items block the camera's move when focusing. 
This system is very compact compare to my earlier setup with a separate guide telescope. Earlier setup: 300 mm lens. This camera and lens is heavier but in total I save a lot of weight. A deeper look at the guide camera's focus:
When I did my first test I couldn't reach focus with the guide camera. More, the adapter body hit the Nikon body (lower arrow). I tested with a 11 mm Pentax M42 extender, this has another thread M42x1.0mm and the threads fitted bad, of course. But from this I can do a calculation. Some dimensions:
Correct distance between the upper arrows:
One of the items of the Off-Axis adapter has to be redesigned. 
New design, total height 20 mm, 10 more than the original. I couldn't design a M42x0.75 mm thread in FreeCAD. I made the outer diameter to 41.6 mm and hope it will self thread when I force it on. If it doesn't work I have to go deeper in Helical design in FreeCAD to master how to do it. Anyway, the 3D-printer doesn't have that high precision. 
The new 20 mm long adapter attached to the guide camera. 
The main camera and the guide camera in place. Now the guide camera has a lot of free space to travel when focusing. The focus system is terrible but it will be a better design later when I find a bigger prism and can have a bigger opening up to the guide camera. Now I must do a new test if I can reach focus for the guide camera, it's promised a clear sky tonight. Early auto guiding test:I was able to get the guide camera in focus and could do a first test. 
This mount is special built and have a custom gear ratio. The driver EQMOD isn't compensated for this yet. Still I can see a big improvement in the guiding. The 50% longer focal length helps but the use of the same optic as the main camera is important. Off-axis is the way to go. The ASI120 camera works much better in the new 64-bit software. New attachement of guide camera:
I started with parts of my T48 off-axis adapter. With that I could do the first tests and get experience. Now I want something much better. More stability, bigger opening, easier focusing. This is the first item, that attach to the barrel where the lens and camera attach to. It has a light barrier around the opening up to the guide camera. Two bars guide it to a movement along the Z-axis. 
The moveable guide camera bracket. It's designed to have a focus travel of +/- 4 mm. Two M3 thread screws used to fine adjust the focus. It doesn't use the T2 thread to connect to the camera, it squeeze around it. 
With the guide camera attached it look like this. Much bigger opening to the guide camera allow for bigger sensors. A mirror or 90 degree prism can be mounted to the brackets that protrude from top. I haven't find any yet so its dimension are not known yet. They have a fixed position, only for full frame sensors. |
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