Remote focussing for MN190

[Gina]

Some success this evening until the clouds came over again Twice I got a star into the QHY5/PHD frame calibrated on it then guiding worked (sort of) With a very bright star (Alp Boo I think) cal was obtained in 4 W & E and 3 N & S. Later with a dimmer star it was 8 WE and 4 NS. So I think that might work

Big problem getting the focuser to move the imaging rig up hill - as 'twere. I could get focus if I applied some finger pressure on the camera. Clearly the focuser needs work on it. When off focus the star image was a perfectly symmetrical doughnut as far as I could see. I think this means collimation is good but I'm only just learning about collimation.

[Chris]

Yep Gina if your out of focus star consists of a concentric doughnut then I'd say collimation is pretty good:)

So do you think you need a more power full motor then for the focuser?

[Gina]

Yep Gina if your out of focus star consists of a concentric doughnut then I'd say collimation is pretty good:)

Good - that's gratifying - thank you

So do you think you need a more power full motor then for the focuser?

No, I need a focuser that doesn't slip.

[Gina]

The clouds cleared again later and I carried on. Guiding was very poor even though I managed to find a star - just the one! Had a go at M51 in LRGB but the guiding problem stopped it being any good. Also, bad dew tonight - need to get dew shield and heater made.

Managed to locate the tower in the image above. It's about 5 miles away

[opticalpath]

No, I need a focuser that doesn't slip.

Does the tension adjustment screw not help, Gina? (why do we call it that btw? It's really a *pressure* adjustment screw) ..... or does it stall the motor if you tighten it enough to stop the slipping?

Adrian

[Gina]

I have the pressure knob as tight as I can get it with my fingers. Maybe try pliers (carefully). I may have a problem when I get the stepper motor installed, have to see. But the real cure would be to improve the Crayford drive. I'll try roughening like Francis did to his. I can see why many people prefer an R&P drive - properly engineered, of course.

[robin-m]

Going to stepper will help you, the reason I built a stepper focuser was that the SW dc focuser couldn't move the rig 'up the hill', the stepper has no problems there. In addition as a stepper has a holding torque if you leave enable high it will eliminate slippage.

[robin-m]

edit: slippage due to the knobs turning rather than the crayford slipping against the tube....

[Gina]

Unfortunately it's Crayford slipping - the belt is easily driving the focuser knob (I have it quite tight).

[opticalpath]

I wouldn't risk over-tightening the screw, Gina. If it's very tight, it begins to act like a lock that will prevent the draw tube from moving at all. Is it possible that's what's happening? If the drawtube is locked tight, the crayford friction drive will inevitably slip. Is there another screw (or inset screws) that allow you to increase the pressure of the drive spindle without locking the drawtube? (Sorry if this is obvious and you've already tried everything.)

Adrian

[Gina]

Well I did manage to get focus last night and I managed to get a star visible in the OAG/QHY5/PHD screen on which I was able to calibrate easily. However, guiding on it was poor and erratic with quite frequent "NO STAR" messages in PHD. But I did manage to get a few half useable subs of M51. I used CdC to slew into the vicinity of M51 then by using astronomy.net to solve my images, got it in the frame with a faint star in the guider frame as I said. I took a stack of LRGB subs but the L subs must have been too short at 2m as they wouldn't stack in DSS. The RGB used 2x2 binning and did stack. I only managed about 8 or 9 runs of LRGB in 1m then 2m subs before I lost the guiding totally - in all just under an hour I calculate. I posted it in "Imaging - Deep Sky" forum but here's a copy.

[Gina]

I wouldn't risk over-tightening the screw, Gina. If it's very tight, it begins to act like a lock that will prevent the draw tube from moving at all. Is it possible that's what's happening? If the drawtube is locked tight, the crayford friction drive will inevitably slip. Is there another screw (or inset screws) that allow you to increase the pressure of the drive spindle without locking the drawtube? (Sorry if this is obvious and you've already tried everything.)

Adrian

AFAIK the screw I'm talking about does this - there's another screw to lock the drawtube, but I don't know the design details of this focuser. I'm wondering if it's possible to take it apart without removing it from the scope and losing collimation.

[Gina]

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.

