So selecting a value of refocusing every 1 degree in temperature change makes a lot of sense. Also the critical focus zone is just a little bit smaller in blue light. I like my stars as nice and tight as possible. In my case, I would rather err on the conservative side. 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.
15.66 * 1.5 = 23.5 which is greater than 19.5 steps, although not badly so. Third focusing every 1.5 degrees risks being outside the critical focus zone before you refocus. 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. So however I was doing it that night, I was doing it a little more frequently than I needed to. In every case at the beginning the difference in steps when refocusing was smaller than my critical focus zone. So now we have all the information we need to draw some conclusions. Taking the worse case from above, my focus changes 15.66 steps/degree centigrade. 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. Fortunately Google to the rescue and it tells me that is 4.064 microns.ĭividing 159 microns for my critical focus zone by 4.064 microns tells me that my critical focus zone is 39 steps. I love converting between inches and microns. Now I need to know how big in microns is each step of my focuser. 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. Now we need to figure out the critical focus zone of the telescope. 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 centigradeĪll of these answers are actually reasonably close. 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 We can approximate the linear fit by taking the first and last values and calculating a slope. But that is what real world data will do to you. In this case the last two points are kind of close together so it spoils it a bit. Normally, a linear fit will approximate the data reasonably well.
The focus temperature and position are right after a refocus has been done. Here are some values I measured from a recent session with NGC 3344. My temperature probe is messed up so that it wasn't really 40.9 degrees centigrade, but it still gets good relative values.
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