FLI Atlas focuser-V shape issue

Sequence Generator Equipment Compatibility
1

Hi, normally I tried looking for a solution on the different forums , but I have no luck to solve this problem.
I have a FLI Atlas , connected to the Proline 16803, and FW, I am using the SGP autofocusing setup and tried different parameters in the SGP setup, and cannot get a normal “V” or even a “U”, My telescope is a FL 1400 , F/Ratio 5.5
I tried first using 2000 steps based on the calculation below:

Autofocus method Data At 1x1 2x2
Telescope FL 1400 Pixel size 9 Read Noise RMS 10e-
Number of steps 105000 resolution per steps 0.083 Micros/step travel 0.35" 8.89mm

                                                    CFZ Critical Focus Zone

Ha 3nm 656.3 95 95
OIII 500.1 72 72
SII 671.6 97 97
Red 650 94 94
Green 510 74 74
Blue 475 69 69

Formula CFZ / Focuser step size Multiply by 1.5 and 2
Ha OIII SII Red Green Blue
(CFZ / Focuser step size) x 1.5 = focuser step size 1,716. 1301 1753 1698 1337 1246
(CFZ / Focuser step size) x 2 = focuser step size 2,289 1734 2337 2265 1783 1662

But when I tried using Ha, with a 2000 initially there were erratic ‘V’ shapes, it started fine on the first point , then followed by a higher point on the second data point, giving a zic-zac shape, I reduced the steps to 500 ,and up to 3000 without success.

This morning I have included a backlash (IN wards) of 500 , but not sure if even the 500 is correct, I read to use the following formula :step size/4 (2000/4), I wanted to see if this will better the V shape as the focuser starts pulling back.

I read in a forum that someone using a FLI Atlas used 200 steps , if the focuser number of steps is 109,000 (say 1 steps = 0.0000846mm), so 200 will move only 0.01692mm which seems too little, but not sure because I haven’t tried it.

My normal work flow is first to try manually the best focus possible, this is my starting point, my SGP setting is the following:

Exposure Time 10sec
Bin 2x2
Data Point 7
Step Size 2000
I thought that the focuser payload could be a problem after having it for four years, but my setup weight is much less than 11 kgs.
Any suggestion will be appreciated
Charles

2

Sorry but I could not understand all the values you did mention but this is what I use in case you want to try:

  • Backlash, a lot, if you use more than it is needed it is OK (I would try 2000 or more and leave it there until I get a decent curve and then the backlash could be re adjusted. Remember that if it is too large it will not afect your focusing curve at all).
  • Remember to set up backlash “in” or “out” depending on your setup and always against gravity. For my refractor the BL compensation ends with the focuser moving “in”
  • Step size: I use 120 micro per step but SGP has a recomendation on how to calculate this value so that max value on the curve is about 2-3 times the minimum.
  • My resolution: 3 micro per step (I wish it was less but …), yours is 0.0889 micro. To get the 120 microns that I use you will need a step size of around 1350 micron
  • And at the end of your email you mention Step size =10 ¿???

Renan

Enviado desde Correo para Windows

5E70C97FBC0F4581A422BA8BE67CEA32.png

3

Renan thanks for answering, the point about the step value was suppose to be 2000, instead of 10 as shown apologies for that, on my calculation is based on finding the step size using different filters nm and CFZ. Well, tried it but with no real result.
I would read you mail in detail and try it tonight.

Thanks again for answering.
Best
Charles

4

A general advice is to learn using your electronic focuser in “manual mode” first, meaning you should be able to find and step through the focus zone using these controls in SGP, without attempting the auto focus:

image

This way you will learn how it behaves, and get an approximate step size. And pay attention to what happens when you change direction.

And as Renan said, once you can manually move in and out of the focus zone reliably, then you can calibrate the step size by stepping out (from good focus) to about 2-3 times the minimum HFR at focus. If you want to have 7 points on your curve, then you should reach this HFR in 3 steps in either direction.

If you can provide a SPG logfile of your auto focusing attempts it would help as well.

Mikael