Recent trouble focusing SCT - lagging mirror?


I have a Rigel stepper motor on the stock focuser of my 8" EdgeHD SCT. With the new focusing system in SGP it has been working quite well but lately I’m not getting consistent focus. The right side of the V curve has much less slope which leads to SGP using the lowest focus point. I then get an out of tolerance warning on the focus.

Also, when watching the focus run there is a consistent shift of the stars towards the left and then a return as the mirror is moved to focus again.

This has been happening more and more lately. It is cooler out now and I’m wondering if this could be due to the mirror lagging behind? I couldn’t find one but is there a setting to put a delay between focus exposures? If not, what would you think of this as a feature request?

Is my reasoning correct or is there some other problem I need to correct or look for? If the mirror is lagging significantly, does that mean the scope should go back to Celestron for service? I think it’s 4 years old now.

Yes, the mirror locks are loosened – I checked that first. :slight_smile:

Here’s the focus pack for that graph:
Autofocus pack

Thanks for any advice,


Steps aren’t big enough or your backlash settings aren’t correct. You can see where the mirror started moving linearly (the good curve) and the stuff in the middle is the backlash.


I’ve been using the same backlash settings for quite a long time. I have it set to IN at 25 steps. I’ll try changing it tonight. I increased my step size quite a bit last night and the curve looked the same.



Based on the right side of that graph, it looks like it didn’t take out some of the backlash. I imagine that these systems change over time depending on the viscosity of the grease? That’s just a wild guess though. I’ve never tried to automate an SCT after watching all the issues people seem to have with it.


Interesting because sometimes I get an mirror image of that graph with my small refractor. The right side will be great, but the left side is very flat. With a refractor and a Moonlite focuser, that is about as basic as it gets. Tagging this so in case anyone has any suggestions on what could cause it.




You have backlash or you’re not looking at enough steps out to HFR 3. My Moonlite produces a very good V. I use ‘smart focus’ and it will expand the data set on either side to ensure a good set of data points.



If someone is having trouble focusing sct with the primary mirror, the first question I would ask is - Is the focus curve being taken with the knob turning counter-clockwise? This is important for repeatability and it is advice given to people who are new to sct’s and using them visually.

The next question is how much backlash compensation there is in sgp. There is no point in talking about it in terms of steps since the steps can be varying size - but it does make sense in terms of how far the focuser knob turns to remove backlash - and for me it is about 1/2 turn of the knob.

I didn’t look at the focus packs here, but the first curve looks fine except for the very first point (which is on the right since focus always goes right to left with sgp - and that could be clockwise or counter-clockwise in terms of the knob).

There is indeed a need for the system - any system - to settle a bit after a focus change, so sct’s are no different. In my case there is an inherent delay when using my atik 383l+ with autofocus - but with other interline cameras such as sx, there is no delay and the focus image happens too quickly and often shows slight blurring. A delay of even 1/2 second would make the difference between autofocus being useless - and autofocus being excellent.

I placed a request for such a delay during autofocus and was told 1) It should be the job of the focus driver to delay appropriately or 2) The need for such a delay indicates poor equipment that is not worth trying to fix. I find both responses unacceptable since the focus driver has no idea what the overall system settle time is - and it is the job of the system integrator (sgp) to handle that - which it does already with delays throughout. This is just one more delay that is also needed. And - sct’s take excellent images when focused properly with the primary - and there is nothing poor about them for that purpose. My long exposure fwhm’s are now in the low 1" and there are few results with sgp on any system to compare with that.

If you or others find you need a focus delay - I would add to that request if it still exists in the system - or make a new one.

So - in summary make sure the autofocus direction is ccw and you have adequate backlash compensation entered for at least about 1/2 turn of the knob. Make absolutely sure there is no slop in the focus driving mechanics. If you still can’t get consistent curves then try increasing the backlash compensation.

If you get things set right - as I have with C11, EdgeHD8, and EdgeHD11, you should be able to get good focus curves and, after taking one curve and finding focus, doing it again should trace out a well-centered parabola with nearly the same focus value.



Interesting because sometimes I get an mirror image of that graph with my small refractor. The right side will be great, but the left side is very flat. With a refractor and a Moonlite focuser, that is about as basic as it gets. Tagging this so in case anyone has any suggestions on what could cause it.

The same principles would apply for a refractor and during focus you should be pushing the imaging train (instead of the sct mirror) up against gravity. Otherwise you are relying on gravity to help it “fall” and that may introduce slop and stalls that appear to flatten out the curve.

So - with any system aim to push up against gravity and allow enough backlash compensation. And beware of focus exposures that happen too quickly after the focus change - which would result in poor star shapes and poor focus curves.



I’ve read through the replies here and I’ve looked at my system. So…

  1. I will increase my backlash compensation steps. However I cannot do 1/2 turn of the knob. My entire focus run from one extreme to the other in SGP only moves about 1/8 to 1/4 of a turn. I have the gear mesh as good as I can get it.

  2. My focus knob rotates CW when SGP is moving “IN”, (decreasing step numbers). Clicking the “reverse focuser direction” in SGP doesn’t do anything, it still moves the same way. If I reverse direction in the driver then SGP gets confused because the numbers start going up, instead of down, when focusing IN and the focuser never stops. I think that due to the mechanics of my system I will not be able to get the knob going CCW for the IN direction.

The thing is, that my focusing was working well a couple of weeks ago, with good, fairly symmetric focus curves. It is only more recently that I am consistently getting the flattened right side which is why I thought there might be a change with the colder temps causing one side of the mirror to move into position more slowly.



I increased my backlash compensation from 25 steps to 50 steps and increase autofocus steps from 125 to 150 and got this curve:

That looks better! We’ll see if it’s consistent now. The focus run still shows the stars shifting to the left from one image to another though.




That looks good and as long as it is consistent you may be ok. You can set the backlash compensation to be as large as you want. It has no direct connection to the width of the focus curve. At the start of autofocus the backlash compensation will cause the focuser to go to very high numbers and then come back down to take the curve. When you don’t have enough backlash compensation, the curve will do weird things and won’t be repeatable. But once you go out far enough to the right and then come back in to take the curve - it should behave well.

Unfortunately SGP only takes the curve from high numbers to low - so changing the meaning if “In” and “Out” directions has no effect on the curve. But you do have to set the backlash direction properly so it only happens when you go to a higher number - and not during the focus curve when all steps are to a lower number.

I’m still not clear if the curve is being taken while the knob is turning counter-clockwise. If it is clockwise, that may explain the shift you are seeing, and the focus may not hold as well as if it were ccw. But you may be ok. The curve certainly looks good.