Focus Assist results

[Howardk]

Why is it I get a lower FWHM number at best focus with bin2x2 as compared with bin1x1

I would have thought that with better resolution at bin1 i would get lower FWHM number

[admin]

Hi,

that's because the FWHM is measured in pixels. If you bin 2x2 then your pixels are (effectively) twice as big. That means that the same size star (same amount of light spread due focus/seeing) will spread over less of the big binned pixels than the small unbinned pixels, giving a smaller FWHM.

It's rare for the full factor of 2 reduction to be seen due to other factors in the measurement.

cheers,

Robin

[Howardk]

Understood
Thanks

I am having difficulties with the focus assistant
It’s a struggle
I often fail to get the valley curve so no automatic best focus
When i start the scan how far away from focus should i be?

[archer1960]

In my personal experience, I have found that I need to start at a focus setting that gives me an FWHM of at least 2 times, and preferably 3 times what I see at best focus.

[admin]

Yes, it has to be a *big* change in FWHM from the ends of the measurement scan to the best position, otherwise you end up with just a small dip in the curve and the random noise in the measurements can make that small dip hard to detect.

cheers,

Robin

[Howardk]

I cannot get a reliable fwhm to stay when running focus assistant
It will jump from 4.9 to 6.1 and is just not reliable

Also when i do manage to get a nice curve it somehow puts the focuser to the best focus but this is way off

I am using the recommended sharpcap backlash setting in hardware

Could all my issues be because i am in altaz with no guiding or polar alignment so tracking is far from perfect??

[admin]

Hi,

I don't think that the AltAz or lack of guiding will make a big impact unless the target is drifting *very* quickly - presumably you are tracking on your AltAz mount, so it largely keeps circular stars and a steady view...

If you are getting jumping measurements then you need to watch the image on screen during the focusing measurements - are you getting plenty of stars highlighted (20+)? Are the same stars being highlighted from frame to frame, or are lots of the stars only being picked up on some frames? Small numbers of stars and intermittent detection can both lead to unstable measurements, which will need resolving before things work reliably... Some things to try in that case include

* Increasing exposure - maybe double the exposure from 1s to 2s or 2s to 4s, etc.
* Increase the star detection sensitivity to try to detect more stars
* Increase the noise reduction a bit to try to stabilise the measurement so that the same stars are being detected in every frame.

All of this needs to be tweaked while the focuser is sitting at one position - once you have it so that the values are fairly steady from frame to frame at a single position, then things will likely work better.

One more thing - I am worried that you mention a 'recommended' backlash setting... While we recommend the use of the 'overshoot' backlash mode, there is no recommendation for the size of the overshoot, since that is a value that needs to be worked out for every combination of focus motor, focuser and telescope. If the overshoot size is set too small then you can end up in a situation where the focus motor numbers are changing, but the focus mechanism is still within the backlash zone and the camera isn't actually moving. You should follow the procedure for measuring focuser backlash as laid out in the SharpCap user manual - see https://docs.sharpcap.co.uk/4.1/#Measur ... 20Backlash

cheers,

Robin

[Howardk]

Was getting 6-10 stars

Will work on getting minimum 20

Backlash is 50 steps…..i literally watched the micro feathertouch focuser on my SCT and saw it move after 50 steps

I am presuming that when the focuser actually turns tgen the mirror moves in an SCT?

Will continue to work this focus assist tool

[Howardk]

Well i increased exposure to 4 secs and increased star detection and noise reduction and now i am locked on 20-35 stars and can get a good valley curve

Problem is when focuser goes to “best focus” although the step count in the focuser is where the curve says best focus is it’s not actually at best focus….for example if best focus is at 46500 with fwhm at 4.8 then yes the focuser goes to 46500 but i get a fwhm of 7.5…

I’ve tried all types of backlash settings but always get this

Any ideas….???

[archer1960]

Do you get better readings if you then move the focuser? Or is taht 7.5 actually the best you can get? On my system, I see the same thing using Voyager: the focus values I see in an actual image are never as good as what the autofocus run says it is giving me, but that is as good as they get. I'm not sure, but it might be that the AF run uses 2x binning, which will always have lower numbers than unbinned images.