Auto focuser - how do you feel about it?

[vlaiv]

I think I pretty much made up my mind on this one, but would like to hear any pros and cons from people using these.

I'll probably go for DIY solution since I have two scopes with same focusers (2.5" R&P TS), and that will help me "split" costs - I'll have two steppers - one on each focuser and single control box (probably attached to mount).

I'm not entirely sure what my concern is, I can only see benefits of using ASCOM controlled, motorized focuser - coupled with SGPro. I just want it to be minimal hassle to connect and use.

Actually here are my concerns about this:

1. I'm feeling that I'm already approaching weight limit for "stuff" on focuser side for my RC8" - last imaging session I noticed change in outer field astigmatism as scope moved around the sky and mirror was in different positions. It was very slight effect but it points to one of these things: mirror is coupled to focuser so total weight on focuser is causing slight mirror tilt at different positions, or weight of all the items on focuser creates "sag" somewhere along the way (threaded connection to focuser, quite a bit of spacers, extension tubes, so something might have some flex in it), or hopefully, collimation locking screws are a bit loose (did not check it, but will do first thing), so tightening up those will help. Stepper+bracket can add up to 300-400g on focuser side of things.

2. DIY design that I looked mentions temperature compensated design - I don't get this one, nor understand if I need it at all. My view is that I can set up SGP to do refocus periodically (like every half hour, or one hour) and I don't need to monitor temperature and refocus on ambient temperature change. Or is that better approach?

3. Since I don't want to mod focusers too much - I plan on using belt system coupled to 1:10 knob. Somewhere I read that this is not the best approach due to wear&tear of micro focusing system - but this approach allows me to use full steps (no need for microsteps to achieve needed resolution), and it does not require modification of focuser - I just attach bracket with motor and put belt on. This is related to my next concern

4. Both focusers are rack and pinion, and have lock screw that I usually tighten after finding good focus position. With motor focuser I presume that I need to leave this lock screw unscrewed. Will motor be able to "hold" position and focuser from changing focus / slipping (although it is R&P and should not slip but rather "unwind" under load) if I couple motor to micro focusing knob (I guess that can slip due to design)?

a penny for your thoughts?

[steppenwolf]

1. Can't really help with this one but the stepper motor and gearbox will be quite light, I suspect?

2. You don't need temperature compensation - I have the feature but don't use it as I simply refocus every 60 minutes which 'compensates' for temperature changes!

3. I use the 10:1 shaft on both my auto-focusers because I like the extra resolution - the only issue I have is that over time there is some slippage that you cannot get with the coarse adjustment shaft on my Feathertouch focusers as they are R & P

4. You do need to leave this bolt unscrewed and the motor should hold the load just fine because you can arrange for SG Pro to finish the fine focus adjustment on inwards travel

You will LOVE auto-focus!

[vlaiv]

7 minutes ago, steppenwolf said:

1. Can't really help with this one but the stepper motor and gearbox will be quite light, I suspect?

2. You don't need temperature compensation - I have the feature but don't use it as I simply refocus every 60 minutes which 'compensates' for temperature changes!

3. I use the 10:1 shaft on both my auto-focusers because I like the extra resolution - the only issue I have is that over time there is some slippage that you cannot get with the coarse adjustment shaft on my Feathertouch focusers as they are R & P

4. You do need to leave this bolt unscrewed and the motor should hold the load just fine because you can arrange for SG Pro to finish the fine focus adjustment on inwards travel

You will LOVE auto-focus!

Thanks!

Do you mind if I ask couple more questions since you seem to have similar setup?

Since there is no position encoder, I presume that it will be ok to use position marks on focuser tube? Focusers have mm markings on them, and for instance, on RC in certain setup (I will probably have different setups depending on use of focal reducers and components in imaging train) focus position is at about 20mm - so I can set range of motor focuser to be between 10mm and 30mm (with appropriate number of steps) - so each time I want to use different setup, I position focuser on 10mm (or equivalent "start" position) before powering everything up and tell it that it is at step 0, right?

How does above relate to focuser calibration in SGP? How often do I need to create full V curve? If there is position change (either due to me fiddling around, or mentioned slip / drift) will SGP cope with that (it should since V curve is just approximation it should find right focus position by half flux radius anyway)?

[steppenwolf]

29 minutes ago, vlaiv said:

Since there is no position encoder, I presume that it will be ok to use position marks on focuser tube?

