Homemade DIY filter wheel for RGB filters

I have used black and white CCD cameras for over a year. The increased sensitivity of a black and white chip is something special. I started with my SC3 modified toucam, and, recently, moved onto an Artemis 285.

To take colour pictures with a black and white camera requires the use of colour filters in front of the CCD sensor. I use a set of red, green and blue filters. Taking a colour picture always been a rather fiddly affair for a number of reasons.

• I have to remove the camera from the telescope to change the filter
• I kept mislaying the filters in the dark.
• The four LRGB images would all be slightly rotated. Although this can be solved in software.
• I would often have to reframe the shot after each filter change because the act of changing the filter would upset the telescope pointing. This leads to corners of the image with incomplete colour data.
• And then I might need to reset the guidestar.

The elegant solution to this is some kind of filter wheel, or filter slide. I went through quite a few experiments before I finally arrived at the Mark One filter wheel below. I had considered a slide system, however, this is apt to be quite long (at least 130mm) and is liable to get in the way of something else.

The key design constraints are to minimise the use of backfocus (ie make the system as thin as possible) and ensure that the camera and filters remain aligned with the optical axis of the telescope. It is also important to keep the filters close enough to the CCD to prevent vignetting. On the other hand, I was not too concerned with being able to swap filters at this stage. One day I will have some hydrogen alpha and other emission line filters, but I will "fall off that bridge" when I get to it.

The final device consists of the following components

• An inner ring with 4 holes cut in it for the filters. The idea is to remove the filters from the commerical holders and sit them in the holes. They rest on small shoulders. a tiny speck of epoxy stops each filter falling out. This inner ring has teeth cut around the circumference, rather like a telescope worm wheel, this would allow me to rotate the filter wheel by turning a screw against these teeth.
• Two outer plates made from 3mm aluminium. The inner filter holder turns against one of the plates, keeping it flat to the plate.
• A thread with handle to rotate the inner wheel
• 4 standoffs to hold the outer plates the correct distance appart. Tiny amount of shimming can help to maintain alignment.
• A male T adapter on one side, and female on the other. This are carefully made and and screwed to the end plates to maintain alignment.

All the parts are made from aluminium except for the thread rod, which is just a bit of studding from B+Q. The aluminium have been anodised varying shades of blue, red and black using my home anodising methods. Anodising is still a bit hit and miss! It sort of matches the Artemis blue. Amazing what you can do with clothes dye.

A couple of rather rough bits... the clothes peg makes a wonderful handle, but looks dreadful, but works rather well! This is a short term measure until I get some stepper motor control. The black ring around the outside is part of a 4 inch drainpipe. It is used to keep the light out. Again, until I think of something better.

The wheel is indexed using 4 holes drilled in the filter holder. There is a hole in the outer plate. Then these line up, the next filter is in position. You just have to keep mental track of which filter is in position. A small hardened still pin allows me to check the holes are lined up. It takes something like 50 turns of the handle to move 1 filter.... a stepper motor is not far off! With a stepper motor I can simple count of the number of revolutions required between each filter.

After a few weeks of using the filter wheel, I realised the obvious way of checking which filter is currently in position, and how much to turn the handle: You simply look down the ota with a torch, and you can see the filters going around as you turn the handle.

Some pictures of the Mark One Filter Wheel

Couple of shots of the filter holder wheel having its circumference teeth cut. The tool is simply a bit of threaded rod with a nick made in it with a dremel cutting disk. This is not a high tolerence worm wheel - its not driving the telescope, just turning some filters.

The freshly anodised back plate is about to be plunged into the blue clothes dye.

Couple of shots of the end plates with the T adapters fitted. The adapters are screwed to the plates with countersunk M3 machine screws. If I want to change them for a different adapter one day I can.

A couple of closeups of the filter holder before and after I anodised it "pale black". You can see the RGB filters themselves attached on the right. I've not put clear glass or IRB into the luminosity hole. There is already an optical window on the Artemis, and I don't want to add to the glass in the way.

THe LRGB wheel mounted on one of the plates. The central axis is just a short steel pin.

Artemis with his new friend. See, the blue nearly matches!!!!