Remote focussing for MN190

[Gina]

Having received a used SkyWatcher MN190 last Friday, I have been looking at setting it up for use for imaging DSOs. Being an earlier MN190 this currently has a single speed focuser. I intend to replace this with a Moonlite CR2 dual speed focuser when they become available. (Currently out of stock at FLO and the only other supplier I've found is much more expensive.) Meanwhile, I'm designing and building a temporary remote focussing setup with a view to changing it to suit the Moonlite when I get it.

As I intend using AstroTortilla to point the scope in the right direction, I don't think I shall be using the finder scope and plan to use the finder scope mounting for attaching the electric focussing unit. For this temporary unit I'm using a DC motor/gearbox. This is a 12v DC motor (reversible) with 4096:1 reduction gearbox (with plastic frame and gears and steel axles, costing a tenner from Maplins). I have yet to add a timing pulley and belt to drive one of the focussing knobs - on order from MotionCo. To control it I will use a variable voltage to control speed and a DPDT switch to control direction. More of this later.

I will replace the DC motor/greabox with a stepper motor controlled by an Arduino when I get a dual speed focuser as the reduction provided gives a suitable focussing speed with just a timing pulley and belt drive.

Three photos :-

1. Focuser and finder bracket
   
2. Wood shaped to fit bracket
   
3. DC motor/gearbox screwed to wood
   

[fwm891]

Hi Gina - why change the focuser, mine is an earlier one and I hang a great deal of imaging kit on it with no problem. I also have a stepper motor to remote focus via a uno and the sgl sketch.

The good thing about the older focuser is that you can collimate and centre the focuser over the secondary with little problem - I know others have had problems centring the moonlites on an mn190 because there are no base plate slots

[Gina]

Hi Gina - why change the focuser, mine is an earlier one and I hang a great deal of imaging kit on it with no problem.

I've found the focuser has a lot of alignment wobbliness (if that's a word ) unless the draw tube clamping knob is screwed in tight. However, I shall probably try the present focuser and see how it goes.

I also have a stepper motor to remote focus via a uno and the sgl sketch.

Is yours a single speed focuser? Do you have any form of reduction drive? With the steppers I have the minimum step size using half phase drive is too big for a single speed focuser. With the Baader SteelTrack on my ED80, using the fine focus knob with a 10:1 reduction plus a 2:1 pulley reduction on the belt drive, I find 5 half phase steps about right for one focussing step. With the coarse knob that would equate to half a step. Maybe you have a stepper with smaller steps.

The good thing about the older focuser is that you can collimate and centre the focuser over the secondary with little problem - I know others have had problems centring the moonlites on an mn190 because there are no base plate slots

Yes, I noticed that. However, the collimating instructions for the 190 tell you to use the centre adjusting screw under the secondary cap to adjust the secondary up and down the tube.

[fwm891]

Gina - I'll give a longer explanation tomorrow re the 'wobbliness' of the focus tube.

The manual does rely on the central and outer screws to position the secondary, but used in combination with the slots in the base plate you can keep the extension of the focusing tube down to a minimum and maximise the returned light cone from the primary captured by the secondary mirror.

I'm using a 400 step motor with (I think) a 10:1 reduction belt drive direct to the single speed focuser. (I think there may be some photos with my DIY OAG thread). I find I'm using 10 step pulses as a minimum adjustment size anyway so the final pulley/belt reduction could be 5:1 or less.

shows how much gear I'm loading the focuser with (hadn't done the remote focus at that stage)

[fwm891]

Gina - some images of the focuser assembly showing the belt mod+stepper motor...

Short description of mod:

One original focusing knob has been removed and replaced with a brass shaft that is secured at the focuser with the same grub screw as the original knob. The other end of the shaft sits in a small ball race

Which itself sits in the ali mounting plate. The knurled brass knob has a machine screw running through it, this goes into the new shaft and the larger pulley is held in place by this machine screw. The purpose of this is to act as a simple clutch, loosening the brass knob as this allows me to make large movements with the opposite original knob without turning the motor.

Looking at the pulleys I think they are 5:1 reduction not 10:1 as I thought.

Don't know if this helps?

Also modified a Moonlite for an SGL member...

[planetdnb]

nice mod!!

[Gina]

Thank you Francis

[Gina]

More progress on the DC motor/gearbox - received pulley and belt today and fitted them. Also adjusted the gearbox fixings to tension the belt.

Tried the motor on variable bench PSU and 12v will do nicely for a fast focussing and 5v runs it slowly enough for fine focussing I think, though can't be quite sure until I get it looking at stars. But as I have 5v and 12v on hand in the warm room I can just use a switch to choose voltage rather than needing an LM317T or suchlike.

[Gina]

I have also been investigating using Arduino control and stepper motor as I'm using for the ED80. If I find the present focuser adequate and I can make an extension shaft I can remove one of the knobs like Francis and use a 100 tooth 64mm diameter timing pulley. Then with a 20 tooth pulley on the stepper I can get 5:1 reduction ratio with the pulleys as compared with about 5:2 on the ED80 gaining me a 2x improvement as required.

