Budget Autofocuser Mod

*Note: This is listed as an original model as, aside from base interface measurements shared with the Leavitt models, no object is derived or modified from the Leavitt focuser assembly.

Preface

In a continued effort to do astronomy / astrophotography on a budget, here is the Budget Autofocuser Mod (BAM). It is designed to seamlessly interface with Leavitt v1 dimensioned focuser assemblies (i.e. also fits the metric conversion). In theory, one could move the bolt attachment points of this mod to fit to other focuser mounting options of other telescopes. 

This model aims to balance performance on a very tight budget, compromising on tilt and smoothness (sound) of operation to enable use of FDM/FFF 3D printing.  This model could be redesigned to use a larger than 2in inner diameter bearing which may be explored in future revisions, but for now the performance of this system is satisfactory for its price point. 

An additional benefit of this design is the independent features that can be used or removed. While the original design focuses and prioritizes autofocusing via a stepper motor, manual focus can be used with similar precision at the same time (when stepper is not energized!) or as a full replacement - no motor necessary. For larger loads, 8mm rod attachments are available for reduced torque and tilt adjustment.

All astrophotography pictures posted here were taken with this (or an older focuser version/iteration) of this focuser. These pictures used ~f/5 optics for their respective FOV with a QHY168C . None of the telescopes, focusers, or off axis guiders are commercial products, all 3D printed. 

DISCLAIMER : The autofocuser portion of this build does NOT provide plug-and-play interfacing with <insert your astrophotography tool of choice>. It is designed around a stepper motor only. You will need to figure out the interfacing from there. Personally, as I use a raspberry pi + Ekos/Kstars I have access to the GPIO pins and with a stepper motor driver I can use rkaczorek's DIY Focuser INDI drivers : https://github.com/rkaczorek/astroberry-diy

Design Explanation

The design revolves around (pun intended) a rotating helical focuser with slightly angled gear teeth to interface with a bevel gear, reaching a 5:1  gear ratio (50 on the focuser ring, 10 on bevel gear). With the focuser ring screw pitch at a 1.5mm this gives a .3mm focal plane movement per rotation of the bevel gear. This is then attached to a stepper motor with dimensions that match 17HS13-0404S-PG5. If using this exact motor which has a planetary gear reduction of 5.18:1 with 200 steps per rotation on the internal shaft, we get 5,180 steps per 1 focuser ring rotation (200 steps x 5.18 planetary gear ratio x 5 bevel focus ring ratio). The final focal plane movement results in 0.0002896mm per step, or around .3 microns per step (41,400 steps for the full focus movement @ 12mm backfocus). This, of course, is actually way beyond the tolerance limit we can achieve with 3D printer tolerances and cheap components but demonstrates the upper limit of what this layout is capable of. It may be overkill, but it also affords us a lot of torque to overcome plastic-on-plastic friction without needing precision parts or much additional hardware. To limit torque and get as much light in view as possible the slim design only allows for 12mm of backfocus travel. Keep this in mind as it could be limited by longer backfocus setups.

Speaking of additional hardware, to at least make an attempt of smooth operation and not brute force rotating the focus ring against a 3D printed plastic retainer, the design uses at least 6 R188 bearings (9 recommended for full assembly without optional features). These are used to suspend the focuser ring and provide smooth-ish operation. The overall efficacy WILL be affected by the load or rather torque on the assembly. Simple astrophotography rigs (cooled camera and maybe a filter or off-axis guider) should work, but be careful when having a hefty cooled camera, coma corrector, filter, and off-axis guider all extending out from this.

To partially address heavier setups, the focuser assembly has places for 2x 8mm rods that can then be used to alleviate some of the weight further away from the focuser ring. This is still a first draft of this part of the assembly so it is crude at best but has thus far yielded decent results. 

Bill of Materials

The BOM items will be split into the main focuser assembly, then the motor and manual focuser modes, and tilt adjuster. For convenience, the most sensible configurations are noted. Choose one or both from the motor or manual focus mode, and whether you want rod/tilt adjusters. The BOM for a full configuration is listed here, but for customization (i.e. removing options) use the link to the google sheet : https://docs.google.com/spreadsheets/d/1mCpJ47S24wq829mHLezN_mhZyBhKr4zk8pekUUQ4x4k/edit?usp=drive_link

Total (All Options):

Build Instructions

Prep

Now for the fun part! Print the options you want in your final assembly. Note whether you want the Rod/Tilt Adjuster option when selecting the motor brace. The following guides through a full build with all options. Skip and/or use the appropriate change when not using an option if noted.

Note: Get the M4 knobs from my other model. It is separate as it is common between various models of mine : https://www.printables.com/model/1084752-m4-knob

Once you have everything it should look something like (inline picture quality may make text difficult to read, see actual project pictures for better quality) :

Please refer to the BOM for the actual hardware featured here.

An alternate angle:

Part 1 : Base Focuser

First, we will be preparing the base focuser body. Start by inserting 3x M3 nuts into the small slots away from the center behind the bearing cutouts. Make sure the nuts go in all the way. You made need to use something to poke them all the way down.

