Recommended Focus System with Celestron C8

Hello, folks.

I want to potentially get a new focuser for my C8. The celestron focus motor is not cutting it for me. I have so far changed the stock focus knob with a feathertouch that was very accurate but a pain in the neck to turn, and then I added the celestron focus motor to the scope.

The next focuser I get will have to have proven success with SGP autofocus system such as my MoonLite has on my refractor. So I am here hoping to get recommendations based on actual experience on an autofocuser that worked well in SGP.

Thanks a lot.



I use a Pegasus Astro FocusCube 2 with my 9.25" Edge HD SCT. It works very reliably. It comes with a bracket that mounts over the focus knob shaft and a pulley that mounts on the shaft. It uses a stepper motor with very fine resolution (about 17,300 steps per focus shaft revolution). I have used it with SGP for over 6 months. It does shift the DEC balance a bit, so some outboard counterweight may be needed. I placed small weights on a camera gimbal clamp attached to my StarSense camera mounted on the opposite side (mainly to act as a counterweight to my CGEM II mount saddle that is horribly imbalanced).

Mark W

Hi, what problem are you having with the Celestron motorized focuser? I have one on a RASA 8 and it has worked very well in SGP.


Mostly I have not been able to get auto focusing to work. I have tried various things such as step sizes and also updated the ASCOM driver and the firmware on the motor.

What kind of parameters have you been using? What step size and how did you determine step size? Have you checked for backlash on the motor and included it in the SGP parameters?

One important thing to know is that you have to start with the scope manually focused good for your seeing that night before you run an autofocus.

With the RASA 8 it’s around 150 for step size, 5 data point and star size 6 or 7. You just have to tune it for the scope. On my 11" SCT I had to use closer to 175 for the step size. Everything else was the same.

The way to tell if the step size is correct is by the graph. If it doesn’t defocus enough to make the graph look like a V shape then turn up the step size. If it’s too steep then turn down the step size. I’ve use two different Celestron focus motors on several different SCT, Hyperstar and RASA systems and they work fantastic on all of them. There are some instructions in the SGP that explain how to do it the correct way but I’ve been doing it so long I just have a feel for what to do. With my Moonlite and Celestron focusers I always start with 125-150 as the step size and adjust it from there to make a V shape.


Thanks for sharing your setup. With all the preparations done, and at various step sizes and recommended 9 data points the curves I am getting are either pretty flat or pretty wacky.


I would never use 9 data points. Change it to 5 points with 150 step size. If the line is not very much like a V then turn up the step size by 10-20 and try it again. Once your third data point hits the bottom of the V then you are done.

Thanks. I will definitely try it out. I was going by the recommendations in the manual.

One other issue that I never could get resolved was SGP calculating negative HFRs as it pulled the Focuser in and terminated in disgust. I don’t know what equations are used to calculate the HFR but I can understand it should not be less than zero.

Do you have logs of this? I have never once seen a negative HFR.

Thank you,

I have sent these logs before. In fact the reason I know of negative complaint is because I have read the log that I sent in.

Thanks a lot. I will look into it.


The log has been shared when focus failing was reported at one such occasion.

I bought a Celestron motorized focuser for my EDGE HD 8 about 8 months ago and it’s worked reliably since I bought it. It’s seems to work flawlessly with SGP. On the odd occasion, when SGP hangs up and I quickly need to reboot, I just need to unplug and re-plugin the focuser USB so it can be recognized by SGP.

Pretty much most of the time, I get good V-curves from which the focus setting corresponding to the lowest HFR is chosen.

Thanks for sharing your experience. What is your settings and setup?

I own the Celestron focus motor for my Edge HD 8. The issue that I had is that you are unable to lock the mirror when using this focuser. Also, the backlash was an issue and I had to use a sizeable backlash setting to get the focus to be repeatable.

I replaced the Celestron focuser with a Moonlite focuser for Edge HD. The procedure I use is this:

  1. Set the Moonlite focuser to the middle of its travel range.
  2. Manually focus the scope with the focus knob and a bahtinov mask.
  3. Lock the mirror.
  4. Run SGP automated focus for the rest of the night, moving the moonlite focuser to achieve focus.

This system works great for me with sharp small stars. Further, I have taken great pains to gather offset positions for each of the filters in my wheel with respect to my LUM filter. Now, I quickly focus with LUM and then switch to my other filter and SGP provides the offset to the focuser. This has REALLY saved a lot of imaging time when shooting with narrow band filters.

Hope that helps.

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Using a focuser on the visual back permits more accurate focusing, remote control and easily reproducible settings. I have an Optec TCF on my Celestron HD1100, which is excellent. I got it

used at a heavy discount to new.

The only concern that I have with this is that I cannot use OAG. How do you guide yours?

I have an OAG in the image train on my scope. It is an Orion thin OAG; I think it is the same as the ZWO. This is for the 11 inch scope, which may have more back focus than the 8 inch. You should see what the back focus is and calculate the usage of all the devices and the camera being used.


In a message dated 4/22/2020 1:14:51 PM Pacific Standard Time, writes:

I use an Esatto from primalucelab with my C8 EdgeHD.
I also use the Moravian OAG that fits the G2-8300 Mark II.
Both work very well.
Only limitation is that you cannot reach the 103 mm backfocus distance when using the .7x reducer with the OAG. Or you have to give up the OAG. I ordered a Moonlite 2.5 CHL for that purpose, lately - I look forward to receiving it.
Regardless, using an OAG at these focal length is a sport in itself, particularly when flipping the mount at meridian: depending which area of the sky, the guider camera often has a hard time finding a guide star.
The best option in this configuration, my experience, is parallel guiding, at the expenditure of potential differential flexure.
I don’t run in any of these issues with the Takahashi 85 EDX, as you can imagine.
Good luck!