Horwig

The Gurley I'm using (or not at the moment) is the 320K pulse R158 encoder, and it's £882 Huw

If I may add my experience here with trying to add an absolute encoder to my Sitech controlled DIY fork, I bought a Gurley encoder as specd for Sitech (not cheap), and mounted it via a flexible shaft coupling to the RA axis, the body of the encoder was solidly mounted by bracket to the bearing block. I must have some axial offset in the centering of the encoder, it gives differing RA speed, dependant on pointing position, so will have to go back to the drawing board in its implementation. EDIT The encoder is not ABSOLUTE, don't know why I called it that!!! Huw

Best of luck with this project, I've been fighting my equatorial fork mounted 400mm f3.5 Newtonian for more time than I care to remember 😀 Not sure, but I think the Sitech system can handle Alt-Az mounts, as well as guiding and de-rotation, see page 22 of this manual http://pgrasc.org/wp-content/uploads/2014/03/SetupManual1-1-a.pdf Huw

Thanks Dave, it feels as if I've been at this forever. I did have triangular stars when I first built this scope (I called them 'mickey mouse ears' I seem to recall), they were caused by mirror edge clips too tight, then I had astigmatism caused by a safety strap araldited to the centre of the secondary. These also could be caused by pinching of the secondary I suppose, but there are no clips, just three blobs of silicone. Guiding was good, don't see many major spikes there. One problem solved causes another one to shine through, hey ho, the life of an engineer... Huw

It started life as a Beacon Hill mount many years ago, looking like this

First of all, a big THANK YOU to Tony Owen for his input on this rebuild. I'll post the latest news in case it is of benefit to somebody else building a mount. The DEC axis rebuild went slightly out of control, I ended up replacing the RA axis as well! Here's the new RA axis in it's bearing block. Here's the worm and wheel re assembled, note the red engine rebuild lube used to lubricate the worm, very clever stuff, the colour shows how well it distributes itself. And here's the fork re-assembled, note the new bearing block on the left hand tine. The hinges on both worm blocks have been replaced with flexors, as can be seen on the DEC worm block to the left of the mount awaiting re assembly. First results are promising, PHD2 is showing guiding of about 0.5" repeatably, and it's applying DEC backlash of about 175 ms. Last night I had a good PEC record run, and did a quick and dirty 300 second test frame before clouds came in. That's a single luminence frame, just stretched and a little noise reduction added. Guiding looks good, but is it my eyes, or do I have triangular stars? Huw

Just what I need. My electricity bill for the dehumidifier in my dome over three months of winter was over £250. £2 for one of these and £15 for a tube heater sounds like a huge bargain: https://www.screwfix.com/p/dimplex-ecot1ft-wall-mounted-tubular-heater-40w/3951f Anybody want to offer me anything for a used dehumidifier?

well said, now if everybody did the same..., well we all know the outcome... Nice one Huw

Oh, Colclene certainly brings back memories, I was a young BBC VT engineer in the early 80s, it was used to clean tape heads back then, both 2" and 1" VTRs.... But back to the PSU in question, I'm using the exact same one in my obsy, and have been since 2009, it's not the prettiest, but mine still plods on. I'd think it's probably just a buildup of crud on the surface of the trimmer pots, doubt very much if there's any lasting damage there. Tweak the voltage set pot back to roughly where it was and everything should hopefully come good, it not give me a PM, I'll take mine to bits so we can compare notes. Huw

Just a quick update on where we're at with this. The new trunion bearing is in, now at least I have repeatable results for DEC backlash, but it's not that brilliant, about 20 arc seconds or there about, with the gearhead on the motor accounting for something like half of that. After a lot of research I found some Japanese zero backlash gearboxes from KamoSeiko, and found one on ebay to play with, actually calling them gearboxes is wrong, they use ball bearings: Here it is on a Pittman motor compared to the Pittman gearmotor that's in at the moment. I just about have space to accommodate the extra length. New timing pulleys are on order, but there's a 15 working day delay, damn. Anybody out there come across the Kamo ball reducers? This shows potential, but is there anything I should know that would make it unsuitable? Huw

Thanks again Tony for putting up with my ramblings. Things are getting serious. New angular contact bearings are in, they make a difference, but have not solved the problem. I'd been noticing that when I dis-assembled and then rebuilt the drive, backlash would be different, and I couldn't work out what was changing, till I rechecked the worm to wheel centering, it was off, a lot of head scratching later, I hope I've found what's causing it, the slop in the stacked pillow blocks is so great that the wheel can twist against the worm, so before I get anything more done I'm going to have to replace the DEC trunion bearings with a pair of tapered rollers. Interestingly, with things as they stand, roughly half the backlash comes from the worm, the rest from the gearbox/belt drive, and most of that I suspect is probably from the gearbox. Huw

That is one of the cleanest ASC images I think I've ever seen, very well done.

