Guiding tips needed: very low RMS and peaks, but stars still not round

[pipnina]

Hi!

I finally got my guidecam (ASI 120mm mini) and performed the belt mod on my HEQ5 pro, and tuned my worm gear spacing. The last two nights I've gotten the chance to test it out and at first I was astounded: Once I had the guidecam at 0.5s exposures it was still picking up plenty of guide stars and my RMS was measuring at 0.2-0.3 arcseconds! But I was experiencing a different story on the main camera that showed very oblong stars and signs of wobbling on a 7 minute test exposure of the leo triplet. Even on 2 minute exposures of my NGC7000 mosaic (on my profile) the stars were still not round despite the guide chart showing error far below my pixel scale.

I suppose there are two reasons I can think of why this may be happening already, but I figure I'd ask here to see if any of the more experienced people around have their own ideas for what might be wrong.

Currently I think it could possibly be flexture, though I don't know how since the guidescope was as tight to the tube ring as I could make it. I have now however affixed it to a dovetail bar that is itself connected to BOTH tube rings, so maybe that will help. I have also made the guide scope align with the main scope's FOV, which in my mind might reduce the effects of turbulence in the guiding (star moves left in main scope, but moved right in guide scope as a result of turbulence??)

I'm also considering that maybe I need to tweak the guider settings? I am using Ekos at the moment and (almost) everything is on default (0.75 aggressiveness, min error 0.1 which I tweaked from 0.2, etc.)

Little sample of my stars after a 4 min sub, with guiding error peaks rating consistently below 0.5 seconds: They're almost sort of becoming square (left-right is ra and up-down is dec, or near enough) I'm imaging at 1.2 seconds per pixel.

Any help appreciated!

[knobby]

Have you entered the guidescope length correctly ?

[pipnina]

4 minutes ago, knobby said:

Have you entered the guidescope length correctly ?

I've just checked the config and the scope's fl and aperture are correct - 200mm fl and 50mm ap

[symmetal]

How good is your polar alignment? If it's too far off your images will exhibit rotation around the guide star. If  your guidescope isn't aligned with the main scope, and the guide star isn't in the image all the stars will show elongation in the same direction.

Alan

[pipnina]

3 minutes ago, symmetal said:

How good is your polar alignment? If it's too far off your images will exhibit rotation around the guide star. If  your guidescope isn't aligned with the main scope, and the guide star isn't in the image all the stars will show elongation in the same direction.

Alan

I'm relatively confident that it's at least close, since polaris was bang on the line in the polar scope, and in the right place on that circle when I set it (accounting for the visual error one expects at least given as the clock turns with RA in the HEQ5....) I didn't measure it but based on times I've measured it following polar scope alignment it's only been between 1 and 3 minutes off total.

[Same old newbie alert]

Check your focus on the guidescope, as you might be guiding on noise...next time get it up and running with the camera on a loop.. then manually move the mount.. if the stars don't move they're not stars.. any reason youre using such low camera loop?

[Same old newbie alert]

Also do a dark library and bad pixel map.. run the guide assistant for at least a worm period and apply the settings

[bottletopburly]

what do you have the spacing set at on the mpcc assuming that your using 130pds withe the baader cc,

[wimvb]

Differential flexure? The main mirror in a Newtonian can move relative to the guide camera. That's why oag is usually recommended for reflectors.

Have you tried imaging and guiding near Zenith? At this position, the mirror rests evenly on its support.

[pipnina]

41 minutes ago, wimvb said:

Differential flexure? The main mirror in a Newtonian can move relative to the guide camera. That's why oag is usually recommended for reflectors.

Have you tried imaging and guiding near Zenith? At this position, the mirror rests evenly on its support.

Last night I think M51 was transiting the zenith during my imaging, and it had similar star shapes to the image i took closer to the horizon the night before of the north america nebula. I heard that OAG was good for FL over 1000 and thought my 650mm/130mm scope would be good for a guide scope.

 

2 hours ago, Same old newbie alert said:

Check your focus on the guidescope, as you might be guiding on noise...next time get it up and running with the camera on a loop.. then manually move the mount.. if the stars don't move they're not stars.. any reason youre using such low camera loop?

2 hours ago, Same old newbie alert said:

Also do a dark library and bad pixel map.. run the guide assistant for at least a worm period and apply the settings

Dark library might be a good idea! I presume I'd be creating a big master dark for the guider to apply? Would it matter in this instance if the outdoor temperature changes since it could be as low as 0 in the winter and 15 in summer? They are pretty short exposures I suppose. Not sure what the guide assistant is though.