Did you need to remove the focuser from the scope tube to get inside it, Francis? I'm reluctant to do this as I think I would loose the collimation setting.

And do you have any details of the internal layout?

[fwm891]

Did you need to remove the focuser from the scope tube to get inside it, Francis? I'm reluctant to do this as I think I would loose the collimation setting.

And do you have any details of the internal layout?

Gina, Again from memory, if you slacken off the lock and tension screws. Then remove one of the focusing knobs the shaft slides out to one side allowing the draw tube to be removed. I can't remember the internal layout too well and its certainly easier to work on off the scope. You can retain your positioning by simply putting masking tape around the tube against the edges of the focuser base plate before you take out the four screws. The retaining nuts are fixed to the inside of the tube so you don't have to worry about them falling off (mine are fixed anyway).

Its the pressure plate arrangement inside I can't picture. I have an Altair Astro 8 inch f4 and I've modified the presure plate on that adding extra ball races so I could tighten the pressure plate very tightly and still have smooth motion. I don't think I did the same to the MN190.

[Gina]

Gina, Again from memory, if you slacken off the lock and tension screws. Then remove one of the focusing knobs the shaft slides out to one side allowing the draw tube to be removed. I can't remember the internal layout too well and its certainly easier to work on off the scope. You can retain your positioning by simply putting masking tape around the tube against the edges of the focuser base plate before you take out the four screws.

Thank you Francis I thought of marking the position with masking tape but wondered it that would be accurate enough. I think it should be as angles will be more important than the distance up/down the tube of the secondary mirror w.r.t. the focuser. Yes, it would certainly be easier to work on the focuser off the scope.

The retaining nuts are fixed to the inside of the tube so you don't have to worry about them falling off (mine are fixed anyway).

I looked at mine and the nuts are fixed on the inside of the tube - welded I think.

Its the pressure plate arrangement inside I can't picture. I have an Altair Astro 8 inch f4 and I've modified the presure plate on that adding extra ball races so I could tighten the pressure plate very tightly and still have smooth motion. I don't think I did the same to the MN190.

I was thinking that one mod would be to add tiny bearings to be able to increase the Crayford pressure without making it stiffer. I think Baader use this arrangement and my Baader SteelTrack on the ED80 is certainly very smooth while being very unlikely to slip.

I take it that the 4mm shaft itself drives the drawtube. If I could move the shaft further out to one side I could mount my 100 tooth timing pulley with just a sleeve bore reducer onto the 4mm focuser shaft. The other idea was to make a new extension shaft with a 4mm hole one end and a 5mm shaft at the other. That would have to wait until I get a lathe

[Gina]

I've been looking into the Arduino control circuit with a view to including dew heater control as well as focussing with one Arduino. Been counting up the digital pins needed and unfortunately there aren't quite enough for a single Nano as I was think of - one short - if I want to have a local display for the dew control (and possible focus count). Digital I/O pins add up like this :-

1. Stepper control 1
   
2. Stepper control 2
   
3. Stepper control 3
   
4. Stepper control 4
   
5. Dew heater PWM
   
6. Ambient temp & humidity DHT22
   
7. 1-wire dew heater temp
   
8. LCD 1
   
9. LCD 2
   
10. LCD 3
    
11. LCD 4
    
12. LCD 5
    
13. LCD 6
    

The Arduinos have 12 digital I/O pins except for the Arduino Mega. So I'm pondering over a number of options :-

1. One Nano and no display - control and display by computer via USB
   
2. Two Nanos - one for focussing and the other for dew control with display
   
3. One Mega which will control everything
   

Nanos have rather a habit of upsetting the USB system and becoming unrecognised plus local control of the dew heater would be both easier and better so I don't like No.1. Options 2. and 3. are both viable. There are pros and cons of both. I'll come to these in a moment when I've looked to see what I've already got.

[Gina]

I have the following available :-

• Arduino Nano x 2
  
• Arduino Uno x 2
  
• Arduino Mega 2560
  
• 20x4 LCD display
  
• SainSmart 16x2 LCD Keypad shield for Arduino Diecimila Duemilanove, UNO, MEGA1280, MEGA2560 board.
  

I have other projects in addition to this one including weather station and CCTV camera selection and direction control but I can always buy more.

