Pancake focus / filter wheel for 10" SCT

[DIYASTRO]

So many threads being started regarding filter wheels and focus devices I thought I would post a design that I have been working on. I mentioned in another thread some days ago that I had some of the components made and when sufficient to assemble I would post up the idea.

The intention of this design is to incorporate with a pancake style assembly the problems of electronic focus and filter wheel assembly as a single construction. It has had its headaches!

Although far from finished the pics should give an idea of what I am trying to develop. The real problem is keeping the 'pancake' small in depth so that the sct will revolve through the forks in dec motion without catching the camera on the raised section of the RA which hold the forks.

I will have to post more pics to this thread in a few mins, my pc is struggling, poor thing!

Boyd

[DIYASTRO]

A few more pics, just a couple more to come!

[DIYASTRO]

Last few pics. Will try and take pics of the 'pancake' in situ on the 10" SCT tommorrow.

As I said, still a long way to go and will give some explanations in the morning, of to my pit!

Boyd

[Reggie]

Very nice work Boyd, you should be proud, it really puts the scrap bit of balsa wood I've just improvised as a pancake focuser attachment to shame although I'm not sure I understand the pancake part, mine is just a large flat bit of balsa that I hope I can get tight enough to help my sct out, only supposed to last the evening though.

[Gina]

Two filter wheels in one housing - interesting Nice job

[DIYASTRO]

Thanks Reggie!

Just taken pic of pancack in position on SCT.

Hopefully this will show the focus drive gears in mesh and the principle of what I call 'Pancake' design.

The objective of course is to keep the depth of the unit as narrow as possible. I can perhaps squeeze a few more mm from the current depth by turning down the belt drive wheels / pulleys by 3/4mm thus saving another 6mm or so. At present the depth of the unit is about 25mm.

You will see from the previous pics that the camera plate will be secured by 'stand off' supports and eventually, a collor around the edge to seal from stray light.

One advantage of this type of design is the space created by the back of the SCT ota. the central 'hub' which has the screw thread for attaching the crayford or visual adaptor leaves a space which has the advantage of being able to place all three motors (two steppers with gear boxes for the wheels and a 12v dc / gearbox for the focus) behind the back plate which enables the depth of the unit to be reduced.

Anyway, hope this latest pic clears up some queries.

Regards

Boyd

[DIYASTRO]

Gina,

Yes, since the 'space' was available and rather than have massive diameter for a single filter wheel I thought that the simplist solution was two wheels.

I need 8 filters, I mentioned to you in answers within another thread that there are advantages to having one filter position blank. well, one of those advantages is that it enables two filter wheels to be used at the same time. One filter wheel, in this instance the lower one, will have the std RGB filters the top one will support the H alpha beta Sii etc. I also have some V band filters and various others that need to be used singly so the blank position is important.

I am not sure how much longer this will take to finish! I have been working on it on and off for months!

Oh, by the way, I noticed in your thread that you like me are using small limit swiches. You will see a micro one in one or two of the pics I have here. They came from PC mouse, there are usually two available along with other usefull bits, including the cable which I use for the Webcam Yes Yes mod!

The limit switch that you can see in the pics here is to 'count' the revolutions of the focus drive gear via the little collor on the shaft with a 10ba round head screw which

does the actual 'depresion' of the micro limit switch.

Boyd

[Gina]

Neat idea I think it must be unique - I haven't come across another dual filter wheel in my research. Yes, I noticed the microswitch and little screw in the collar

[Gina]

Taking on board your mouse microswitch idea I have taken two old rolling ball type mice apart and yes, tiny microswitches. Being change over type ie. SPDT I realised that this type of switch is what I really want for my focus control. In fact the whole control circuit can go in a mouse. Wonder if I can arrange the scroll wheel to turn a pot and use it as speed control

I'll report results in the appropriate threads

[DIYASTRO]

Done a little more work to the unit.

Hopefully you can see the idea a little more clearly with the two plates together and supporting the DSLR . The trick is to allow the camera to pass through the forks without snagging on the fork base.

You will see from the pics that thankfully it does!

