Project:
Pentax 645 300 mm ED f/4 lens

Contents:

1. Overview
2. Internal filter holder
3. Install the lens on the 300 mm rig and setup USB-focus for reverse
4. Temperature compensating and tilted focus plane
5. Autofocus with APT and N.I.N.A.
6. Move the equipment to the observatory
7. A second try at the observatory
8. Light leakage
9. Optical analyze
10. Fix of tilted sensor
11. Control of aperture and deformed stars
12. Update of Ekos auto focus driver
13. Transport box
14. To be continued

Related projects:

1. HEQ5 as portable mount
2. Pentax 645 300 mm ED IF f/4 lens
3. KStars / EKOS and Raspberry Pi4

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

2: Internal filter holder

I have some plans to filter modify my Canon 6D camera or maybe a newer mirror less model. If I do it I remove all internal filter and place the filters I need outside the camera. With normal Canon lenses it's difficult because of the limited space between the camera and the lens. But now with the medium format lens I got a lot of space.

As you maybe have seen I have made some 3D-printed details to my medium format lens, more details here: Pentax 645 300 mm tube rings. When I bought the Pentax lens I also ordered an adapter, from Pentax 645 system to Canon EF system.

This adapter is 27 mm long and let me place things inside it. One idea is to 3D-print a filter holder that fit inside this adapter. One of the filter can be a UV - IR cut off filter, if it clips at 700 nm I get full H-Alpha radiation to the camera sensor after it has been modified.

To the right is a normal UV camera filter with its holder. It's a 48 mm filter and the outside diameter is 58 mm. The openings diameter inside the adapter is:

1. 57 mm, entrance
2. 56 mm, edge inside
3. 48 mm, opening to the camera

The 56 mm opening will not let the filter come inside the adapter. But an unmounted 50 mm filter will. I can easily 3D-print a filter holder that fit inside this adapter. This adapter fit DSLR cameras, if I instead use a mirror less camera I will get a lot of more space. Then I don't need the adapter, I 3D-print an adapter and design it to have a filter drawer.

The camera side of the adapter.

Three different adapters: Pentax 645 to Canon EF, M48 to Canon EF and T2 to Canon EF, the last one only have a a 42 mm opening.

With the adapter installed on the Pentax 645 lens. Very stable connection between them which is important to get good results.

Before I decide to do this work I must test the lens that it's good enough for astrophoto, these lenses are about 30 years old now.