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Old 13-01-2014, 04:58 PM
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Focusmax and Atlas focusor

Last night I got to do my v curves run with the FLI atlas, GSO RC12 and STXL.

I thought it went ok until with a couple of exceptions.

The v curves were sort of lop sided towards the right with the center line. With the slope being shorter on the right than the left which starts at the top left. I was using 30 points and a 40HFD setting. The focusor would go from 21700 to 101500 with focus being at 68,000ish. There is plenty of travel left, but I am considering doing a 30HFD to make V curves easier. Perhaps it ought to be less.

The second thing and as can be seen in the attached image. The V curves don't look right. The PID's are not consistently close to each other. Peter.M suggested that this could be seeing related but not sure if that is the problem.

When I went to focus it would get to near focus and then it would endlessly try to get focus convergence. I had it set for 10 samples with the same but not one was the same as the other.

So aside from putting up a post on the Yahoo group, does anyone have any suggestions as to how to solve the problem. Am I missing something?
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  #2  
Old 13-01-2014, 05:22 PM
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your critical focus zone is 148000 nanometers, the FLI's resolution is 8 nm per step so 2000 * 8 is 16000 nm, well within your CFZ. The differences look big, but they arent its just the resolution of your focuser is better.
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Old 13-01-2014, 05:27 PM
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RickS (Rick)
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Paul,

My V curves on the Ceravolo are fairly symmetrical. I'm getting more (but not incredibly) consistent PID and much closer agreement between left and right slope on each curve than you are at a similar focal length. I doubt it's my exceptional seeing

I'm sure you've already checked for any mechanical issues but worth check again for anything that might be flexing or moving.

How's your collimation? Does a defocused star look the same inside and outside focus? Something weird going on there could give rise to odd looking asymmetric curves.

Cheers,
Rick.
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Old 13-01-2014, 05:30 PM
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multiweb (Marc)
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Lopsided could be linked to some kind of optical aberration in and out of focus?
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Old 13-01-2014, 05:36 PM
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one thing springs to mind, might be wrong, what start focus value have you set up in focusmax, the last good focus?.
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Old 13-01-2014, 05:47 PM
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Collimation is ok but may need looking at. In and out looks ok but collimation could be a valid consideration.

Fred the start focus value was set by the wizard but it is 51340. If that makes any sense.
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Old 13-01-2014, 05:50 PM
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Another thing that can make the v-curves lopsided is when the focus is changing due to a temperature induced focus shift. I find that if I am using a scope that shifts badly due to temperature change, I have to wait till the temperature stabilises before I can do v-curve calibration.

Brett
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Old 13-01-2014, 06:00 PM
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Quote:
Originally Posted by bert View Post
Another thing that can make the v-curves lopsided is when the focus is changing due to a temperature induced focus shift. I find that if I am using a scope that shifts badly due to temperature change, I have to wait till the temperature stabilises before I can do v-curve calibration.

Brett
Hmmm yeah could be that, but night time temps are not stabilising until much later in the night. Point taken. I don't know what the temps are in the dome but 40+ today and 36 yesterday.
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Old 14-01-2014, 09:37 AM
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Did another run this time with 30HFD and V curve was still slightly shifted to the right but better. Temps were in the 20's at the time, so scope might not have cooled properly. I will need a night of lower temps.

Using Focus convergence in Focusmax results in no focus being obtained. I let it go for about 45 or so iterations and then cancelled it. I unchecked focus convergence and focusmax obtained focus. I had a read about focus convergence, but wonder what is going wrong. It is supposed to be the ducks nuts for focus control. Any ideas out there?
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Old 14-01-2014, 11:13 AM
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Quote:
Originally Posted by Paul Haese View Post
Did another run this time with 30HFD and V curve was still slightly shifted to the right but better. Temps were in the 20's at the time, so scope might not have cooled properly. I will need a night of lower temps.

Using Focus convergence in Focusmax results in no focus being obtained. I let it go for about 45 or so iterations and then cancelled it. I unchecked focus convergence and focusmax obtained focus. I had a read about focus convergence, but wonder what is going wrong. It is supposed to be the ducks nuts for focus control. Any ideas out there?
Depends on what focus convergence settings you are using. The standard settings will be several magnitudes out fort he atlas's resolution. You set how many iterations of convergence you want and a target range that you want those focus points to be grouped. You have my number, give a call if my incoherent rumblings don't make sense.

Brett
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Old 20-01-2014, 10:36 AM
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Ok just to wrap up this thread for anyone using an Atlas and focusmax in the future, this is what I have done with focus convergence settings.

I now have the steps at 100 and doing 10 samples and the scope focuses in 60 seconds. 105 seconds from start to finish and back onto the target.

I worked out the CFZ has 1870 steps in it and using 100 steps for focus convergence gives about 15 samples with the 10 being in the same step. I tried using 7 samples but found the focus not as totally accurate.

To work out your critical focus zone you times your focal ratio by the size of the pixels on your scope. That gave me 159 microns. There are 85nm to each step on the Atlas. I converted the 85nm to microns which is 0.085 microns and that makes 1870 steps.

I am still not 100% happy with the V curves and I am going to thoroughly check the collimation once I get hold of a glatter RC collimator. I suspect the V curves are being tainted by collimation issue. However at this stage things are working well.
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