Home Made Remote Controlled Filter Wheel

[Gina]

Here is a more detailed design for a 6 hole filter wheel using a 100mm diameter by 5mm thick acrylic disc plus a 110mm x 3mm aluminium disc. Both these discs are available ready made from eBay Top Sellers. That will save me the most difficult part of cutting round discs. I can cut the filter holes easily enough with a trepanning tool, which I already have.

The aluminium disc has holes to take the threaded part of the filter and the acrylic one takes the outside. I think this arrangement will help hold the filter straight - at right angles to the light path. I have shown a ball bearing as the main centre bearing but I'm not sure this is necessary. A plain hole and bolt would make construction easier.

I've also been giving some thought to the casing. A circular casing would probably look nicer but a rectangular (or square) one would probably be easier to make. To connect to the scope I will need plates with 1.25" standard thread or a suitable size tube to fit a 1.25" adapter or Barlow. The camera side will want something to match the camera so I don't think I can design that until I see what camera I eventually get.

[Gina]

I've worked out that the wheel and motor unit will fit inside a casing made of 150mm discs. I'm thinking of a pair of 2mm thick discs like these :-

Aluminium Disc 150 mm Diameter 2mm Thick | eBay

This would make a much neater and safer unit than two 150mm squares - I don't like corners, particularly in the dark.

I'll post a drawing tomorrow. I think 2mm should be a suitable thickness. A compromise between weight and rigidity. Any engineers out there have an opinion?

[Reggie]

I love the DIY forum, it just keeps on giving Gina, thanks for the link to precut circles, it's got me thinking about an SCT solar filter now

I dug out my prebuilt filter wheel yesterday, last time I looked at it, it wouldn't relinquish the nebula/planetary filters that are pre-fitted, yesterday they came out very easily, nice 1.25" filter threads

only one issue, I have to dismantle it to swap out filters, so it looks like I'm probably going to have to take a hacksaw to it so the filters are accessible.

[Gina]

The aluminium disc came today and it's beautifully cut - nice smooth edge

I'm planning to keep my filters in the filter wheel but if I get more than 6 I'll think about it then. Probably make a 8 hole one

[Merlin66]

You may have a problem trying to thread the holes for the filters..The M28.5 isn't really a standard....maybe a bit of metal epoxy and grease on the filter threads??

[fwm891]

Hi Gina, All looking good. Remember one thing with the filter wheel and any other attachments that you have a very limited space in which to put them - especially if your a Newt user, fractrs user fair better as they tend to have longer back focus distance.

I made a 28.5 x 0.5 mm pitch tap to thread an ali disc sometime ago. If you want to send me your disc I can cut the holes and thread it for you... (PM me)

Francis

[Gina]

I wasn't going to thread the ali disc as I haven't got the facilities but that is clearly the best way. So thank you very much for your offer Francis - I'll PM you

I have an ED80 refractor so I don't think I'm short of space. I am more concerned about weight.

[Gina]

With the horrible weather I've been working on my filter wheel design again. I've decided to go for 3mm thick aluminium discs 145mm diameter for the casing back and front plates. This would give a solid and rigid base for the attachments to scope and camera, plus enough to cut threads in for screws. Separating these will be a number of 14mm long M3 hexagonal spacers with Allen key machine screws holding it all together.

The diagram shows the motor drive consisting on motor with pulleys and belt to an intermediate wheel which has a toothed pinion driving a larger gear. Then to drive the filter wheel disc I'm using a rubber tyred idler wheel. The axle for this being on a linkage with spring to pull the idler onto the gear and filter wheel. I've decided that a plain bearing would be sufficient for the filter wheel main bearing having light and slow loading.

[steelfixer]

Gina It is looking good.

If it is any help to you I have literally hundreds of plastic gear wheels here.

They range in size from about 8 teeth to 120 teeth and are all the same pitch.

They are all about 4mm thick.

If you let me know you can have any that help you with your build.

Graham.

[pmlogg]

6 holes would not be enough to have an LRGB set and 3 narrowbands fitted at the same time. My Truetek wheel has 8 positions. 8 seemed a lot but I've just had to take out the Baader clear filter to make way for a Hydrogen beta.

[kookoo_gr]

i've been reading this thread with a lot of interest and at the moment i have to make some suggestions.

1) for centering the filters at the camera hole i would suggest the use of opto-couplers. You could also use hall sensors.

2) since you are going to use allen spacers to close the whole wheel i would suggest to make the hole towards the telescope a bit bigger so you can remove the filters without unscrewing the whole wheel

3) i would go for a 9 slot filter carousel LRGBC-NB and one for future use assuming the focuser can handle the weight of the camera/fw. Also in the future the size of the enclosure can support a 2'' filter carousel. You need to consider some space for the electronics.

4) if the filters are permanently fitted at the fw carousel you could mount the axis of a small motor at the center of the filter wheel carousel in order to reduce size and weight. You could use a small stepper motor or a servo motor that are used at remote controlled models

[yesyes]

Regarding number of positions, it might be a good idea to keep one position empty for when you want to image without filter but don't want to remove the whole contraption....

