My astronomy project:
Duo narrow band filter

Content:

1. Duo narrow band filter
2. To be continued

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

1: Duo narrow band filter

Today (2025) the duo narrow band filter I ordered has arrived. Now I can measure the exact dimension of it and judge if it's possible to fit into my adapter. My plan is to use it with my Nikon D800 full frame camera and the Pentax medium format 300mm f/4 lens when I want to catch nebulosity.

The little jewel box that arrived today.

The filter is well protected in a plastic box and protection films over the filter's surfaces. It's the model SV220, a duo 7nm H-Alpha and OIII filter.

Included was this graph over the filter's curve. Two pass band centered over 500 nm (OIII) and another over 656 nm (H-Alpha).

M48 (2") filter are on the limit to be too small (44 mm clear opening) for a full frame sensor. It's important to mount it as close as possible to the sensor. In a DSLR camera this distance is relatively long, in this case 50 mm if I place it in the bottom of the adapter. That's the only place where it's placed behind the off-axis guider. Above is the 3D-printed Nikon DSLR to Pentax 645 prototype adapter.

Drop the filter into the adapter and of course it didn't fit directly. The filter's outer diameter is 50.9 mm. The threads outside diameter is 47.8 mm. The thickness of it is 7 mm and of that the thread is 2 mm. It's the thickness of it that cause the problem, 5.5 mm had been okay, but 7 mm is problematic. This will take long time to get it working, but before I spend time on that I will IR modify my Nikon D800 camera, remove the filters to increase its IR response.

An alternative placement of the duoband filter will be in front of the sensor, inside the camera. The original filter stack which I remove is thick so maybe possible. But looks like one edge of the filter has to be cut.