Home Made Remote Controlled Filter Wheel

[Gina]

Having gate-crashed another thread about CCD cameras with my ideas on a home made filter wheel, I thought I'd start a new thread in what I think is a more appropriate forum. These are only ideas at present as it will probably be some time before I can afford a dedicated mono CCD camera with which to use a filter wheel.

I'm at it again with my cheap and cheerful, simple designs I used to be happy programming microprocessors to handle this sort of thing but feeling a bit past it now so tending to use simpler and more straightforward ways of doing things - the "Blue Peter" approach In spite of most of the associated things like filters and cameras costing a lot, I like the challenge of making at least part of the system on a shoestring - using cheap components and recycled bits from broken equipment and household items.

So I'm rejecting stepper motors, Arduinos and suchlike and going for the basic approach. If someone else would like to take some of my ideas but use a stepper motor with their associated driver circuits please feel free. All my ideas are in the Public Domain

Having set the ground level principles I'll stop now and post again when I get some diagrams together to show my ideas.

[Reggie]

Hi Gina, nothing wrong with keeping things simple, Search the forum, there was a very recent post where someone has built a DIY filter wheel, it may give you some inspiration

Personally, I like computer automation, purely because I want to be able to sit in the warm when I'm imaging, I really don't need to be there for anything other than initial setup and observing, that's the theory at least

[Gina]

Hi Gina, nothing wrong with keeping things simple, Search the forum, there was a very recent post where someone has built a DIY filter wheel, it may give you some inspiration

Yes, I remember reading that - a manual wheel if I recall

Personally, I like computer automation, purely because I want to be able to sit in the warm when I'm imaging, I really don't need to be there for anything other than initial setup and observing, that's the theory at least

I like computer automation too but like to keep it as simple as I can.

[Gina]

Been stripping down a broken Blu-ray player and the tray mechanism has a very neat little 5v DC motor with a pulley and belt drive to a geared step down drive to open and close the tray and move the heads/motor unit up to the disc. I can remove motor and reduction pulleys/gears which I could arrange to drive the edge of the filter wheel. I'll post a photo later.

Another source of drive motor would be old computer optical drives with a similar tray mechanism.

[Gina]

DC motor drive to tray open/close mechanism. The motor is just under 30mm diameter and 18mm thick (not counting pulley). The whole reduction drive is about 22m thick and 75mm x 38mm of framework required to hold motor and reduction mechanism.

[jnp]

I posted the DIY filter wheel a few days back. Don't think I have seen any other threads. Feel free to use laminate flooring, I saw it advertised at £4 per square meter.

By sheer coincidence I considered using a motor salvaged from a DVD player. (24mm diameter and 12mm deep excluding shaft in my case). You would probably also need a micro-switch, or detector of some kind to know when to stop the motor etc.

Be interested to see how this develops. I wont be motorising the wheel yet as I have too many other DIY builds on the go.

John

[Gina]

Thank you John

I've been doing some research on commercial wheels and also seen one write-up on a home made one. Most drive the rim of the wheel by either friction (Starlight Express and some others) and a few have teeth on the outside of the wheel driven by a pinion on the motor/gearbox shaft.

In general they use magnets and either hall effect sensors or reed relays to determine the wheel position and stop it in the right place. I was going to use reed relays to detect which filter was in use but didn't think a magnetic sensor would be accurate enough to use to stop the motor for precise positioning, but apparently it is. At least Starlight Express and many others think so. I guess the accuracy is less important than repeatability - a mm out in the absolute positioning wouldn't matter as long as the same filter finished in the same position each time. This would mean I could dispense with the escapement mechanism and rely entirely on magnetic detection.

[yesyes]

I would probably drill a small hole per filter position in the filter wheel disk and then use one of these opposing LED / photo transistor contraptions (not sure what the correct English term is) that you would find in an old ball mouse.

[Gina]

Yes, I used that method for my weather station wind vane to read wind direction. Much more accurate than magnet/reedswitch combo. The only problem with LEDs is the possibility of stray light getting into the scope/camera light path.

[steelfixer]

Hi Gina.

Could you not drill a small hole into the edge of the wheel.

Not large enough to effect the friction drive but big enough to allow a micro switch to drop into.

Hope that makes sense.

[yesyes]

something like that?

MicroSwitch with Roller Actuator (Micro Switch) | eBay

[Gina]

Yes, that's something to consider I think I have a micro-switch or two with roller lever. I guess a hole with a steep drop in and shallow slope out to avoid loading the motor drive too much. I'll have to check how quickly the motor stops. That could be used to stop the motor and magnet/reedswitches could read which filter is in use.

[Gina]

Technobots are cheaper for those microswitches :- 3A Roller Lever Microswitch V4

To make construction of the wheel easier without a lathe or other way of making accurate discs, I'm thinking of using a belt rather than friction drive. I can get a 150mm diameter belt from Technobots for about a quid. I think that would make for something like a 5.5" wheel but will check - have to sort out the geometry

Can someone tell me the outside diameter of the Baader LRGB 1.25" filters please? I need to know how much space to allow.

[yesyes]

That ebay listing was meant to show the type of switch I meant. I didn't do any research as to where it would be cheapest...

I should have mentioned that....

[yesyes]

If you're not in a rush and want it really cheap....

10 Roller Lever Arm Micro Switches AC 250V HV-156-1C25 | eBay

[Gina]

This is a diagram of a 6" wheel with 8 1.5" holes. I think the filters might be about that size (but guessing).

[Reggie]

the filters should be 1.25" unless you want to go expensive or need bigger ones, which in general are 2".

[Gina]

the filters should be 1.25" unless you want to go expensive or need bigger ones, which in general are 2".

But is that the overall size or the size of the filter itself or the thread size? I haven't yet bought any 1.25" filters to measure.

[Psychobilly]

The filters are slightly smaller od than the push fit tube on the end of the EP...

[Reggie]

Indeed, and all 1.25" filters are threaded to fit standard 1.25" eyepieces and barlows etc.

[jnp]

1.25" is based on the outside diameter. Will be the same as the barrel of an eyepiece.

Based on Revelation Astro filters, the dimensions are:

Outside diameter = 31mm

Outside diameter of threaded section = 28.42

Total Height = 11.13mm

Depth of threaded section = 11.13-7.69= 3.44

Actual depth of thread = 2.25mm approx

Actual Filter aperture = 25.92

You should actually have a microswitch on the Blu ray tray, there was one on my DVD?

8 x 1.25" filters is quite a lot, although I did contemplate making my wheel bigger to support 1.25" and 2" filters seeing as I have a mixture.

[Gina]

Very helpful info - thank you very much

As for number of holes in the wheel, while 8 is just about on, I'm thinking I may go for 6 or even 4 to start with and if, at a later date, I find I want more I could build a bigger one. I already have a belt that I can use for a smaller wheel. Must say, I'm not really happy with the small amount of material in the "spokes" between the holes in the 8 hole design.

Further designs coming up soon...

[Gina]

Here is a 4 hole wheel, 4" diameter, together with belt and pulley.

[Gina]

6 hole wheel.

[Gina]

I think a 6 hole wheel would be a good option. Only slightly bigger than the 4 hole version whereas the 8 hole one needs to be quite a lot bigger.