Advice on autofocusing?

Hi all,

I’m using autofocus for the first time (Focus boss) and so far it’s working beautifully. My question is settings, and I’m just looking for advice. I don’t have it temperature compensated yet, although that will be coming sometime. I’m planning on obviously checking autofocus with each filter, but my question is how often do you autofocus other times? Do you do it between every frame? Every 10 minutes? Every 30 minutes?


I refocus every 0.5 degree in temp change and every filter change and meridian flip.


Hi Stuart

I re-focus on every filter change and on every sub. I have 2 imaging refractors, one at 500mm FL and one at 925mm FL native. The longer FL refractor will contract proportionately more than the shorter as temperature drops and will require a re-focus after 5 minute or longer sub, especially in the maritime environment where I live on the south coast of the UK. The shorter Esprit 80ED refractor has obviously less material to contract and the change in focus point throughout an evening is noticeably less than the longer WO FLT132.

I thought about setting a temp parameter for both scopes but decided on balance that the time taken to re-focus is better than the disappointment of a discarded sub when I came to processing.

So the factors I have taken into account in my decision related to the thermal characteristics/experience of my two scopes, the prevailing weather and humidity in the UK, scarcity of imaging time vs time taken to AF for each sub. I have especially noticed the softer focus-drift when taking 20 min + Ha subs.

A factor also is that I’ve recently changed to imaging sequentially through each event rather than completing an event before moving to the next: e.g., LRGB, LRGB, LRGB rather than all the Lums, then all the Reds etc. In this way building a dataset for each channel (and therefore distributing that evening’s conditions evenly throughout each channel rather than concentrated in any one channel) and so there is a need to re-focus on each filter change.

There is no right or wrong approach here, just a trade off of the factors affecting your equipment and imaging conditions.



Thanks guys. What’s interesting to me is that I’ve gotten answers from the extremes of the spectrum. I have one friend who is an advanced astroimager who told me, “If your focuser is temp compensated you should theoretically only need to autofocus once”. And then there’s Barry who focuses before every sub.

This gives me a range to work with. :smile:
Thanks again.

By the way, since I have the floor (and I can create a new topic if necessary), how can I see the autofocus graph for the last autofocus run? It’s there when it’s focusing but then the window disappears. Is there a way to pop the window back in view so I can see it?

Once it’s gone it’s gone. However you can open up the image history:

You can also set the delay for how long the AF dialog will be shown as well, but keep in mind this will block the sequence from continuing.

As for my settings I focus every 1 degree change and every filter change. I prefer this method as I’m not focusing when I need to. The image history shows that this is quite adequate with my setup as the HFR is very close between the image right before and right after AF runs. And you are correct that if your temp compensation is setup correctly you should never have to refocus due to temperature. However I haven’t gotten to the point of trusting this.


I ran into an interesting situation last night. I was setup to focus on
every 1 degree temperature change. As I neared a meridian flip, the
autofocus routine started running. I did not get a meridian flip, however
this was because I had it disabled :-(. It did raise a question in my mind
which is:

Which has the highest priority Auto Focus or Pier flip? This is important
because if Autofocus takes a while and leads to the mount reaching its
limits because we failed to pier flip, that could be problematic.


Hi Jared and Ken,
Can you tell me what I may be doing wrong tonight? The last two nights I’ve been working on M101, a dim target. I’m imaging with a TEC 140 and have a QSI 683, with a Focus Boss focuser using Focus Lynx. Last night I had no trouble with my lum data and it focused fine. Tonight, I am imaging color, and I’ve had a devil of a time trying to get the focuser to get a curve. It just squiggled all over the place, and I was never able to get focus with the red filter (I didn’t try the blue or green). I was registering stars so I know it was at least getting some data.

I increased the binning to 2X2, and I tried increasing the integration time in the focus setting to as much as 30 seconds a shot. But software didn’t seem to be integrating that much time–that is, increasing the integration time didn’t increase the amount of time per autofocus exposure. It was still only doing a few seconds per shot.

I gave up on focusing with the red filter and I set the lum filter as the default focusing filter and came inside as I wanted to get some data.

So my question is: What am I doing wrong?


What length exposure are you using?

Generally when I get good AF performance one night and bad the next it’s almost always because of a change in seeing. In which case upping the exposure helps to level things out.


Well, that’s the thing. I tried increasing it up to 30 seconds but it
didn’t make a difference. But this question occurred to me much later:
When you set the autofocus exposure box, is that for the whole run, or is
that per exposure. For example, if I set 10 seconds, with, say 5 data
points, Am I getting five 10 second exposures for a total of 50 seconds, or
five 2 second exposures for a total of 10 seconds? I was assuming it was
the former.

When you’re focusing with, say, a red filter, what is your exposure time?


You can set the exposure in 2 different places. But if you set it at 10 seconds that’s 10 seconds per exposure, regardless of how many samples you have.

For LRGB I generally set my exposure to 5 seconds. For narrowband I focus through my lum and have offsets for each filter relative to my lum.

Also red, being the highest frequency of light, will also be the least sensitive to focuser movement. Thus your curve won’t be as pronounced as it would be with say blue.


Also you should be setting up your focus exposure time here:


Oh, that must have been my problem. When I was setting the exposure time I
was putting it in the “no filter auto filter exposure”.