Home Made Remote Controlled Filter Wheel

[Lonestar70]

Hi Gina,

You certainly get yourself involved in some interesting DIY stuff.

I like the idea of the inverted belt, however, you appear to have a groove around the outside of your carousel... was this for a drive belt?

One problem you might run into is securing the inverted belt since it will only be supported at 2 points across it's width... placing external tension on this (with another, larger dia belt from the motor) will almost certainly lead to the rim belt collapsing into the groove... I think you would need to fill this with something (car body filler or epoxy) before you fit the inverted rim belt... I presume you will be glueing this in place?

How are you intending to Index the filter positions using the Arduino? are you intending to have a sensor at each position with a home position sensor as reference?.. or perhaps some form of quadrature encoding?

I can see, what appears to be a single hole for a reference (pos home position sensor) but do not see any others.

How thick is your carousel wheel? and how deep is the recess for the 36mm filters, also what is the (clear) inside diameter of the filter holes?... I am assuming that the outside dia of the filters you have is 36mm.

I may be able to help you with the required 1 1/4" - 36mm adaptors, and any other bits you need turning.

Pm me a sketch of what you require and your email address and we can discuss it off-site in more detail.

Best regards.

[Gina]

Here's another diagram - this shows roughly using the SX adapters to attach camera and lens/scope. Can't finalise it until I get the adapters.

[Gina]

Hi Gina,

You certainly get yourself involved in some interesting DIY stuff.

Thank you

I like the idea of the inverted belt, however, you appear to have a groove around the outside of your carousel... was this for a drive belt?

Yes, I had originally intended to use a round belt.

One problem you might run into is securing the inverted belt since it will only be supported at 2 points across it's width... placing external tension on this (with another, larger dia belt from the motor) will almost certainly lead to the rim belt collapsing into the groove... I think you would need to fill this with something (car body filler or epoxy) before you fit the inverted rim belt... I presume you will be glueing this in place?

Yes, It needs something - maybe a round rubber belt that just fits tight and then the top ground off. Not really thought about it much yet.

How are you intending to Index the filter positions using the Arduino? are you intending to have a sensor at each position with a home position sensor as reference?.. or perhaps some form of quadrature encoding?

I can see, what appears to be a single hole for a reference (pos home position sensor) but do not see any others.

I thought that if I used a timing belt and stepper motor I could just index the home position and reach each filter position by counting steps. Saves having to have indexing for each filter. Still a bit undecided on this.

How thick is your carousel wheel? and how deep is the recess for the 36mm filters, also what is the (clear) inside diameter of the filter holes?... I am assuming that the outside dia of the filters you have is 36mm.

Carousel is 3mm thick, recess about 1.9mm and diameter about 36.1mm.

I may be able to help you with the required 1 1/4" - 36mm adaptors, and any other bits you need turning.

Pm me a sketch of what you require and your email address and we can discuss it off-site in more detail.

Best regards.

I'll do that - thank you

[Gina]

I have received the SX filter wheel adapters and they're a bit different from what I expected so some modifications have been made to my design and the side plates will be 150mm square rather than 125mm. I'll post details later.

[Gina]

Here is the design using the SX filter wheel adapters to provide T2 connections for camera and scope/lens. These will require a hole to be cut in both plates of 60mm diameter. The adapters both add 4mm from the outside of the FW box or 7mm from the inside. I have ordered 3mm thick plates for the sides (150mm square). The EFW2 is 22.3mm thick with the Atik T2 adapter so to get the same, mine would have to be 8.3mm between the inside surfaces of the side plates. This is getting very tight considering the Baader 1.25" filter I've just measured is 8mm thick.

I may have to do so something different for widefield use. One possibility would be to attach a Pentax screw adapter direct to the side plate. One thought is to use the mounting plate as side plate for the FW with built-in M42 adapter and a separate 150mm square plate with SX T2 adapter for use on a scope. In other words, take the filter wheel apart to move between scope and widefield rig.

Here is the design for the stand-alone filter wheel using male and female T2 SX adapters.

[Gina]

Let's see if I can link to my SketchUp file...

https://dl.dropboxus..._Cameras_07.skp

[Lonestar70]

Hi Gina,

Correct me if I am mistaken, but I seem to recall (from one of your many threads on camera spacing) that you had to add an 8mm or perhaps a 10mm extension spacer between the EFW and the Atik to obtain the correct back focus distance.

It may have been in relation to using the Atik a EFW with your ED80... which requires 55mm behind the FF/FR.