[Gina]

Gina It is looking good.

If it is any help to you I have literally hundreds of plastic gear wheels here.

They range in size from about 8 teeth to 120 teeth and are all the same pitch.

They are all about 4mm thick.

If you let me know you can have any that help you with your build.

Graham.

That's very kind of you, thank you very much I'll let you know if I want any.

[Gina]

6 holes would not be enough to have an LRGB set and 3 narrowbands fitted at the same time. My Truetek wheel has 8 positions. 8 seemed a lot but I've just had to take out the Baader clear filter to make way for a Hydrogen beta.

I was thinking of LRGB plus Ha and OIII only and maybe upgrade later but I take your point. Thank you for your reply, I'll give it more thought

[Gina]

i've been reading this thread with a lot of interest and at the moment i have to make some suggestions.

Thank you - always glad of suggestions

1) for centering the filters at the camera hole i would suggest the use of opto-couplers. You could also use hall sensors.

Yes, I must admit I would prefer opto too just need to make sure no stray light gets into the scope light path. I'll look into hall sensors.

2) since you are going to use allen spacers to close the whole wheel i would suggest to make the hole towards the telescope a bit bigger so you can remove the filters without unscrewing the whole wheel

I'll give that some thought.

3) i would go for a 9 slot filter carousel LRGBC-NB and one for future use assuming the focuser can handle the weight of the camera/fw. Also in the future the size of the enclosure can support a 2'' filter carousel. You need to consider some space for the electronics.

9 is beginning to get pretty big but I might look into that or at least 8. 8 would be easier. But I am very conscious of the weight.

4) if the filters are permanently fitted at the fw carousel you could mount the axis of a small motor at the center of the filter wheel carousel in order to reduce size and weight. You could use a small stepper motor or a servo motor that are used at remote controlled models

Yes, I thought of that. Having the motor drive outside the filter wheel casing would mean the casing would only need to be only big enough to allow for attaching front to back plates, saving weight. I'll give it more thought, thank you

[Gina]

Regarding number of positions, it might be a good idea to keep one position empty for when you want to image without filter but don't want to remove the whole contraption....

Yes, or even better probably, have a parfocal clear glass filter (an extra £21 won't break the bank). Good thought

[Gina]

Been looking into some of the above suggestions and investigating availability of things. I agree with the idea of future-proofing by making a wheel with expansion possibilities (same principle I used when I bought the NEQ6 Mount). For more filter holes I could use a 145mm or 150mm ali disc for the wheel and 180mm discs for the casing. I already have 2 145mmx3mm discs ordered so could use one of those for the wheel and get 2 180mmx3mm for the casing.

I like the idea of a centre motor/servo driving the wheel directly. That should be possible. I also like the idea of non-touching position sensing with no friction. That would mean friction from centre bearing/motor only. Then there would just be the inertia of the wheel and filters to consider plus unbalance due to not having all the holes filled with filters (at least, not at first). That motor drive I've got with idler wheel is rather complicated and I'm none too happy with it.

Back to the drawing board then

[kookoo_gr]

I measured one of my Baader 1.25 LRGB filters and i post the dimensions of the filter. hope it helps

Total Height=7.73mm

Outer Diameter=30.98mm

Threaded outer diameter (this side screws at the FW/EP)=28.37mm

Thickness of metal=1.51mm

Height of unthreaded metal=6mm

Inner Diameter of Unthreaded side=28.30mm

Inner Diameter of threaded side (this side screws at the FW/EP)=26.58

[Gina]

I measured one of my Baader 1.25 LRGB filters and i post the dimensions of the filter. hope it helps

Total Height=7.73mm

Outer Diameter=30.98mm

Threaded outer diameter (this side screws at the FW/EP)=28.37mm

Thickness of metal=1.51mm

Height of unthreaded metal=6mm

Inner Diameter of Unthreaded side=28.30mm

Inner Diameter of threaded side (this side screws at the FW/EP)=26.58

Thank you Yes, very helpful and different from some other filters in height.

[Gina]

Been looking around for servos. There's Maplins but I can't seem to find another firm I found before that did them cheaper. Maplins also sell opto sensors so I could make up an order with them, but I usually find other sources of the same things at lower prices. Maybe I'm just tired Had a busy day.

P.S. One thing I thought of is that with an external motor drive I could also have external optical sensing without the problem of stray light getting to the camera. I've found before that old/useless CDs or DVDs are easy to make into optical encoders either by scraping the reflective coating off or using paint.

[Gina]

Anyone have suggestions for the best servo for this purpose, please? Obviously it must be able to rotate 360 degrees. Amazon have some but I don't know what to look for - is the cheapest alright or is a 6" wheel full of filters a bit much? If a simple servo would do the job, I guess the filter wheel could be attached direct to the actuator disc/cross thingy and use the servo's bearings. That would make things very simple

Would the wheel continue to rotate after it was asked to stop? Would it need a brake of some sort? A geared DC motor would probably just stop, virtually instantaneously.

