kbrown

Like I mentioned before the fourth isn't used at all. I just put it in just in case as I wasn't sure how the magnets and sensors would play together so close to each other.

Another idea I had in my mind was to 3D print a 9 x 1.25" filter carousel that would fit in the same housing. That would require four sensors. I haven't done this yet though. Don't even know if it would be physically possible

Yes the sensors are aligned so that the magnets on the wheel travel right underneath them. One sensor detects one magnet at a time so basically I encode the slots with the presence or absence of the magnets as seen here:

Notice the variation of the magnet placements on each slot. This is what I tried to explain in my first reply with the truth table. Hope this helps?

This photo shows the sensors soldered on the PCB (Click on the arrow icon on the top right to get to the right post). They're the four transistor like devices in the middle with bent legs to get them close to the surface of the wheel housing.

This photo shows the additional bracket I made to hold the screws on top of the sensors.

2 minutes ago, msacco said:

Well, I don't. But it seems like for a reason there is high shipping fees for them, like 10-20-50 would cost me $5 for shipping alone, so that ends up at around the same price.
On ebay it is a bit cheaper, but still not cheap enough for me to pick it, e.g 25pcs for $3.88 including shipping. I guess I'll just take the higher amount in that case.

And thanks, I did notice now, last before I purchase that, is there anything I should check in terms of scales in case my filter wheel is different from yours and it might not fit? Can you tell from the pictures?

And I have 1 more question about the hall sensor if you could explain more about it, do you use a single hall sensor? Where did you put it in your build? And what is the logic used behind it? I understand what is the purpose and how it works, but I'm wondering about what you did exactly according to the position(e.g stop when reaching that position, move x steps from that position, etc, etc).
If you could just write an extremely simple pseudo code for the logic of it I think it would be very easy for me to understand

Thanks!

Your wheel looks exactly the same I have as far as I can see.

No there's four sensors (U2, U3, U4 and U5 in the schematic. Have another look in my old thread you linked in the first post. There's tons of photos showing how I put everything together. Do you have a CNC mill/router? Not sure if I would have attempted this without mine...

The magnets you found look correct but do you really want 200 of them? Cheap though so why not...

Hope you noticed the edits I made to my previous reply?

As for the Allegro A3144  hall effect sensors you could get them from flea bay (eg item no. 274508408913). The magnets I used were 3mm diameter 4mm thick neodymium magnets. I think I got those from flea bay too. I used two component epoxy glue to mount them on the wheel. Just need to glue them one at a time per filter slot. Otherwise the magnets may dislodge from their intended position while the glue is still curing...

Ran into some trouble resurrecting my old project files. I run the development branch of KiCad these days (kicad-nightly) and it doesn't seem to like my 3+ year old project files. I managed to get the schematic displaying correctly but getting the PCB working would require a bit more work. I've attached a pdf copy of the schematic so at least you can see which components I used. I've also attached an .svg file of the PCB which I used to etch the PCB with.

If you want to make your own version you can skip R1 and R2 if you like. This was an attempt to implement a software based low voltage monitor that didn't really work as trying to run with a flat/weak battery introduces a lot of unpredictable issues.

EDIT: Just checked the Arduino code and I did actually use the R1 / R2 voltage divider to detect whether the Arduino is running from external power as it doesn't make sense to try to run the motor if it's only powered from the USB connector. However they are not on the PCB as it was an afterthought. I just soldered the resistors directly on the arduino pins.

You can skip  GPIO_A and GPIO_B if you wish. I just added these connectors for debugging purposes. When I was developing this I had a small serial LCD display connected on the GPIO_B which was a handy thing to display various things going on in the Arduino. GPIO_A I never really used for anything...

Happy to explain more if this is a bit confusing... 