I do not actually use SG Pro, I use MaxIm DL which assumes that I have absolute positioning (i.e. I am using a stepper motor rather than a open loop DC motor which has relative positioning) but I don't have an encoder on my motors. I know that my system (SharpSky) also works with SG Pro so as you are using stepper motors too, you should be able to define your start position for the telescope configuration in use at that session and produce a 'V' curve on the fly. Again, this is what MaxIm DL does and SG Pro as well to the best of my knowledge. It is only FocusMax that retains the values for the 'V' curve run for use in subsequent sessions.

[noah4x4]

This information might assist, or assist a future reader seeking similar information.

I have an MKIT20-WL wireless autofocuser and can recommend it.  If using its direct wireless route you need no local Network (it works like Celestron direct WiFi). The wireless receiver is about the size of a pack of cards (easily velcroed to OTA).  So the added weight at the scope is minimal. It requires 12v, but draws little amperage. 

Its hand controller is larger, about the size of a small paperback book, and it also  requires 12v/2 Amps. That connects to your laptop where for autofocus you need to be running FocusMax. However FocusMax only works with SG Pro or Maxim DL. You can see how the cost might escalate. I also found SG Pro unnecessarily complex for my EAA needs.

But more simply, the MKIT20-WL itself has a learning mode and auto-temperature compensation adjustment. To be frank, I havn't used Focusmax or Temperature Adjustment. I always use Hyperstar so my exposures are extremely short and in the UK temperature adjustment isn't a major challenge unless doing extreme long exposures (which I don't). 

But what I did discover was whilst this wireless set up was great for focuser control when doing remote EAA from my indoor 'mission control' it wasn't ideal when doing visual astronomy whilst stood at the scope with the controller indoors. Eventually I put an Intel NUC at the scope and connected that to another Intel NUC indoors that controlled the former using Windows Remote Desktop.  This might be over 802.11ac wireless (which I found flaky) or cat 6 cable. I could then locate the focuser hand controller at the scope and link that to the NUC at the scope by USB. I could then use its up/down  buttons during  visual astronomy at the scope or use its Microtouch software to control focus remotely from the NUC indoors.  So save some money, as if you have two computers available you don't need the wireless version of this Microfocuser!  The standard version is adequate. 

I now focus using only the simple focus tool in Atik's Infinity Software. This uses FMHW methodology and I consistently get down to a low number below 3. If 'Infinity' is then set to reject stacks with focus under 5 it works superbly.  

 

 

[bottletopburly]

Have a look at www.deepskydad.com auto focuser  small and light  uses a Nema 14 stepper motor , costs around £120 ish  or make your own plans on his website,  best thing I fitted focusing so easy I use APT , holds rock solid .

 

[vlaiv]

5 minutes ago, bottletopburly said:

Have a look at www.deepskydad.com auto focuser  small and light  uses a Nema 14 stepper motor , costs around £120 ish  or make your own plans on his website,  best thing I fitted focusing so easy I use APT , holds rock solid .

 

This is pretty much how I thought it will look like - I'm also thinking about Nema 14 motors in combination with Arduino nano and a driver. Found where to source most of the items, just need to see about the belt and pulley, and to figure out a bracket for attaching motors (that will also probably be DIY out of piece of metal sheet - little drilling and bending).

Found this open source project:

https://sourceforge.net/projects/arduinofocuscontrollerpro/

 

[bottletopburly]

9 minutes ago, vlaiv said:

This is pretty much how I thought it will look like - I'm also thinking about Nema 14 motors in combination with Arduino nano and a driver. Found where to source most of the items, just need to see about the belt and pulley, and to figure out a bracket for attaching motors (that will also probably be DIY out of piece of metal sheet - little drilling and bending).

Found this open source project:

https://sourceforge.net/projects/arduinofocuscontrollerpro/

 

Another project here https://www.cloudynights.com/topic/594658-pnp-focus-the-simplest-ever-arduino-focus-controller/

[blinky]

They are dead easy to make but the bit I find tricky to impossible is making a DIY bracket!  Wish I knew somebody who could make an adjustable one for me.

[Thalestris24]

I have a retail usb-focus kit from ts https://www.teleskop-express.de/shop/product_info.php/info/p6773_USB-Focus-v3-Set--controller--temperature-sensor--strong-motor--for---1-5-kg-payload.html

It was the only one I could find with a bracket that fits my ts 115mm frac but it works well with APT's ascom focus control. It has a manual controller too. Also very high resolution in terms of distance/step.

