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Starting focus for SGP. Ideas?

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#1 kathyastro

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Posted 17 August 2019 - 10:30 AM

I have a permanent setup in an observatory, and I am trying to get it fully automatic.  I recently started using SGP's autofocus, and have it calibrated for good results.  However, the autofocus needs the camera to be close to good focus before it starts.  How do folks ensure that?

 

I could go out to the observatory and do a Bahtinov mask manual focus, but I want to get away from manual intervention, especially if it requires my physical presence.

 

I could do a manual focus remotely without the mask.  That worked last night, but I still want to get away from manual intervention, in case I want my session to start at 3:00 am.

 

I hoped that the focus would hold from one session to the next, but it didn't.  The focuser seemed to have slipped between sessions.

 

So, I am looking for ideas to establish focus at the beginning of a session.  The best I can think of right now is to lock the focuser in the morning when I install the covers, and hopefully remember to unlock it when I remove the covers for the next session.  The ultimate solution is to replace the Crayford with a rack and pinion, but that's expensive: I'd need a new focus motor, too.



#2 f430

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Posted 17 August 2019 - 10:40 AM

Maybe park you scope in a East/West horizontal position, then there will be no reason for the focuser to slip.



#3 bobzeq25

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Posted 17 August 2019 - 10:43 AM

I have a permanent setup in an observatory, and I am trying to get it fully automatic.  I recently started using SGP's autofocus, and have it calibrated for good results.  However, the autofocus needs the camera to be close to good focus before it starts.  How do folks ensure that?

 

I could go out to the observatory and do a Bahtinov mask manual focus, but I want to get away from manual intervention, especially if it requires my physical presence.

 

I could do a manual focus remotely without the mask.  That worked last night, but I still want to get away from manual intervention, in case I want my session to start at 3:00 am.

 

I hoped that the focus would hold from one session to the next, but it didn't.  The focuser seemed to have slipped between sessions.

 

So, I am looking for ideas to establish focus at the beginning of a session.  The best I can think of right now is to lock the focuser in the morning when I install the covers, and hopefully remember to unlock it when I remove the covers for the next session.  The ultimate solution is to replace the Crayford with a rack and pinion, but that's expensive: I'd need a new focus motor, too.

I assume you have a stepper motor focuser.  You can tighten up on the focus hold screw a bit to give some friction, the stepper will bang things along.  If that won't hold focus "good enough" you have a serious problem, focus is likely to shift between autofocus events.



#4 entilza

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Posted 17 August 2019 - 10:45 AM

Hmm.  How far is it out of focus the next day it shouldn't be that far off that the auto focus can't correct itself..  Previously I had a masking tape on the focus tube for rough focus.  Have you tried tightening the pressure on the crayford?

 

Can you get a screenshot of your focus curves, make sure it's going far out enough, you can tell by how much the HFR reading changes from the low and high point.  should be enough of a range say 4 from high to low for example.



#5 Alex McConahay

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Posted 17 August 2019 - 10:52 AM

In an observatory mounted scope, the last focus from the time before is probably good enough for SGP to start a new focus for tonight.

 

In a temperature compensated focuser this will almost certainly be true, since the motor will move the focuser back to the ideal focus for that temperature.

 

In a scope with a low coefficient of expansion (like a carbon fiber tube or truss) the focus would not have moved that much anyway.

 

If using Smart Focus, SGP will compensate if the focus has shifted too much. 

 

Now, I hear you say " I hoped that the focus would hold from one session to the next, but it didn't.  The focuser seemed to have slipped between sessions." (By the way, is the focuser really slipping, or is it just in a different position because it was colder at the end of the last sessions than it is tonight at the beginning of this?)

 

Then, of course, fix the focuser. If it is slipping between sessions, it is slipping at other times also. 

 

What I would do if I were you would be to try it for a while, sitting there for the first focus of the night. I think you will find in general that the focus is good enough to allow SGP to work.

 

That has been my experience on both my obs mounted scopes. If that is not true, come on back and maybe we can have more ideas. 

