zuts The maximum range on the EAF is around 60,000 steps, however the full range of focus on my Edge is around 180,000 steps.

The only recommendation I can give you is to basically treat the EAF steps as a number mod 60000 (or something smaller than 65536). And keeping a note on the number/60000.

For example, if you are in the range between 0 and 60000 and want to go to step 110000, make it go to 60000, and then reset the step count to 0. Make a note that n/60000 is now 1. And then move it to 110000-60000 = 50000. Rinse and repeat to go to other ranges.

To go backwards for more than one range, just ask it to go to 0 and then set the step count to 60000. Etc.

It would be much simpler if ZWO would just change the firmware to use 32 bit integers (other manufacturers' focusers have larger ranges than ZWO). They obviously have not thought through the problem. Using 16 bit integers is so 20th century.

As to the shallow V curve, there is a field that you can set in the more recent releases of ASIAIR: open the Focuser Setting window of ASIAIR app. Near the top, there is a menu item called "Auto Focus." In the AutoFocus Settings window, change the Step Size to a number that is a couple of times larger than your instrument's Critical Focus Zone.

Chen

zuts I have found a formula for the critical focus zone

Hi Paul,

Check (Google) also for "New Critical Focus Zone" that takes into account things like "seeing" etc.

With my FSQ-85 (f/5.4 system), the original CFZ comes out to about 55 microns, and the NCFZ comes out with typical conditions to 32 microns.

With the Takahashi R&P mechanism and the ZWO EAF, this equates to a critical focus zone of between 8 and 14 EAF steps. It is natural that your CFZ is greater since the CFZ goes by the square of the f ratio. Takahashis are known for their small spot sizes.

It is easy to check the EAF steps relationship. Just rack the EAF by 1000 units, for example and measure how much your drawtube has moved.

With my FOA-76Q and the 5V EAF, for example, I see 21.07mm of drawtube change for 5000 EAF steps, or about 4.2 microns per EAF step . The FSQ-85 and the 12V EAF gave about 4 µ per EAF step.

Every situation will be different, so you need to measure your own set up.

FWIW, a WhiteCat51 with a belt driven EAF connection gave me 2.2 microns per step, and a belt driven Askar ACL200 gave even much smaller movement per EAF step.

Chen

zuts It's a bit difficult with a Schmidt Cassegrain as there is no drawtube.

Ah, I forget there are people using SCTs.

You can use the Detect Star tools to get approximate HFD numbers. As you get into the critical focus zone, the HFD will no longer change, or changes erratically. Just plot the HFD (from Detect Stars) vs EAF and you should see a nice V (not the parabola that ZWO draws -- that is wrong; away from focus, geometric optics predicts an asymtotic hyperbolic curve with two straight walls, not a parabolic curve). The flattish area is the Fourier optics region ("diffraction limited") that deviates from a hyperbola. Ask the autofcous routine to step at last half the span of the flat region.

Ideally, you will want the HFD to start from around 7 to 12 pixels, down past the minima, and then back up to about 7 or 12 pixels again. All with no more than 12 or so total steps.

Chen

PeterH Can you say it in simple words?

OK, go back to high school physics and geometric optics... notice all the nice straight lines forming similar triangles for a simple lens.

What happens there is that if you look at the star size (say HFD) as you go from intra-focus to focus, or extra focus to focus, you will see that the HFD from geometric optics form straight lines (with minimum HFD of zero).

As you get close to focus, diffraction starts to make the curve deviate from geometric optics. But further away from focus, where there are more points, the "curve" should take on a V shape (thus what autofocus is called).

However, in their infinite wisdom, ZWO assumes that it is a general 2nd order curve, ignoring Physics.

So they blindly use linear least squares (I guess that is what they learned in math, but not physics), and what you end up is the part of the conic section that is a parabola (what you usually see from ASIAIR).

Now, when I use a constrained 2nd order curve to model as a hyperbola, and thus get the straight sides of a V-curve.

(I had use minimum mean square to solve, since it is easier to constrain the solutions to hyperbolas.)

Sorry, I can't make it simpler than this since it involves mathematics of conic sections. You either know it, or you don't know it. But look for the parabolas from ASIAIR, vs the V shaped curve above. I have a program that takes HFD for different eaf steps, and then spit out the focus EAF steps, if anyone is interested... Xcode project for macos.

Notice the very distinct point of the hyperbola (that is the estimated focus) instead of an indistinct rounded cusp. 52940 EAF steps, reported on the graph.

ZWO then makes another mistake of taking a second pass. They should not haven done that, but go straight to the minimum of the curve! Not only does it waste time, they are using singular data points and therefore has noisy data.

Two mistakes, compounding one another, and you end up with flakey autofocus in ASIAIR. When I use ASIAIR, I stop it after the first pass, look for the minimum, and go straight there. That way, there is only the inaccuracy of parabolas. On the second pass, you will notice that ASIAIR does not always end up at the parabola's minimum; and that causes big star blobs. Best is just to abandon ASIAIR and use something else, or Bahtinov masks.

Chen

PeterH as a MacUser for nearly 30 years I didn't wanted to spend money to a WINDOWS Solution

I was still working at Apple 30 years ago :-). I retired in 2005 -- first joined in 1988.

I keep an old Surface Pro around, just in case some devices' firmware update needs it, but other than that, I don't use Windows. My spouse retired from Intel, and she has not touched a Windows machine since she retired :-).