Louise

[vlaiv]

I just realized that I will not be as straight forward as I thought.

Although both focusers are of the same name - TS 2.5" - they differ in fine focus knob and travel per turn. On RC I believe that fine focus knob will have to be replaced with either pulley or coupling, depending on mounting position (below or to the side). On TS80 APO fine focus knob already has proper grooves that I can use to put belt on.

I was hoping to leave focusers "intact" and just mount motors to them, but it appears that I will need to mod at least RC focuser.

I've found good "summary" resource on this one that helped me understand differences between mounting options:

https://astrojolo.com/astrolink-4-0-mini/focusing-stepper-motor-solutions/

According to that page, coupling to fine focusing shaft is pretty good option. Did some calculations and it looks like there will be enough resolution with 200 step motor and fine focus reduction to be able to go as low as F/4 - which is enough for my needs. Now I just need to find where to source pulleys, belts, couplings (I can do it online, but would rather like to inspect items for suitability prior to purchase).

[Thalestris24]

FWIW my usb focus motor has gears and a direct coupling to the focus shaft:

As you can see, the motor is held in place with a simple L-bracket. It would be easy to duplicate the hardware but some effort to write an ascom interface, I think. I imagine it has a simple usb-serial interface with some sort of microcontroller and stepper motor driver in the little control box. There is an external temperature probe which plugs into the box (with the blue manual control buttons). It does 8000 steps/turn and is powered from an 8v dc mains adapter. My version handles  ~1.5kg load - there's also a ~4.5kg version, I think

 

Here is motor from other side (you can see the end of the geared shaft that goes to coupling):

Hope that's useful

Louise

Edited November 16, 2018 by Thalestris24

[steppenwolf]

2 hours ago, vlaiv said:

Although both focusers are of the same name - TS 2.5" - they differ in fine focus knob and travel per turn. On RC I believe that fine focus knob will have to be replaced with either pulley or coupling, depending on mounting position (below or to the side). On TS80 APO fine focus knob already has proper grooves that I can use to put belt on.

I have to say that I prefer a non-belt driven connection if possible and use a direct connection with the drive shaft and focus shaft in line as this way, there is no sideways pressure on the shaft bearing:-

[vlaiv]

1 hour ago, Thalestris24 said:

FWIW my usb focus motor has gears and a direct coupling to the focus shaft:

As you can see, the motor is held in place with a simple L-bracket. It would be easy to duplicate the hardware but some effort to write an ascom interface, I think. I imagine it has a simple usb-serial interface with some sort of microcontroller and stepper motor driver in the little control box. There is an external temperature probe which plugs into the box (with the blue manual control buttons). It does 8000 steps/turn and is powered from an 8v dc mains adapter. My version handles  ~1.5kg load - there's also a ~4.5kg version, I think

 

Here is motor from other side (you can see the end of the geared shaft that goes to coupling):

Hope that's useful

Louise

Thanks for that info. I did look at usb focus as an option, but I can't justify the cost at the moment. My dealer has kit listed at around 180e without holding bracket, brackets are around 85e and additional motor is around 90e

That would total to about 570e with VAT and customs fees - pretty steep. I believe that I can motorize both focusers for quarter of that price if I choose DIY route.

[vlaiv]

1 hour ago, steppenwolf said:

I have to say that I prefer a non-belt driven connection if possible and use a direct connection with the drive shaft and focus shaft in line as this way, there is no sideways pressure on the shaft bearing:-

More I think about it, more I'm in favor of "in line" approach vs belted side by side configuration. That would mean removing both fine focusing knobs, but I guess that would be ok as it's "non invasive" operation - I can mount them back without much hassle.

[Thalestris24]

5 minutes ago, vlaiv said:

Thanks for that info. I did look at usb focus as an option, but I can't justify the cost at the moment. My dealer has kit listed at around 180e without holding bracket, brackets are around 85e and additional motor is around 90e

That would total to about 570e with VAT and customs fees - pretty steep. I believe that I can motorize both focusers for quarter of that price if I choose DIY route.

Hi Vlaiv

I wasn't suggesting you buy one, lol! I was just showing you how mine is put together - it's quite simple, and as I said, probably not difficult to duplicate. It's just a geared stepper motor, L- bracket and controller. If I'd had the ability to do the mechanics I might have tried to make one myself at the time. For one thing, they charge 100 Euros just for the L-bracket on its own which is nothing to make, if you can do such things.

