CFZ - don’t suppose you are working on some way of putting the central focus area towards the back of the cfz so those who either don’t have flatteners or whose flatteners don’t quite work have more of their image in the cfz? Or is that a daft thing?

How are you working out the cfz? I’ve seen different ideas for working this out which for my setup seem to range from over 150 microns down to 20.

Regards

Nigel

There will be several algos for CFZ so you'll be able to choose what you want.

Not sure I completely understand your question but if you want to focus on a certain part of the sensor you could try setting the Annulus field. Only stars in the annulus will be used for focusing.

The Annulus itself is centered, so it's not really possible to analyze, e.g., tilt this way. (Which would be a great addition by itself, like splitting the area in several fields and process them separated to look for systematic drifts...)

Do you mean like the FWHMEccentricity script in PI?

Oh, and I completed testing on the sim for the "Overscan" change we were discussing. I can't test on my rig at the moment due to bad weather. The code is in...
invent.kde.org/education/kstars/-/merge_requests/795

Its not merged at the moment as I need to test it some more on my rig and also as there's a lockdown on new code ahead of next week's 3.6.2 release.

ok this is just an idea based on a theory based on reading various websites… so may be completely stupid….

In the C6 manual when it goes on about how to focus, it says to go counter clockwise as the last step (ie lifting the mirror). Then when you have got your target in sharp focus (I’m sort of assuming the target is in the middle here as I’ve not got the manual to hand) to give it an extra turn of 1/12th of a turn for visual use and 1/24th of a turn for photographic use.

What I believe the idea is, is that given the sensor is a flat plane but the image focus point is in a curve with the edges further ‘in’ , that moving it a bit further moves the central part towards the back of the cfz and in doing so brings more of the edge into it.

For my setup C6 with a Celestron focuser we are on about approx 30 microns or 40 steps. This sort of matches somethings I’ve read (specifically the article from sky and telescope in 2010 by Don Goldman and Barry Megdal - In perfect focus - link astrodonimaging.com/tutorials/ ) the cfz for an f/10 is around 40 micron and f/7 around 20 micron in one direction (so I assume 40 and 40 wide).

So once in focus on the target in the middle of the image a step out of 40 would bring more into the cfz.

Possibly this is a bit of a old idea and that doing the focusing using the whole or large part of the frame makes it irrelevant.

I guess this has gone off topic but anyway...

You could do an autofocus run and then manually move focus to where you want. Alternatively, as I mentioned above, use Annulus to use the part of the sensor you want to be in focus, or pick a single star that you want to be in focus and use that.

If you want to see how good/bad your setup is and have PI, load a frame and run FWHMEccentricity on it.

Do you mean like the FWHMEccentricity script in PI?

Not sure, as I don't use PI. I know ASTAP has a feature to show FWHM variation over the FOV, and compute that into a tilt. That's what I was thinking of. Depends of course on detecting enough stars....

Ah yes...
www.hnsky.org/astap.htm#inspector

The contour map is the same idea as PI, but looks like it does a bit of maths to convert to a "tilt" %. Very nice!

The following website maybe of interest www.astrobin.com/forum/c/astrophotograph...-objective-analysis/

goes on about analysing tilt and back focus using astap

A few observations:

I think the overscan/backlash separation is the right move. Allow separate determination of whether the driver or focus routine manages backlash. In my case I have a Nitecrawler which works very well and doesn’t seem to care - it works fine as is with the linear 1 pass routine even if if doubles up somehow. My second rig has an Esatto and the driver backlash seems not to work well, it introduces randomness having it enabled. Linear 1 pass doesn’t even run unless the backlash box is unclicked - this does leave some residual backlash in the system. I think the overscan option in this case might help.

Regarding the ASTAP tilt/backfocus - this is a very quantitative approach to dialing in best possible image. Having an option in the focus routine where you could automatically scan through the focus position unidirectionally while saving the images would be handy. At present you have to monitor and step the focus position then take a 30 second image, step and image etc until you have all the images to load into ASTAP. Specifying a starting point, step size, and number of steps and walking away for 15 minutes or so would be a nice convenience.

If you turn on Developer->Save Focus Images then when you do a Linear 1 Pass focus run it will save your focus images automatically in a sub-directory in the same place as your log files. The first image until the last but one will be a sweep from out to in. The last one is after it has moved to the point of optimum focus (of course, when we've fixed your problem with the Essato).

I've not done this myself but you should be able to take these images and load into ASTAP.