Kunming United Optics focuser appalling backlash

stephen2615 (Stephen) · #1 20-03-2021, 12:49 PM

Greetings,

I have a Sky Rover refractor that uses the Kunming United Optics Crawford style focuser. It has appalling backlash and I don't have much luck trying to get my autofocus routine (in NINA) to consistently give me a nice V shape with my Pegasus Focus Cube 2. More often than not, it ends up with a saw tooth pattern. Sometimes the autofocus routine just gives up as it cannot find decent HFR especially when changing filters or HFR has changed due to the scope finding better skies towards meridian flip.

I am wondering that given the backlash that it is so bad that I might be better off getting another quality focuser to help with the autofocus routine. There are various grub screws under the focuser but I cannot find anything that tells me what screw does what so it is very frustrating trying to determine if there might be a simple fix to the backlash or it is just a piece of garbage and I need to look at something better.

The backlash is very obvious as the Focus Cube 2 locks the focuser so it cannot be manually turned. When turning the focuser knob, the focuser moves up to two mm before the Focus Cube stops the movement.

Any ideas or suggestions would be very welcome. TIA.

Stephen

gregbradley · #2 20-03-2021, 01:14 PM

Try posting on Cloudy Nights. Focusers in general, have adjustment screws so it may be improved by tightening up those screws.

Greg.

Robert_T · #3 20-03-2021, 01:58 PM

Hi stephen, is your Focus Cube driving the micro-focus or is it on the coarse focus side? If using the microside it may be fixed by moving to coarse focus side.

I had struggles with the 3.7in focusser on my recent APM LZOS 130/f6 purchase (note to self if shelling out big $ on top optics dont scrimp on the focuser). Bad slippage in the micro-focuser which was a problem for me as I used Sesto Senso 2 focusers that are setup and with adaptors to work on the micro-side only. I tried tweaking the focuser to get this to focus in NINA to no avail. In the end I tried a ZWO EAF on the coarse focus side and problem largely solvered...nice focus curves possible. I do prefer the Sesto as a focuser though so a bit miffed by this.

I think mine may also be a Kumning United Optics focusser that APM use as their standard focuser. Like you I saw there are a number of grub screws etc under the focusser with no indcation or clue as to what does what. I'm constantly staggered by how expensive items (even this kumning focuser is 8-900AUD) in Astronomy which inevitably require adjustment can continue to come out with zero...really zero info and none available online. You may have some luck with Cloudy Nights, but if you do probably someones trial and error.

Good luck. I hope you can get this resolved. Me I'm still thinking wether I should bite the bullet and get a FT3.5.

appiice (Ed) · #4 20-03-2021, 02:41 PM

Hello Stephen

Are you aware that NINA now has the ability to go past the point of backlash, hopefully this will come out right.

You can set NINA to " dial out " backlash so that it always comes to a new point from the same direction, so say you have a backlash equivalent to 15 degrees, NINA will move the focuser more than 15 degrees away from the last point then move to it, so the backlash is taken out in the first part of the move.

The people writing NINA introduced that about 6 months ago, not sure where it is but most likely in the equipment setup.

Hope this helps.

Ed

stephen2615 (Stephen) · #5 20-03-2021, 04:25 PM

Quote:

Originally Posted by gregbradley

Try posting on Cloudy Nights. Focusers in general, have adjustment screws so it may be improved by tightening up those screws.

Greg.

Thanks. The problem is I have no idea what the screws do. I will post on CN and see what happens.

stephen2615 (Stephen) · #6 20-03-2021, 04:32 PM

Quote:

Originally Posted by appiice

Hello Stephen

Are you aware that NINA now has the ability to go past the point of backlash, hopefully this will come out right.

You can set NINA to " dial out " backlash so that it always comes to a new point from the same direction, so say you have a backlash equivalent to 15 degrees, NINA will move the focuser more than 15 degrees away from the last point then move to it, so the backlash is taken out in the first part of the move.

The people writing NINA introduced that about 6 months ago, not sure where it is but most likely in the equipment setup.