If this is so, and your camera lenses are of similar backfocus requirements... then you could add the 8mm-10mm to the width of the new filter wheel and do away with the external extension... just a thought.

Best regards.

[Gina]

Hi Gina,

Correct me if I am mistaken, but I seem to recall (from one of your many threads on camera spacing) that you had to add an 8mm or perhaps a 10mm extension spacer between the EFW and the Atik to obtain the correct back focus distance.

It may have been in relation to using the Atik a EFW with your ED80... which requires 55mm behind the FF/FR.

If this is so, and your camera lenses are of similar backfocus requirements... then you could add the 8mm-10mm to the width of the new filter wheel and do away with the external extension... just a thought.

Best regards.

It was with the ED80 FR/FF. Yes, that needs 55mm. I forget the exact figures now but after including the EFW2 and all the adapters, I had to add 11mm in T2 extension tubes to bring it up to the required distance.

I found with the Asahi Pentax lenses that by the time I'd included various adapters and the EFW2, I needed just 0.6mm to provide infinity focus for the worst case wavelength. So distance from M42 to T2 adapter to camera front face was 22.3 (EFW2) plus 0.6 delrin spacer ie. 22.9mm.

[Gina]

I've bought a filter tray [removed word] adapter for the second camera and lens in the widefield rig. This has a female M42 Pentax/Praktica thread one end and a male T2 thread on the other. This will be instead of using a filter wheel. So my filter wheel will be used with scope only and this will be an ED80 with FR/FF so the spacing will be 55mm less 13.5 for the camera and whatever adapters I need to use. I already have the 48mm to T2 adapter which takes up about 5mm as I recall. I think that's about 36.5mm if my arithmetic is right.

[Gina]

Dratted swear filter The removed word is a latin one which means with, plus or stroke - spelt see-you-em.

[Lonestar70]

Hi Gina,

I had a look on the internet and found some details of focal length for the ASAHI PENTAX LENSES.

This is specified as 45.5mm Flange focus distance... This being the distance required between the rear flange of the lens and the front face of the camera sensor, and the lenses use an M42 x 1mm attachment thread.

The ATIK site states 13.1mm from front of camera flange to sensor with T2 thread (M42 x 0.75mm)

So, with these figures in mind I drew up a possible filter wheel housing using 3mm thick sideplates and has 20.4mm of available inside width... this should be more than enough to fit the carousel and drive mechanism even with the 8mm thick filters.

Auto Filter wheel_1.pdf

The lens side adaptor should allow you to screw your lens in directly without need of any other adaptor.

The Camera adaptor is a direct attachment for the ATIK.

Both are custom adaptors which could easily be made.

What do you think?

Best Regards.

[Gina]

Yes, that was pretty much what I had in mind for the Pentax lenses before I bought the filter tray adapter. I might yet do something like that. The camera attachment you show is much like the SX adapter I've got and the lens adapter could be a standard M42 to EOS ring attached to the side wall by it's flange and bolts into tapped holes in the side plate.

[Gina]

Been looking at this some more in view of recent developments. I've bought a second Evostar 80 ED DS Pro scope plus FR/FF so I will have two identical telescopes (except that I changed the SW focuser on my present one to a SteelTrack). So I have a pair of APOs and can use any filter on either - mix and match. So my filter wheel wants to match the OAG plus EFW2 in optical length. Furthermore the FW will be attaching to the 48mm male thread of the FR/FF. This gives plenty of space between the side plates.

The current corousel now seems unsuitable for a couple of reasons - it's too thin to take an inverted timing belt effectively and there isn't room around the edge of the wheel to take photoelectric indexing as the filter holes are too near the edge.

I'm pretty much sold on the idea of an inverted timing belt around the carousel to act as a large timing pulley and use an Arduino controlled stepper motor and count pulses to determine the stopping positions and the identitity of the filter. So I will need a "home" index detector only. I did reconsider the ide of direct centre drive with stepper motor but thought the impulse drive might strip the gears in the internal gearbox - I doubt they're designed to take the sort of dynamic load the wheel and filters would impose. Timing belt and pulleys will absorb the pulsing motion of the stepper motor quite well.