I used the lower camera loop because it was providing better RMS, while still tracking stars. I believe it wasn't tracking noise since my mount has always drifted much more considerably during longer-than-minute exposures, and it stayed put during a 7 min exposure (albeit with the star being oblong and fuzzy, can't find the image now though...) It also performed the DEC and RA star cross calibration fine which makes me think it was definitely detecting real stars.

Does a longer exposure (1 or 2 or even 3 seconds?) tend to result in better in-practice guiding?

[Same old newbie alert]

31 minutes ago, pipnina said:

Last night I think M51 was transiting the zenith during my imaging, and it had similar star shapes to the image i took closer to the horizon the night before of the north america nebula. I heard that OAG was good for FL over 1000 and thought my 650mm/130mm scope would be good for a guide scope.

 

Dark library might be a good idea! I presume I'd be creating a big master dark for the guider to apply? Would it matter in this instance if the outdoor temperature changes since it could be as low as 0 in the winter and 15 in summer? They are pretty short exposures I suppose. Not sure what the guide assistant is though.

I used the lower camera loop because it was providing better RMS, while still tracking stars. I believe it wasn't tracking noise since my mount has always drifted much more considerably during longer-than-minute exposures, and it stayed put during a 7 min exposure (albeit with the star being oblong and fuzzy, can't find the image now though...) It also performed the DEC and RA star cross calibration fine which makes me think it was definitely detecting real stars.

Does a longer exposure (1 or 2 or even 3 seconds?) tend to result in better in-practice guiding?

What was the guidestar snr reading? 

The guide assistant will help iron out the tracking errors a little bit, it switches the guiding off and tracks the star to see how it performs then it calculates the min mo, aggression etc, in the tools tab.. the camera loop depends on how well the mount performs

[pipnina]

15 minutes ago, Same old newbie alert said:

What was the guidestar snr reading? 

The guide assistant will help iron out the tracking errors a little bit, it switches the guiding off and tracks the star to see how it performs then it calculates the min mo, aggression etc, in the tools tab.. the camera loop depends on how well the mount performs

I found a screenshot of what I was seeing in the guide preview screen

Also, just in case it turns out that this is the issue...

When I select the scope here, am I supposed to pick the guide scope or the primary scope? I figured it was supposed to be the primary since that would give me the arcsecond rms reading I was after... If I was supposed to pick guide scope it might explain why it thought my RMS was lower than my images showed?

[Same old newbie alert]

Sorry I assumed you to be using PhD, but looking at the screenshot was you on the simulator?

The pulse values haven't been filled, so I'd assume you was guiding with the simulator..

 

Edited April 10, 2022 by Same old newbie alert
Added info

[pipnina]

13 minutes ago, Same old newbie alert said:

Sorry I assumed you to be using PhD, but looking at the screenshot was you on the simulator?

The pulse values haven't been filled, so I'd assume you was guiding with the simulator..

 

Oh no no, this is me jkust bringing it up on my desktop in simulator mode, my kit is down in the shed so I can't load up what it was showing last night or what the kit shows in use atm.

I found a partial screenshot of what I was doing last night but sadly not the whole screen.

[wimvb]

25 minutes ago, pipnina said:

I found a screenshot of what I was seeing in the guide preview screen

Also, just in case it turns out that this is the issue...

When I select the scope here, am I supposed to pick the guide scope or the primary scope? I figured it was supposed to be the primary since that would give me the arcsecond rms reading I was after... If I was supposed to pick guide scope it might explain why it thought my RMS was lower than my images showed?

You need to select the guidescope. If that scope is 200 mm fl, then your guide rms is abt 3 x larger than what is reported when you have selected the imaging scope (650 mm).

Edited April 10, 2022 by wimvb

[pipnina]

2 minutes ago, wimvb said:

You need to select the guidescope. If that scope is 200 mm fl, then your guide rms is abt 3 x larger than what is reported when you have selected the imaging scope (650 mm).

Woops!

I guess I know what was wrong now then haha. I presume the guider was sending signals to the mount that were either too big or too small?

[wimvb]

Reported rms @650 mm was 0,32", which corresponds to 3.25 x 0.32" = 1.04" if your guidescope fl = 200 mm

[wimvb]

3 minutes ago, pipnina said:

Woops!

I guess I know what was wrong now then haha. I presume the guider was sending signals to the mount that were either too big or too small?

The guider works in pixels, but the mount moves in arc seconds. Otoh, if you have calibrated with the same setup as you use for guiding, it will still work. But the numbers in arc seconds are nowhere to be trusted.

Which also means that you have to recalibrate now that you've changed fl in the settings.

Edited April 10, 2022 by wimvb