We will now prep the focus ring riser bearings by inserting the 3D printed shaft buffers:

If you are struggling to get these in, or the bolts in the following tips may help:

• use an M3 screw and M4 washer on the flat side

• place an M3 nut after the washer

• with pliers hold the nut and then gently screw the bolt in until fully set

• To extract use one of the knobs turned nut-side up to face the bolt head

• place on hard surface and tap the screw

Now place an M4 washer in the circular recess where the bearings will go. This serves to space the bearing from the 3D printed surface. Once in place, slide the bearing in and then screw in an M3x8 bolt. This will need to be done one at a time so the washer is held in place. Do for all 3 bearing slots.

Check!

If we now place the focus ring and retainer on the main body, we should be able to slide the ring smoothly with little to no friction from the bearings or printed parts. We should have clearance between the ring and retainer.

Part 2 : Motor Prep

SKIP IF NOT USING MOTOR

Place the motor brace 3D printed part on the motor. Note that the original design places the motor on the left-hand side (see the picture below how it is not symmetrical). If you want it on the other side AND you are using the rods/tilt adjusters, make sure to mirror the part in the slicer. If not, then the motor brace can be placed on either side assuming you printed the correct part (which is symmetrical).

Line up the holes and place 2x M3 washers on the top holes, along with 2x M3x8 bolts. Screw them in.

Next, get the bevel gear with the D shaft opening (NOT HEX - that is for the manual focus). Align it onto the motor shaft and gently press in. It will be a tight fit and you will have to use some force, but be careful to not damage the motor and gearing.

The end result will look like so:

Check!

Place the motor + brace against the focuser ring retainer where the motor fits into the large curved cutout, and the bevel gear into one one of the smaller curved cutouts toward the center. The bevel gear should almost touch the retainer. If it is farther than .5mm (basically any noticeable amount that isn't “barely”) you will need to press more.

Part 3 : Motor + Retainer Assembly

SKIP IF NOT USING MOTOR

We now need to attach the motor to the retainer. If you are also using the rod/tilt adjuster you will need the appropriate side of the 3D printed RodAdapter.stl

Start by placing the rod adapter into the mounting hole of the retainer. Choose the corner that you want your motor to be on. Skip this if you do not intend to use the rod adapters.

Next, place the motor and brace into the respective spot on the same side. The lower part of the brace (where the rod adapter isn't) should slot into the mounting hole similar to the rod adapter. The top should have space for the adapter to fit. Make sure the top lines up well as there is nothing holding it in place.

You should now see the two remaining M3 screw holes of the motor when viewing the interior of the retainer.

Screw 2x M3x12 bolts, making sure they go in all the way. Do not over do it! As we are screwing parallel to the layer lines, overtightening may cause cracking of the pieces. 

When viewed from the bottom, you should not be able to see the bolts at all.

Part 4 : Main Assembly

!!! IMPORTANT !!!

If you plan on using the rod/tilt adjuster you need to place 2x M4 nuts into the focuser body NOW in either the top OR bottom slots (not both). This should be aligned with where you want the rod adapters. I prefer the top as that allowed me to use a hanging string harness before I had a tilt adjuster.

First, place the focuser itself (the threaded 2 inch cylinder with winglets) into the focuser ring (the other threaded component that has angled gears all along the top). This may be difficult at first, so fully thread it back and forth a few times. It was designed with extremely tight tolerances in mind to reduce backlash and tilt.

Tip: If it is very difficult to turn, you can place the winglets into the focuser body and use that to hold the focuser in place while you turn the ring. Be careful to not go too far on screwing in as it may become stuck.

Now with the focuser, ring, and body in place, put the retainer on top. You should see a small gap between the bevel gear of the motor and the threads of the focuser. If not, go back to the motor prep and adjust the bevel gear.

From here we will insert the other R188 bearings around the retainer. It may be rather difficult at first to insert them as you need to make sure the bearing goes UNDER the lip of the focuser ring such that the flat of the bearing presses into the flat of the ring. Below is a conceptual example of how they should fit (don't mind the slightly different angle of the focuser ring, this is an older version):

To get the bearings in, first place the bearing straight down (flat) into the largest part of the circular cutouts on the top of the retainer.

Then, press the bearing down into the focuser ring while pressing the top of the bearing away from the center.

Another angle:

Great! Now do this for the desired amount of bearings / sturdiness you want (minimum 3).

With a 3 bearing configuration, the ideal placement is (O=bearing X=no bearing)

With a 4 bearing configuration, the ideal placement is (O=bearing X=no bearing)

With a 5 bearing configuration, the ideal placement is (O=bearing X=no bearing)

Where the up direction of the charts correlates to the up direction of the way you intend to mount the focuser assembly (up is out away to the sky, because it's pretty hard to look down at the sky…). The intent is to evenly distribute the weight, but put extra support bearings on the bottom portion that will be load-bearing (pun intended) while the top will mostly be torque-retaining whatever you have sticking out from the focuser. *** For loads that are far away the weight isn't the primary issue anymore and the charts should be flipped. Better yet, just use all the bearing spots.     

