Possible feature request for focus


#1

I have been thinking (based on a problem we are looking at on one scope in a remote setup) that it would be nice to have the ability to have focus proceed in the opposite direction. Instead of always starting on the same side of focus, it would be nice to be able to tell SGP to start on the other side. This would also reverse the final move to focus. This might be useful depending on the degree and nature of backlash, if any, in the focuser.

If there is a way to do this now, let me know, but I am not aware of it if there is.


#2

If there is the way I’d implement it is to make the autofocus step size signed. If it’s negative then autofocus moves in the opposite direction.


#3

Makes sense.

Chris wrote:


#4

@CCDMan

Since the telescope is always pointed “up” when focusing, gravity is trying to pull the camera away from the scope. So best practice says the focuser should be moving against gravity when focusing. This ensures that backlash does not affect the final position of the camera.

So, let’s say the camera is at focuser position 1500 when the AF routine starts. SGP would move the focuser to 2000 and then begin taking exposures and moving in. Let’s say it finishes at 1000 and computes best HFR at 1600. The optimum move would now be for SGP to move the focuser to 1700 and then reverse and move to 1600. This way, the focuser is moving to its final position traveling in the same direction as it was when best HFR was recorded. This completely eliminates backlash and ensures the camera has the best chance of being precisely at the calculated best focus point.

This logic works great for refractors and catadioptric scopes. It gets more complicated if the focuser is on the side of a Newtonian reflector because the camera position is unknown to SGP and the camera position (up / down) can change depending on where the scope is pointing. A common practice with Newtonians, is to rotate the tube so the focuser and camera point down the DEC axis toward the counter weights. This helps with both dynamic balance of the Newtonian OTA and generally keeps the camera and focuser pointing down.

Charlie


#5

I guess this is the problem with posting to a forum, The people answering a post have no idea what one’s knowledge or experience are so often answer with basic information. I knew all of the above. Between myself and my imaging partner (who is a retired engineer) on the system at Deep Sky West, we have over 40 years of imaging experience, having both started in the mid 90s. We also both feel this feature would be useful.

In addition to systems with side focus, there are also systems that focus by moving the secondary, BTW…

I am not going to get into a discussion on the subject, that would be pointless. There are situations where the feature would be useful, I will leave it at that.


#6

How about using the reverse focuser direction feature?


#7

Not certain but I think that only works with the manual push button mode, not during autofocus.


#8

I think this is an useful feature. Ideally I want the auto focus to be the in same direction as the focal length change when temperature drops.


#9

I guess that SGP assumes that big numbers are further out. It does not really know what is in and out. Is there any way to change the cable connectivity of the motor, so it reverses direction (assuming it is not one of those that simply fits on a Crayford or R&P shaft, otherwise, one might try to mount it on the other end…


#10

I may be misunderstanding - but I think this is a very old issue that did get solved in a way.

SGP insists on autofocusing from right to left, high numbers to low - but they recently added a reversal mode that makes sgp see your focuser numbers in the opposite way. So there is no change to the focuser itself - but when SGP takes the numbers from high to low, the focuser will be going from low to high.

This is somewhat confusing - but should allow autofocus to happen in the preferred direction pushing against gravity - and you can also set backlash appropriately.

In the past I needed to switch the wires on my stepper motor for SGP to do focus the right way - there was no other way to make it work right. But now there should be some kind of “reverse” button in sgp that allows it to work.

Frank


#11

Are you talking about “reverse direction” checkbox?

I have two telescope, one focal length decreases while another increases when temperature drops. So having an option is really useful.


#12

Here is the relevant thread regarding changes to what “reverse” means:

If I read it correctly - originally as in before last year - “reverse” meant to reverse the directions In and Out meant. But there was no change to Autofocus because Autofocus always goes from high numbers to low.

I think the new meaning of reverse - since last year - is actually to change the numbering internally. And that should accommodate various needs for autofocus. Because Autofocus doesn’t use “In” or “Out” - it just goes from high to low.

So I think this will now work for people who need autofocus to go the other direction.

But I haven’t tested it myself because I changed my motor wiring to make things work.

Frank


#13

Thanks! I will have to digest it since it is not intuitive how it works.

All we need is just to change auto focus direction, as @Chris pointed out, if SGPro supports signed step then everything should work. But it looks like SGPro internally change the reference point of focuser position (0 used to mean rack allway in, but now it means rack allway out), it is confusing, because it might affect everything related to focuser move.


#14

My understanding is that “reverse direction” is more a driver/firmware level support, so it can move focuser in opposite direction (eg due to connection/mounting limitation where normal direction can not be used). In application level, it should not change the meaning of focuser position, it will cause lot of confusion.


#15

Yes it is confusing - but I think it is all currently set up so autofocus can go the desired direction for any focuser.

A signed step would make sense - but that’s not how they did it.

In this case there is a request for a feature - and I think that feature already exists - in a form.

I recommend experimenting with the reverse button and autofocus while you are at the scope and can see the motor turning to know which way it is going.

Once you get the autofocus direction going the right way and backlash compensation going the right way - you are done and don’t have to worry about it.

Frank


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