New Feature : Improved Star detection in Multi-star FWHM Focus

Discussions, Bug Reports and Issues related to Beta versions of SharpCap
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Re: New Feature : Improved Star detection in Multi-star FWHM Focus

Post by admin »

Hi,

thanks for the report - I've been testing this out over the last couple of nights too and have had 100% success rate - looking good so far!

cheers,

Robin
Anthony Quintile
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Re: New Feature : Improved Star detection in Multi-star FWHM Focus

Post by Anthony Quintile »

So several days ago I started searching for some help (again) with focus settings because I had moved to using my SCT for the first time with my ASI2600MC. Previously I had been using a stock Pentax, and the ASI had arrived around the same time I got a new Newtonian astrograph, so the SCT had been sitting, and the Newtonian, and my refractor, were both easier to get the focus settings to work (faster scopes, wider fields, inherently more visible stars in the FOV).

I stumbled across this thread, downloaded v4.xxx, and as soon as I opened the new FWHM focus settings, it began to work significantly better. Perfect timing on the launch of this for me!

This is so much more consistent, ***MUCH*** easier to use, less impacted by seeing variations, and far more confidence inspiring. Previously, even with the faster, wider field scopes, I might have to run several curves sometimes yielding several different results. I was not able to get anything consistent with the SCT as it would not see enough stars far enough out on the edges of the curve to generate a wide enough curve to be useful. The improved version "just works".

Huge improvement! Thanks Robin!
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Re: New Feature : Improved Star detection in Multi-star FWHM Focus

Post by admin »

Hi,

thanks for the feedback - the 'just works' is what I was aiming for :)

Actually you might notice that sometimes SharpCap will adjust the star detection sensitivity for you automatically - if there are too many stars (more than about 800) then it will turn the sensitivity down. If there are too few it will turn it up. This is another part of the 'just works' goal :)

cheers,

Robin
howardgrams
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Re: New Feature : Improved Star detection in Multi-star FWHM Focus

Post by howardgrams »

Hi Robin,

My results with the new Multi-star FWHM focus have been disappointing. I use a ZWO ASI183MC Pro camera and a RASA 8 with the Celestron focuser. Even with SC 3.2, I never got multi-star focusing to work very well compared to single star.

Observations:
- the collection of which stars are being used is remarkably transient - at any instant the stars being used (as marked on the display) can be completely different from those being used a second or so earlier or later. Is this normal or is it a problem?
- When I initiate autofocus by clicking scan out, the green triangle focus values plotted on the graph are very noisy from one step to the next instead of forming a clean curve. But when I accidentally tried using the focuser button to fine step out, the noisiness mostly went away and a proper graph with an expected minimum appeared after a dozen steps.

Does this suggest some kind of settling problem where SC is measuring a FWHM/HFD while the focuser is still moving or the telescope has not stabilized?

Questions:
- What are your recommendations for the Averaging panel? If I just use one frame, is the result dependent on good seeing, or does the averaging process over multiple stars tend to eliminate the possible effect of turbulence on a given star?
- When I use use the automatic process to Step Out, the step size it uses seems to be 4. Is this related in any way to the Coarse and Fine settings for my focuser (which are step size 50 and 10 respectively)? It seems like the ideal step size should be somehow specified by the user. (I have another focuser for another telescope that requires many fewer steps to make a significant focus change.)
- Is proper operation of multi-star focusing somewhat dependent on how bright the stars in the frame are?
- Is proper operation somewhat dependent on exposure time? Is having bright stars and short exposures equivalent to faint stars and longer exposures?
- Should it make any difference if I try to refocus using the same exposure and dark frame as I've been using, or should focusing be done with no dark?
- Should it make any difference if I try to focus using binning versus using the full camera resolution?

Regards,
Howard
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Re: New Feature : Improved Star detection in Multi-star FWHM Focus

Post by admin »

Hi Howard,

it does sound like the bulk of your problems are being caused by noisy images - you can try the following:

* Consider using longer exposures and less gain while focusing - this will reduce the noise levels in each frame
* Turn up the noise reduction control in the FWHM settings a little - this should reduce the skipping of stars
* In the focus scan settings, make sure that you are taking a number of samples (3-5 or more) at each position. This will also help reduce the randomness
* put the camera into still mode rather than live mode - this ensures that images are not taken while the focuser is moving

To answer your specific questions...

* Averaging I would suggest to leave at one frame, but collect multiple samples at each position - probably 3 to 5, possibly more
Capture.JPG
Capture.JPG (14.66 KiB) Viewed 296 times
* Step size can also be adjusted in the options (highlighted above).