From here, we will now put in the retaining bolts through the bearings. Start by prepping as many M4x20 bolts as you have retainer bearings with a washer and a 3D printed R188 shaft with a flange.

Insert these into the retaining bearings and place an M4 nut on the corresponding hole in the bottom of the focuser body.

Check!

For some of these, the bolt length is critical to being able to mount this onto the OTA. These are the corner bearings closest to the mounting bolts. When looking down the screw hole, you should not see the bearing retaining bolt intersect at all.

Tighten all bolts such that they are snug, but they could reasonably move with the focuser ring rotation (this will be a little hard to tell with just a motor setup, so err on the side of caution, you can always tighten later).

You should now have something like this:

Note: If you are also using the rod/tilt adjuster install 2x M4x12 bolt now, one on the top left corner and the other on the top right after placing the other 3D printed rod adapter part.

Part 5 : Manual Focuser

SKIP THIS PART ENTIRELY IF NOT USING THIS MODE

Put 2x M4 nuts in only one of the pairs of slots between retainer bearing spots. No bearings need to be on either side of this location to work. There are four pairs of slots around the focuser. I've chosen to use the top right spot so as to focus with my right hand. You can easily move this around afterwards so feel free to experiment with placement location.

Get the 3D printed M4 allen key guide and one R188 bearing. Fully push one end of the 3D printed part through the bearing.

Place this into the manual focus brace with the bearing side away from the opening. Press in as far as it will go. Don't worry if it pokes out as we go through other steps.

Place another R188 bearing on the other curved side of the brace. Press this in all the way. It will be difficult so I recommend using the allen key bevel gear as a jig to press fit it past the curve and indented section. 

It should fit in such that the bearing is recessed in 3D printed part mostly and the bearing edge is at most tangent to the edge of the part.

Now place an M4 allen key through the guide and the bevel gear on the end of the shaft, with the shaft and and bevel gear on the concave side of the 3D printed brace. Don't worry about exact length. We will fix that in the next part.

Now when you place the bevel gear in the circular indent on the retainer such that the brace holes radially line up with the pair of M4 nuts you placed earlier, you will notice that the shaft length may be off (in this example too long):

Adjust the length until the brace fits snuggly against the retainer whilst the bevel gear sits in the circular indentation.

Check!

Just like the motor bevel gear, there should be a tiny gap between the focuser threads and the gear when viewed from the side.

Recall that the allen key here is part of the hardware BOM! You will need another one to finish screwing in the 2x M4x16 bolts.

Slide the knob onto the other end of the allen key, and optionally use an M4 nut and M4x10 screw to clamp that in place (I find mine is tight enough that friction fit is sufficient). 

You should now easily be able to rotation the focuser ring with the manual focuser even with the motor attached. If not, check if your retaining bearings are too tight or something is misaligned.

Part 6 : Focuser Clamp & Rod adapters

This is pretty straight-forward. Put an M4 nut in one of the knobs. Put the knob on an M4x30 bolt (nut-side facing the top of the bolt. Add some washers to space it out (I use about 8). Place an M4 nut in the clamp and screw in the bolt from the other side.

Check!

There should still be a small gap between the bevel gears and the focuser with the clamp on.

Now do more or less the same procedure for the rod adapters. If you are not using them, skip this step.

Part 7 : Mounting

Line the focuser onto the top UTA assembly and place 2x M4x30 bolts on the motor side, 1x M4x30 bolt with a washer on the corner with the rod adapter (washer not shown in this picture), and an M4x20 on the corner with nothing. If you are not using rod adapters or the motor brace refer to the BOM for the relevant instructions (or intuitively copy the bolt usage from a similar corner setup).

Note: The UTA shown below is not a Leavitt but does show that this focuser can be adapted to any UTA that can accommodate the mounting holes. 

Your final setup should look something like so:

Because this took very long to write up, I will not be leaving instructions on the tilt adjuster build for now. I will leave a photo of the setup but it is far more self-explanatory of how to put together than the focuser. It also mostly works but is not as well tested as the focuser.

When finishing your setup, ensure your nominal focus is done when your focuser is at the midpoint (like any other focuser). I'm saying this because the assembly only has 12mm of backfocus, which is enough for most things but will not be as forgiving if you are off by a finger's breadth. This is a deliberate compromise of the system to make it as compact, lightweight and powerful (in weight tolerance and precision) as it is. One could modify the design to lengthen the focuser, but this was initially designed to be as slim as possible. If this is taken into account, then you should find this focuser assembly quite nice to use whether in astrophotography or visual observations.

Performance

I use Ekos/Kstars + astroberry DIY focuser driver to control this autofocuser. Below are some charts from previous sessions (I unfortunately forgot to get screen captures during sessions) autofocus routines.

Sometimes the first focus of sessions can be rough if things have slipped or there were drastic changes from last session's focus. This will come down to tuning the autofocuser in your software. For my case, I use 500-750 steps as the initial step size and a factor of 5-10x when starting the routine (effectively stepping out ~2k-4k steps as seen below) with a refinement step size around 250. These are used with the linear algorithm which does a dual-pass and has worked fairly well for me. These numbers are approximations as I don't have my setup up at the moment to pull the exact numbers.