Additional autofocus trigger condition - HFD change

Would it be possible to add a new focus trigger condition in sequences or mosaic/research and survey?

One of my imaging scopes is an SCT, and it is strongly affected by temperature change, but the focus change lags behind the temperature change, if I trigger focus runs based on temperature delta it was often possible to see the focus continue changing after a focus run, the first exposure after focus would be sharp then the successive ones would go out of focus until the scope stabilised. My only resolution was to focus after every third exposure to keep it in focus early in the night as the air temperature fell, but this obviously leads to lost time repeatedly focusing through the night if the temperature is stable.

Given the image statistics Voyager generates, would it be possible to use percentage HFD change since the last focus run as a trigger? I imagine cloud could fool this measurement, but in that case your data will be poor anyway.

in this way you just follow the problem without solve.
Leave the telescope stabilize the temperature before use. My RC need 1.5h before use, my FSQ 1hr.

If you want you can inject a focus request from monitor menù when you want during the sequence or dragscript.

All the best

The issue I run in to where I live is the temperature gradient over the evening in the warmer part of the year. It is often steep enough to need regular refocusing (Again, the tube lags behind the air significantly) but not usually enough to cause air currents inside the tube. It can be nearly midnight before it all settles down. The only other way to deal with it is to ignore it so to speak, set up focus on both temperature delta and time and know that in the summer, subs shot before about midnight may be throwaways.

I don’t image a lot with that scope at the moment but I am just about to put it on a second mount so I can image with both scopes at the same time.

I will add that if it is not a feature that can be done, I will just keep imaging as I was before, just that a scope where the focus point can shift by over 100 microns per degree and nights where the temperature delta can be over 10 degrees is a tough combination.

Without stability any chance to get data is ruined. If you dont want to wait for stability you can activate the focus trigger by temperature but in anycase you will put in the basket probably a lot of sub.

Voyager not trust in focus compensation but you can activate it in your focuser driver if you have.

All th ebest

I am 100% with you on not trusting or using auto temperature compensation. I tried it out early and found it less reliable than leaving the focus alone altogether, and that was after wasting a couple of nights training the system. I found the same issue as focusing the SCT by temperature delta in fact, the focus compensation usually led the OTA so it moved and then the telescope caught up with it over time. I got more out of focus subs that way than by focusing on temperature delta alone.

The issue I have run in to is that I can have nights where the temperature changes by 10 degrees or more through the night and with the SCT design the foal plane moves by around 140 microns per degree of temperature change. You can get good data, but you have to focus often until the air temperature stabilises and that can be midnight sometimes.

If the idea of a HFD change focus trigger is not going to go anywhere (I thought it might be a useful trigger in it’s own right, better than based on time) I might have to do something with a dragscript to stop the sequence and focus at increasing intervals as the air temperature change slows down. In this case I am using a telescope that I bought ten years ago as a visual instrument here and it performs remarkably well for imaging, but it has some drawbacks.

use temperature trigger, this is the solution for you

I am going to come up with a combination of triggers for when I use that particular scope in warmer weather, as I wrote at the start, the issue with this scope is that in hot weather when the outside air temperature is changing significantly for much of the evening the scope can lag the air temperature and not stabilise until well into the night, so a focus run based on that will occur and then the scope continues to go out of temperature for a while longer. If I was going to keep imaging with that scope in the long term I would be brave and disassemble it to fit cooling fans and air filters to it to equalise it more quickly.

I think I will create two sequences for each target, with both having temperature and time triggers, but one of them having the time set for a much shorter interval, then manage changing between the two using a dragscript. That way I can keep it in focus in the earlier part of the night where the scope cant keep up and slow down the focus runs later when they are just wasting imaging time.