10 replies to this topic

[Neinball]

An older guy in my club has built himself a 14" dob with goto and is trying to use Servocat to control it, he's been struggling with it for 2 years now and it's never pointed with less than a 5 arc second error. He's feeling pretty helpless with it at this point and I'd like to try my hand and see if I can't figure it out, but I'm not familiar with ServoCat and don't really know where to start with it. Would any of y'all have any advice on where to start tutorial and knowledge wise? Any advice y'all can give would be greatly appreciated as I can't really find anything that doesn't assume I already know how servocat works and how to navigate it.

[Dale Eason]

The best place to go is the Servo cat groups.io  at  https://groups.io/g/ServoCAT/messages

[astrokeith]

An older guy in my club has built himself a 14" dob with goto and is trying to use Servocat to control it, he's been struggling with it for 2 years now and it's never pointed with less than a 5 arc second error. He's feeling pretty helpless with it at this point and I'd like to try my hand and see if I can't figure it out, but I'm not familiar with ServoCat and don't really know where to start with it. Would any of y'all have any advice on where to start tutorial and knowledge wise? Any advice y'all can give would be greatly appreciated as I can't really find anything that doesn't assume I already know how servocat works and how to navigate it.

Do you really mean " 5 arc seconds"? You shouldn't expect ServoCat to point much better than 5 arc minutes as a norm. Is the error regular, ie mostly azimuth?

 

First step, is how does it 'feel' without the drives engaged? Needs to be smooth and no appreciable stiction.

Start with the ServoCat backlash compensation set low, say 1 arc minute. 

Next looking at a target, how responsive is it to small handbox movements. You want to see little backlash on changing direction, and very little movement in the 'other' axis - ie when moving in altitude you shouldn't see an azimuth movement.

This test should expose a  common problem in the wire altitude drive. It can lift one side off the bearings, and/or cause the scope to skew in azimuth.

Check the encoder installation. Shafts gripped firmly and tangent arms not loose.

Check for collimation change as the scope moves. A collimation laser is good for this. I've seen secondary mirrors sag a lot as telescope altitude is changed.

[eye relief]

I just picked up a used dob the other day with a Servocat installed. Have only limited use one night before the clouds rolled back in. Altitude drive looks to not be disengaugeable without the drive cable becoming a tangled mess. not friendly as push to that I can tell yet. Following....

[Dale Eason]

I just picked up a used dob the other day with a Servocat installed. Have only limited use one night before the clouds rolled back in. Altitude drive looks to not be disengaugeable without the drive cable becoming a tangled mess. not friendly as push to that I can tell yet. Following....

You should not full disengage drive cable.  You are supposed to just loosen it enough so that you can then manually push.  Works well on my scope that way.

[Dale Eason]

I had a post that never made it through last night.  I too said 5 arc seconds go to is way more accurate than I have ever seen any servo cat systems I have used can do.  I'm happy if the object makes it into my lowest power eyepiece. That makes it accurate to about .1 deg or so.   Then I fine tune it and it tracks the object well for visual.  

[eye relief]

You should not full disengage drive cable.  You are supposed to just loosen it enough so that you can then manually push.  Works well on my scope that way.

Thanks Dale, and your reporting of the "tracks well for visual" and for the servo cat group link.

Scott

[Neinball]

I had a post that never made it through last night. I too said 5 arc seconds go to is way more accurate than I have ever seen any servo cat systems I have used can do. I'm happy if the object makes it into my lowest power eyepiece. That makes it accurate to about .1 deg or so. Then I fine tune it and it tracks the object well for visual.

I'm going off what he told me is the error, it's probably arc minutes, the scope will point to the general vicinity of the selected object but it won't ever be in the eyepiece, no matter what corrections you make during alignment it's always off and by how much is variable.

[Neinball]

Do you really mean " 5 arc seconds"? You shouldn't expect ServoCat to point much better than 5 arc minutes as a norm. Is the error regular, ie mostly azimuth?

First step, is how does it 'feel' without the drives engaged? Needs to be smooth and no appreciable stiction.
Start with the ServoCat backlash compensation set low, say 1 arc minute.
Next looking at a target, how responsive is it to small handbox movements. You want to see little backlash on changing direction, and very little movement in the 'other' axis - ie when moving in altitude you shouldn't see an azimuth movement.
This test should expose a common problem in the wire altitude drive. It can lift one side off the bearings, and/or cause the scope to skew in azimuth.
Check the encoder installation. Shafts gripped firmly and tangent arms not loose.
Check for collimation change as the scope moves. A collimation laser is good for this. I've seen secondary mirrors sag a lot as telescope altitude is changed.

Meant minutes not seconds, the scope will point to the general vicinity of the target but it'll never be in the EP, and it's not responsive to alignment corrections. As far as feel goes it's hand made but the wood and build quality/attention to detail rivals an Obsession to the point that that's what I thought it was when I first saw it, so it feels perfect in the hand, and the mirrors I believe are Hubble Optics research grade and were custom made for the project. Only part that he kinda cheaped out on is the generic focuser that I think came off an old Orion. So as far as I can tell there's no physical issue with the scope that could cause the accuracy issue and the likely problem is software/user error, but I'm not familiar enough with the system to figure out what it is.

[John Miele]

IIRC, there is a setting on the stepper motors where you have to calibrate them based on the diameter of the altitude bearing and azimuth bearing. If this is set wrong, you will never get good pointing. Perhaps that needs to be adjusted. The SC user manual tells you how to do it. I did it on mine. You need to load some software on your computer and connect the SC main box with a cable to your computer to make the change and then upload it back to the main control box.

 

cs...John

Edited by John Miele, 19 June 2025 - 02:59 PM.

[Dale Eason]

There are many places that can affect accurate pointing.  The best thing to do is get the manuals usually on the CD that came with it or ask on their GoupsIO forum.  The designer of the system is there and willing to help.