Hope this helps.

Ed

Thanks. I have been using the overshoot method with NINA. I measured the focuser backlash with some callipers and got a good estimate of the numerical value to use with NINA. I put in a number times 120 percent so it should be ok. The correct number gives me that sawtooth outcome in NINA autofocus diagram. If I put in a very high number, I can get a V shape but the edges are with HFR's of 10 plus. My current system gives me a HFR of around 4 at max with a generally good outcome of around 2 to 2.5 depending on the camera/filters. My vendor washed his hands of this issue when I thought it was my Sesto Senso V 1 auto focuser but it turned out not to be it. I did manage to keep the Sesto and he refunded me to shut me up.

stephen2615 (Stephen) · #7 20-03-2021, 04:46 PM

Quote:

Originally Posted by Robert_T

Hi stephen, is your Focus Cube driving the micro-focus or is it on the coarse focus side? If using the microside it may be fixed by moving to coarse focus side.

I had struggles with the 3.7in focusser on my recent APM LZOS 130/f6 purchase (note to self if shelling out big $ on top optics dont scrimp on the focuser). Bad slippage in the micro-focuser which was a problem for me as I used Sesto Senso 2 focusers that are setup and with adaptors to work on the micro-side only. I tried tweaking the focuser to get this to focus in NINA to no avail. In the end I tried a ZWO EAF on the coarse focus side and problem largely solvered...nice focus curves possible. I do prefer the Sesto as a focuser though so a bit miffed by this.

I think mine may also be a Kumning United Optics focusser that APM use as their standard focuser. Like you I saw there are a number of grub screws etc under the focusser with no indcation or clue as to what does what. I'm constantly staggered by how expensive items (even this kumning focuser is 8-900AUD) in Astronomy which inevitably require adjustment can continue to come out with zero...really zero info and none available online. You may have some luck with Cloudy Nights, but if you do probably someones trial and error.

Good luck. I hope you can get this resolved. Me I'm still thinking wether I should bite the bullet and get a FT3.5.

Thanks. The Focus Cube 2 only sits on the course side as it has a micro stepper motor that doesn't need to use the micro focus. It is designed to use the course side. I have a Sesto Senso V 1 autofocuser that IMHO was a dumb design as it only used a grub screw to attach to the round shaft of the focuser instead of screwing something into the flat part of the shaft which would not let it turn itself. I asked the vendor for some information about how to maintain the focuser and he said that he didn't know of anything and it was well, tough "poop". We are not vendor/client anymore.

The Pegasus uses one large particular screw to attach to the body so as to stop it spinning and it may actually play an important role with the focuser but I think it's use is to lock the focuser in position. As I don't want to lock it, I am happy to use it to attach the Pegasus to the scope. The other screws are a mystery to me. It sucks that you are not able to get a manual for something like that.

I did experiment a lot on my WO Z61 and the Sesto Senso with one particular grub screw. I got a decent result but the margin of error with tightening this one screw was very very small. It was either too hard to more the focuser or it would just slip. I wasted a day of my life with that so that was when I went the Pegasus way.

The focuser looks identical to what William Optics uses for their bigger scopes. Maybe I should ask WO for some guidance?

Stephen

The_bluester (Paul) · #8 21-03-2021, 02:05 PM

Backlash compensation should not result in the HFR at the ends of an autofocus run being increased. It is to remove mechanical backlash in the focus system itself. Generally with any scope moving a drawtube to focus, all focus moves should be lifting the load. If you need to move focus inward by 50 steps it moves inward by 50 steps, if you need to move outward by 50 steps and had (As an example) 200 steps of backlash compensation, it will move outward by 250 steps then come back in by 200, so the last move was against gravity. The end result is still only a move of 50 steps.

If you have backlash compensation set up right, if you trigger an outward focuser move you should see the focuser move out, then in again. An inwards move should be in only.

