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Old 20-09-2015, 08:20 PM
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Originally Posted by peter_4059 View Post
Ray,

Thanks for sharing. Here's a few of my better v-curves. Although the curve looks ok and correlates quite well with CCD Inspector FWHM, the real test for me is the look of the stars and the Bahtinov mask results.

Peter
looks fine to me Peter - with curves like that I would have thought that you should be getting good star images. good topic for discussion though.

Slawomir, will try longer step settings, but I had to reduce it previously to get the best possible stars.
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  #22  
Old 20-09-2015, 08:32 PM
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Here's an animation of the AF frames from the AFID20 sequence.
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Old 20-09-2015, 08:47 PM
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Ray, I'm happy the curve resembles a V however I'm not too convinced the slope of the LH vs RH side of the V is correct. If I fit regression lines to the data either side of focus I'm getting quite different slopes if I include all the points. If I restrict the data to points close to focus but not in the flat part of the curve I get much closer gradients. I'm not sure what SGP is doing behind the scenes with the data but if it is simply the intersection of the two lines then the different slopes impacts the focus position. I've tried to demonstrate this in the attached image which shows two sets of regression lines - one using all the data points and the other selected ones.
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Old 20-09-2015, 09:55 PM
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Originally Posted by peter_4059 View Post
Ray, I'm happy the curve resembles a V however I'm not too convinced the slope of the LH vs RH side of the V is correct. If I fit regression lines to the data either side of focus I'm getting quite different slopes if I include all the points. If I restrict the data to points close to focus but not in the flat part of the curve I get much closer gradients. I'm not sure what SGP is doing behind the scenes with the data but if it is simply the intersection of the two lines then the different slopes impacts the focus position. I've tried to demonstrate this in the attached image which shows two sets of regression lines - one using all the data points and the other selected ones.
Ah OK, understand. Similar questions led me to use small steps and rely more on the fallback statistical estimator rather than the intersecting lines fitted either side of focus. The other issue was that out-of-focus stars can drop below the detection threshold and one way around that in sparse starfields was make sure they did not go too far from focus. Having said all that though, I tried upping the step size as Slawomir suggested and it is working fine with tonight's target region.

Last edited by Shiraz; 20-09-2015 at 10:05 PM.
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Old 21-09-2015, 09:19 AM
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I am glad that my advise was helpful for you Ray

Peter, when I looked at your graph it would make sense to me to draw a straight line of best fit for the four or five data points on the left hand side of the focus graph. With only two points that are relatively close to each other the line of best fit is much more likely to be affected by errors that each data point is unavoidably inflicted with (seeing, tracking etc).
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Old 21-09-2015, 09:28 AM
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I agree Slawomir however that gives a gradient inside focus that is twice that of outside focus and that makes no sense to me. I understand the V should be reasonably symmetric around the focus point. I think the answer is to reduce the number of steps between points to something like 10 and aim to get 4 or 5 points either side of the focus point but close enough so the gradient isn't influenced by strange star shapes or donuts.
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Old 21-09-2015, 09:37 AM
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Good point Peter.

I must admit I never thoroughly checked the focus after a focusing routine, except of running it again or checking with a bahtinov mask. Perhaps maintaining correct focus at f5.6 is much easier (double the CFZ = twice as easy?) relative to f4.

Good discussion that got me thinking more carefully about focusing
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Old 21-09-2015, 02:23 PM
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Peter
Not sure this will be of value in your discussion but at Astrofest I had a problem in getting the autofocus to work properly with the refractor in that I found with the latest software that the scope was going so far out of focus on either side of the CFZ that spurious hot pixels were being registered as stars and messing up the HFR calculations. This threw the focus off or made the focus routine repeat itself with adding extra points to the curve and making it worse.
So I shortened the step value and increased the number of steps to 9. The step value would be dependant upon your scope and I found that it never went out of focus so much that hot pixels were a problem with the HFR.
Just throwing that in there.
Allan
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  #29  
Old 21-09-2015, 05:33 PM
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Last week I was manually focusing my camera lens (300mm f/2.8) with next to zero luck.

I fitted up a home built stepper system based on the SGL automation teams designs, and use a 5:1 gearing ratio and despite the theoretical 17.4um cfz I am able to achieve focus quite easily and repeatably. I do not however have to contend with focusing for different filters at this stage so it's hard to say what impact that will have and I'm yet to do any serious testing with regards to temperature (other than knowing that I have to refocus every 10 minutes or so because focus shifts noticeably every 1 degree of temperature change.