* Multi star FWHM needs stars that are bright enough to be easily detected, but not saturated - saturated stars give misleading results and are ignored by the algorithm. The more stars the better as the averaging helps reduce errors

* Yes, the exposure matters because it influences how detectable the stars are (and if the exposure is too long then they will be saturated, which means the algorithm will have to look for fainter ones)

* You should probably not need to use darks for focusing as the exposure times are usually short enough that they are not necessary. If you do want to use them, it will do no harm as long as the settings for the dark (exposure, gain, etc) match the settings you are using to focus - do not use darks for 180s exposures when focusing with 5s exposures.

* Binning is fine as long as in-focus stars still have noticeable spread. If you were using a big pixel camera (say 6 micron pixels) and then bin 2x2 you might end up in a situation where in-focus stars would be a single bright pixel, which would not allow the focus algorithms to work properly.

Hope this helps,

Robin
Anthony Quintile
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Re: New Feature : Improved Star detection in Multi-star FWHM Focus

Post by Anthony Quintile »

I am posting this in response to this thread because it *may* be related to the new FWHM/HFD calculation, but I am grasping at straws a bit, so take this question as that...

Is it possible that SharpCap focus calculation could get a lower number in FWHM/HFD for stars suffering from some sort of astigmatism-ish type elongation that is resulting from being slightly out of focus? Maybe stars could be significantly enough skinnier in one direction than they are longer in the other than had they been round? (Is my theory at least clear even if flawed?)

I am chasing a star elongation issue with my reduced (Starizona SCT Corrector II .63x) Celestron 8SE SCT on only one side of meridian. I am assuming it is related to mirror shift a bit. Since it occurred I have been trying to formulate reasons and one idea is that better focus might have resolved it, even though I had refocused at least a couple of times while reversing meridian flips in an attempt to diagnose.

As I said, just spitballing here at this point to try to formulate a solution. Several other more probable theories are in play.

Thanks for considering...
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Re: New Feature : Improved Star detection in Multi-star FWHM Focus

Post by admin »

Hi,

at least in SharpCap 4.0, the HFD/FWHM measurement should be relatively insensitive to star shape - let me explain...

SharpCap first identifies bright blobs in the image as stars (identifying a bounding rectangle for each that contains all of the star and some surrounding black area). Boxes that contain multiple bright peaks are flagged and not used for FWHM calculations (double stars, etc).

In the case of single stars, the first thing to do is work out the background brightness level in the region of the star, subtract that and then add up all the pixel brightness values - that gives the total brightness of the star.

SharpCap then takes the pixel values in descending brightness order and counts the number of pixels needed until the total brightness of the pixels counted is equal to half the total star brightness (this can give a result that is a fractional number of pixels). Now we have a number of pixels that contains half the brightness of the star - effectively that's an area, and the area can be converted to diameter using

[math] and [math]

As you can see, this pixel counting approach shouldn't really care whether the stars are perfectly round or if they are distorted - it is just looking at how tightly confined half of the star brightness is.

cheers,

Robin
timh
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Re: New Feature : Improved Star detection in Multi-star FWHM Focus

Post by timh »

Just to comment that whereas I used to have to struggle with digital gain and blackness level etc before I could get multistar FWHM to work it now does seem easier as well as more robust and intuitive than before - especially the fact that adjusting the histogram so that you can see the stars - as you do anyway- also makes them more 'visible' to the focusing software.

Except perhaps for very short noisy frames the absolute FWHM values themselves do also seem to be consistent with those generated in other software (PixInsight subframe selector)

Overall a very worthwhile improvement, thankyou

TimH
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Re: New Feature : Improved Star detection in Multi-star FWHM Focus

Post by admin »

Excellent news, thanks for the report :)

cheers,

Robin
Anthony Quintile
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Re: New Feature : Improved Star detection in Multi-star FWHM Focus

Post by Anthony Quintile »

admin wrote: Tue May 04, 2021 12:45 pm Hi,

at least in SharpCap 4.0, the HFD/FWHM measurement should be relatively insensitive to star shape - let me explain...

SharpCap first identifies bright blobs in the image as stars (identifying a bounding rectangle for each that contains all of the star and some surrounding black area). Boxes that contain multiple bright peaks...

Robin
Robin-

So, to (maybe) finish answering my question with a very dumbed-down example, stars that showed as four pixels in a row because of some sort of optics anomaly might show as much "in-focus" as stars that were 2x2 pixels? (Removing the less saturated surrounding pixels for the sake of simplicity...)

Please understand that I am not faulting anything that SharpCap is doing, just trying to figure out another issue and how I may be able to use SharpCap to address it.

Thanks for the reply!
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