I have reduced the thickness of the toothed pulleys to 8mm which is just large enough for the 7mm belt width.

I now need to do the collor for the outer rim to keep it light tight.

Then the electronics to drive the two wheels and the focus motor drive which will be PWM.

Hope to have some of that done in the next couple of weeks and will post up when I have something to take a pic of!

[steelfixer]

Boyd First things first I am well impressed with the design and the engineering involved.

My question - How are you going to drive the stepper motors. I ask because I am going to build a super EQ mount that will be driven by Nema 23 stepper motors.

My problem is going to be driving the motors.

Electronics is way outside my field so home built stuff is out of the question for me.

[DIYASTRO]

I use stepper motor drivers from www.electrozoneuk.com/

They are very helpfull to people who arnt quite sure what they need. They make very nice proggrammable boards, so simple to use and very well constructed.

Hope this usefull.

Boyd

[DIYASTRO]

Fogot to mention, if you are looking for pc control for steppers I use boards from www.pc-control.co.uk. There are lots of other suppliers of this type of board, but I can understand how confusing it may be if you are not sure what you need in terms of power requirement etc. Do try emailing electrozone, cant remember the guys name but he has been very helpfull to me in the past.

Boyd

[steelfixer]

Thanks for the info.

You seem to know about this stuff so can I ask you this.

The synscan on my EQ5 uses stepper motors.

When I build my EQ mount will it be possible to use the Synscan box of tricks to drive my Nemas insted of the toy town motors it already drives.

I understand there is going to be a jump in the voltage needed to drive the Nemas but can this be got round with electronics.

[Reggie]

Steelfixer, It's not so much the electronics that would be an issue, it's the gearing and motor steps. Your synscan box will be looking for specific gear ratios otherwise it just won't track properly, so you'll want those to match too.

It might be possible to use the synscan board but really we'd need to see a hi-res image of it (both sides) to see what's going on. There are a couple of threads on here that look at modding HEQ5 pro gears to a belt and pulleys system, that would probably be a good place to ask stuff as well.

[steelfixer]

Many thanks Reggie.

I do not want to hijack this brilliant thread so I have started another one.

Appolagies to Diyastro, cant wait for your report on how this works.

Graham

[DIYASTRO]

Just done a trial bit to see how it works out.

this is the reason why I wanted to leave a blank position in both of the 5 station filter wheels.

The rear of assembly now showing a 'filter draw / slide' unit to take 50mm sq. filters. Of course, it will also allow the option of other slides which may have 2" or 1.25" filters, these will not be parafocal but gives options for 'non std' filters (ie v band etc) to be implimented without need to mess with the wheels once set up.

I have had some thoughts on the electronics and hope to have some pics of circuits up on the thread next weekend.

Boyd

[Reggie]

Looking good as usual boyd, I particularly like the slide option, I guess that effectively means you can buy random square filters of appropriate size as well as the standard 1.25"/2" fare?

Looking forward to seeing the circuit that will be driving it too

Regards,

Reggie.

[DIYASTRO]

Thanks Reggie, sorry for delay in reply.

Have some options which std. filter wheel configs dont offer. one of the reasons for DIY!

Although my ideas for electronics for this may be as antiquated as I am I believe it will all work.

based on the old cmos circuits, counters, dividers, etc. and the old and trusted PWM 555 timer circuits for dc modulated drive i have some hopes.

Kind Regards

Boyd

[DIYASTRO]

I have been trying out various circuits to test my filter wheels and focus drive and as promised some pics of how things are so far. Note that the boards are just trials and I will make some neater ones for the actual control panel... eventually!

You will see from the pics of filter wheel that I have notched out the edge of the wheel to take the limit switch actuator and signal the new position when driven, to the led readout board.

This is the board with rectangular leds... red for one wheel and green for the other. The circuit for this is a simple Cmos 4017 1 of 10 decoder.

By using one of the outputs to to reset the Cmos device it basically counts the 5 filter positions reseting the leds to 0 (blank filter every complete revolution and gives indication of which filter position is in optical train.

The pic with the three circuit boards are the two stepper motor drivers for the wheels (Electrozone programable and I like them!)

