Project:
Nikon D800 for astrophotography ?

Contents:

1. Nikon D800 overview
2. Adapter Nikon to Pentax 645
3. Lid, Dove tail, Tube ring
4. Focuser mechanism, driver
5. Off-axis guide camera
6. Main bracket
7. Wiring & USB-HUB
8. Test with Windows and APT
9. First Light
10. Balance system
11. Nikon vs Canon, Pentax FA vs Pentax
12. Tilt adjuster
13. KStars/INDI setup for Nikon
14. Correction of tilt
15. To be continued

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

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• KStars / Linux commands and udev rules
• KStars / Raspberry Power
• Setup astroserver with Raspberry Pi3
• Setup astroserver with Raspberry Pi4
• Setup astroserver with Raspberry Pi5

2: Adapter Nikon to Pentax 645

To have a lens to do some test with I use my Pentax FA 645 300mm f/4 ED lens. To connect the camera to the lens I need an adapter between them. An adapter with built in tilt adjuster. An off-axis adapter. A holder for a permanent 2 inch IR/UV block filter. There is no one to buy and I design one in 3D-CAD and 3D-print it later.

My old Canon camera that I have used all the years has a back focus of 44 mm. This new (used) Nikon D800 has some longer back focus, 46.5 mm. Shorter is better, give you more space to add other equipment.

Cameras back focus information:

• https://en.wikipedia.org/ wiki/ Flange focal distance (Flange focal distance, Wikipedia)

The Pentax 645 lens is a medium format lens. With that I get a bigger image circle and longer back focus. This Pentax system has a back focus of 70.87 mm. From that I found that the adapter should be 24,37 mm long.

Pentax 645 bayonet:

I design the adapter in more than one unit and then glue them together. Start with the end that connect to the Pentax 645 lens.

Later I add devices that let the tilt eliminated. If there is space enough I also add an off-axis guider that will be built in. If the bayonet doesn't work well I can design a clamp that grip around the lens barrel which will be more stable.

This is the second version, still a prototype, the final version will be in black.

It fit very well on the lens and it feels very stable too.

Nikon bayonet (alternative 1):

At the other end of the adapter I have the Nikon bayonet. This isn't good, the opening diameter is only 40 mm, not good enough for a full frame sensor which has a diagonal size of 44 mm.

Alternative to the Nikon bayonet:

Can I use these five screws to attach my adapter ?

Before trying to detach this bayonet ring I must have a screw driver that fit perfect to the screws. Of the tools I have this Pozidriv #00 screw driver fits best. The screw is 2 mm in diameter and 4.8 mm long. What thread it has I don't know.

With the bayonet ring away I can see what's behind. A leaf spring for the bayonet, the shaft at lower left is for the focus motor, and the bar at right is the release mechanism of the lens.

With the leaf spring away I can see what's behind and the fondation where I want to attach my adapter.

Now I must figure out the position of these screws, they look to be attached to a 50.8 mm circle, that's 2 inches. The thickness of it is 2.4 mm. Today Japan use the metric system but this bayonet system was constructed long time ago.

After some work with the CAD I came up with this design. I have it designed for a 50.4 mm unmounted filter. The free opening is now 46.5 mm, much bigger than the 40 mm I had when using the bayonet. The short side size of a full frame sensor is 24 mm. Half of that, 12 mm protrude away from the optical axis. I have this off-axis adapter at a safe distance of 14 mm from the optical axis, or maybe even less. I glue it in a fixed position when I have tested it out to get maximal stability.

3D-printed:

The first version of the 3D-printed camera / lens adapter with off-axis guide and 2 inch filter. The red part is the bayonet for the Pentax 645 lens, it will be glued in correct position later.

The backside that connect to the Nikon camera with its five screw holes.

The adapter screwed to the camera. Very satisfied with this so far, big opening, low weight and sturdy.

The older Pentax 645 300 mm ED f/4 IF lens connected to the Nikon full frame camera.

And the newer Pentax 645 FA 300 mm ED f/4 IF attached. The lenses is about the same size but the FA model is much heavier. The newer lens has two more lenses for a higher correction and is of better optical quality, but is it better for astrophotography ? More lenses can cause more reflections and lower contrast. The yellow arrow points at the balance point.

With the guide camera attached for some visual inspection. There are free spaces around it to not collide with any other part.

New bayonet:

After some test of the first version of the adapter I felt I must developed the bayonet that connect to the Pentax 645 lens further. First I have added three tabs that I can use for a tilt adjustment. The bayonet tabs has also been improved.

The bayonet tabs have a stop now, with that the lens and camera always sit together at the same angle. What's still missing is a lock mechanism for the bayonet.

The new items:

Tilt adjuster, Off-axis adapter and the bracket for the guide camera. The O-ring is to be used as spring when adjusting the tilt. The springs to the right is for the focus of the guide camera.

Guide camera bracket:

There are two brackets for the camera, one with a height of 18 mm, and the other with a height of 23 mm. To cover any combination of focus distances.

Mounted together, everything fit together now, later I replace the O-rings with something better, shims maybe for the stability reasons. Need a first surface mirror, my idea is as simple as to glue it on the bracket that protrude. But until I foind that mirror I use the prism from my old off-axis adapter.