 

Alex


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#6 guyroch

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Posted 17 August 2019 - 11:47 AM

Is it really slipping?  I bet this is more a temperature change from the early hours of your previous session form the early evening in your next session.

 

In my remote setup I see a focus change going inward as the night progresses and the temperature goes down several degrees, which moves the focus position inward as the night goes on.  I have it setup to refocus every 1 degree change.  

 

All this means that I need to reset to focus position to a known starting point at the beginning of each session. I just use a goto command.  I don't have a focus slip issue so in my case I do a goto to 45200 (my known starting point) and it always works out for me.

 

Guylain


Edited by guyroch, 17 August 2019 - 11:48 AM.

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#7 kathyastro

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Posted 17 August 2019 - 11:51 AM

Maybe park you scope in a East/West horizontal position, then there will be no reason for the focuser to slip.

Possibly true.  But the east-west park position is just the right position to ensure that I get bonked on the noggin when I stand up after entering the observatory.  And as Bob and Alex say below, if it slips between sessions, it is probably slipping during a session too.

 

I assume you have a stepper motor focuser.  You can tighten up on the focus hold screw a bit to give some friction, the stepper will bang things along.  If that won't hold focus "good enough" you have a serious problem, focus is likely to shift between autofocus events.

I am not sure the stepper has enough torque.  With the Crayford tension turned up high, it couldn't move the focuser.  There's only about a 1/16th of a turn difference in tension between too tight and too loose.  And it doesn't help that the tension knob is hidden under the motor and can only be reached with pliers.

 

I realize that you are referring to the lock knob rather than the Crayford tension.  I had assumed that the lock knob would make it harder to move the focuser.  At least the Crayford tension is contributing to the drive traction.  The lock knob is just straight drag.  But it is worth a try.  There may be a combination that provides enough traction without excessive drag.

 

Hmm.  How far is it out of focus the next day it shouldn't be that far off that the auto focus can't correct itself..  Previously I had a masking tape on the focus tube for rough focus.  Have you tried tightening the pressure on the crayford?

 

Can you get a screenshot of your focus curves, make sure it's going far out enough, you can tell by how much the HFR reading changes from the low and high point.  should be enough of a range say 4 from high to low for example.

As noted above, the Crayford tension is very finicky.  Too tight and the focuser won't move.  Too lose and it won't move.  The difference between them is very slight.

 

I am not concerned about the focus curves.  I calibrated the curve as per the SGP instructions and it works.  If I start with the focus anywhere close to correct, it autofocuses just fine.  However, my starting position last night had bright stars showing up as nickle-sized donuts and dim stars not visible at all.  That is not a focus curve issue; it is a way-too-far-out-of-focus-to-begin issue.  The autofocus worked fine after I manually focused.

 

In an observatory mounted scope, the last focus from the time before is probably good enough for SGP to start a new focus for tonight.

 

In a temperature compensated focuser this will almost certainly be true, since the motor will move the focuser back to the ideal focus for that temperature.

 

In a scope with a low coefficient of expansion (like a carbon fiber tube or truss) the focus would not have moved that much anyway.

 

If using Smart Focus, SGP will compensate if the focus has shifted too much. 

 

Now, I hear you say " I hoped that the focus would hold from one session to the next, but it didn't.  The focuser seemed to have slipped between sessions." (By the way, is the focuser really slipping, or is it just in a different position because it was colder at the end of the last sessions than it is tonight at the beginning of this?)

 

Then, of course, fix the focuser. If it is slipping between sessions, it is slipping at other times also. 

 

What I would do if I were you would be to try it for a while, sitting there for the first focus of the night. I think you will find in general that the focus is good enough to allow SGP to work.

 

That has been my experience on both my obs mounted scopes. If that is not true, come on back and maybe we can have more ideas. 

 

Alex

 

Yes, I assume that the temperature change would not be enough to prevent autofocusing in a new session.  The total focus shift over a temperature drop of 10 degrees is only something like half a millimetre.  So I don't think that temperature is an issue.  It is the slipping that is the problem.
 



