Autofocus help

Sequence Generator
41

Hello freestar8n,
I am new here and have a small question. I typical download of the whole image lasts 30s with my STL11k and 15-20s with my Moravian CCD. Than with medium or bad seeing the FHWM values change much without any focus change, so there is the need to sum more images per step. I would take very long. Is it possible to define an ROI for autofocus?
Thomas

42

Hi-

I believe this issue came up earlier and there is currently no way to crop on download. All you can do to reduce download time is bin.

You can restrict the area used for measuring stars - and I do that to improve the measurement and avoid the edges.

I want to save time everywhere when I am not exposing an image - and autofocus takes a long time due to backlash unwinding and image download. So I agree that you should be able to use a cropped region of interest. There already is a setting to limit the region used for autofocus analysis - and I think it should also be used to crop the actual image download - to save time.

I am using an 8300 sensor and when using binx2 the focus time is mainly limited by backlash unwinding on an sct - but if I could crop the images, I would.

As for the question prior to yours - there is a mistaken impression that focusing an sct is equivalent to “focusing at f/10” - but that isn’t true when you focus with the primary. When you focus with the primary you are effectively moving an f/2 (very fast - much faster than a Newtonian) optic - and the rest of the imaging system simply magnifies the resulting view of focus. This is very different from using a crayford and moving a sensor back and forth in a stationary f/10 cone to focus.

So when I focus using the primary of an sct - I am doing something much more critical than focusing an f/4 Newtonian with a crayford. Yet this method of focusing an sct, with autofocus software and backlash compensation, does work well - and it simplifies the imaging train.

Frank

43

Slated for 2.5:

44

While this discussion is concentrating on the methodology of auto focus routines, let’s not lose sight of the need to tie the OTA temperature to every aspect of automated focusing. In “SCT” style OTA’s, the amplifying secondary requires temperature compensation to be taken into account when implementing focusing routines since the effect of the change in OTA length is also amplified by the secondary.

According to Roland Christen of Astro-Physics, short focus APO refractors with aluminum tubes have a nearly zero temperature coefficient since the APO lens changes focal length with temperature change almost identically with aluminum. This is why AP does not sell refractors with carbon fiber tubes.

My system may have good focus with the luminance filter with the focuser at position 22550 when the temperature of the OTA is 20.4 degrees Celsius but it will need to be at 23200 when the temperature drops to 19.4 degrees Celsius – a change of 650 steps.

So when designing the system to support preset focus positions, such as for changing filters, those recorded focuser positions must also have the matching OTA termperature recorded. This permits temperature compensation to be taken into account when actually moving the focuser to a new filter position.

cm

45

While I understand what you are saying, this does not seem to be entirely necessary if you record your focus positions within a reasonable period of time. Originally, SGPro had this functionality, but added a complexity we didn’t feel was entirely necessary. This is in the docs… if you are recording your filter offsets at random times over different nights, you are not going to be happy. Trying to walk the line between usable and powerful is sometimes a tricky problem.

1 Like
46

Ken:

I certainly understand the desire to keep features as simple as possible. However, with my setup the issue is that there is no such thing as “where does the Red filter focus” that does not include a corresponding temperature reading. The Optec Gemini focuser on my 12" Meade LX850 OTA has a temperature coefficient of 650 steps / degree C.

  1. At 10PM the temperature is 20 C and the Red filter focuses at position 22000.
  2. At 3AM the temperature is 15 C and the Red filter now focuses at position 25250.

I would prefer that SGP not do a full auto focus routine on a filter change – it takes too long. I am suggesting a method whereby SGP could “adjust” the focus position based on a temperature calculation. Since an initial focus calibration routine could span several minutes, you could have a temperature change during the session. So the filter table might look similar to:

Lum: 21850 @ 21.3 C
Red: 22000 @ 20.0 C
Grn: 22115 @ 19.9 C
Blu: 21930 @ 19.7 C

My goal would be to perform a full auto focus routine for each filter at the beginning of the evening and then rely on “focus adjustments” for the remainder of the night.

Charlie

47

With an sct I found no clear correlation with temperature that could be used to help focusing. I just want to focus periodically and do it well and as quickly as possible - so I agree with the SGP approach to do accurate focusing with many stars and without switching to a bright star for focus - in which case you don’t need to include temp. as a factor except perhaps as a guide to when you need to change focus.

The filter offsets should be constant with temperature - if not that is pretty strange. So you should be able to focus relatively quickly with L and then know the focus accurately for the other filters using their offsets - until the next time you need to refocus.

Frank

48

Charlie,

In a certain sense SGPro already allows for this. Set your filter offset positions within a reasonable time period… positions within a degree or two will likely not lead you out of the CFZ. Then run AF on you favorite filter (lum usually). In the sequence uncheck “use AF” option. Turn on temperature compensation. Filter changes will change the focuser pos by the delta of the current and new filter offsets and temperature compensation will help adjust them in real time.

We’re probably not looking to change the AF methodology anytime soon (with the exception of better algorithms for secondary obstructions).

49

Ken:

I would be very happy with just the new auto focus algorithms for SCT style scopes and rely on SGP’s temperature compensation routines for holding focus between auto focus steps.

Charlie

50

sorry, but this is optically non sens at all… your critical focus zone is for F/10 not F/2.

51

No - it is optically correct. The typical cfz calculations show a fixed setup and an image plane moving through it. That is not what is happening when you focus an sct by moving the primary. You are not moving the image plane - you are moving an f/2 optic - and resulting size of the star spot will change dramatically as you move it. Very slight movement of the primary will result in large changes of the star spot - just as when you focus with a fixed f/2 primary by moving the sensor.

It is optically correct to view it as I describe - because that is how it works. It is incorrect to view it as a crayford moving a sensor in and out of an f/10 cone - because that is not how it works.

The main point is - focusing an f/10 sct or f/2 hyperstar by moving the primary - does work well. They both rely on critical positioning of an f/2 optic with a fixed sensor - and they both work well with the multistar focusing of sgp - as long as backlash is handled properly.

Frank

52

You should already be able to do this:

  1. Turn on temp compensation in SGP or use the temperature compensation built into your focuser (it sounds like you currently are).
  2. Setup focus positions (which will be treated as offsets) in the filter setup.
  3. Turn on “Auto adjust focus per filter”
  4. Disable auto focus if you don’t want it, or set it to run AF between targets, or whatever.

Now focus your scope and run the sequence.

In between frames (or even during if it’s setup in your focuser) the temp compensation will keep you focused. When you change filters the focus position will be updated using the data setup in step 2, keeping you in focus.

I think this is what you’re looking for? The only “gotcha” is that backlash compensation may not be invoked if you need it (or it may, I can’t recall 100% here).

(Just realized this is the same that Ken recommended above…which should do exactly what you’re after)

Thanks,
Jared

53

Jared:

Thanks for these suggestions. I will give them a try on my next imaging run.

Charlie