[Bizibilder]

Try these: Servo Shop

or these: Hitec HS-311 Standard Servo - HS311 - £8.99

and this may be useful: Hitec HS-311 Standard Servo - HS311 - £8.99

You can make a model type servo turn through 360° by opening it up and finding the moulded "stop" tab on one of the gears - or possibly the casing - and cutting it away.

[Gina]

Try these: Servo Shop

or these: Hitec HS-311 Standard Servo - HS311 - £8.99

and this may be useful: Hitec HS-311 Standard Servo - HS311 - £8.99

You can make a model type servo turn through 360° by opening it up and finding the moulded "stop" tab on one of the gears - or possibly the casing - and cutting it away.

Thanks

I've got my brain in gear now and found some on Amazon and eBay.

This one looks much the same as the one in your links and is cheaper :-

3kg High Torque Steering Servo Nitro Models RC Buggy | eBay

And this is a smaller one and cheaper still but would it be strong enough I wonder. I think it would be quite strong enough to rotate my little security CCTV cameras (it would only have to resist the springiness of the cable). :- 9g SD90 RC Micro High Speed Servo for Model Car/Truck | eBay

BTW - I see these servos have gears in and I guess they should stop at the right place and not overrun.

[kookoo_gr]

you could use a simple dc motor. the weight of the carousel won't be so big especially when you cut the holes for the filters, so i think even a small 12v dc motor will be able to rotate the carousel. if the wheel continues to rotate after you give the order to stop then you need to program/adjust the circuit to give the stop order sooner when it rotates but bear in mind that if this happens then you will definitely going to need a brake since the carousel will slip during imaging

eg. assume the time to change a filter is 1sec, you will probably give the stop order at 0.8-0.9 sec and you want the wheel to stop dead at its tracks. if you see the the wheel to rotate after the stop command then you need to program it to give the stop command a bit sooner, let's say 0.5sec. that way you can take advantage of the 0.3 sec into the change time of the filter wheel. possible drawback of this method is that the change time between filters may not be the same especially if the the filter holes have not the same spacing between them.

You could make a permanent tension at the carousel so it won't slip after the stop command and keep the filter steady at its place when imaging. You can place a rubber at the periphery of the carousel and place a thin rotatable axis that will touch the rubber and give you the tension you need. You could also use a spring brake (just like the switches that are at some previous links) that will slot itself into a small hole at the carousel and will allow the wheel to move for the next filter and stay locked during imaging

another thought is to place the motor towards the side of the focuser so you won't have any problems when you mount the camera on the filter wheel (assuming you place a motor at the center of the carousel). If the motor is touching the focuser then you will need to place the motor at the camera side. If you own a ccd camera i suggest to measure its external dimensions and place the motor at the camera side. Of course this means you may have to make a larger carousel or go with one of your earlier designs

[Gina]

you could use a simple dc motor. the weight of the carousel won't be so big especially when you cut the holes for the filters, so i think even a small 12v dc motor will be able to rotate the carousel. if the wheel continues to rotate after you give the order to stop then you need to program/adjust the circuit to give the stop order sooner when it rotates but bear in mind that if this happens then you will definitely going to need a brake since the carousel will slip during imaging

eg. assume the time to change a filter is 1sec, you will probably give the stop order at 0.8-0.9 sec and you want the wheel to stop dead at its tracks. if you see the the wheel to rotate after the stop command then you need to program it to give the stop command a bit sooner, let's say 0.5sec. that way you can take advantage of the 0.3 sec into the change time of the filter wheel. possible drawback of this method is that the change time between filters may not be the same especially if the the filter holes have not the same spacing between them.

You could make a permanent tension at the carousel so it won't slip after the stop command and keep the filter steady at its place when imaging. You can place a rubber at the periphery of the carousel and place a thin rotatable axis that will touch the rubber and give you the tension you need. You could also use a spring brake (just like the switches that are at some previous links) that will slot itself into a small hole at the carousel and will allow the wheel to move for the next filter and stay locked during imaging

Yes, I think it needs a definite mechanical stop, thinking about it. As you say, back to a roller micro switch with a definite detent on the filter wheel.

another thought is to place the motor towards the side of the focuser so you won't have any problems when you mount the camera on the filter wheel (assuming you place a motor at the center of the carousel). If the motor is touching the focuser then you will need to place the motor at the camera side. If you own a ccd camera i suggest to measure its external dimensions and place the motor at the camera side. Of course this means you may have to make a larger carousel or go with one of your earlier designs

I haven't yet bought a CCD astro camera so I'll make allowance for the biggest I'm likely to get. It might be better to put the motor on the focuser side as I know what space I have. Um... Or do I? What if I upgrade my scope? More to think about than first appears. Thank you for bringing this to my attention