The other is the PWM for the dc motor which does the focus gears.

The last circuit is the counter for the drive spindle of the focus motor drive. Eventually I will need a clutch for the gears on this but also wanted to have some form of visable indication as to how much forward / back focus is being used. The problem being that the focus on sct is around 42 turns of the focus knob for moving the mirror back and forth and the limit switch on the drive spindle will allow me to calculate just how far the mirror has moved.

The filters in the wheel are just simple red yellow and blue ones for testing purposes.

Regards

Boyd

[Gina]

Very interesting I too am going to use micro switches and a counter to detect the filter number and stopping position on mine.

As for the focussing motor, I'm thinking of arranging a position indicator for mine. Used in earnest I have found some deficiencies. The slowest speed is too fast, the fastest is too slow and I find it difficult without a position indicator to guess the middle setting from the two limits of when I can detect noticeable out-of-focus. I'll have to check which motor/gearbox I used for the focussing motor but with one of them I added a reed switch and a tiny magnet in the first gear in the train.

Have you thought about interfacing to computer and control your wheels and focussing via ASCOM? I'm thinking that ideally I would like to control everything from APT in time.

[DIYASTRO]

Gina,

After much trial and error I found that the micro limit switch idea to be the simplist and my use of cmos circuits although bulky and not particulaly very trendy is simply my limited experience with 'modern' components.

the advantage of cmos is the supply voltage 3-15volts, so dirrect connections to the usual supplies that we all use for our scope drives.

I have used reed switches for many things over the years and in some applications ie slow switching, they are very usefull. Will be fine in your application for example. They can present problems when faster detection is reqiured giving spurious signals occasionally.

I have thought about using ascom, and have a couple of pc stepper motor boards and a dc motor drive board so that in the future i have that option. For now, I think I would like a 'seperate' control panel. I like to have complete individual units for each opperation, for example I use a small netbook for guiding and a seperate pc for telescope control, and then another pc for the imageing side of things, yes, a lot of extra gear but each system independant of each other. I think I spent too much time watching the Gerry Anderson productions as a child!

You wont have as much time as I dont with all the other things to do, but I hope you get some work done on your filter wheel, look forward to seeing your progress on your thread. I find it quite stimulating to see other peoples ideas and progress on various threads on SGL it forces me to get on with mine so that we all continue to share these thoughts with each other!

Regards

Boyd

[Gina]

Thank you Boyd And I think the same way as you - I too love to see other examples of astro DIY And I agree, seeing others give you the encouragement to progress your own. I like the simpler ways of doing things like CMOS and agree about the supply voltage. Quite insensitive to interference too.

[DIYASTRO]

Not a great deal done this weekend, non astro stuff making demands!

Have managed to hard wire the bits and pieces ( limit switches, stepper motors, DC motor outputs etc.

Hopefully you will see from the couple of pics that I have made a channel for the cables to run around the edge of the pancake so as to avoid and foul up with moving parts!

The cables that are required (25 in all, will perhaps eventually be connected to a 25 pin D skt) you will see protruding from the junction box on the left side. All connections have 1mm pitch plugs and skt's so easy to take apart.

I have also moved the Red, Blue, Yellow and Green filters to the lower filter wheel.

Hope to be able to make a start on perm. connections to the various circuits very soon, I have 'plugged' it all in and every thing seems to be working, which is a wonder, In my haste I usually cross wire something!

Regards

Boyd

[DIYASTRO]

Have been quite busy this weekend housing the electronic boards within a console case.

You will see from pics that I have decided to leave the wire loom intact, and in fact extended it with further plugs and skts until I decide what type of single / multi plug / skt to use.

I will try and discover how to post a video on this thread when I get time so interested people can see the focus motor and filter wheels in motion and the corresponding led readout to show what filters are in optical train.

One further thing I need to work on is the digital readout for the focus. Intention here is to give a reading of movement of the mirror and to avoid maybe fitting a clutch. At the moment each count or one rev of the drive gear is equal to 0.18mm.

Hope to have some more completed next week and will post up results.

Kind regards

Boyd