Hi Jared,
Here are all the AF packs I have currently available. None especially problematic but interested to know if a) wings of curve get more values b) much change to the final focus position.
Hope of some use:
https://www.dropbox.com/sh/ozsbsbhzyx5mnx7/AACuu9UESMgwav9ghxKYLnQha?dl=0
Hello Jared,
The current setup in ASTAP is such that it can’t measure above around HFD=14 so HFR=7. That is related to the measuring rectangle window used. Beyond that value the measurement will break down more or less completely because the star donut is bigger then the measuring window/rectangle.
So for focusing you should stay below this value.
Han
Ah, good to know. I’ll let them know and give that a shot.
Thanks!
Jared
The limit is a little higher. HFR=10 is the absolute maximum.
Hello Jared,
Here are some AF packs that have poor autofocus results, plus a few with good results. Four metre focal length and native pixel scale of about 0.193", so around 0.77" per pixel @ Bin x4:
Some of the poor results may be due to windy conditions (the Meade 16" tends to be a bit of a sail), but most will be due to poor seeing and limited stars I suspect. On nights of very good seeing conditions, HFR tends to get down to just below 2 (at bin x4). On bad nights it can be up around 4, and the focus curve becomes quite flat. I can’t increase the tail HFR much more (to obtain a deeper curve on nights with such poor seeing) because sometimes no stars are detected at the tails and AF fails.
By the way, would it be possible to include the RA and Dec in the Fits header of the autofocus images? These images make great trial images for testing/tuning plate solvers like ASTAP, but without hints the blind solves can take a lot of time.
Sorry Jared, no good examples at this time except for what I posted here…
If you’re not seeing the HFR outliers caused by small galaxies and double stars, that’s a very good sign. (-:
Hi Guys,
We’re going through a period of poor weather here in the UK so rather than be totally idle I’ve had a think about how one might test whether this candidate change is actually an imrovement.
I’m thinking:
a) If the star detection method can be a switchable option (ie current SGP AF and ASTAP AF) then for a chosen filter (ie L) one might run several AF runs in quick succession so as to minimise impacts of changing temperature and atmospheric conditions. Hopefully one would see a tighter bunching of results with one or other option which would demonstrate greater consistency.
b) As for a) run a sequence of AF runs for a chosen filter but using only a small step size so as not to deviate too far from the anticipated best focus. Hopefully this test would still see a shallow U shaped AF curve with ideally the previously identified best focus point still showing as the lowest point. The better AF method would be the one with the starting AF focus point showing the least deviation from the lowest point of the curve.
c) The ASTAP analysis method seems able for a given AF image to make considerably more star detections than the current SGP method. The value of these extra detections is not yet proven - I accept a point that Han made earlier that the quality of the detections is a least as important as the volume. However I’m currently of the opinion that perhaps 10-15 detections at the extremities of the AF curve would be about right but if sufficient detections can be made with a shorter exposure time then I think this would also be a good thing.
@Ross_Walker , I’ve taken a look at two of the good AF run results you posted and I see that ASTAP generates c5x more detections from each image compared with current SGP.
@Ross_Walker, @Jared , I’ve only had a quick look at your poor AF runs but from what I see I think a change in star detection method alone will not benefit runs made during say gusty conditions. A single poor exposure may well be covered up by the curve fitting routine but probably not much more than this. The answer here I think might be be to have (as an option) SGP to make say three exposures at each AF step and then to stack these exposures before doing the star analysis. ASTAP appears on its UI to have the capability to do this but I suggest the idea needs some further investigation before proceeding further. This option of making more than one exposure per AF point might benefit imagers working in poor conditions/locations and also those with high-end equipment seeking the ultimate from their work.
Regards
Mike
Han, KG, Ross, Jared
Hi KG,
I’m not familiar with PWI3 but assume that it adopts a similar approah to focusing (ie AF Curve fitting) as SGP. I’m wondering if the method they use is proprietary or described in publicly available material. I’m not sure about copyright but if it is described in publicly available material then maybe the method could be incorporated with SGP if it is demonstrably superior and benefits the wider astroimaging user community. Do you have time to collate some material (AF images etc) that could be analysed and compared PWI3 versus SGP)?
You assert that SGPs focus point is frequently out by c 100 microns. My focuser set-up by comparison does c6000 steps per inch of travel, equivalent to 25400/6000 or approx 4 microns per step, so a 100 micron error would to me be c25 steps. Looking at my AF curves, this level of focus error would be noticable in HFD measurements and I think would certainly be rather annoying if it happened on more than say 5% of occasions.
You suggest that this problem is due to the inclusion of ‘double stars, small galaxies and other non-star objects’. I’m not sure that this ‘feature’ would be so much of a problem when taking a median HFR/HFD value on a larger population of detections as will hopefully be provided by ASTAP. To illustrate my point I noticed a small number of such occurences when looking at the data provided by Ross:
Do you think say a 3 sigma clip proior to selecting the median value would give a more robust HFD calculation?
Regards
Mike
In the past I had in ASTAP a stricter algorithm for oval stars and galaxies but it excluded too much if the guiding was not perfect or aberration near the edges. Your PN is round making it even more difficult to exclude on oval shape.
Stars are normally more abundant then galaxies so statistics help.
Sigma clip exclusion could could work but then you have to do a few repeat till the values are stable. So exclude outliers in the first round and do again a mean and standard deviation calculation. But I think this work less well for a small number of values.
