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Old 23-09-2015, 10:30 AM
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Hi Peter,

I measured the angles in the second image between the lines in imageJ and the angles are not equal - the red line should go down as Bahtinov grabber suggests. The difference is nearly 2 degrees (about 8%). But that would also include some measurement error...
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Old 23-09-2015, 11:13 AM
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Interesting - must be manufacturing error in the mask!

I wonder how that impacts the diffraction pattern accuracy in depicting correct focus?
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Old 23-09-2015, 01:13 PM
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When I think about it, would it be reasonable to expect that the mask should be orthogonal to the optical axis and precisely cut for the image to be sharp and symmetrical as well?
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Old 23-09-2015, 05:12 PM
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When I think about it, would it be reasonable to expect that the mask should be orthogonal to the optical axis and precisely cut for the image to be sharp and symmetrical as well?
I was hoping you had the answer
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Old 23-09-2015, 08:10 PM
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I was hoping you had the answer
I guess sometimes answers (and new questions) are generated by threads like this one

I think one way to verify which one is more precise for your system - manual focusing with Bahtinov mask or SGP focusing routine (or offsets) would be to take a few shortish exposures of the same area of the sky after focusing with all three methods and then to compare the stars. But the outcome from such investigation might be different for different setups, so it might not necessarily apply to everyone.

In my case, I would still go with autofocus in SGP for temperature compensation. For some strange reason I do not totally trust preset offsets, and Bahtinov mask is just not practical for long imaging sessions.
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Old 24-09-2015, 01:53 PM
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In my case, I would still go with autofocus in SGP for temperature compensation. For some strange reason I do not totally trust preset offsets, and Bahtinov mask is just not practical for long imaging sessions.
The only problem I see with that argument is you could spend a long imaging session relying on autofocus and find your stars are out of focus when you come to process. I'd rather spend a bit of time during the image capture phase making sure the raw data is as good as it can be. There's not much point having a fully automated capture process if the results are not satisfactory.
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Old 24-09-2015, 05:02 PM
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The only problem I see with that argument is you could spend a long imaging session relying on autofocus and find your stars are out of focus when you come to process. I'd rather spend a bit of time during the image capture phase making sure the raw data is as good as it can be. There's not much point having a fully automated capture process if the results are not satisfactory.
This 100% depends on your situation, When I go to pauls house to image for new moons. I will watch the focus routine and make sure it picks the best focus. From home, mid week, if I do a run and it gets 3 shots that are out of focus at 3am and then goes back into focus. Its way more data than I would have got without setting up because I have work and cant babysit it all night.

I will fire up the lappy and see if I have any logs one of these days to send you .
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Old 24-09-2015, 05:48 PM
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I totally agree with Peter.M, and that's what I mentioned in my post - it depends on your situation/set-up.

SGP's autofocus routine has been working for me very reliably and it consistently (always) gives me a good focus. I think my recent photos are a good testimony of that. Whether focus in my case could be a tad better leading to sharper images, I do not know that, but given decent yet limited tracking accuracy of AZEQ6, I doubt it.

Also, unless you are constantly checking temperature changes, I bet correctly set autofocus would yield superior results to one or two manual focus adjustments throughout long imaging session. On clear nights temperature can shift quite significantly, usually with my gear I get 5-7 automated focus adjustements per night. It would be impractical for me (too lazy) to manually interrupt imaging session so many times, slew to a bright star, refocus, slew back to the target, plate solve...waste of imaging time, in my opinion.

Maybe one night I should do a test and compare the stars with autofocus and manual focusing with a mask, and that way I would know for sure
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Old 24-09-2015, 08:22 PM
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Here's a curve for my refractor rig Peter.
At f5 using a Hurst stepper on my robofocus setup (believe it has bit more steps than normal robofocus motor and more torque):
- CFZ 62 microns
- Each focus step is about 3 microns (20 steps across the CFZ)

In SGP I've found best reliability using:
- 11 data points
- Step Size: 15


Sometimes with 9 steps the curve might not centre well, especially if I didn't star very close to focused. 11 steps takes a smidge longer but seems more reliable. On some filters I find the HFR is falsely low if the focus is way inside our outside of focus and that could confuse things. 11 steps with 15 step size seems to almost always avoid this by give a nice steep V curve.