Prior to gearing it 5:1 I had it geared 2:1 using a drive when instead of a belt drive and found I had nowhere near enough resolution in the step sizes to find a 17um focus point.
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Old 21-09-2015, 05:59 PM
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Quote:
Originally Posted by peter_4059 View Post
Ray, I'm happy the curve resembles a V however I'm not too convinced the slope of the LH vs RH side of the V is correct. If I fit regression lines to the data either side of focus I'm getting quite different slopes if I include all the points. If I restrict the data to points close to focus but not in the flat part of the curve I get much closer gradients. I'm not sure what SGP is doing behind the scenes with the data but if it is simply the intersection of the two lines then the different slopes impacts the focus position. I've tried to demonstrate this in the attached image which shows two sets of regression lines - one using all the data points and the other selected ones.

I did a calculation in photoshop using the distance between the 10400 and 10350 marks and the distance between the 2 intersections. The difference I get is 6 steps. Being that your CFZ is 10, do you think that the difference causes the scope to be out of focus?
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  #31  
Old 21-09-2015, 07:17 PM
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Hi David - what software are you using to focus? SGP autofocus doesn't slew to a bright star - it calculates the average HFR of the field that is being imaged.
I'm using Focusmax - version 3 (free)

I tell it to slew to a nearby mag 5-6 star, focus and slew back to the target - all scripted in CCDC. The focus star will be in centre of the frame.

DT
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  #32  
Old 21-09-2015, 08:51 PM
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I did a calculation in photoshop using the distance between the 10400 and 10350 marks and the distance between the 2 intersections. The difference I get is 6 steps. Being that your CFZ is 10, do you think that the difference causes the scope to be out of focus?
That's what I'm trying to figure out - the Bahtinov mask shows a difference but I am yet to work out if this is noticeable in terms of star sizes.
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  #33  
Old 21-09-2015, 09:11 PM
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Here's some more tests from tonight with autofocus and Bahtinov mask.
1. Manually focus with the Bahtinov mask - 10350 steps
2. Run autofocus - 10362 steps - 12 steps or 48 microns difference
3. Re-run autofocus - 10359 steps - within 3 steps of previous result
4. Check current focus position with the Bahtinov mask (10359)
5. Manually focus with the Bahtinov mask - 10349 steps (10 steps in on AF position = 40 microns difference
6. Re-run AF with 20 steps per point - 10370 steps - 20 steps difference
7. Check current focus position with the Bahtinov mask (10370)
8. Manually focus with the Bahtinov mask - 10350 steps (20 steps in on AF position = 80 microns difference
9. Run autofocus - 10363 steps - 13 steps or 52 microns difference
10. Re-run autofocus - 10367 steps - within 4 steps of previous result
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Old 21-09-2015, 09:12 PM
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here's the last one
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Old 22-09-2015, 04:59 AM
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Your steps are too small, and you are using too many points. I use larger steps but only take 5-7 points. There must be discernible differences in the HFR of every point according to the SGP boys. The reason why I say you are using too many samples is because if you make the step size larger it will defocus too much.
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Old 22-09-2015, 05:14 AM
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Quote:
Originally Posted by Peter.M View Post
Your steps are too small, and you are using too many points. I use larger steps but only take 5-7 points. There must be discernible differences in the HFR of every point according to the SGP boys. The reason why I say you are using too many samples is because if you make the step size larger it will defocus too much.
Hi Peter- would you care to share one of your SGP log files from an imaging run where you were using autofocus (email peter_4059@yahoo.com)? I've got some VB code that extracts the autofocus data.

Peter
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Old 22-09-2015, 06:01 AM
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Hi Peter,

Interesting results indeed - I have been following your findings with great interest.

Not sure whether you are using Bahtinov grabber - it's free software that helps to manually find optimal focus with a mask: http://www.njnoordhoek.com/?p=325

I quite liked it and from memory it was simple to use.

Sincerely
S.
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Old 22-09-2015, 07:39 AM
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Thanks Slawomir, I do have that application however it ends up taking a lot of screen space up so I tend not to use it. I think the mask is quite easy to interpret visually though.
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Old 22-09-2015, 03:51 PM
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Sure. I was thinking it might be an interesting experiment to use this app, as it could possibly ensure stronger repetitiveness of the method by eliminating human factor altogether
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Old 22-09-2015, 07:55 PM
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Slawomir,

Here's an interesting result - Bahtinov Grabber offline on the two bahtinov mask images. According to BG neither image is in focus!
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