My new EQ mount for wide fields

[bojan]

Recently, in Melbourne Bintel shop, on the specials pile, I stumbled on interesting found: equatorial mount, EQ3 or so.
I did not think much of it.. and then, couple of days later it occurred to me that this may be an excellent addition to my gear, for wide field shots.
I already build something for that purpose, but because of relatively high PE, I was using EQ6 for this job, a gross overkill.
So I gave Bintel a call and they told me they sold that one.. but there was another one available for me if I wanted it.
Apparently, they obtained those mounts very cheaply, and since mounts were absolutely horribly assembled, they were modifying them (new grease etc) and then selling them functional. But I could have one out of the box, unmodified, for $50 !!! With tripod and all!
Of course I bite in! :-)
Well, after coming home, and dismantling it, I found the following problems:

- Grease used was too thick.
- RA worm did not move
- Also, RA worm appears to be from different mount (? different colour.. and somehow it did not fit to worm gear, 2.mm off )
- The cast was damaged at one place (probably due to fall on the floor) causing grinding between moving parts
- DEC worm moved but very hard (because of thick grease) and there was a lot of backlash

All in all, renovator's dream

DEc axis was easy to fix - just new grease (Lithium based green stuff) and a bit of tightening of retaining rings inside did the trick.

RA axis proved to be a bit harder nut to crack.
After cleaning and reassembly, I found out that the RA upper bearing (actually, this was just a hole in the worm gear (made of aluminium)) was not co-axial with the worm itself. Also, the diameter was too big for the shaft (plated steel pipe, not machined at all), so there was a lot of play here (obviously, manufacturer tried to solve those problems with thick grease).
My way of solving this problem was to insert a brass collar with tighter tolerances, as a new internal top bearing and then to machine this on lathe, to have everything coaxial and tighter (0.05mm tol.)
The worm cage needed to be machined a bit (I took off 2mm, to improve meshing with gear).
There was also some play in bearings (again, just holes in aluminium cage.. but I left it as it is for now. If there will be problems, I will put in ball bearings).
After RA shaft assembly with additional plastic spacers (to mesh the worm gear properly - I cut them out of foil for printing) and after addition of plastic cushion under the tightening screw which serves as a clutch, the mount behaved as it should.
I also had to deburr the groove tips of worm a bit, it seems they were damaged during the first assembly in the factory.

Next, the drive unit. In one of the boxes in the shed where I keep various mechanical parts, I found a stepper motor with gearbox (2048 steps per rev), and timing belt pulleys, just for the purpose. I already had a PCB for processor and stepper motor driver, the only thing remained to do was to program the processor with suitable firmware (gear ratio) and drill couple of holes in a piece of aluminium to mount the motor next to RA worm.

Also, I used smaller counterweight, since this mount is not capable of carrying much more over 1-2kg, this was pretty obvious when the mount was in pieces.. it is mostly Aluminium cast, and very light.
But, for $50.. an absolute bargain !!!

The whole project took couple of days of work...
Now, I only need a cloudless night to try it

[h0ughy (David)]

wow you have the knack for tinkering - excelent work and i look forward to the results

[bojan]

Thank you :-)
Yep, I am also looking forward to see how it performs..
So far, it moves.. and at apparently right rate :-)

[[1ponders] (Paul)]

but at least it came with a bubble level for the tripod.

I take my hat off to you guys that can multitask in different disciplines; engineering, electronics, programming etc AND know what you are doing.

[bojan]

Well, it works..
The elevation is not perfect (did not bother to do drift alignment.. tomorrow..)
And I think I managed to capture Nova Centauri on test images.
Here it is, stack 4x1min, Canon FD 50mm F1.4 @f4, ISO200.

[scopemankit (Chris)]

A true ATM! Fossick, find, adapt and use.

[bojan]

The time has come for a bit deeper follow-up on performance of this cheap mount..
Last night I did some imaging with 200mm lens and Canon 400D.
The series of 20 45-sec shots every minute was used for the analisys below..

Here it is:

PE (p-p) was found to be 132 arcsec.
The fastest change was about 79" in one minute, pretty bad.

The pixel sizes for various lens FL:

200mm = 5.88"
50mm = 23.51"

From the above, the conclusion is: this mount is good for 50mm lens and exposure times up to 1 minute, if we want 90% of the images in series to be good (that means, elongation of star image size smaller or equal to pixel size).
However, due to the shape of the PE curve, 4-5 minutes exposures are feasible, with perhaps 50% of them un-usable.. Still pretty much OK.

For 200m lens (with pixel size 5.88") it is not so good - max exposure will be 30" ) for 90% good frames), but this may be quite enough for some (serious) purposes (photometry, spectroscopy etc).

So, all in all, not too bad for the price ($50 plus a bit of tinkering). Especially because the mount itself is very light weight (11kg, with lens, camera and counterweight).

But, definitely not for the perfectionists