for attaching ROBO-FOCUS to SCT-Focuser 2"



The story began with my desire to improve the CGE CELESTRON 11" setup so that I decide to use ROBO-FOCUS remote focus driver attached to a Schmidt-Cassegrain SCT-Focuser 2".


As many other users I consider the primary mirror focusing mechanism of Schmidt-Cassegrain Telescopes weighty to use. It was the main reason for me to buy Cryford style focuser and to attach it to rear tube of my Celestron 11". The focuser is Schmidt-Cassegrain SCT-Focuser 2" with 1:10 microfocuser. It has practically zero image shifts and a smooth focus motion.

For remote and PC control focusing ability I ordered to Technical Inovations a ROBO-FOCUS system both for CST Cryford style focuser and for ordinary Schmidt-Cassegrain main mirror focusing knob. Thereby I obtained two motors. Now I am sure that I will not use the second one for SC main mirror focusing but instead of that I will add it to my SCOPOS TL APO 906 (90 mm - f/6.7 cemented Triplet APO refractor) which has the same SCT-Focuser 2".

When the ROBO-FOCUS arrived I examineed the included hardware. At a first glance the robust ROBO-FOCUS stepping motors were supplied with some funny hardware things. There were some aluminium flat brackets and plastic bars with double back tape. As producer recommends in a short web based description one can use these things in a way “that the user bends (by hand) to provide the motor mounting surface”.

At first I was trying to reveal the right way for attaching ROBO-FOCUS to my telescope hardware. It was hard to be done because supplied hardware did not offer many possibilities. To use the plastic bar with double back tape and aluminium brackets was uncertain and did not look serious. There was no way to permanently attach the ROBO-FOCUS stepper motor to the Cryford focuser without permanently attaching both of them to the telescope back plate. That converts the setup to not flexible ones. Naturally every one would like to use the telescope visually and for astrophotography which means the setup has to be flexible and durable.

On one hand the "gluey" setup impeded ability of my focuser to turn in 360 degrees for adjusting frame orientation. On the other hand the only way to disassemble these items was to get unstuck the plastic bar from telescope plate. One could guess how “useful” and “friendly” is this.

All this provoke me to search more convenient way to correct my hardware setup. I decide to design and construct special adapter (holder) for permanently attaching ROBO-FOCUS stepper motor to the Cryford focuser. The aim was to not disturb 360 degrees rotation system of the SCT Crayford style focuser and to make the setup independent from the telescope back plate. This would allow unscrewing focuser from telescope rear tube without disassembling the holder mounted permanently to the stepper motor.

I started to examine my Cryford style focuser and checked two of small Allen head hex bolts near to the 360 degrees rotating ring. I realized that they were used only to block up the small holes and for nothing else (marked with red circles).

Therefore I decide to use this M4 treaded holes to mount my future holder. After some carefully measuring and sketching I was able to make the whole mechanical designs of this adapter holder (see next two images).

As my desire to make some additional items (rings for my old f/4.4 Newton etc.) I choose 16 mm thick aluminium plate. Aluminium parts were sawed off by water jet automatic machine. That was amazingly fine. The surfaces became smooth with nice nacreous glitter.


After anodizing they got deep mat-black colour.


A few words about focusing:

The difficulties in manual focusing have two sources: Firstly our eyes suffer from lack of precision and subjective interpretation and secondly, through the night as the ambient temperature goes down, the correct focus position of most telescopes will change with the temperature differential. Therefore you need to correct the focus to adapt to those changes. The extent of these focusing problems will vary depending on the focal length of your setup. Short focal ratio instruments like a wide field refractor will have a much smaller “critical focus zone” (CFZ) than will a long focal ratio as Schmidt-Cassegrain and other similar. Precise focus is even more important for short focal-length scopes than for long. The calculation for the CFZ is equal to “the f-number of the optical system squared x 2.2”.

CFZ = f-number^ 2 * 2.2

For my f/6.7 SCOPOS TL APO 906 CFZ would be 6.7^2 * 2.2 = 99 microns or 0.099 mm and with field flattener x 0.8 or f/5.4 it would be 5.4^ 2 * 2.2 = 64 microns or 0.064 mm. For my longer f/10 focal-length CELESTRON 11" it would be 10^2 * 2.2 = 220 microns or 0.22 mm.

In practice, the effective CFZ is often much larger due to atmospheric seeing, under-sampling on the CCD, inaccurate guiding, or any other effect that blur the image. For my scopes, the CFZ is accordingly 0.064 mm, 0.099 mm and 0.22 mm. For perfect focusing, I need to be able to make focus movements at least as small as the CFZ. With RoboFocus on an R&Pfocuser or Cryford style focuser, the least move is approximately 0.0026 - 0.005 mm, or about 12 to 44 times smaller than the CFZ. For effective use of automatic focusing, it is not necessary to know the particular CFZ value for the system, only that the step size is smaller than the CFZ.

Thereby I decide to use ROBO-FOCUS in both of my main optical systems furthermore they have the same Cryford style 2” focusers.


What you see below are the pictures of finished MAX 5 adapter (holder).

Any opinions and suggestions are welcome ...


View 1

The raw parts (see the nacreous glitter)

View 2

The two raw parts with holes.

View 3

The assemble simulation.

View 4

The items assembled before blackening.

View 5

The same after blackening.

View 6

Front view

with ROBO-FOCUS stepping motor.

View 7

View from above

with ROBO-FOCUS stepping motor.

View 8

Lateral view

with ROBO-FOCUS stepping motor.

View 9

Notice that the parts are not attached to the back telescope plate.

View 10

Notice that the parts are not attached to the back telescope plate. This would allow unscrewing focuser from telescope rear tube without disassembling the holder mounted permanently to the stepper motor.



To obtain drawings for free please e-mail me. For non commercial use only.


All Contents copyright Velimir Popov unless otherwise noted.