I'm slowly getting a workflow together for jobs to be done. The bearings for the worm linked to above were larger diameter than the present ones, so would need a new worm block to house them, so I've found some angular contact bearings that match the size of the present ones: https://simplybearings.co.uk/shop/Bearings-Angular-Contact-Only/c3_4592/index.html If the worm block shows any sign of flex, I will the go to larger bearings and fabricate a new stiffer block. I've some left over 4 inch ali tube section with 3/8 wall thickness which should make a housing for the tapered bearings, and you are right Tony, I should address the single pillow block on the far side, I'll probably just replace it with the pair I'm taking off the driven trunion, should be better than what's there now. Huw

An update, the preload was not as tight as I'd thought, tightened it, backlash is now closer to 16 arc seconds, getting there I think. I am still going to change the worm block bearings to angular contact, these are what I've been looking at : https://www.bearingboys.co.uk/SKF--FAG/7201BEP-SKF-Single-Row-Angular-Contact-Ball-Bearing-22415-p Also, the pillow block pack on the driven trunion is going to be replaced with a pair of taper roller bearings, these look possible: https://www.bearingboys.co.uk/Imperial-Taper-Roller-Bearings/HM-801346-X2310QVQ523-SKF-Imperial-Taper-Roller-Bearing-82351-p Any thoughts from anybody? Huw

Thanks for taking the time to give such full answers both, lots to consider. Firstly, the mount is stripped back, the OTA removed, and the fork removed from the polar axis for ease of measurement. So all tests are done off load. Initial results this morning. The worm bearings are plain ball, but preloaded. Measurement just now shows end float of about 20 microns when changing directions on the motor, on a 21 cm dia. wheel I think that equates to about 40 seconds of arc. Could be the principal source of error. Also the pillow blocks show appreciable slop, resulting in about 100 microns displacement at the worm when push/pulling the trunion hard by hand, this also needs addressing, but is only going to give problems under heavy load. Engineers blue was applied to the drive, nice even pattern, no bottoming out exhibited. Bearing research starts now. Much appreciated both of you. Huw

This is really a followup to a thread I started last year: Firstly, a big thank-you to everybody that helped me out then, the mount started behaving for the first time. HOWEVER... I tried to image on a few of night last winter and got pants guiding, so I sulked and left the problem for another day. That other day arrived this week. The mount is stripped out again, and a problem was found. The motor shafts are locked to the timing pulleys with grub screws against flats on the shafts, the DEC grub screw was loose. Will replace with a split spring pin, hopefully problem solved. Trouble is, I can't leave well alone. The new sprung worm blocks work well enough, with PHD2 sorting out the (nice predictable) backlash, but it would be nice to mechanically minimise if at all possible. Upshot is I've gone back to measure DEC backlash, and I can't get it under about 30 arc seconds whatever I do. (I measure it by using a dial gauge on a long metal rod clamped to the DEC axis, Sitech is set at 15 arc seconds per second DEC speed, there's 2 seconds dead time, seems to agree with 30 arc seconds). The DEC worm wheel has been re-checked to be square and centered to the worm. I have two obvious adjustments, the setscrew to set clearance, and the spring plunger to set worm to wheel pressure. I've checked the worm block hinge, it's free to move, and has absolutely minimal play, the backstop setscrew is set at 50 micron clearance, and I've experimented with spring plunger tightness over its full range. Nothing I do seems to change the 30 arc second dead zone, minimising the backstop clearance shows no improvement, upping the spring force to max makes it worse if anything. Grabbing the DEC axis trunion reveals no slop, but I still get the 2 seconds backlash delay. The only change I've done that improves matters was to change from thick lithium grease to a thin aerosol white grease, but the change was minimal. I've tried attaching a rod to the end of the worm as before, there's no dead zone there, it must be just the worm/wheel interface. Could it be that they just don't mesh? HELP! Huw

May I add my twopence worth to this thread. I've built a couple of light boxes for flats, here's the thread on the one I use on my refractor. To measure the evenness of illumination, I used a stills camera in a darkened room, not filling the frame with the lightbox, so as not to get affected by vignetting on the lens. Then stretch the contrast on the image in photoshop, a good visual way of doing it, but not as accurate as Oddsocks method above. Just my opinions?, naturally there are more ways than one of skinning this feline. Huw

Following the above discussion, I was reminded of the mug a friend drinks tea from at work (hope it at least raises a smile): ?

Pentax 67 lenses are the way to go for widefield astro and mono camera because if the long back focus and the sharp optics, especially since we only use the centers, I have a selection that I use with a full frame astro CCD: 50mm f4: 300 mm f4 Huw

Oh come on, why so negative, 20 inch Newtonian maybe, nothing like another challenge. After this obsy nothing will seem impossible? Huw

I got a motor from Germany next working day, what's not to like?

Another vote for OMC (https://www.omc-stepperonline.com). have used them many times, really good firm to deal with. they hold stock in Europe as well, so long delivery times are not a problem. Huw

Having looked at it again, yes indeed, there are FOUR in total

What's wrong with doing a belt reduction in two steps? isn't that what Avalon are doing here? https://astrogarten-shop.de/de/zubehoer/montierungen-zubehoer/avalon/avalon-muno-fast-reverse-montierung-synscan.html#lg=1&slide=1 (OK, they have three stages) H

WOW, what kind of axle dia. have you got for the polar axis, have you seen the range of small bearings on the Technobots site, even the Igus plastic/nylon bearings would probably work for a 5 Kg load: https://www.technobotsonline.com/bearings.html H