Nitty-Gritty Process for Filter Offset/Temperature Calibration?

I tried a Sequence last night to develop filter offsets. LRGB Ha-O3-S2. Set all for 2x2 and a few seconds for LRGB and 90 seconds for Narrowband and 3 repetitions for each filter (alternating between filters each time. I have an f/4.5 5" refractor and set step size to 100, with 3.7 micron pixels camera ASI1600. I was getting a lot of variance within individual filters (100+ microns) and the NB struggled to reach focus. Plus by the time I was halfway through the temp had dropped almost 2 degrees C, so that probably had an impact.

I was thinking it was too ambitious to try to do all the filters at once - maybe the LRGB together at 1x1 and then Narrowband at 4x4 But how would I account for temperature differences? Should I do a Temperature calibration first with the Lim filter and then apply that to the values I get on the offset run? Would appreciate practical advice from those who have gone before.


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Your suggestion about doing temp calibration with just Lum and then filter offsets later is a viable option.

If your temps drop a lot overnight (as mine do), I think it’s best to record three numbers for each successful focus: filter, position, and temperature.

Take a lot of data points (dozens), particularly if you have a lot of scatter for the NB filters. For me this takes at least all night, sometimes several nights.

Then after the fact create a spreadsheet with the three columns (filter, position, temp) and plot focus position against temperature for each filter.

You should get fairly linear behavior between focus position and temp, with the slope of each filter line being very similar to each other (parallel lines). The lines for each filter should be offset from one another.

Run a linear regression for each filter, to get a slope and intercept.

The average of the slopes is your temperature coefficient.

Subtract the y-intercept of each filter’s line from the y-intercept of the Lum filter. These are your focuser offsets.

Here’s a program in R that does all this, and an example of the output for my system.

Scott Denning


That’s cool Scott. I had already started entering that data in a spreadsheet but I see I don’t have nearly that much data yet.

Mr. Denning,

Is it, do you suppose, reasonable to assume that the offsets that work at summer evening temperatures are likely to work at winter evening temperatures? That is, do you believe that a given OTA’s expansion and contraction is linear over a range of, say, negative 10 degrees Fahrenheit up to 80 degrees Fahrenheit?

(I’m thinking your R program is a brilliant bit of work.)



With some scopes I have found that the temperature dependency is very tricky to get right. The temp sensor reacts quickly to falling temp but the mechanics do not. Steady state is different to dynamic state. For that reason I gave up on temp dependent focus positions and used a focus mask to quickly establish relative positions and then have SGP trigger an AF run for every 0.7C change.

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It would be a great SGP feature if this information was logged and could be displayed easily the way Scott has done above. I think the focuser runs are logged in the normal log file but it takes a fair amount of searching to extract the results. I guess the one catch is the focuser position (at least in my setup) is not absolute so it might be difficult to compare from one run to the next.

My understanding is that the filter offsets are a result of the optical properties the filters themselves, so yes I hope they are robust to changing temperatures. But I’m no expert, just trying to suss out how best to use my equipment and software just like everybody else!

In the case of refractors (or anything with glass optics in the light path) the offsets are a result of both the optical filters (optical path length at different wavelengths) and the optics themselves, which may not focus all wavelengths at the same point.