#8 Alex McConahay

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Posted 17 August 2019 - 12:17 PM

If you have any control over your focuser (step size, pauses, speed etc.) slow your focuser down. You have been trying to correct the "slippage" thinking it is mechanical slippage and can be cured with just the right setting on the tightness, etc. But it may be that the stepper motor is mis-counting a step now and then because it is moving too fast for itself, thus causing what appears to be a slippage.

 

Alex



#9 kathyastro

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Posted 17 August 2019 - 12:21 PM

No, I'm pretty sure it is mechanical slippage.  I have seen the motor driving the gears and rotating the shaft, with the focuser tube not moving.  The focuser just doesn't have the traction for the weight of the camera, filter wheel and Paracorr.

 

I have run across some links to Crayford tune-up tutorials.  I might have to look up one of those and do some tinkering.



#10 pfile

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Posted 17 August 2019 - 01:05 PM

i have very slow focus creep with my moonlite; after a few months eventually the stepper is at the stop, but of course the actual focus point hasn't actually changed over that time. i just have to go in and reset the driver every once in a while to put the in-focus position at around 32000 counts. i do see morning-to-night shifts as guylain describes and sometimes SGP can't recover from this the next night. it probably depends on what filter i finished on vs. what i'm starting up on the next night as well.

 

if you have an OAG you can get near correct focus by looping PHD exposures and driving the focus around while eyeballing the selected star HFR display. i can usually do this while pointed at polaris (in the AP "park3" position). if no OAG, you could do a couple of things - first would be to just kick off an autofocus run manually at the start of the night, before you start your sequence. another would be to just find a bright star (again polaris might be good enough) and draw a region around it so that you're only downloading a small subframe. this makes the loop mode in the frame and focus tab really fast. then just sort of eyeball the focus by driving the focus controls manually. you just need to be close for it to work...

 

rob



#11 Alex McConahay

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Posted 17 August 2019 - 01:09 PM

>>>>> No, I'm pretty sure it is mechanical slippage.

Remember, at the distances these things move, it can be very hard to see anything moving. 

 

But, if it is mechanical, then it is mechanical. And it has to get fixed in the long run. 

 

I know you want to fully automate, but until you get things worked out, you can just watch your focuser working for the first run each night while SGP does an autofocus. If you are satisfied that the curve goes down, and then starts back up (starting a "V" you can leave it at that. Otherwise, just click "Run again." With Smartfocus you should not need to do this. But it does give you an idea of what is happening. 

 

Alex



#12 kathyastro

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Posted 17 August 2019 - 02:11 PM

Thanks for the responses all.

 

I think that, for the short term, I will have to have to do a manual focus before starting the sequence.  I have confirmed that it doesn't have to be Bahtinov-quality to be good enough, so I can do it remotely.  I will also check the physical position of the focuser when I remove the covers before a session.  I know that my "normal" focus position is around 39.7mm on the focuser scale.  Hopefully that will be close enough to get the autofocus started in the event that it is too cloudy to focus properly (assuming clearing later in the night, obviously).

 

Long term, I probaly need a better focuser.



#13 Alex McConahay

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Posted 17 August 2019 - 02:24 PM

>>>>  a manual focus 

 

If by manual focus you mean starting the autofocus while you watch, yes.....but just watching an autofocus run should be sufficient. That means you do not have to actually do the bhat mask, or even watch the star. Just tell SGP to run the autofocus, and watch the lines. If they are running downhill, and never turn around, then run it again until it does. If it is running uphill after a couple of exposures, stop it, and move the focuser out (I think.....it is either in or out, obviously, but you can figure that out easily enough) a hundred spaces (or whatever is needed) and try again.

 

Another thing you learn pretty quickly is that your stars pretty much always focus at about the same HFR. So, that helps you know where you are on the first focusing run.