The Sesto probably has a backlash compensation setup in the driver and it sounds like Nina does too now, only use one of them, not both.

stephen2615 (Stephen) · #9 21-03-2021, 07:35 PM

Quote:

Originally Posted by The_bluester

Backlash compensation should not result in the HFR at the ends of an autofocus run being increased. It is to remove mechanical backlash in the focus system itself. Generally with any scope moving a drawtube to focus, all focus moves should be lifting the load. If you need to move focus inward by 50 steps it moves inward by 50 steps, if you need to move outward by 50 steps and had (As an example) 200 steps of backlash compensation, it will move outward by 250 steps then come back in by 200, so the last move was against gravity. The end result is still only a move of 50 steps.

If you have backlash compensation set up right, if you trigger an outward focuser move you should see the focuser move out, then in again. An inwards move should be in only.

The Sesto probably has a backlash compensation setup in the driver and it sounds like Nina does too now, only use one of them, not both.

I am not using the Sesto Senso device on my main scope. I am using the Pegasus Focus Cube 2. The point I am trying to make is that I rarely get anything other than a saw tooth result with my autofocus routine. During any movement of the autofocuser routine (with a setting of 550 step Backlash OUT as I measured about 440 step backlash) setting, I see the overshoot working. I recall seeing some discussion about why a saw tooth result was seen but I can't recall why it was that way. It is rather annoying as sometimes the autofocus routine seems to go nowhere and it will give up with the result out of focus.

The_bluester (Paul) · #10 24-03-2021, 08:58 AM

Sorry, misread and conflated the early posts where a Sesto was mentioned!

I am not familiar with NINA's focus routine, but the only time I had sawtooth HR readings happening it was wrong backlash setup or other looseness in the system.

Having just re read your first post, Is the focus cube properly attached to the shaft? being able to move the focuser two millimeters by the other focus knob before the mechanics of the focus cube stops it does not sound at all right. On my refractor you cant move the focuser manually. The only movement of any sort is the small amount of springiness introduced by the flexible coupling where the focus motor attaches to the shaft. If the cube is attached to the 1:1 shaft directly (Assuming it has a 10:1 or similar knob on one side) and you can move the focuser a couple of mm by the other knob there is likely another issue there rather than backlash in the focuser itself.

Have you got any photos of how the cube is attached?

stephen2615 (Stephen) · #11 24-03-2021, 09:09 AM

Thanks for the response.

I am certain the Focus Cube 2 is attached correctly. It is pretty hard to not attach it to the flat part of the spindle. I will recheck it again just to be sure.

The thing that really bothers me is that if I grab the telescope tube itself and then grab the camera, I can move the camera in and out by that couple of mm. It is very visible. This suggests that the focuser backlash is the actual problem.

Karlz (Karl) · #12 24-03-2021, 10:32 AM

Following with much interest, I've got a Pegasus focus cube coming. I'll be running it on a Moonlite

stephen2615 (Stephen) · #13 24-03-2021, 11:00 AM

Important update.

Mounting the Pegasus Focus Cube 2 on my focuser is almost foolproof. The focuser's shaft has a small V shaped groove in it at the course end so it is virtually impossible to install the Focus Cube 2 the wrong way if you screw the grub screw into that groove. Somehow the little grub screw managed to loosen itself just enough so that it wasn't sitting in the groove. I pulled the Pegasus off and had a good look at it and I noticed the grub screw was loose. I am certain that I tightened it during the installation.

Anyway, it is very tight again and there is no huge backlash apparent anymore. I am uncertain if it was my fault with the installation or the grub screw just loosened itself over time with movement. This has been a somewhat recent phenomenon so I might have missed the V groove and the general usage just compounded the looseness. I feel a bit stupid now. I hope this helps someone in the future.

The_bluester (Paul) · #14 24-03-2021, 12:42 PM

I would not feel silly, it is certainly not unknown for things to come loose. My Celestron focus motor just about ate itself internally as a lock nut that I had to part disassemble it to get to came undone.

Regards being able to move the focuser drawtube while the grub screw was loose, that is not abnormal. It would probably just spin the focus shaft until it came up against whatever part of the flat was stopping it.