If I can get away with not spending tons of pounds on a Moonlite focuser I shall have more funds for something else - like a little lathe

[robin-m]

I don't think you need to worry about the reduction too much. On my homedade focus driver I have an 18T primary and 48T secondary giving slightly less than 3:1 reduction. With the stepper running in half step mode I get 0.0022mm (0.0009") per step (measured with a dial guage). One thing I would suggest is a microswitch set to trigger at your home position. This gives an absolute reference that can be used for repeatability.

[Gina]

I think I've worked out how to use the Arduino controller but I think I'll do initial testing with the DC system as I won't be getting the parts for the stepper drive until next week and may get an hour or two of clear night sky before then (hopefully).

I've been testing the OAG with QHY5 too and I think I may have got that sorted out. Had the scope and attachments on the living room table pointing out of the window and using trees for focus tests. I was having trouble with the QHY5 focus earlier - turned out it needed to go further in than it was able to, to get focus. I've solved the problem by inserting 11mm of T2 extension tubes between the OAG and filter wheel. This is the setup I've been using with my ED80 plus SW 0.85x FR/FF but without the 48mm to T2 adapter.

Here are photos and images from yesterday, using a tree at the bottom of the field to focus on about 100 yards away, in poor visibility. I'm hoping the visibility will improve enough today to try on trees around 5 or 6 miles away, which will be closer to star distance focus. It's already much better than yesterday when visibility was little more than half a mile. Using manual focussing, I was able to roughly focus the scope on some twigs, imaging with the Atik 314L+ and Artemis Capture. Then I tried the QHY5 with GCVideo on the OAG and eventually got an image of branches which I was then able to focus roughly by sliding the QHY5 gradually further out As can be seen in the photo, the scope focuser is about half out and guider at about 8mm out, so I think there's a good chance of the focussing being alright for stars etc.

1. Imaging setup
   
2. Tree used for tests
   
3. 314L+ image improved a bit in PS
   
4. Screenshot of a single frame of the AVI video capture by GCVideo from the QHY5 guide camera
   

[Chris]

I'm glad your new toy arrived safe and sound Gina, OAG as well! And I see it didn't take long before the DIY started

I really wish you all the best for first light:) I think it will kick bottom on those smaller targets!

Chris

[Gina]

Thank you Chris

[Gina]

I've got the scope out on the mount now and pointing towards the hill to the east with it's trees, a couple of houses and road signs. The weather is showery but fortunately the wind has gone round to the south and I can have the east wall flap open with the roof closed and no rain is coming in. The eastward view from the obsy is actually slightly north of east and the rain is well clearing the opening

I've been focussing on the trees etc. on the far hill and the focuser draw tube is now almost right in so I think I shall need to reduce the spacers between OAG and FW. Unfortunately the 8mm and 3mm extension tubes seem "cold welded" together I may have to order another 8mm spacer (or 5mm). We are having clear spells between the showers so I might be able to get onto a star in the east this evening.

I've found that the focussing movement is noticeably greater with this scope than with the ED80 - of course it's twice the FL compared with ED80 + FR/FF. I'd forgotten that aspect - means the Arduino focussing system should be quite fine enough without needing a dual speed focuser. Only problem is that the current focuser is having trouble lifting the weight of the imaging rig without slipping and I've tightened the knurled know to increase the Crayford pressure. The focussing is now quite tight and the small stepper motor I've got might struggle - have to see, might need a bigger one.

[Gina]

[Gina]

DC remote focussing is now working fine from the warm room. I've been unscrewing the QHY5 to fine focus the the guide cam Now having found where focus is I can adjust the tube in/out and try again. Then repeat until decent focus is obtained with the QHY screwed in finger tight.

[Gina]

Typically, since it's got dark we have had no more gaps in the clouds Half an hour would be enough to find a star and focus on it And now the clear sky that was forecast for tomorrow night seems to have evaporated into partly cloudy And no more clear forecast for 10 days at least Hmmmm... the scope shows nice views of trees, houses and road signs I got a great image of the buds on the twigs

[fwm891]

It's coming together Gina

[Gina]

Thank you Francis - yes, coming along quite nicely

[Zakalwe]

I love your ingenuity Gina! Great idea on the motor focuser.

Unfortunately the 8mm and 3mm extension tubes seem "cold welded" together I may have to order another 8mm spacer (or 5mm). We are having clear spells between the showers so I might be able to get onto a star in the east this evening.

I feel your pain. I hate these threads with a passion.

My new camera now has a nosepiece stuck solid. I spent 20 minutes with veins bulging in my forehead trying to unscrew another T-thread from my filter wheel. Failed miserably. Then I had to give up on trying to seperate two other extension tubes...even after heating them on the gas cooker. blumming things are the work of Satan. :mad:

[Gina]

I love your ingenuity Gina! Great idea on the motor focuser.

Thank you

I feel your pain. I hate these threads with a passion.