I wouldn't normally post the Ha curve, but that's all I'm shooting tonight, so I have attached the most recent refocus if of any value. Makes sense different OTA systems will have different shaped V curves though I would have thought. I'm yet to get enough confidence with offsets to focus on a broadband filter in SGP, so let it run off 30 sec exposures. I don't totally agree with the final point on the curve below - usually its a nicer fit for LRGB.
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Old 24-09-2015, 08:27 PM
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Slawomir,

I'm not suggesting you should do something different - autofocus seems to be more reliable/forgiving on slower refractors than faster Newtonians and if it was working for me I'd use it.

What I'm saying is it doesn't make sense for me to ignore the inconsistent results I'm getting with autofocus on my rig. I'd rather set the sequence to automatically pause for autofocus when the temperature drops 1 or 2 degrees (TBD) and get a good focus manually with the mask than capture a night worth of rubbish. I can pre-set a focus star position and slew to that with one click. When focus is done I can use centre target to re-centre the target. If I use filter offsets I only need to re-focus when the temperature changes.

Last edited by peter_4059; 24-09-2015 at 08:38 PM.
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Old 24-09-2015, 08:36 PM
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Rob - that's a good example of how the "wings" at the extremities are influencing the slope of the best fit lines and hence the intersection point - in particular on the LHS of the curve. If the result at 6216 steps was not included the LHS best fit line would have a steeper gradient and the intersection of the two lines would be further to the left. It probably doesn't matter for your setup with 20 steps across the CFZ. It would be interesting to see a shot with a Bahtinov mask on your rig if you autofocused and then moved the focuser 15 steps and took another shot.

I can see a difference in the bahtinov mask with a 5 step movement on mine.
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Old 24-09-2015, 09:43 PM
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Quote:
Originally Posted by peter_4059 View Post
Slawomir,

I'm not suggesting you should do something different - autofocus seems to be more reliable/forgiving on slower refractors than faster Newtonians and if it was working for me I'd use it.

What I'm saying is it doesn't make sense for me to ignore the inconsistent results I'm getting with autofocus on my rig. I'd rather set the sequence to automatically pause for autofocus when the temperature drops 1 or 2 degrees (TBD) and get a good focus manually with the mask than capture a night worth of rubbish. I can pre-set a focus star position and slew to that with one click. When focus is done I can use centre target to re-centre the target. If I use filter offsets I only need to re-focus when the temperature changes.
Agree totally the left most point should have been left out ideally. We need Juan (of PI fame) to start writing the maths/code for SGP autofocus
"Linear fit" only the most linear parts of each side of the V, or have a parameter for how many points to include/exclude.
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Old 25-09-2015, 06:48 AM
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Slawomir,

I'm not suggesting you should do something different - autofocus seems to be more reliable/forgiving on slower refractors than faster Newtonians and if it was working for me I'd use it.

What I'm saying is it doesn't make sense for me to ignore the inconsistent results I'm getting with autofocus on my rig. I'd rather set the sequence to automatically pause for autofocus when the temperature drops 1 or 2 degrees (TBD) and get a good focus manually with the mask than capture a night worth of rubbish. I can pre-set a focus star position and slew to that with one click. When focus is done I can use centre target to re-centre the target. If I use filter offsets I only need to re-focus when the temperature changes.
Hi Peter,

I looks like carefully selected filter offsets might be the way to go in your case That would certainly be the quickest way of adjusting focus. I know that some networks of robotic telescopes (such as iTelescope.Net for example) have an option for using filter offsets.

It shouldn't take to long to set up filter offsets. I would love to help with experimenting, but my camera has been sent for upgrading, so I am out of action for a while.
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Old 25-09-2015, 09:16 AM
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I have no experience at all with SGP, but I do have quite a lot with FocusMax V3 and V4. I believe that FM results are excellent. Given the discussion re masks where the precision of the cut of the mask may influence focus whether or not grabber software is used I personally wouldn't have any faith in comparing SGP focus to that of a mask, grabber software included. Why not install free FMV3 and use that to compare? I think the results would be much more illuminating.

Peter
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Old 25-09-2015, 09:26 AM
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I have no experience at all with SGP, but I do have quite a lot with FocusMax V3 and V4. I believe that FM results are excellent. Given the discussion re masks where the precision of the cut of the mask may influence focus whether or not grabber software is used I personally wouldn't have any faith in comparing SGP focus to that of a mask, grabber software included. Why not install free FMV3 and use that to compare? I think the results would be much more illuminating.

Peter
Peter, I would if it worked stand-alone however I'm not planning to purchase Maxim, TheSkyX or CCDSoft and I understand these are the only image acquisition packages it works with.
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