Can FWHM monitor indicate periodic errors of the mount?

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GoFraRo
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Can FWHM monitor indicate periodic errors of the mount?

#1

Post by GoFraRo »

During my last session, where I used Livestack, I monitored FWHM constantly. I ran with an exposure of 15sec over a longer period of 60 - 75 minutes.
I could see that the FWHM was around 4.5 to 5.5 with some peaks of 6.0 to 6.5
Over the time, it looked like ---^-----^-----^-----^--- (unfortunately, I do not have a screenprint)
It seems to me, that every 90 sec I have to deal with an periodic error (PE). Or is it something else ? Any idea ?

If I set the FWHM limit in Livestack appropriately, could this improve my images significantly ?

Regarding the mount... iOptron Cube Pro (Alt/Az mode) with TS Photoline 60/360, Neptune-C

Regards
Gottfried
TS Photoline 60/360, TS Photon 150/750 Advance Newton, Bresser 90/500, ZWO ASI120MC-s, Player One Neptune-C, SVBONY SV165 Guidescope, iOptron Cube Pro. Bortle 4, Germany-northwest region
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oopfan
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Re: Can FWHM monitor indicate periodic errors of the mount?

#2

Post by oopfan »

90 seconds seems short. Mine is 8.5 minutes. That's the time it takes for the worm gear to turn once.

However, I have seen short period oscillations on the order of 2 minutes, however, it was due to flexure in the mount. The RA axis and DEC axis were not locked 90 degrees. I corrected that by tightening a large 1-1/2 inch nut.

My mount is not the same as yours, but still I would recommend checking everything. When in doubt...tighten but don't strip. Check the manual or contact the manufacturer.

Do your images show elongated stars?

Brian
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Re: Can FWHM monitor indicate periodic errors of the mount?

#3

Post by admin »

Hi,

Brian is right to suggest checking for elongated stars in the frames that have the higher FWHM - that would definitely point to a tracking glitch during the capture of those frames.

It's harder to pin down PE with an Alt/Az mount, since the rate of movement in each direction changes with the part of the sky you are looking at, meaning that the period length of any irregularity will also change. With an EQ mount the periods stay constant. It certainly sounds plausible as an explanation though.

cheers,

Robin
GoFraRo
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Re: Can FWHM monitor indicate periodic errors of the mount?

#4

Post by GoFraRo »

Thanks for your reply.
I checked all images of the session with Astrosurface FWHM-measurement tool. And indeed, there are elongated stars in it. Only very slightly. And I cannot find any certain structure. It seems to me, that in cases, where there is more field rotation, I have more elongated ones. But I can live with that.

My thought is, if I see such a pattern in the FWHM monitor, I could skip this frame and improve the stacked image (to some extent).

Another problem I'm faced with is drift. In my case, it seems that the sidereal and even king rate doesn't fit. Even with less field rotation, I can see the shift in the stacked image. Without guiding, it is a challenge for longer exposures. I still need to run some tests with an adapted rate.
Unfortunately, the iOptron Cube Pro doesn't support ST4 or PulseGuiding.

Gottfried
TS Photoline 60/360, TS Photon 150/750 Advance Newton, Bresser 90/500, ZWO ASI120MC-s, Player One Neptune-C, SVBONY SV165 Guidescope, iOptron Cube Pro. Bortle 4, Germany-northwest region
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Re: Can FWHM monitor indicate periodic errors of the mount?

#5

Post by admin »

Hi,

all I can suggest on the drift is that my experience with AltAz mounts (Celestron Nexstar in my case) is that they are more sensitive to poor alignment in terms of getting bad tracking than their EQ cousins.

cheers,

Robin
GoFraRo
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Joined: Thu Sep 16, 2021 8:45 pm

Re: Can FWHM monitor indicate periodic errors of the mount?

#6

Post by GoFraRo »

Hi Robin and all others,

Regarding the drift, I did some homework. I'm not an expert and maybe I'm wrong, but...
I checked a couple of images (the fits-files from Livestack) and measured the number of pixels from the physical edge to the latest visual edge of the image. Dividing it by the integration time and multiplying it by the exposure time, I came to an average drift of 1,4pxl/15 sec exposure time (I used in this case).
The pixel size of the imx178 is 2.4 Micron. Resolution given by astrometry is 1.37 arcsec/px.

After some readings in various forums and websites, it seems I have two options.
- reducing the exposure time, to mitigate elongated and blurry stars (which is neglibigle in this case).
- but this requires also an adaption of gain and offset settings and could have an impact of stardetection for the Livestack alignment
- adjusting the tracking rate (in this case, the mount is slightly too fast), to avoid lost space beside the field rotation.
- it seems, one drawback of Alt/AZ mounts is the fact, that the AZ-drive cannot work against gravity. I will try to set a slower custom rate and test it.

Unfortunately, there are different statements regarding the tracking accuracy of the mount. In the manual, I could only read "Resolution = 0.5 arc minute", on their website "Resolution = 0.5 arc seconds", where the latter sounds more reasonable, whatever it means.

Now, I only need better weather. It seems, this will be my first project in the next year.

Gottfried
TS Photoline 60/360, TS Photon 150/750 Advance Newton, Bresser 90/500, ZWO ASI120MC-s, Player One Neptune-C, SVBONY SV165 Guidescope, iOptron Cube Pro. Bortle 4, Germany-northwest region
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oopfan
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Re: Can FWHM monitor indicate periodic errors of the mount?

#7

Post by oopfan »

Hi Gottfried,

I've been a happy SharpCap user, I guess going on 4 years. In that time, I changed cameras twice, telescopes twice, but I've had the same mount for the past 50 years. All three components have worked, more or less, to my satisfaction. However, I wish I had cut my losses short by selling some of the equipment. Instead, I accepted it as a personal challenge to overcome and solve some of the more serious problems. I am proud of myself for that. However, looking back at it, I ask myself "what am I trying to accomplish?" Am I here to fix engineering problems or have fun with astrophotography? If I could do it all over again, I'd choose the latter. Having said that, know that there is a robust secondary market for used equipment.

Brian
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