The Uno seems to have little advantage over rthe Nano excep I'v never had any problems with these upsetting the USB system.

[Gina]

I'm thinking the Mega with LCD/Keypad shield looks interesting. The keypad has SELECT/LEFT/UP/DOWN/RIGHT/RST buttons with the up above the down button. RST resets the Arduino. I could use SELECT to bring up a menu for setting the degrees above dew point for the heater temperature or displaying Ambient temp, dew point, set temp and actual heater temp. 16x2 chars will be fine for that. I don't think it would be particularly useful but I could also use it as a local focus control by adding that to the menu.

[Gina]

One thing I've been thinking of is that I could use the same control unit on both imaging rigs - viz. the MN190 and the ED80/WF rig. I was originally thinking of having a separate one for the 190. A single one could be attached with Velcro and swapped between scopes - just a few plugs to change over. As such the Mega with LCD/Keypad shield would do everything relatively easily.

Anyone have any thoughts of any reason not to go this route?

Here's a block diagram :-

[Gina]

Come to think of it, I don't need two stepper controls, I could plug in one or the other stepper motor for either the ED80 or the WF - I have only the one camera and filter wheel so will only want focussing control of one or other focuser at a time. I need two dew heater controls on the ED80 rig for ED80 and ST80 scopes (or camera lens dew heater for WF). Selecting parameters that differ between imaging rigs can be done in the menu from the keypad on the shield.

[SnakeyJ]

Gina,

I've finally got the bits and soldering iron out of the cupboard and started my focuser build (Nano, 28BYJ-48 and ULN3003 control board). All is working and turning using the modified sketch that you posted on the previous thread, though I have yet to sort some momentary contact switch/buttons for direct control at the box. Any idea where to look for belts and drive pulleys?

I'm also quite keen to have some sort of clutch in the system, if you have any suggestion - though engineering is not my strong suit and the lathe is currently languishing at my brother in laws. May have to settle for something quick and simple like slipping the belt.

[opticalpath]

Regarding plugging one or other motor to the control box ....... That's what I've just set up, Gina, albeit with a different motor driver. I'm using an Arduino + EasyDriver in a control box with a 4-pin locking plug connector (http://www.maplin.co...iway-connectors) that connects to the stepper cable. I've just put a stepper on a second scope with a similar cable and connector, so I can easily unplug one scope/ stepper and plug in the other when I need to. The only consideration for me is that the motors have to be similar because there is a motor current preset on the EasyDriver and it would be fiddly to have to adjust that if a second stepper had different drive current requirements.

Adrian

[robin-m]

The analog pins on an UNO (14-19) can be used as digital by setting pinmode.

Robin

[Gina]

Gina,

I've finally got the bits and soldering iron out of the cupboard and started my focuser build (Nano, 28BYJ-48 and ULN3003 control board). All is working and turning using the modified sketch that you posted on the previous thread, though I have yet to sort some momentary contact switch/buttons for direct control at the box. Any idea where to look for belts and drive pulleys?

I'm also quite keen to have some sort of clutch in the system, if you have any suggestion - though engineering is not my strong suit and the lathe is currently languishing at my brother in laws. May have to settle for something quick and simple like slipping the belt.

I get my timing pulleys and belts from motionco - the MXL range. I haven't got a clutch myself but the ED80 has a spring tensioned stepper motor and I can turn the focus knob. On the 190 I'm slipping the belt off the focus knob ATM. If/when I get a lathe I may make a clutch similar to Francis.

[Gina]

Regarding plugging one or other motor to the control box ....... That's what I've just set up, Gina, albeit with a different motor driver. I'm using an Arduino + EasyDriver in a control box with a 4-pin locking plug connector (http://www.maplin.co...iway-connectors) that connects to the stepper cable. I've just put a stepper on a second scope with a similar cable and connector, so I can easily unplug one scope/ stepper and plug in the other when I need to. The only consideration for me is that the motors have to be similar because there is a motor current preset on the EasyDriver and it would be fiddly to have to adjust that if a second stepper had different drive current requirements.

Adrian

Ah yes I'm using the same stepper motor model for all three of mine. These have 5 wires and I've ordered some 5 pin mini XLR connectors for them.