View Full Version here: : An 8" on a portable home built Fork Mount
07-12-2011, 01:15 PM
Thought I'd post some pics of an 8" F4 scope on a wooden fork mount that I've been working on for a while now.
I love DIY so have used most of what I could build or re-use myself.
This is still a work in progress but I've just completed the drive, working on the electronics and autoguiding bits, hope to start taking some images soon.
It will have optical encoders and drives on both axes, autoguiding with an ST80 or a finder guider, lx200 emulated picaxe based stepper controller using the DRV8824 microstepping driver. It will be wireless as well.
I struggled a bit for the clutch, but I found a very good solution for the clutch plate - hard disk platters. They're aluminium but have a very smooth coating, and I used two pressure plates. hard disk platters on one side of the timing pulley, and an ikea cutting board on the other. this is fixed onto the shaft and the only bit that rotates free is the timing pulley. pressure is adjusted by the three bolts mounted on T nuts. works superbly.
for the gearbox, I wanted to stay away from worms, so went with a 250:1 gearbox from mclennan. $50 on rs-online. but the input gear pinion had a 2mm bore and my stepper motor a 5mm shaft.
so I opened up a toy motor which had a 2mm shaft, removed the coil, cut an opening in the side, fixed a small timing pulley to the shaft, and used a belt for the stepper. its almost 1:1 so not much reduction there. but overall reduction is around 1085:1. backlash is pretty low, and it has more than sufficient output torque for the load of the fork, ota, dslr, counterweights, st80.
the output timing pulley sits on a 6mm steel shaft on two 6mm ball bearings in the wooden housing.
the fork fits onto a 50mm shaft, delrin at the moment but I'll be replacing it with a hollow steel rod as it flexes too much. I made the wedge out of 2x18mm ply glued and screwed on so its pretty strong. still has a bit of flexure, so I might replace it with aluminium. but does the job for now.
I routed 4 holes on the base of the wedge so I can adjust for polar alignment. I will be adding a tangential screw drive for fine adjustment.
The pier is a straightforward design but is foldable and light and very rigid. it easily takes my weight (100kg!!) and I've added adjustable antivibration feet. all the pieces are joined with glue, screw and 20mm dowels drilled through. so very rigid.
my main objective was to make a portable fork mount, so i've kept things as light as possible. I'll be redoing the fork with a truss design using carbon fibre as I'll be adapting it to a 12" F4 that I'm starting soon.
I've been refining it a fair bit and with autoguiding and redesigning a few parts to minimize flexure, am confident I should get some basic imaging out of it.
The worthwhile part is that I can say I built the whole mount and electronics from the ground up, it may be far from the standards of modern rigs, but I'm not aiming for that. Just something I've enjoyed building and i've learnt a lot along the way.
comments and criticisms welcome.
Alistair your ingenuity is amazinglooks awesome - well done!looking forward to seeing the imaging resultshow do you ensure dew doesn't get in to the ply and eventaully swell it...?niko
07-12-2011, 01:35 PM
Thanks Niko. I've applied clear marine grade varnish for the pier so it stays waterproof. Since its only at nights and the temp varies between 5 and 25C, the varnish should keep it protected from dew similar to a lot of the Big wooden Dobs.
I'll be making the fork out of carbon fibre trusses, so its still rough.
07-12-2011, 02:32 PM
A few pics of the OTA cradle. Niko, this was driven by your comment in snake valley last year. All good, will explain when we meet.!
I needed a quick release mechanism to adjust the ota or balance it, so I ended up using suitcase latches on the two halves of the cradle.
Just Cut off the sides so it stays slim, centering of the two pieces is taken care of by the ota circle as the tolerance is very small.
I drilled a 25mm hole 3/4th the way on either side for the delrin shaft and a screw locks the shaft to the cradle. that way, the motor will drive the delrin shaft and ota.
the delrin shaft rotates on a 25mm teflon coated bushing thats inserted into the fork arms. that way I save weight on traditional bearings and housings and kept friction low even with load.
to keep the cradle centred, I made two shaft collars and used a hard disk platter as a bearing surface. so the cradle stays centred at all positions. as long as the fork arm flex is low, the ota axis should be aligned to the RA axis.
07-12-2011, 02:43 PM
Fantastic, I bought a couple of pillow blocks to look at making and equitorial mount myself. I was going to use Aluminium though being lighter but ply could be a good alternative. Good thought in using marine ply too for water resistance.
07-12-2011, 06:10 PM
It's good to see your setup taking shape and I like your idea of keeping it light. This should help to overcome some of the torque problems I,m having with my drive.