KiCad_test.pdf KiCad_test-B.Cu_F.Silk_B.Mask.svg

1 minute ago, msacco said:

Thanks for the reply! That's more great information, but what I'm wondering about is how to actually design it correctly? Is there any formula I can go by to know what magnets I need, calculate the distances and other similar crucial information?
Or is it simply trial and error until I just get it right?

Trial and error in my case. Although in my case the first sensors and magnets I bought worked eventually after I found the configuration described above. I'll do some digging tonight to see if I can find my project files and more details for you...

I'll see if I can find my old KiCad files for the electronics. Have to do some digging around...

For me the 28BYJ-48 has been enough as is. I designed the mechanics so that I can rotate the motor around one of the mounting screws that allows me to adjust how tight the rubber O-ring is pressing against the edge of the filter wheel. I haven't had any issues with the motor turning the wheel. There is (was) a sort of a spring clip inside the housing that "locks" the wheel into the slots. I removed it completely so the wheel is held in position entirely by the motor and the motor doesn't have to do any extra work to get the wheel moving again. Again this has worked well. The slot is held in position even when the motor isn't powered.

The hall sensors are a bit like transistor switches that turn on when a magnetic field is present. They are hooked up to the Arduino digital input pins which I read in as an integer value. I have four sensors in my design although only three are used as three bits is enough to encode up to 7 slots. The truth table for the filter slots is straight forward:

• 000 = Not in a slot
• 001 = First filter slot
• 010 = Second filter slot
• 011 = Third filter slot
• 100 = Fourth filter slot
• 101 = Fifth filter slot

There was a couple of physical/mechanical issues with these though. First I couldn't get them to read the magnets from the required distance. This was fixed by placing ferrous material (screws in my case) behind the sensors. They had the effect of redirecting (aiming) the magnetic flows from the magnets towards them (=through the sensors). The second challenge was to deal with the interfering magnetic flow from the adjacent magnet as they were pretty close to each other. This was fixed by having every other magnet facing north side up and the rest of them south side up. The sensors on the PCB had to reflect this polarity as well. With these changes I haven't had any problems reading the magnets reliably. In the arduino code and driver I implemented programmable offsets for each filter so they would always stop at the right position regardless which direction the wheel was rotating.

Hope that helps...

2 hours ago, tooth_dr said:

Keep us updated on the outcome 👍🏻

I sure will. Another thing that might tip this idea over is the exposure time. With such bright objects they'll probably be in the milliseconds range. Not sure how the guiding algorithms will like that as I typically guide with about 2s exposures on DSOs.

I like your camera lens and filter adapter 3d prints btw! Is that filter not colliding with the back of any lenses?

2 minutes ago, tooth_dr said:

I had thought about this using type of setup up for tracking but thought it wouldn’t work.  Calibration will require a fairly hefty calibration step for sure. 

That's probably where it will fail first. Hoping to work around it or cheat it somehow.

Took this test image of the sun through my bedroom window. The bright white dot is the sun. The dark rings are probably light leaks and reflections between the solar film and sensor. This is with the above lens set to its widest 1.4mm.

This one is with the same lens set to its "tele" end @ 3.1mm.

Haven't tried guiding yet but by the looks of these images I think PHD2 should be able to guide. How useful this will be, I don't know. I calculated that with this lens I'll get about 250 - 550" / pixel so massively under-sampled as expected. I'm only hoping it to be enough to keep the target (moon or sun) in the frame without having to do manual corrections. I'm most likely going to use this with my ZWO ASI183MC Pro on a 500mm refractor which gives me about 1.51° x 1.01° FOV natively and about 21.09' x 14.09' FOV with my Daystar H-Alpha Chromosphere that has a 4.3x barlow built in.

All I need is some time and clear skies to test

For higher load threading I just use metal thread inserts like in this case for the tripod adapter thread and the M3 thread for the nylon screw.

5 minutes ago, tooth_dr said:

Those look nice!  I’ve been dipping my toe in 3D printing and have been surprised how nicely threads etc can come up. 

