Hi John,

I greatly appreciate your approach since I face the same issues with my EAFs. Only "linear" works, but not very reliable in the 2nd pass.

Question about the standard deviation weighting of the fit: The standard deviation will be calculated from all HFR values obtained from the field, correct? The HFR distribution depends (1) on the distribution of the star brightnesses in the field and (2) on local effects (seeing) on individual HFR values. So (1) will be nearly constant for a given focus field, but (2) varies and is a measure for the "quality" of that particular focus frame. Or am I making a mistake here?

Nevertheless, I would be very happy to test your routine!

CS, Bernd

Hi Bernd,

Thanks for replying. On the standard deviation:
1. If single star mode is selected, the SD option is disabled.
2. For multi-star... the stars go through an extraction process to find suitable candidates (I haven't changed this). There are several options but if you use the default SEP there are many parameters to control this, for example removing clipped stars and those below a certain brightness. After this process has run there will be a set of stars that meet the criteria and each has its HFR calculated. I then calculate the SD of this set.

The star set will vary from frame to frame and there is some existing work in Linear to try and use the common subset of stars for consistency. I am trying to use this in Linear 1 Pass, but there is still some work to do. Variation in brightness of the stars should not affect the HFRs too much (assuming the extraction and processing parameters are set reasonably).

The variation in HFRs will depend on focus effects, seeing, etc. but I think its a reasonable measure of "quality" for the focus point. What I've observed on my equipment is a trend whereby SD increases with distance from optimum focus but then certain points deviate from this with transient effects like seeing, wind gusts, etc etc.

As to whether applying weighting makes enough of a difference to the calculated optimum focus point to be noticeable, I don't know. We'll just have to give it a try.

Thanks for the explanations, and your project sounds really promising, I'm very curious!

Regards, Bernd

John,

Thank you. I understand the process... but in my case, when it does the "backlash hokey-cokey" it moves out then in so quickly, that I don't think the focuser has reached the full "out" setting. I think it would benefit from a move out, pause 1 second, move in... to assure that it getting the full effect.

Thanks,

Ron

Hi Ron,

A couple thoughts for you. I like the approach, it has a BIG advantage over the current linear alg in that it doesn't "pull up short" which has previously been described by prior posts on the subject.

My only suggestion is to consider whether you want to make the same mistake Linear Alg made in the Mechanics tab. Picking an "Initial Step", and "Out Step Multiple" has created a lot of unnecessary confusion for folks. In reality, the "proper" way to define the step size and out step multiple is to calculate those parameters from other system info as it relates to CFZ. Knowing the focus drawtube screw pitch and motor scale would ensure the correct step-size is used. Alternatively, not knowing these parameters creates the conditions for step-size exceeding the CFZ (jumps over), or too small (wasting focus time). This is completely avoidable. You might want to consider at least having an option to calculate the step size, correlating to CFZ, motor scale, and focuser thread pitch size. See the following info which might help:

www.goldastro.com/goldfocus/ncfz.php

Cheers, and good luck, Doug S

Doug,

Thanks for the thoughts. I have read this article and find it interesting. I guess that I have used "trial and error" for my different scopes and focusers.

Like others, I find that the "one pass" does a pretty good job at finding the best focus. The second pass process, while theoretically valuable, does little to improve the focus. In fact, it usually was the cause of creating an error, aborting and restarting the process.

I'm looking forward to using John's new process!

Ron

Thanks Ron, I'll take a look. I see you have a Celestron Focus Motor and ZWO EAF. Do you think both have this issue?

Thanks Doug, I'll take a look at this.

John,

I only use the Celestron focuser with the linear method of my Celestron 8HD Edge. I'm not sure that I would describe it as an issue with the mechanics... the current process just doesn't give the focuser time to complete the "out" movement before it issues an "in" command. It may be working fine and I'm simply experiencing what Doug is describing. However, when I'm sitting at the scope it sounds like it starts reversing before it finishes the out movement.

This is one of those situations where I would think it wouldn't hurt to have a very short (1 sec) pause between commands.

BTW... I'm not fond of the Celestron focuser. I sometimes experience connection issues with INDI... seems unstable. I've ordered an ZWO EAF to replace it on the 8HD Edge, but have been waiting several weeks for the back order to clear.

The ZWO EAF on my refractor works perfectly with the polynomial process.

Thanks for your effort!

Ron

Would it be possible to have the system determine what the step size should be in some way each session? The starting step for determining the final step size could even be calculated from the user's equipment configuration. Store this value and use it as the seed for the next time an auto-configure of step size is run.

Thanks for the reply Jim, let me know if you have any suggestions on what the step size algorithm should be.

Hi John, IMHO, the only step size that is "appropriate" is one that is tightly correlated to the CFZ size (in microns) for the instrument. This is what the Critical CFZ discussion I previously posted is all about. It's seeing and focus tolerance dependent after factoring f/ratio and aperture size. Whether one wants to be conservative and use a step size that is 1/2 CFZ (to always be sure to land in it), or less conservative (~1 CFZ) could be a user choice (but with potential consequences).

It would be best if the algorithm calculated step size from user input of how many microns per revolution the focus motor has, and how many microns of focuser drawtube travel occurs per revolution of the focusing mechanism. Not knowing these two values, and just "winging" a generic step size is just guessing. Folks should try to get some understanding of their CFZ and then set a step size to match the instrument. Since some will not want or know this info (or won't want to go to the trouble to find it), the guess may need to be allowed. But a better solution would be to allow for a precise calculation when the parameters are known.