Live stacking with a large bright PN in the field

[lowenthalm]

In live stacking, is there any way to configure the star detection controls to not detect the a bright planetary nebula as a giant star? If the field of view only has a few stars (NGC 7662 is a great example, but there are numerous others), the PN is always detected as a star and wildly distorts the FWHM filter measurement to such an extent that it becomes useless for filtering frames with bad tracking. I am imaging IC 4593 and Sharpcap thinks the stars are wildly gyrating between 8 and 10 pixels, when in fact they are 6 ranging from to 6.5 pixels.

Sharpcap 4.1 has a dimmed out "Ignore stars filter size" box, but its dimmed out and doesn't seem to do anything and seems related to comet tracking. The sensitivity control seems to be a relatively blunt instrument that doesn't change the detection of bright/large stars. A few versions of SharpCap back, there was an ignore stars larger than X pixels, which did the trick perfectly, but the option is gone now.

[admin]

Hi,

yes, once upon a time there was a maximum star width option (back in about version 3.2). However since that time I completely re-worked the star detection settings to make them simpler, and the maximum star width vanished in the change

The ignore stars filter size is an adjustment for comet tracking mode, so remains disabled unless you turn that on. It wouldn't help in your situation anyway even if you could turn it on... Along the way when sorting out the star detection I also made it capable of detecting very out-of-focus stars to help with focusing, and that is probably backfiring here and picking up the PN.

It sounds like the only way to sort this would be a new option. I will put it on the todo list.

cheers,

Robin

[lowenthalm]

Live stacking doesn't need to work in an out of focus state like the FWHM focusing routines do. It might be enough to just disable the detection of wildly out of focus stars in live stacking. Maybe a checkbox isn't needed there. Or simply looking at the distribution of FWHM in the field and ignore stars that are clear outliers. There should be no more than one PN in the field!

Addendum: This evening I was trying to do FWHM autofocus at IC4593 with a dual band Ha/OIII-Hb filter. The star field around it is sparse (made worse by the narrow field of view of my camera), and the FWHM focus was utterly confounded by the bright PN core being included in the average, causing the FWHM to jump around wildly, especially as it got close to focus. I had to manually intervene and put a selection box around stars to one side of the PN to get FWHM focus to work properly. I'm thinking the outlier rejection suggested above might be a good idea in both the focus case and the live-stack case.

[admin]

Hi,

yes, the outlier rejection might be a good idea - or to simply use the median rather than the mean of the FWHM values? I will have to think about that and do some testing.

For now I have added a limit adjustment to live stacking (defaulting to 50 to keep the current behaviour as default), but you can dial that down as required to avoid picking up extended objects.

cheers,

Robin

[admin]

Hi,

just checked how this all works as I was looking to see how easy/hard it would be to change to medians or some other approach.

For the Multi Star FWHM focus, it already performs sigma clipping to 2 standard deviations each side of the mean providing that the sigma clip still leaves at least 6 stars in the sample. I wonder if you were right on the edge of this and were therefore sometimes getting the sigma clipping, but if the frame had a couple less stars then the sigma clipping would not be applied giving a different score?

cheers,

Robin

[lowenthalm]

I’ll test this some more this evening with this in mind to see if I can give you more data. I know both love stack and focus views show the PN as a detected star pretty much all the time. Are the detected star box marks displayed before or after sigma clipping is performed internally? My interpretation of SC behavior is the answer to this is after.

Tonights testing Addendum:

Tonight, I found that with my UVIRCut filter on my OSC ASI294 cooled camera, FWHM focus measurement rejected IC4593 consistently throughout focusing from near focus to good focus. HOWEVER, when I switched to my dual band (Ha, OIII/Hb) filter, it did fail just enough to be annoying. See the attached screen shot. Everything looked fine and then suddenly as it got near focus, it picked up the nebula and blew out the average (3 samples are being taken per step here).

I discovered tonight that my PEC playback randomly, and with no warning at all turned off after after slew. It was on during the test tonight for sure, however it may have turned off on me last night and made the effect even worse. Maybe very out of round stars during a bad frame can trigger the problem? The attached screen shot does not seem to have out-of round stars, so this does not seem to be a prerequisite to detecting the nebula as a star.

[admin]

Hi,

it's interesting that in your screenshot the star count is 6. This implies that the sigma clipping is not being applied because that would lower the star count to 5 which is below the threshold. Were you getting more stars when slightly out of focus? I wonder if the dual band filter knocks down the illumination level enough to reduce the number of stars being found and trigger this effect...

The idea of using the median is appealing to avoid the effect of outliers, but I'm not comfortable with using it for small star counts, as in those situations I worry that adding or removing one star may move the median quite a bit.

For now some suggestions that might help

* Focus with the telescope offset a bit so the nebula is out of view
* Increase exposure a bit (or maybe gain) to detect more stars

cheers,

Robin

[lowenthalm]

Now that I am aware of the effect, I can combat it as you suggest. But I will have to change my autofocus sequences so that they don't use the "SET PLATESOLVE/FOCUS SETTINGS" block, since this sets the exposure to 4 seconds. I'll experiment with bumping gain too, to avoid slowing down focusing procedures too much. I will also try using the the "Add Exposure/Gain shift control" that you snuck into SharpCap 4.1 - an interesting idea!

By the way, on a lark, I tried to change the 4 second exposure to 3 seconds (to speed things up) inside a "SET PLATESOLVE/FOCUS SETTINGS" block, expecting it to restore to what it had saved. The result was - not what I expected. I believe it just left the exposure at 4 seconds. Should I expect this to work? Now that I think of it, this might just have been a case where the original exposure setting isn't always restored, as I reported in an another bug thread.

[lowenthalm]

The max star size option works great! Thanks for the quick fix. The FWHM measurement is much more consistent from frame to frame and it keeps the brightness from bouncing around too.