Calibrating the Atlas Focuser

I have to recalibrate the new focuser:

1. Autofocus parameters in SGPro
2. Temperature compensation
3. Filter defaults
4. Filter offset

Before I could start, I noticed that it sometimes took several attempts to connect to the focuser. And later, SGPro would sometimes hang when it needed to move the focuser out for the first measurement. I asked on the SGPro mailing list - need to find a way to generate debugging information for the Atlas Focuser. Asked FLI about it.

Next, I wanted to calibrate autofocus in SGPro. But the very small resolution of the Atlas focuser means that even with a step size of 1,000, I have to take more then 20 (better 30) data points to create a good V-curve. Unfortunately, SGPro doesn't allow step sizes greater then 1,000. I asked on the mailing list and the maximum will be raised to 10,000 in the next SGPro release. Yei!

So, for now, I have to use a step size of 1,000 and 31(!) data points to get a good V-curve. Which of course takes quite a while :-(

While I was playing with the step sizes, I also played with binning. And I noticed that there isn't much of an accuracy improvement from binning 1x1 to 2x2. But of course, the exposure times are going down significantly. But when I set binning to 3x3 or even 4x4, the accuracy in the focus region gets very bad. So, binning 2x2 is what I will now be using.

With regards to temperature measurement, the Atlas focuser has 2 measurements: internal and external. The internal one looks much more reasonable (the external one reads very high). So, I'm using that one for now, but also asked FLI what these two temperatures are about.

For the next night, I created an SGPro sequence to do the focus measurements. In the autofocus settings I set "Refocus after each frame", so that SGPro would run the autofocus routine every time. As SGPro writes the temperature and the focuser position into the FIT file, I could run this sequence all night and evaluate it the next morning.

1. Sequence

8 Luminance images

3 Red images

3 Green images

3 Blue images

3 Ha images

3 SII images

3 OIII images
(I set this sequence to end at 3:30, so that I would take another series of Luminance images)

2. Sequence (to be started at 3:30)

8 Luminance images.

All images are 0 second exposure time.

With the Luminance images at the beginning and the end of the night, I can determine the temperature coefficient of my focuser. With the other images, I can determine the focus position differences. With the temperature coefficient I can then calibrate them all to the same temperature.

The next morning I noticed a) it wasn't enough time to capture all filters. The narrowband filters take 30 second exposures. With 31 exposures that's 16 minutes per images!

And b) it also got cloudy at the end of the night and I only had 5 luminance images at the end of the night.

Nevertheless, I think I had enough measurements for the temperature coefficient:

	Evening Measurement			Morning Measurement
	23.5625	25251		19.5	23104
	23.6875	25202		19.75	23159
	23.4375	25010		19.625	22963
	23.375	24886		19.6875	23118
	23.4375	24380		19.63	23265
	23.25	24688
	23.0625	24606
	23	24337
Average	23.35	24795		19.64	23122
	Temperature Difference		3.71
	Focus Position Difference		1673
	Focuser Steps per 1 degree		451

So, 451 steps per 1 degree (which shows the small resolution of the Atlas Focuser - the Robofocus focuser had 15 steps per 1 degree!!!)

Next, the individual filter measurements.

From the Averages (you can see all measurements in this spreadsheet):

	Lum		Red		Green		Blue
	Temperature	Focuser Pos.	Temperature	Focuser Pos.	Temperature	Focuser Pos.	Temperature	Focuser Pos.
Averages	23.3516	24795	22.813	24139	22.6250	23942	22.1875	23349
Diff to Lum Temp			0.5391		0.7266		1.1641
Adjustment				243		327		525
Adjusted Pos				24382		24269		23874
Diff to Lum Pos				413		526		921

So, these are my filter offsets and filter defaults for the RGB filters. Next, I need to take the defaults and offsets for the narrowband filters. But I'll do that after the Oregon Star Party (where I will use the LRGB filters only).

In the next nights, I will still focus before each frame to compare how well the temperature compensation works.