I see you have the same output gear box that I'm using, at least it looks the same. As you have seen in my previous posts, I'm using direct friction drive onto the R.A wheel from this gear box, it's struggling with the weight! even though it's balanced. So as a consiquence, the geear box builds up torsion which gives me a 40 arc second wobble approximately every 2 minutes.
I'm just using a reostat to control it, but it's easy to over correct. As I said before, I wish I had your skills with electronics so I could use a stepper motor with computer control.
Still there's plenty I can do with a clean two minute track.
Here's a couple of images. 200mm f/6 prime focus.
Orion showing the error.
Tarantula a 2minute track, desregard bad focus or the slight wobble when taking a closer look, this was wind blowing the scope around.
07-12-2011, 07:38 PM
Nice and solid work, Alistair :thumbsup:
My only concern would be a very probable periodic error from gear box.. I would go with friction reduction or, even better, worm gear on the RA shaft (and stepper coupled to worm directly).
However the proof will be in the pudding...
Looks great Alistair.
I always liked fork mounts. I look forward to hearing how it performs under the stars.
07-12-2011, 10:03 PM
Top job Alistair. Real craftmanship. Looks great.
07-12-2011, 11:40 PM
I will be testing it shortly, unguided to start with and will post results.
Justin, I'm more than happy to help with your motor drive if you're interested.
Do you know what your total reduction is and what the reduction on just the gearbox is? also what is the speed of your motor.
there are a couple of options for your drive.
you could use a dc motor with a higher ratio gearbox than what you have (higher torque) and control its speed with a pwm motor controller, readily available for under $20. they usually come with a rheostat.
you could also use a DC motor as a closed loop system, that is an encoder on the motor shaft, and get constant speed, bit more complicated but doable. I do have spares lying around pulled from printers.
else you could use a stepper with a gearbox and I could send you one of my stepper driver boards with a picaxe controller.
The one I'm testing right now, I've programmed two buttons. if you press one button, it adds 10 microseconds between each pulse or step, so slows it down for each press.
other button reduces 10 microseconds, and thus speeds it up in small increments. it can even display the current value.
manual guiding if you like, just till i get the QHY5.
you'd get much more control this way.
challenge will be to get a 1.8 degree stepper with a matching gearhead with sufficient reduction as most nema steppers with gearheads are very expensive, that's why I went with the mclennan gearboxes and my kludge drive.
With the 1.8 deg stepper and with 1/32 microstepping, I use a frequency of 87.3Hz to achieve sidereal rate with my gear ratios. (1178:1)
PM me if you'd like to explore and I can help with the electronics.
the other thing would be to use shorter focal length mirrors, I tried imaging with an F6, and moved to an F4 and noticed huge reduction in exposure time required, and added benefit of a smaller OTA. downside is you need a coma corrector, but well worth it. F4 will reduce your flexure problems and decrease torque required.
Bojan, I agree, there might be significant PE, but unlike a worm wheel, since this is a 3 stage reduction gearbox with very small pitch gears, would PE be noticeable as the periodicity of the PE in the first stage gear would be very small, and might not even be reflected in the output as its a 250:1.
I'm waiting to get a guidecam, that would give me a much better indication.
will carry out some tests and see.
08-12-2011, 10:14 AM
Hard disc platters must be one of the most useful things ever thrown away.
Alistair, I'm flattered to think that anything that's come out of my mouth has inspired anything - particularly at Snake Valley :lol:
Hey Justin - just a question on the tarantula. When I used to use my film camera I got that lovely green colour, now I use digital it's all red. Is your camera modded to get that great colour?
08-12-2011, 02:09 PM
Hi, Alistair and Niko.
Niko the camera I'm using is a new Canon 1100D, Have'nt changed any settings, that's the colour from the camera. Don't know enough to say why there would be a difference. Perhaps the ccd chip is more sensitive in the green?
Some one on the forum should know.
Alistair, your help would be greatly appreciated. I'll look up the specs on my motor, I should be able to get them from Crouzet, the makers.
I take your suggestions on board, and will design a new mount and OTA from the ground up.
The observatory scope has been through so many changes, it's identity is rather confused. It started life as a portable truss dob and has been adapted ever since.
( See images)
08-12-2011, 02:46 PM
I tried a quick imaging trial last night just to see how it behaved.
this is a 20s unprocessed single frame with a 1000d.
very rough polar alignment, eyeballed collimation, manual focus, no coma corrector (F4).
I'm happy with the response of the drive train to speed change commands, but I need to increase the clock speed of my mcu to 64Mhz, as the current minimum increment for pulsewidth changes is too big at 32Mhz clock.
