I am using a Optec Direct Sync focuser with the FocusLynx controller. When temperature compensation is used the focuser is moving in responce to the temperature changes as it should. However it is trying to follow changes in the hundreds of a degree. I have confirmed with Optec that their temperature probe isn’t that sensitive, and also the FocusLynx controller algorythm is also using tenths only. The focuser does move with hundreds of a degree change and is correct for a while, but then I find I have to do a auto focus routine to get it back in line. I believe the focuser is responding to noise as well as some actual temperature change
Could we get a option to allow a choice of tenths? Some of the fancy focusers may have accuracy in the hundreds, but it appears mine does not.
Many Thanks
John.
I fully agree: we don’t want that the noise in the temperature signal causes permanent corrections of the focus position, IN and OUT. Each reversal in direction will even cause the backlash compensation to operate. Thus unnecessary corrections of the focus position are causing a lot of traffic. This situation is unfavorable.
I made a related feature request in november 2017: Focuser Temperature Compensation - Focus Position threshold
Unfortunately, at that time there was no discussion based on facts. I hope that this subject will be revisited.
I think a focus position threshold is even better than setting the number of the significant decimal places of the temperature value. If we are allowed to set a focus position threshold we could account for the Critical Focus Deviation (CFD) of the optical system that is used - this is the relevant quantity for the decision whether a focuser temperature compensation has to be applied or not.
Bernd
To recap, there are two methods of performing focus temperature compensation in SGP. The first is to let SGP do it and the second is to let your focuser do it.
When you enable SGP’s TC, it will move the focuser before each exposure is started based on the temperature change since the start of the previous frame and the defined temperature compensation amount (steps / degree C). The downside to this is that on a long, narrow band exposure, say 30 minutes, no TC is performed during that 30 minutes. On a night of rapid temperature change, the camera can drift out of critical focus during the exposure.
When you enable temperature compensation in the focuser (but not in SGP), then the focuser will respond immediately to a temperature change and keep the camera in focus during the exposure. The downside is that a focuser that is not highly rigid may cause small image shifts when it moves the drawtube.
I use an Optec Gemini focuser that has very small steps and is very rigid; so I always enable TC in the Optec driver. When I am in my observatory during an imaging run, I can hear my focuser making small adjustments almost continuously when there is a rapid temperature drop. The Optec driver and firmware only operate at the tenth of a degree resolution, which is way more than adequate. For reasons unknown, SGP records the temperatures reported by the Optec driver to the hundredth of a degree although Optec support says their driver only reports tenths to the ASCOM client.
One note – the temperature compensation coefficient (steps/degree C) can be either positive or negative depending on which way the focuser needs to move to keep the camera at the optical focal plane as the temperature changes. If you get the sign wrong, you will be doubling the error instead of cancelling it.
Charlie
Thanks for the clarification. I will try the TC through the focuser and see how it does. I really do prefer the convenience of the SGP window though. I like having all the controls in one spot. Hope the GURU’s can fix it. As for long exposures…no problem there, I have a OSC and coupled with the huge light gathering the scope has I can’t take more than a couple minute shot before I start blowing out the stars. Finally decided to only image at ZERO gain. I can get to 3.5 minutes and only saturate a couple of the big stars. Also no worries about moving the tube, my focuser turns the focus knob and is not inline with the optics. I’m really back focus limited, and wouldn’t be able to use one inline.