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Old 24-01-2021, 05:12 AM
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Question Celestron Internal Focus Stop

I've purchased a Celestron Focuser (https://www.celestron.com/products/focus-motor). Did some review on installations and potential problems, thought I'd ask here.



When I remove the rubber knob off of the OTA, I noticed that the travel focus rod only has a Philip head screw on the end with no washer. I was curious as to whether this is the case for all SCT, as I've seen photos where some have a washer (presumably as a stop) and others without. Iím not certain of the design of the internal focuser and how the travel stops work, so I thought Iíd ask here if I should consider placing a washer on the end.

Iíve trolled variously places without luck as to the design by generation of SCT, as I donít know if adding a washer will adversely affect the focus. My main concern is calibration, which might result in the mirror dropping off the focusing rail. I've read about some disasters with respect to this.


https://www.cloudynights.com/topic/6...ehd/?p=9296201



https://www.cloudynights.com/topic/6...ehd/?p=9303961



Attached image of my focuser rail system (I'm asking about the screw at the end of the black threaded rod, which has a screw, but no washer).


Cheers,
OIC!
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Old 24-01-2021, 06:00 AM
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Well, the engineer in me took over and said "Do something now!"... so I did.

Placed an 1/8" nylon washer at the end of the rod... resulted in a nice hard stop rather than a soft one I was experiencing manually. As the focuser has a lot of torque, I was concerned that it would go past the screw and drop the mirror.

As an FYI, stop to stop is just over 42 turns. I read that the best way to calibrate the focuser is to run it in half way.

Wish me luck, about to calibrate.
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Old 24-01-2021, 08:10 AM
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Well, small problem.

Focuser would stall on calibration. Took it apart four times, finally decided to get a square out and sure enough, the cutout for the focuser on the OTA itself was not square. No big deal if you are manually focusing as the orange ring is decorative, not structural.

In order to square things up I had to put two nylon washers under one screw and one on another.

The focuser can now turn although it runs fast/slow as it makes a complete turn, but I was able to calibrate it. Needed to perform this as INDI has no calibration routine. Once calibrated, the values are stored in the focuser so that INDI can access it.

https://www.indilib.org/forum/focuse...?start=0#63373
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Old 24-01-2021, 08:38 AM
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Merlin66 (Ken)
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Interesting read.
I canít remember the washer - Iíll check the next time itís apart.
I use the JMI focus motor on the C11 and have no problems running the focuser through (what I thought) full travel. From memory about 30 turns.
I do this semi regularly to reduce any potential mirror slop by respreading the grease on the baffle.
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Old 24-01-2021, 10:28 AM
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Quote:
Originally Posted by merlin66
I canít remember the washer - Iíll check the next time itís apart.
I do this semi regularly to reduce any potential mirror slop by respreading the grease on the baffle.
I used the washers between the orange plate and the OTA to get the baseplate to square in relation to the brass focusing rod. It was definately off and the rod would rotate in small circles (still does, but less), which prevents the focuser from stalling.

I may take the OTA apart and give it a good service, as weather has set in and it's starting to snow (of course... I just bought something)...
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Old 24-01-2021, 01:13 PM
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Mine has a stop washer under the screw on the end of the focus shaft.

The calibration routine you did is for the Celestron motor, you need to do that to get a valid reading on the end stops so that the motor does not try to run it past the ends. You only need to do it again if you remove the motor (So you loose the fixed positions if you happen to move the focus knob manually) or maybe after a firmware update. I assume if you turned the focus motor manually while powered off you would need to re calibrate too.

One thing I did was reverse the focuser direction in imaging software (I assume you can do that in Indi?) as in Celestron speak, "Out" is moving the mirror outwards from the rear cell of the scope, which is optically equivalent to "In" on an external focuser such as pretty much everything else has. I had to do that when using SGP as the focus runs took longer and were not as effective as they could be as it would move the focuser "Out" for the initial focus image and then the 8 steps inward, all of which required backlash compensation as they were with, rather than against gravity so it added a fair bit of time to the run and actually impacted focus quality as the mirror had not always settled by the time it shot the focus sub.
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Old 24-01-2021, 01:26 PM
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The research continues... I'm still not satisfied with the induced circular motion of the focusing rod. After some research, other's have it but call it a "wobble". Looks like it may be related to the plastic insert not being a tight enough fit, which introduces a displacement off-axis (there's only one set screw to tighten the center plastic insert).

I'm taking it apart again (for the sixth time) to see what I can do to center the OTA brass with the focuser.
https://www.cloudynights.com/topic/7...e-calibrating/
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Old 24-01-2021, 04:02 PM
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Actually I had that too, I ended up loosening everything off so the focus knob inner could spin in the plastic insert, loosened the bolts that hold the focus motor on to the scope and just wriggled everything about to allow it to find it's own center. It is still not a great setup and you can hear it has a tighter spot once per focuser revolution, but it has been working for the last year or so.


I have been thinking of changing mine to the ZWO EAF now that they have a bracket for the C925, not likely to be stranded investment if I change scopes given it is a generic focus motor.

Last edited by The_bluester; 24-01-2021 at 04:24 PM.
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Old 25-01-2021, 03:45 AM
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Another update...
Decided to take everything apart again and use the original decorative backplate and the focuser baseplate, after reading a few posts on CN (fyi, I used longer 3mm bolts).

End result is that I didn't need any washers, but the focusing rod still wobbles. It appears that this is indeed related to the grub set screw which tightens the plastic insert onto the brass rod.

Looks like I'll need to solve this before proceeding. I'll probably get a piece of thin plastic to wedge into the opposite side of the set screw. Others have used electrical tape around the brass, but I don't like using electrical tape as the adhesive tends to degrade over time, resulting in a mess when removed.


https://www.cloudynights.com/topic/6...o-8/?p=9543853


https://www.cloudynights.com/uploads...3249_thumb.jpg
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