Alex



#14 cfosterstars

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Posted 17 August 2019 - 03:56 PM

I had a slipping crayford that was really bad in the winter. The way you can tell is point to near zenith and run and autofocus. Note the position of best focus and run it again. Repeat this about three times. If you are slipping, the focus position will be move outward to the full draw tube direction. I had this issue with my EON APO and its stock crayford. No amount of adjusting the tension would prevent it. If I went too far, then focuser would lock up and not be able to move in both directions. I replaced my stock crayford with a starlight instruments R/P and never looked back. Its a stable as a rock. For finding the initial focus, I just use the frame and focus utility and manually adjust to focus point until is close. You get a digital reading of the focus position so this is easy. I never use a Bat mask even though I have two of them. I just use the star diameters since then I dont need to take to mask off. Once it resonable, SGP just takes over and I run autofocus. With my Starlight Instruments R/P, The focus is always close from session to session.


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#15 pfile

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Posted 17 August 2019 - 04:15 PM

actually i did misspeak - the system i'm using now has a feathertouch focuser which looks to me to have a rack & pinion. however, i am driving it with a moonlite controller along with some step-down resistors, so it is possible that the 'slippage' is electronic in nature.

 

rob



#16 freestar8n

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Posted 17 August 2019 - 05:46 PM

I would definitely try to fix the focuser if possible.  It may not be mechanical slipping if it is missing steps and doesn't realize it.  That would mean the SGP autofocus might not actually come to focus - because even if the curve looks good to identify the minimum - when it comes back through focus it may miss steps and not hit the minimum.  I don't know the focuser involved here, but many of them don't have encoders that make sure steps really happened.  There may be a way to slow down the steps to improve it if that is the issue rather than mechanical slipping.

 

In my case with EdgeHD11 it holds the focus well between sessions so that focus curves can be taken directly.  I can dial it in better just by using ambient temperature and the measured focus drift with temperature - but I usually don't need to bother.  I was very surprised that I could predict focus pretty well just based on temperature - but it appears to work for me and may for other systems.

 

Frank



#17 kathyastro

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Posted 17 August 2019 - 06:33 PM

I would definitely try to fix the focuser if possible.  It may not be mechanical slipping if it is missing steps and doesn't realize it.  That would mean the SGP autofocus might not actually come to focus - because even if the curve looks good to identify the minimum - when it comes back through focus it may miss steps and not hit the minimum.  I don't know the focuser involved here, but many of them don't have encoders that make sure steps really happened.  There may be a way to slow down the steps to improve it if that is the issue rather than mechanical slipping.

 

In my case with EdgeHD11 it holds the focus well between sessions so that focus curves can be taken directly.  I can dial it in better just by using ambient temperature and the measured focus drift with temperature - but I usually don't need to bother.  I was very surprised that I could predict focus pretty well just based on temperature - but it appears to work for me and may for other systems.

 

Frank

Thanks, but I don't have trouble with the autofocus.  It autofocuses just fine, so no missed steps (or at least so few that it is good enough).

 

However, when I parked it last week, it was at a step count of around 18,000.  When I powered up last night, the stepper was still at around 18,000, but I had to drive it to about 14,000 to get it to focus.  Trust me, that is not missed steps.  That is slippage: the tube had extended without any stepper motor input.



#18 guyroch

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Posted 17 August 2019 - 08:26 PM

No, I'm pretty sure it is mechanical slippage.  I have seen the motor driving the gears and rotating the shaft, with the focuser tube not moving.  The focuser just doesn't have the traction for the weight of the camera, filter wheel and Paracorr.

 

I have run across some links to Crayford tune-up tutorials.  I might have to look up one of those and do some tinkering.

Should be easy to confirm if it is mechanical. Take a caliper (preferably digital) and take a measurement in the morning when you close up and then take another measurement in the evening when you're getting ready for another session.

 

Edit:  I just read your post above where you said you have to drive it up to 14000 steps to get back where is was.  That is a lot. Visually though, do you see a difference of a few cm?

 

Regards,


Edited by guyroch, 17 August 2019 - 08:29 PM.



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