I'm now thinking of making a new carousel of thicker material - probably about the same thickness as the width of the timing belt viz 6.3mm or thicker. At the same time I can decide whether I want to use those Ha and SII 36mm filters or stick to 1.25"

[Gina]

The problem with using an inside out timing belt as a pulley is that it's virtually impossible to get a continuous belt that fits the wheel tightly. A belt with a join could be used but unless the size works out to a whole number of teeth it can't be used as a timing pulley where the rotation is more than the free space between pulley and belt. Another possibility would be to drive the inverted belt directly with the motor pulley - like a spur gear. In this case the drive would not be continuous but would reverse instead, leaving one part of the 360 degrees for the gap. The home position could now be a physical stop - maybe a peg operating a micro switch. To allow for inaccuracies in the mechanics, the stepper motor could be spring loaded.

[Gina]

I've been looking on ebay for suitable materials for a new carousel but there's nothing suitable but OTOH I do have an acrylic disc of 100mm diameter and 10mm thick which I think would be suitable. I've been working out details. At most I think I wiil want 4 filters such as LRGB and 4 could be accommodated quite easily. Furthermore, with that thickness I could accommodate either 1.25" or 36mm in any position. I can cut the stepped holes with a trepanning tool in a pillar drill.

I've drawn a rough model in ShetchUp. I've shown only two holes with the full detail to take both sizes of filter. Also I've shown a cut inverted timing belt (couldn't do the teeth ) but I think it could be retained by taking it down into the notch/hole and peg it. I plan to drive this directly with a timing pulley on a stepper motor. I'll add more details later.

[Gina]

Here's a drawing...

[Gina]

A bit more added...

[Gina]

I think I need to do some calculations to see how long my wheel will take to change filters. I'm using the ubiquitous 28BYJ-48 5v DC step motor - as readily available from ebay. I use these for everything They have a 64:1 reduction gearbox giving 4096 steps per revolution. Max step frequency is 500pps. Thus a revolution would take around 8s at full speed.

My current design uses a 16 tooth pulley on the stepper and about 120 teeth around the circumference of the carousel. So at full speed the pinion moves at 2 teeth per second. So the wheel would take 60s per revoultion - one whole minute And that's at full speed. Hmmm... 15s to change filters with a four hole wheel. I suppose that's just about acceptable considering we're talking of hours of exposure. OTOH 15s only applies to moving from one filter to its neighbour. In the worst case we would be turning 3 filter positions or 45s.

Taking the motionco pulleys as a sample (and other suppliers seem to have have the same pulleys) an alternative would be a 48 tooth aluminium pulley giving three times the speed. This has a 5mm bore which matches the stepper motor shaft. I'll take another look at the mechanical design.

[Gina]

Here's a rough drawing showing the bigger motor pulley.

[Gina]

I'm none too enamoured with that idea Have had another. That 48 tooth pulley would fit in the middle of the original carousel and I could open out the centre hole to fit the hub. Then there would be a 1:3 reduction if I used the 16 tooth pulley on the motor shaft. Thus at full speed a revolution would take 24s. I could run the stepper at half speed and have 48s per revolution. This seems far more suitable. Furthermore there wouldn't be the restriction of a cut belt/pulley and the wheel could work in the usual way of always going in one direction - not that that should make any difference as it's all catered for in the sketch.

[Gina]

Here's the engineering drawing roughly to scale.

[Gina]

This filter wheel is to be used with the FR/FF on the ED80 so wants to connect to the M48 male thread on the camera/eyepiece side of the reducer. Now I could use the TS metal M48 to T2 adapter and the wide flanged SX female T2 adapter, bolted onto the side plate. OTOH I have an M48 to EOS adapter which comes apart to give a ring with an M48 female thread in it and I'm thinking of having a 48mm hole in the side plate to take the reducer male thread. Then the ring could be used to hold it all together. The thread on the reducer is about 6.5mm long so with a side plate thickness of 3mm there's 3.5mm of thread holding things together. That's a lot better than the 2mm of thread on the TS M48 to T2 adapter.

[Gina]

Next thing to work out is the optical spacing. Reducer to camera sensor wants to be 55mm. 13.5mm of that is taken by the 314L+ leaving 41.5. SX adapter is 7mm camera face to inside - 34.5mm. Taking a mm off to allow for a filter gives 33.5mm. Filter wheel side plate is 3mm giving an inside spacing of 30.5mm. I wonder if I've got that right. The drawing above has an internal spacing of 20mm.

If I were to use the TX M48 to T2 adapter plus SX T2 to flange adapter I would have about 9mm for the TX and 7mm for the SX to the inner face. So 16mm instead of 3 giving an internal spacing of 17.5 which would not accommodate the timing pulley. So that's out anyway.

[Gina]

Here's the drawing...

[fwm891]

If you mount both rotors at the same time you will effectively reduce the 36mm filters to 1.25"