WO RedCat Autofocuser

The brand new William Optics RedCat telescope is an alternative to the widefield lens shooting with 250mm of focal length. With the helical focuser, focusing can be quite time consuming process even with the Bahtinov mask (especially with narrowband filters). Our autofocuser will save you time and frustration by providing pinpoint stars on your beautiful photos of the cosmos. Our AF3 unit is mounted via 3D printed adapter to the utility holes on the black compression ring. It is also includes modular guide scope shoes, which are compatible with v1 and v2 ring. We designed the bracket so that the whole setup is nicely balanced even when the autoguider is mounted.



  ASCOM supported
  custom step size
  speed control
  current control
  hand controller (optional)
  temperature probe (optional)
  settle buffer option
  compact design

You are getting

  AF3s unit
  WO RedCat bracket
  timing belt ring
  20T pulley
  B151MXL timing belt
  Mounting bolts
recommended power adapter: 12V (1A min) with 5.5mm x 2.1mm plug (not included)
connectivity: Micro USB cable (not included)

Development & field testing

Focusing with the helical focuser can be tricky to nail by itself even with bahtinov mask. When you add the focal point shift due to temperature changes throughout the night, it can make an all-night session quite exhausting.

That is why at Deep Sky Dad we decided to prepare an autofocusing solution. There are some tricks to the settings, so read on.

If you want a technical-only guide on how to set everything up, please read our FAQ.

Helical focuser ring stiffness

One thing I noticed immediately is the stiffness of the focusing ring. It is bearable in the middle range, but extra stiff at first 5mm or so. To provide sufficient torque, we have used a stronger motor for the RedCat autofocuser. IMPORTANT: even with stronger motor, we recommend that you use the autofocuser after 5mm mark on the drawtube, to prevent problems due to stiffness as more force is required to turn the ring in that area.

The slipping timing belt

First prototype included only autofocuser housing without the extra ring and 3D printed pulley. I mounted the MXL timing belt to the 20T metal pulley on the motor part and directly to the helical focuser ring on the RedCat part. I was sceptical about this solution due possible belt slipping, but gave it a try anyway.
It was quickly obvious that this solution will not work. Ring stiffness caused belt slipping, especially during the direction change and in first 5mm of the focusing range. That is why we added the ring with MXL teeth pitch and put it on top of the helical focuser ring.

Timing belt play. Or is it?

Once the ring was added, autofocuser worked much better. So it was time for first field test on M101. The V curve was nice, but the graph indicated that the ring does not respond immediately after direction change, which indicates focuser play. Since it was already late I used SGPro backlash compensation option to counteract the issue and shot an hour or so of M101 (after that the clouds rolled in). This is the result:
When I investigated the autofocuser closer the next day, I noticed that there is slight play on the 20T metal pulley on the motor side, when motor direction is changed. That is why we designed a 3D printed pulley in a similar matter as the ring and it worked - no more play during the direction change. IMPORTANT: timing belt ring has to be tensioned just so that the ring doesnt turn freely and the belt fits to the teeth. If it is too tight, the belt might be too loose. Also, we have added the guide scope shoe in this phase. The orientation of the components results in a nicely balanced setup.

Galaxy zoo

Since at the moment here is a "galaxy season", the widefield target choice for a all-night test was quite obvius to me, as I am a sucker for galaxies - Markarian's Chain :) This field test was planned so that the autofocuser is tested heavily - refocus on stars every 15min. It's an overkill for standard usage, but I wanted to make sure that focal point is nailed every time. I ran the autofocus, perfect V curve but stars still slightly out of focus. I checked if everything was assembled properly and the conclussion was that the helical focuser ring has some internal play to it - at least in the unit I had at hand. But nothing a backlash compensation could solve, check our FAQ, section "What SGPro settings should I use?". This is result of field test on Markarian's Chain:


The final solution makes utilizing this little refractor a breeze. If you have any additional questions, check our FAQ or contact us