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My astronomy project:
DIY 3D-printed medium format off-axis adapter
with two filter drawers


  1. 3D CAD with ideas
  2. Optimizing the openings
  3. to be continued

I take no responsibility or liability for what are written here, you use the information on your own risk!

2: Optimizing the openings

Now when I know that everything fit in the space between the lens and camera I can start optimizing the openings in the off-axis guider. Not too big because it can weakens the construction (it can not have too big outside diameter) and not too small because it cause vignetting. I have to implement a tilt function too, but not a big problem.

Spread sheet:

I setup a simple spread sheet to calculate the needed openings at different distance from the focus plan. This is based on the TS130 telescope, the 300 mm lens is not that big problem because the back lens doesn't have that big diameter. If it works for the telescope it works for the lens.

Astronomy project: Medium format off-axis adapter 3D CAD

If looking in the table above, at the distance 70 mm which is the back focus we can read that the opening must a least be 51 mm wide and 40 mm in height. Adding one or 2 mm to have some safe marginal.

One important thing about the telescopes, its internal baffles against he camera must have a hole big enough for a medium format sensor. It's not so easy to measure how big the hole in the baffles are, but I can look through the telescope from the front lens and have the camera's shutter open to get an idea. My full frame camera's sensor has plenty of free space around it from the baffles, the telescope is also delivered with the bigger 3" focuser. This 3" size focuser is good when have a 0.75 reducer/field flatter and full frame sensor.

How good the optical quality is this far away from optical center, no idea, I have to test it. The field flatter is a big 1x 3", an earlier variant of this was used on the big Meade APO refractors (1980s) and was sold to be used with Pentax 67 system, a film with size of 70x60 mm, for sure not all the field was used.

The mirror that pickup the light beam to the guide camera must be in correct place, too far in and it cause vignetting and too far out the signal will be weak because of vignetting or out of focus.

Here is an idea how it works: Astronomy project: Medium format off-axis adapter 3D CAD

Both the camera sensor and the guide camera sensor must fit inside the image circle the telescope can deliver including the field corrector. Field correctors with a reducer have much smaller image circle and maybe doesn't work.

Astronomy project: Medium format off-axis adapter 3D CAD

How it can look for a specific optical system, for short focal length the vertical beam came in an angle.

More details here: Setup an off-axis adapter.

Tilt mechanism:

Astronomy project: Medium format off-axis adapter 3D CAD

Where the arrows are there is a problem. These are the joints between the bayonets, it's a wiggle between them that I want to get rid off. If I can attach the off-axis adapter to the bracket that hold the lens it will solve that problem. Three supports 120 degree apart like the rectangle could be the attach point for the off-axis adapter. I can add shims here to adjust the tilt, or maybe some more complicated mechanism. The silver foils at the middle arrow is the shims today, there are four layers that add up to 80 my. Here is how I found the optimal thickness of the shims: Pentax lens tilt.

Astronomy project: Medium format off-axis adapter 3D CAD

With a small modification of the bracket that hold the lens and focuser I could add three supports where I can attach the off-axis guider. I also have moved the bracket for the control unit for the focuser 10 mm to the right. A medium format camera needs more space. Looks good and much more stiff compare to how it's today.

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To page III, to be continued

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