Louise

[vlaiv]

32 minutes ago, Thalestris24 said:

Hi Vlaiv

I wasn't suggesting you buy one, lol! I was just showing you how mine is put together - it's quite simple, and as I said, probably not difficult to duplicate. It's just a geared stepper motor, L- bracket and controller. If I'd had the ability to do the mechanics I might have tried to make one myself at the time. For one thing, they charge 100 Euros just for the L-bracket on its own which is nothing to make, if you can do such things.

Louise

But I did consider buying one at first - until I calculated total costs and saw that it is quite easy DIY solution - very nice for a small project in winter months when weather does not cooperate.

At the moment I think I'll look into building L bracket - lego way - out of two or more pieces. Retailers that deal with stepper motors and "robotic" accessories have generic mounting brackets - so maybe I can figure out some way of fashioning suitable L bracket - bolt here nut there ...

 

[Thalestris24]

I've never done it myself but you can just bend some aluminium bar e.g.

 

[iapa]

pegasus actually have a diagram for their l-bracket on their web sight.

You could start with that...?

It has multiple slots for the focuser side

For an RC, can you 'lock' the mirror, and add a crayford inline with optics? - I did not you think you are at weight limits, so, may not be viable. 

[vlaiv]

11 minutes ago, iapa said:

pegasus actually have a diagram for their l-bracket on their web sight.

You could start with that...?

It has multiple slots for the focuser side

For an RC, can you 'lock' the mirror, and add a crayford inline with optics? - I did not you think you are at weight limits, so, may not be viable. 

You can't lock the mirror since it is not movable. Problem with 6" and 8" models is that mirror cell and focuser attachment are in one piece. There is tilt mechanism for focuser - it is "squared in" by design and manufacture. This as a consequence has a problem if too much weight is hanging of the back of the scope (focuser included) - as it will move the cell together with the mirror.

I've checked my scope and one of the collimation screws was indeed a bit loose (1/8 of a turn) - but I think I now wrecked collimation - I forgot that smaller screws are collimation screws and larger ones are locking screws so I tightened up smaller screw (instead of just making sure cell is locked by larger screw). Will need to check it under stars.

Don't know if I'm over weight limit, really should not be - using ASI1600, OAG, and filter drawer - all of those are fairly light components (in their class). Putting lightweight step motor and bracket shouldn't really be a problem.

I've decided on "serial" configuration, and made sure both fine focus knobs are removable - and indeed they are, simple set screw. Fine focus shaft looks like 4mm (or maybe 3mm I need to finally get myself a caliper one of these days) - and this seems to be problem on its own . Checked all local retailers and they have elastic shaft couplers in every possible combination except one that I might need (3-5mm or 4-5mm, neither is available, every other combination under the sun is in stock).

[tomato]

Here is another ‘DIY’ take on a motorised focuser. As you can see, provision was made to retain the use of the lock on the focuser, but in practice it is not needed, even with a heavy camera set up pointing at the zenith.

[steppenwolf]

9 hours ago, tomato said:

Here is another ‘DIY’ take on a motorised focuser. As you can see, provision was made to retain the use of the lock on the focuser, but in practice it is not needed, even with a heavy camera set up pointing at the zenith.

I love this solution!

[tomato]

12 hours ago, steppenwolf said:

I love this solution!

Thanks Steve, I have Tomatobro to thank for this, a very competent (now retired) engineer. It really does perform, will go the full auto focus route when the permenant observatory comes together.

[sloz1664]

I have built 3 focusers for my set-up. One in-line & two belt driven, all running in SGPro. The source for my DIY focusers:- https://sourceforge.net/projects/arduinoascomfocuserpro2diy/

All working exceeding well

Steve

[david_taurus83]

On 15/11/2018 at 18:59, bottletopburly said:

Have a look at www.deepskydad.com auto focuser  small and light  uses a Nema 14 stepper motor , costs around £120 ish  or make your own plans on his website,  best thing I fitted focusing so easy I use APT , holds rock solid .

 

Ordered! I never thought I'd need one as I used to just focus at the start of the session and it held solid all night. But since switching from DSLR to mono, and checking focus for each filter, it's a bit of a faff to go out each time. How do you find APT works with auto focusing? Or do you manually check HFD and adjust yourself?