Pancake focus / filter wheel for 10" SCT

[DIYASTRO]

I have been doing some prelim. trials with the focus drive, which is controled by the pulse width modulation circuit.

It seems that at its highest speed I will be moving the mirror at a rate of .01636mm / sec. The slowest speed is .0001592mm / sec.

This should be good enough to get an accurate focus. I will set the main mirror close to focus with the gears unmeshed first so that time is not lost using such small movements.

My two filter wheels which are stepper motor / gearbox driven, move at about 1 rev per 40 sec, which equates to around 8 sec. per filter, I have option to increase or decrease this speed so for the moment I will leave that alone.

So far so good, I think I shall try and get the casing finished this week if time allows, need to make the 22mm edge to block light getting in.

Whilst doing that, I will be thinking of new electronics for it, I may after all control this from pc, although the console will be ok when it goes into my obsy! but when that gets built heaven knows!

regards

Boyd

Regards

Boyd

[Gina]

If you use computer control you won't need to build a control box. Maybe a bit of programming though. That is why I have decided to use a computer USB interface rather than a home made control box. The one interface card will control both filter wheel and focussing motor. The Velleman K8055 USB interface card has 2 analog and 5 digital inputs and 2 analog or PWM outputs and 8 digital outputs capable of driving an inductive load of up to 30v and 100mA. Demo/test software is provided which could be used as is for control though a special UI would be advantageous.

[EBU]

Nice job You are doing. It would be really nice to see Your filter wheel in action. Maybe You could store video on YouTube and give us link to it?

[DIYASTRO]

Gina,

Yes, I have had a look at the velleman ( I think I used one of these some years ago, but perhaps it was an earlier model) I am not sure if I will eventually go pc control. We will see. I have to admit that I like seperate hardware to do some functions and my comp. progg. is very poor. although I do have some ideas how to use some existing devices to do some of these functions, if I go down that route I will add it to this thread.

EBU,

Thanks for your kind comments.

I was waiting for some kind soul to give instructions on how to post a video direct to this thread, perhaps thats not possible. Its such a pity, since the whole concept becomes much more apparent when you press a button or two and all the wheels start moving smoothly!

As always, would welcolm any suggestions that would improve the design or its functions.

Regards

Boyd

[Reggie]

Boyd, you should take a look at arduino and pic units if you go for pc control, I can't vouch for pic chips as such but they are well respected in the electronics community, the arduino units are the same, I can definitely vouch for them and programming is not as much of an issue as you think, there are tons of tutorials for controlling motors, there are loads of code libraries for pretty much everything out there they of course do serial comms. and user interfaces (buttons) are pretty simple, there is also a mature community surrounding them.

[DIYASTRO]

Reggie,

I shall have a look at ardinino in more detail when time allows and thanks for your input, always usefull as ever!

I have not had the time this week to attend to much on the filter wheel assembly.

I have however started the gear arrangement for the digital readout for the focus control.

By putting a small gear combination from the drive gearbox to the detecting limit switch I can callibrate the readout to .1mm or indeed .01mm.

Simply using a copy of the focus gears which has a drive gear of 27 teeth and a driven gear of 150 (ratio 5.5555/1) the other gear train which is much smaller in tooth size/pitch, is drive gear 18 teeth and driven gear of 100 teeth (ratio 5.5555/1).

The limit switch is positioned to the teeth of the 100tooth gear to give a .01mm readout or, use a cog with ten teeth attached to the 100 t gear to give .1mm readout.

I will post some pics of this when it is re-attached to the unit, pics always easier to follow!

Regards

Boyd

[Reggie]

Hi Boyd, they're both very valid platforms, pic/arduino, although I think generally the arduino edges it on price and software, especially looking at the velleman kit, pic chips have been a mainstay for hobbyists for quite some time as well, whichever way you choose, there will be lots of code examples and support out there.

[DIYASTRO]

Reggie,

sorry for delay lots of other things going on.

I will look into these when time allows and ponder.

I have assembled the gear parts that I was describing and post some pics to show what is going on.

The ali 'cog' has 10 indents in it to give me a digital readout of .1mm mirror movement.

I am working on a gear that would be optional to give me .01mm movement but it may not be neccesary.

I hope to be able to complete some more to the contraption this weekend if other family members dont have other ideas!

The gear assembly in these pics is the same as the drive to the gears for the mirror focus, its just the size / pitch that is different which have identical ratios.

Kind regards

Boyd