I found that the overall stiffness has improved a lot with this pier design, only flex is in my RA shaft and the fork arms, so will be redesigning them shortly along with a steel hollow shaft.
clutch works well, gearbox is also pretty responsive. there is a bit of backlash that shows when the motor is driven slow, but once engaged, it doesn't drift.
the good thing with the clutch is that I can keep tracking on and move the scope to another target manually and leave it, it continues tracking.
But its very sensitive to balancing so it took me a while to get my head around how the weights affect balance. good thing with forks is that the counterweight requirement is very small. hardly 1 to 2 kgs just to counter the camera.
Can I measure PE with a guidecam on a guidescope? How do you'll get the PE graphs with deviation in RA and DEC?
I also have to learn to drift align.
Can someone tell me what the orientation of the dslr body should be with respect to the ota? perpendicular to the optical axis, parallel, 45 deg?
reason is I don't understand what the star drift direction should be across the screen in live view with tracking off. once I understand that, I can learn to drift align.
Next bits are dec drive, encoder on both axes, st4 interface, lx200 goto. I hope to have them done soon, else I'm going to have to live in the garage after all the cutting and drilling that my wife's been patiently monitoring and enduring!!
08-12-2011, 04:15 PM
go to k3CCD (http://www.pk3.org/Astro/) and download lite version of K3CCD tools (it is free, however requires a key).
Software is here:
Key is here:
This will give you the ability to precisely measure PE. Of course, longer the FL, the better. Also, this software can be used for guiding (you may need it, considering your gearbox).
Orientation of the camera is not critical, but it should be such that the drift (no tracking) produces trails parallel to X or Y axis of the frame.
08-12-2011, 06:11 PM
will give it a go.
08-12-2011, 06:41 PM
Software for PE analysis can be found here:
08-12-2011, 08:54 PM
BTW, could you let me know where did you find those clamps?
I was looking for something like this for quite a while.... I would like to use it like on bw image of my OTA.
08-12-2011, 08:59 PM
Luggage case clamps should ba available from your local hardware stoe eg; Mitre ten.
They are uasually made by zenith products and come in several different styles.
08-12-2011, 09:03 PM
Hi Justin, thank you for this info.
The clamps I was using (on bw pic) were from military radio equipment, used to hold the top cover in place - VERY tight. The whole OTA was held in place with two of them..
Hopefully Bunnings may have them?
08-12-2011, 09:17 PM
I doubt the quality of the clips will be the same as shown on your image, but they worked well on my two piece planetary scope.
08-12-2011, 09:33 PM
I bought them from Mitre10 and Bunnings. there are a few types but this one suited the best.
10-12-2011, 04:23 PM
Where abouts are you getting flex in your fork?
I've just emailed plans of mine, to you.
Hope they are of use.
10-12-2011, 04:33 PM
Thanks for the plans Justin.
I've just bought 18mm ply.
While balancing, i noticed that without any load, it had a tendency to swing more on one section than the other. not a sideways bias, but a sectional bias.
on measurement, i found that the central 50mm hole is actually off centre by about 2mm. hence the bias and errors in tracking.
not too difficult to fix.
so I'm redoing the fork, this time with a different approach, but maintaining focus on stiffness and lightness.
will email some plans through.
If I get the chance, I'm going to try out some truss designs as well similar to these. a lot more difficult but should be a lot lighter and stiffer if i get the truss calculations and joints correct.
07-03-2012, 09:26 PM
Finally got the ST4 autoguiding working with Phd, an St80 guide scope and a QHY5.
A control picaxe monitors the state of the 4 ST4 pins from the QHY5 optocoupler outputs and sends appropriate RA corrections to the picaxe controlling the stepper via I2C.
Since the picaxes run at 64Mhz, corrections are instant.
I added LED's as well so I can visually see when guide pulses are sent.
Each correction pulse changes the tracking speed by 0.5x for the duration of the pulse to reduce or increase the tracking speed.
I was only able to test it for a minute before the clouds rolled in but the Phd RA graph was within one division of the centre line and I got perfectly round stars. So its looking promising.
I mounted the ST80 on the opposite side of the QHY8 and focuser, so it acts like a counterweight.
Just waiting for the skies to clear and will try it again.
08-03-2012, 03:43 PM
Its a great looking setup.
You will have to get your cabling organised next. :D
Bring on the clear skies.
08-03-2012, 04:51 PM
Yes, I have 6 cables coming off the scope and have to route them properly along the fork arms so they don't snag.
Skies aren't looking too good this month. weatherzone shows a high probability of rain almost the whole of this month.
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