For focusing the best approach is taking the median of the collected HFR values for each focuser position. So you first sort the values on value. Then take the middle value. So if you get something like
0.1, 7.0, 7.1, 7. 1, 7.2 , 7.2, 7.2, 7.3, 22.0 the median is 7.2
or if you have even number of values:
0.1, 7.0, 7.1, 7. 1, 7.2 , 7.2 , 7.3, 22.0 then the median is (7. 1+ 7.2)=7.15
ASTAP reports the median value.
Han
With ASTAP it is possible to create artificial star images (tab Pixel math 2) containing Gaussian shaped stars against a noise background. This was great for testing. I have made one with a HFD=3 /HFR 1.5 and loaded it in SGP and ASTAP. The SGP reported values are not so stable even for high flux values and drop out earlier a low intensities. There seems a fixed ratio difference in the reported values of 1.7 / 1.5.
You can see ASTAP struggling at the very low flux intensities but still reporting a little lower values up to the 14th row. SGP is starts to fail at the 7th row at an SNR of about 20.
Han
For HFD=10 so simulated donut shaped stars SGP values are simular as ASTAP. Both achieve good reading up to column 14 (from the right).
Regards, Han
If a human can do it like you folks are above, a computer can too. (-:
High level…
I’ll see if I can get more details for the Planewave PWI3 logic. It may be getting some help from Maxim. Its V-curve is very consistent. If I run it 5 times on the same star field, my results are within +/- 20 microns on a 5800 micron focus. It detects more stars as you get closer to focus. It also seems to work well when detecting only 10 stars.
KG
One more measurement with artificial stars with a HFD=2.0 /HFR 1.0. The variation in the SGP measurements becomes much higher. Also for the brightest stars with a very good SNR value.
I tried something I didn’t do before. In the star simulation the star center of gravity is at a pixel center. I have modified the simulator such that the star center is at a pixel corner. This results in four equal illuminated pixels in the middle. Then for the simulation HFR=1 SGP reports the correct HFR and values are very stable. But the star detection algorithm should be able to detect the stars at any location. ASTAP report for this artificial image a HFR of 1.15 In practice the star can be at any location and the algorithm should ideally report the same value for each location.
Han
I have issued an ASTAP development version 0.9.439 with an improved artificial star image creation tool (tab pixel math 2). The center of the star is placed random within a pixel. The Gaussian signal dropoof is calculated on sub pixel level. Download:http://www.hnsky.org/astap_setup.exe It will show Gaussian shaped stars up to HFD= 5 (HFR=2.5). Above this value it will produce star donuts. The produced artificial images should be very close to reality.
After loading and measuring these artificial images in SGP, the measured HFR values are varying more then they should and this could effect focusing. Especially if only a few stars are visible. I see the similar variation in HFR values if real images are tested in SGP.
Han
Hi Han,
fyi: I’ve just downloaded your latest version 0.9.439 (I think I was on 0.9.418 previously) and have so far reviewed a couple of the AF packs that I reported on in my Excel spreadsheet. From this so far limited analysis I’m seeing a) a small improvement in #stars detected slightly moreso in the extremeties of the AF curve than near best focus; b) a small reduction (0.01-0.02) in the reported median HFD values reported - I assume as a result of the few extra star detections. All this I feel suggests we (or more precisely you!) are helping make some further minor improvements to SGP AF.
*** In view of above I’m thinking that if/when Jared releases a beta with this proposed change that the release note should also include a recommendation to upgrade ASTAP to latest release ***
I’ve also been investigating some subs I have of targets that I thought might cause ASTAP star detection issues, such as Stephan’s Quintet compact galaxy group and M33 with its numerous Ha emission regions, but apart from that one PN discussed above I’ve not so far noticed any other objects or artifacts erroneously counted as star detections. Only a very small sample I readily admit but most encouraging I think.
On the topic of false detections I am wondering if it would be simple for you to include a menu item to request output of say a .csv file containing an ordered list of the HFD/HFR measurements calculated for an image? I’m thinking that in any future discussion on the merits of e.g. simple median versus sigma crop plus median, then numerical evidence could be readily obtained to support/reject a proposal.
Regards
Mike
The best approach will be to get SGP star detection and focusing better. Use of ASTAP should be intermediate solution.
My apologies if I have somehow gotten ahead of the game.
I feel that with your generous assistance, investigating with ASTAP has produced a solid body of evidence to show SGP’s AF star detection could be somewhat improved.
I’m naturally hoping that Jared will soon report confirmatory feedback from the friends to whom he released his prototype to for testing and that this feedback will also be positive.
As a keen user (weather permitting) I am naturally interested in seeing seemingly solid improvements released as soon as possible. However it is clearly for the SGP devs to decide their long-term product direction.
I have no idea of the time and effort needed to get to the level of capability offered by ASTAP but as a user it seems you have in-depth knowledge and a product (or a component therein) that it would seemingly be somewhat short-sighted not to capitalise upon, with your mutual ageement.
Apologies again if I have accidently trodden on toes
Regards
Mike
Mike, I don’t mind if they use ASTAP. The topic has my technical interest and there is always something to learn.
Jared, I have SGP running together with my “Sky Simulator for ASCOM. v0.1.19” so I can run focussing in simulation. Can I experiment with the SGP version capable of using ASTAP for HFR measurements to see the difference?
Han
Han,
Yes, I’ll PM you.
Jared