I also had a massive tilt issue

Because of the way it implemented "autofocus," tilt is bad news for ASIAIR. The tilt causes stars at different parts of the frame to be at different focal distances. And ASIAIR does not restrict the region of the star search -- it seems to jump all over the entire image frame looking for appropriate stars . To be able to compute HFD (cannot take saturated pixels), when it is very out of focus, ASIAIR picks bright stars; and when it gets in focus, it picks dimmer stars. Instead of doing this, it should be smarter and change exposure and gain so that it can use the same star (say 1/3 of the away from the oprical axis).

I pretty much never use ASIAIR's "autofocus." It is too amateurish. I either use my own V curve if I am too lazy to go outside the swap a Bahtinov mask in and out, or I use a Bahtinov mask, which is the fastest way to achieve accurate focus.

ASI AIR is not able to a) focus well and b) provides not a Filter Offset

In case you are not aware, basing focus on filter glass thickness ("filter offset") does not work if you have a flattener or reducer that requires an accurate backfocus.

I don't know who first suggested it, but this old wives' tale has been around for ages now, and it is plain wrong for people who use flatteners and reducers! The only time it works is 1) you have a simple doublet or triplet without a flattener/reducer, or 2) you use a Petzval scope.

I have a Petzval (FSQ-85) and I still have to adjust backfocus when I change filter thickness because I use the 1.01x flattener and the 0x73x reducer.

When the filter is in the path where backfocus distance is critical, you need to change the backfocus. You cannot change just the focus!

My EFWs uses 36mm and 2" Chroma filters, which thankfully have constant 3mm thick glass. But not all manufacturers are consistent. The Optolong UV/IR cut filter, for example is 1.8 mm thick, while their L-Pro filter is 2 mm thick.

I tend to use filter drawers from my color cameras, and the different filter thickness is all over the map -- and when I change filters with them, I also have to change backfocus. I have been using this backfocus adjuster on my different OTA just so that I don't have to fiddle with spacer rings in the dark when changing filter glass thickness:

https://agenaastro.com/askar-m54x0-75-thread-spacer-ring-backfocus-adjuster-with-variable-16mm-20mm-extension.html

Chen

PeterH pls explain again how you use your own V-Curve. How do you create it and how do you deal with it in the
a) middle of the night, if you are sleeping

Hi Peter,

I don't do astronomy when I am sleeping. Habit from my days of actually looking through a telescope's eyepiece. IMHO, you really cannot enjoy astronomy or instrumentation if all you do is tell a computer to make exposures for you and then leave the telescope alone. Watching how things work is one of my enjoyments in this hobby (started when I ground my first 5" mirror in high school back in 1963).

if you change a filter

When you change a filter of a different thickness, you need to go outdoors to change the backfocus anyway, if the filter is located between a flatterner/reducer and the sensor - you have to be awake if your filters do not have the same thickness.

You cannot sleep through a filter change if you are not using a Petzval scope. Even with a Petzval, if you have a flattener or reducer, like the 1.01x flattener on my FSQ-85, you still need to change the backfocus.

if the temperature changes

This one is simple. I use a ∆EAF vs ∆T curve for temperature changes (which the FSQ needs every 0.5ºC). Since there is no need to change backfocus, this can be done from indoors.

I have an FOA60Q (not the FS60Q, like you), but I only use that for solar work. We don't really need such high quality optics for H-alpha solar, but I just wanted to own the commercial scope with the best Strehl ratio. I have stack of two 60mm Lunts to go with it.

are you still fine with the ASI AIR or what do you think of stellar mate?

I use ASIAIR and Stellarmate when I am lazy, and I write my own macOS code to work with INDIGO Sky on a Rasberry Pi. I am planning to load INDIGO into a Mac Mini M1 that I have modified for 12V (added a buck boost converter after removing the 110 volt power supply). Because Peter Polakovic is a Mac nut (he even bought a Mac Studio before I bought mine), INDIGO is perfect for people who use Xcode and Objective-C (I retired from Apple in 2005). My All-sky telescope is a complete INDIGO setup (Ethernet PoE), also with my own code.

Stellarmate user experience has improved quite a bit after they intoduced the iOS app. So, somewhere between ASIAIR and INDIGO, IMHO. Not as brain-dead as ASIAIR, and not as flexible as INDIGO.

I am licensed for INDIGO A1, but don't really use it, since I use INDIGO primarily to develop and test some of own algorithms.

how can a perfect evening using the ASI AIR look like ?

Not too bad if you have been doing astrophotography for a long time. ZWO takes too many ignorant short cuts, and I pity the beginners who don't know how to work around the simplistic stuff and default settings. The biggest downside is it does not support any quality devices. It only supports the cheap ZWO devices. Its support of PHD2 sucks, but I do get consistent 0.35" RMS type autoguiding with my RST-135e.

I also do not use auto meridian flip by the way, nor any "auto" in ASIAIR (like auto centering). All the "auto" stuff from ZWO are complete nonsense junk, with zero understanding of celestial mechanics, and time keeping.

Chen

PeterH dann können wir uns ja in Deutsch unterhalten

Nein Peter, meine Deutsche ist zu schlecht! :-)

Chen

P.S.: There must be at least two typos, and five grammatic errors in that one sentence :-).