My new camera now has a nosepiece stuck solid. I spent 20 minutes with veins bulging in my forehead trying to unscrew another T-thread from my filter wheel. Failed miserably. Then I had to give up on trying to seperate two other extension tubes...even after heating them on the gas cooker. blumming things are the work of Satan. :mad:

Yes, these fine threeads are a pain! I'm using Olly's boot polish idea on all new one's but those were put together before Olly posted his advice

I'm still hopeful of some clear sky tonight to test it on the stars - this hobby sure needs a lot of patience!!!

[opticalpath]

Hi Gina. I have a Moonlite CR2 fitted to my MN190 and offer a couple of thoughts:

I use a 400-step motor and a 1:3 belt reduction on to the main focuser shaft (not the 1:10 reduction shaft) and I find that full steps give me the resoulution I need. I've tried going to half-stepping but it just becomes harder to judge the sweet spot. Possibly if using auto-focusing software, a finer resolution would be advantageous, but if doing it manually whilst monitoring FWHM, full stepping works for me. 400-step motors are not so easy to find but if you can half-step a 200-step motor, I think you'd find that about right with a 1:3 or 1:4 belt reduction on the main focuser shaft. It's generally recommended to apply drive to the main shaft rather than to the reducing shaft since the reducing drive can slip under load.

I would reiterate Francis's point about the lack of adjustment on the Moonlite base plate; the nature of the base plate fitting does not allow for any lateral adjustment (no equivalent of the slotted holes in the base of the SW focuser). I found that when I replaced the SW focuser with the Moonlite, the new focuser axis pointed slightly below the centre of the secondary. Not being able to shuffle the focuser up a bit, I chose to lower the secondary by loosening the central attachment screw and screwing in the tilt adjustment screws further. WARNING: if you do this, beware of running out of threads on the central attachment screw - it's shorter than the tilt adjustment screws and will run out of threads before they do ..... releasing the whole secondary assembly with potentially dire consequences! I ended up replacing the central screw with a longer one to be safe. WARNING2: I had a heck of a job re-collimating the scope after moving the secondary down - I mean REAL problems! This is a lot harder than a Newtonian to collimate, I discovered. I ended up moving the secondary back up a bit and compromising on a not-quite-centred position that works reasonably well.

I swapped to a Moonlite because I have a heavy imaging train (QSI 583wsg) and the SW focuser was struggling to hold it square. The main culprit is that horrible telescopic draw-tube extension that is a very loose fit in the main draw-tube and depends on only one clamp screw to secure it. If you can find a way (as Francis did) to eliminate that weak component, and if your imaging kit is a moderate load, I would really suggest seeing if you can live with it. The Moonlite is a lovely bit of engineering, but the collimation issues it gave me were a nightmare. To be honest, if I'd known what was in store, I would have persisted with the SW focuser and tried to fix the weak points. If your scope is in good collimation, my advice is to change as little as possible!

Adrian

[Gina]

Thank you very much for that Adrian I will persevere with the SW focuser. If I eventually treat myself to a small lathe I might look at re-engineering the SW focuser. A few things spring to mind including tighter tolerances and maybe relpacing the Crayford system with a rack and pinion. R&P don't slip! Another thought would be to replace the 2" tube and clamp system with a screw connection.

This afternoon I have had the scope pointing south at a far hill 5 or 6 miles away. The visibility is very good after the rain and I have had a nice sharp image of tree branches on the top of the hill. I have found that if the draw tube is horizontal the focussing works alright. My present DC motor remote focuser is working well - if anything rather on the fine side. I have ordered a 100 tooth timing pulley and 5mm to 4mm reducing sleeve that will allow me to attach it to the focuser shaft. Then with the 20 tooth small pulley that fits my stepper motor I will have a 5:1 reduction ratio. I'm pretty certain this will give fine enough focussing.

The draw tube is about 4 or 5 mm from fully in so I think I should be able to focus at infinity - maybe the skies will play ball tonight Current weather is sunny with about 50% cloud cover. I also managed to focus the QHY5 on other trees at a reasonable position so I'm hopeful. Guide cam focussing is much less critical with this setup with 1000mm FL than it was with the 400 mm FL ST80 guide scope.

[fwm891]

Gina - from memory (not a good thing!) the only things I did with the focuser to improve the transport in/out was to :

1, roughen the contact faces on the draw tube and roller (approx #360 wet-n-dry dry).

2, there are two screws to the draw tube, a lock screw and the tension screw. The tension screw I left as is, the lock screw I seem to remember I flattened the end of the screw then made a small PTFE cap to fit over the end. Although it meant I couldn't lock the drawtube it did allow me to add so extra tension which was enough to remove any slop from the assembly when the 383L + OAG and FW were mounted.

To find out I would need to dismantle the focuser assembly and I don't really want to do that ATM.

To help with reducing counterweights and help with the focuser loading I keep the focuser axis in line with the dec axis. For me it means the load is always to the side never underneath the OTA.

[Gina]

That's useful info - thank you Francis

I have been seeing how good I could get the focus. Using binoculars I found a tower on the far hill and have focused on that - I could hardly see it with the naked eye. Didn't even know it was there Must look on Google and see if I can find it. Meanwhile, here's the image.