Yeah. That took me by surprise a couple of years ago too. Takes a bit of experimentation on the tolerances as it's (at least in my case) too easy to print too tight fits.

Today I designed and printed a 2" filter adapter for this particular CCTV lens and a 2" filter housing to have a piece of baader solar white light film in. The adapter accepts any 2" filter so I can experiment with lots of things.

Nice. This has been on my to-do list as well. I think I even have all the components already too. Just haven't done it yet. Did you calibrate yours somehow?

What started as a pondering about how could I keep the object better in the centre of the frame when doing lunar or solar imaging ended up realising as these two prints. They allow me to attach 1.25" things on either a camera tripod or a SW finder shoe holder.

My idea is to try to make the moon and/or sun appear as a star for PHD2 so I figured I'd just use a really wide CCTV lens on my QHY5L-II and try to guide with that.

For solar guiding I will print another adapter for holding a piece of baader solar film in front of the lens.

Haven't had a chance to test any of this yet. It'll be a fun experiment if nothing else 🙂

Just a quick update on this. The ronchi screen arrived. Mounted in a 35mm slide holder. The quality of it looks good but I was a little disappointed the grating was only about half inch square in the middle, not the whole film.

2 hours ago, skybadger said:

This further simplifies if the source is small enough to not need a slit.

Am I correct you still need a slit in front of the sensor?

On 09/10/2020 at 20:14, John78 said:

This place appears to have them in stock at sensible prices and despite the 1998 looking website it does have a COVID19 notice up so would appear to be in buisness still

http://www.awrtech.co.uk/awr_cat.htm

I contacted Alan at the above website. He said he's got them in stock so I ordered one from him. He accepts paypal. Should arrive in the post soon. Fingers crossed

I've never done a Focault knife edge or Ronchi grating test but I'd like to learn how to do both in the view of maybe one day grinding my own mirror(s). I've read a lot about the subject online and in books and I've now decided to make a tester so I can start experimenting with my existing 10" F/4.7 skywatcher mirror. However there's a few basic questions I'd like some clarifications on:

1. Some sources say the light source should be cut by the knife edge or go through the ronchi grating and then reflect back from the mirror. But I swear I've seen designs that have the light source to the side or above or below knife/grating. What's the deal here?

2. If the light source is not cut by the knife edge or by the grating, should it be behind a narrow slit, like I've also seen in some cases?

3. Some designs use laser diode without the collimating lens as a point light source. Surely this doesn't need to be cut by the knife edge or the grating or even be behind a slit? I am aware that this is not suitable for visual testing especially if the diode is actually lasing...

Just tried printing on an OHP acetate with rather poor results. Think I'll just buy one to save me from the headaches of a sub-par screen...

17 minutes ago, Gina said:

Goodness me, you telescope builders are very clever!!  I would have liked to build my own but never got further than looking at it and joining the M-O-M group and learning a bit about grinding mirrors.  I realised though that it's a very long-winded job and wondered if I'd have the patience let alone the skill.  I very much admire you all.

This is all just a day dream for me at the moment. I first have to arrange a place to do anything like this in i.e. reinstate a garage in our back garden which in itself will be a bit of a project and probably not going to happen until spring/summer 2021 earliest. Anyway. I'm not in any sort of rush with this as I can happily continue using what I already have and do a bit of learning and planning on this as a slow back burner... Got a lot of useful feedback from you guys already so thank you for that

3 hours ago, John78 said:

600dpi is 42um dots, so if you aim to print the lines in the 600dpi direction, perpendicular to the way the paper feeds, and print screens in 42, 84, 126, 168um lines you should get a usable result.

The way 2400dpi is achieved wont work for printing 10.5um lines, its not really 2400dpi.

Another factor is the "resolution" of the film itself and how well the toner sticks to it. I thought I'd just give it a go and see what happens. Ordered some 100 micron film sheets. Couldn't find any better for a reasonable price.