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My astronomy project:
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10: Prism bracketIt has been many weeks with cloudy skies now. I use that time to update some design problems in my equipment. First I have to shorten the adapter between the camera and telescope once again to get a less curvature on the focus plan. I want it to protect from dust better at the same time. The other problem is with the off-axis adapter, not with the telescope but with the Pentax 645 300 mm lens. The back lens block the light ray partly, too small diameter. Strange because it's a medium format lens. 
This half of the off-axis adapter is the same for the Pentax 645 camera system and the telescope. A rubber gasket will be installed between the camera and adapter to protect from dust. I made some small modification to the prism outlet to fit a new design of prism bracket. I also increased the over all diameter to have more space to work with. 
The 67x2 mm O-ring installed for better dust protection and get it light proff. It will also be easier to take the system apart with this. Ray trace over the off-axis and Pentax 645 300 mm lens problem:
A front view of the Pentax 645 300 mm f/4 lens. The small rectangle is the full frame sensor, the big rectangle is medium format film, 60x45 mm. The square 10x10 mm prism and at top the guide camera sensor 6x4 mm. Small circle back lens, 36 mm and big circle, front lens 70 mm. As seen the back lens has too small diameter and partly block the light ray to the guide camera sensor. Ray trace of prism:
A simple ray-trace over the off-axis guider. Many of the light rays are outside the back lens rim. This is not good, it's working but must be corrected later. 
By offset the guide camera sensor 5 mm towards the main camera it tilt the light ray down. With that the light rays can pass through the back lens. Never seen a design like this in a off-axis guider. It could give some problem when focusing, the guide stars will translate up and down. But during the auto focusing it don't exposure and the temperature drift is slow and the auto guiding will handle it. This is not difficult to built it I and must do it for a test. New prism bracket design:
For the telescope the back lens of the field flatter is 65 mm in diameter so this isn't a problem. But still the prism bracket could be more developed. The new bracket to the right. I have made the walls thinner to use the size of the prism better, 10 mm. 
The prism is moved downwards, that was easier than cut of the prism 1 mm. But now it protrude 1 mm outside the bracket at bottom side. 
When the prism is lowered I can use the upper part of it which is closer to the optical axis. That's more important for the Pentax 645 lens. 
I had to make a pit in the adapter where the prism edge could protrude into. 
Prism and its bracket installed, with these problems solved I can now use the same bottom part of the off-axis guider to the lens and telescope. Below the prism is the filter holder. It take a M48 mounted and 50 mm unmounted filter, the first is a bit small for full frame cameras. But I could only find M48 mounted filter, later if I find 50 mm unmounted filter I will use them. When I have found 50 mm unmounted filter I can redesign the outlet to the guide camera, expand the opening by 1 mm if needed. 
Finished, but all these works got a lot of dust into the camera and it had already a lot off dry stains from earlier. I have cleaned the senor with Isopropanol now and blow away all dust. For sure I have to take it apart one or more times until it's perfect, it's difficult to clean it. The guide camera sensor could be seen in the little window. The reflections I had problem with earlier is much reduced now with the rectangular mask. Later I will investigate the design with a ray trace analytics to see if I can get it even better. Changing the angles of the walls and maybe made some structure on surfaces. But first I must master how to use a tool like that, there is one in FreeCAD. |
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