How Often to Autofocus
It is fairly common knowledge among astrophotographers that focus shifts through the night. As a result many automation packages that support image acquisition give you options for when to refocus. There are various reasons why refocusing may be necessary. A common one is that filters are not necessarily par-focal, so when you switch filters, you may need to refocus (or use filter offsets).
Another common reason you may need to refocus is because the temperature has changed significantly. Many of the pieces that make up a telescope expand or contract as the temperature changes. Because of this, image acquisition software often gives the user the option to refocus whenever there is a temperature change of some delta amount that you specify.
Here is what those controls look like in some common software acquisition packages:
It is fairly common knowledge among astrophotographers that focus shifts through the night. As a result many automation packages that support image acquisition give you options for when to refocus. There are various reasons why refocusing may be necessary. A common one is that filters are not necessarily par-focal, so when you switch filters, you may need to refocus (or use filter offsets).
Another common reason you may need to refocus is because the temperature has changed significantly. Many of the pieces that make up a telescope expand or contract as the temperature changes. Because of this, image acquisition software often gives the user the option to refocus whenever there is a temperature change of some delta amount that you specify.
Here is what those controls look like in some common software acquisition packages:
As you can see, the controls in these various packages are quite similar. Great, the capability is there. But that tells us nothing about how often we actually need to do that. Most often astrophotographers just try refocusing every 1 degree centigrade, or if they are really cautious, every .5 degree. But there isn't actually any science behind what they are doing. They get on the internet and someone said they refocus about that often and it is astrophotographer see, astrophotographer do.
Basing it on something more than a hunch or so and so said such and such is actually not all that difficult, especially if your acquisition software writes the focuser position and temperature into your FITS header for each file.
Basing it on something more than a hunch or so and so said such and such is actually not all that difficult, especially if your acquisition software writes the focuser position and temperature into your FITS header for each file.
My temperature probe is messed up so that it wasn't really 40.9 degrees centigrade, but it still gets good relative values.
Here are some values I measured from a recent session with NGC 3344. The focus temperature and position are right after a refocus has been done.
5200, 42.905
5186, 42.01
5178, 41.5
5177, 40.9
Normally, a linear fit will approximate the data reasonably well. In this case the last two points are kind of close together so it spoils it a bit. But that is what real world data will do to you.
We can approximate the linear fit by taking the first and last values and calculating a slope.
(5200-5177)/(42.905-40.9) = 11.47 steps / degree centigrade.
If I actually do this in a more sophisticated way using linear regression I get a very similar answer of 13.001 steps/degree centigrade
If I just decide the last point is anomalous and throw it out I get (5200-5178)/(42.905 - 41.5) = 15.66 steps / degree centigrade
All of these answers are actually reasonably close.
Now we need to figure out the critical focus zone of the telescope.
There is a calculator at this site:
http://www.wilmslowastro.com/software/formulae.htm#CFZ
Plugging in my values of focal ratio o f/8 and camera pixel size of 5.4 the answer 159 microns pops out in green light.
We are almost home free. Now I need to know how big in microns is each step of my focuser. I have a Moonlite, and my step size is .00016" in full step mode.
Great. I love converting between inches and microns. Fortunately Google to the rescue and it tells me that is 4.064 microns.
Dividing 159 microns for my critical focus zone by 4.064 microns tells me that my critical focus zone is 39 steps. If I was in the middle of my critical focus zone then I could move 19.5 steps before I was on the edge of the critical focus zone.
Taking the worse case from above, my focus changes 15.66 steps/degree centigrade.
So now we have all the information we need to draw some conclusions. In every case at the beginning the difference in steps when refocusing was smaller than my critical focus zone. So however I was doing it that night, I was doing it a little more frequently than I needed to. Second, if we wanted to refocus based on temperature change, it is clear that refocusing every 1 degree change in temperature is often enough with this system. 15.66 steps is less than 19.5 steps. Third focusing every 1.5 degrees risks being outside the critical focus zone before you refocus. 15.66 * 1.5 = 23.5 which is greater than 19.5 steps, although not badly so. If my real rate is actually more like the linear regression rate of 13.001 * 1.5 = 19.52 that would actually be a pretty good match to my 19.5 step critical focus zone.
In my case, I would rather err on the conservative side. I like my stars as nice and tight as possible. Also the critical focus zone is just a little bit smaller in blue light. So selecting a value of refocusing every 1 degree in temperature change makes a lot of sense.
Now the whole point of this article is that measuring this does not take a degree in higher math. A little subtraction, division, and a conversion using Google gets you the answer as long as you can plug in your focal ratio and camera pixel size into the critical zone calculator.
By the way, if you should so choose, the number calculated for steps/degree centigrade is also what you need if you decide to use focuser temperature compensation.
Here are some values I measured from a recent session with NGC 3344. The focus temperature and position are right after a refocus has been done.
5200, 42.905
5186, 42.01
5178, 41.5
5177, 40.9
Normally, a linear fit will approximate the data reasonably well. In this case the last two points are kind of close together so it spoils it a bit. But that is what real world data will do to you.
We can approximate the linear fit by taking the first and last values and calculating a slope.
(5200-5177)/(42.905-40.9) = 11.47 steps / degree centigrade.
If I actually do this in a more sophisticated way using linear regression I get a very similar answer of 13.001 steps/degree centigrade
If I just decide the last point is anomalous and throw it out I get (5200-5178)/(42.905 - 41.5) = 15.66 steps / degree centigrade
All of these answers are actually reasonably close.
Now we need to figure out the critical focus zone of the telescope.
There is a calculator at this site:
http://www.wilmslowastro.com/software/formulae.htm#CFZ
Plugging in my values of focal ratio o f/8 and camera pixel size of 5.4 the answer 159 microns pops out in green light.
We are almost home free. Now I need to know how big in microns is each step of my focuser. I have a Moonlite, and my step size is .00016" in full step mode.
Great. I love converting between inches and microns. Fortunately Google to the rescue and it tells me that is 4.064 microns.
Dividing 159 microns for my critical focus zone by 4.064 microns tells me that my critical focus zone is 39 steps. If I was in the middle of my critical focus zone then I could move 19.5 steps before I was on the edge of the critical focus zone.
Taking the worse case from above, my focus changes 15.66 steps/degree centigrade.
So now we have all the information we need to draw some conclusions. In every case at the beginning the difference in steps when refocusing was smaller than my critical focus zone. So however I was doing it that night, I was doing it a little more frequently than I needed to. Second, if we wanted to refocus based on temperature change, it is clear that refocusing every 1 degree change in temperature is often enough with this system. 15.66 steps is less than 19.5 steps. Third focusing every 1.5 degrees risks being outside the critical focus zone before you refocus. 15.66 * 1.5 = 23.5 which is greater than 19.5 steps, although not badly so. If my real rate is actually more like the linear regression rate of 13.001 * 1.5 = 19.52 that would actually be a pretty good match to my 19.5 step critical focus zone.
In my case, I would rather err on the conservative side. I like my stars as nice and tight as possible. Also the critical focus zone is just a little bit smaller in blue light. So selecting a value of refocusing every 1 degree in temperature change makes a lot of sense.
Now the whole point of this article is that measuring this does not take a degree in higher math. A little subtraction, division, and a conversion using Google gets you the answer as long as you can plug in your focal ratio and camera pixel size into the critical zone calculator.
By the way, if you should so choose, the number calculated for steps/degree centigrade is also what you need if you decide to use focuser temperature compensation.