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blinky

Can my guiding be improved (cheaply/free!)

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I now have a dual setup with my F4 8" Newt and WO ZS71 side by side, I don't have any problems with the ZS71 but do get trailing when looking at the starts @ 100% from the Newt.  My imaging/Guiding is Lodestar on an OAG and a ZWO ASI1600mono with filter wheel, the mount is an EQ6 Pro that has been stripped and regreased a couple of years ago (more like 5)  I bin the Lodestar X2 as suggested but can never figure out why!  I have the predictive algorithm in PHD and have PEC recorded in EQMod.  So.….. when imaging with the Newt im at 0.97 arc secs/Pixel and I think this is my problem, can an EQ6 Guide that accurately?  I am drift aligned as well, so its not that either - just wondering if really, im at the limit of my trusty old EQ6!

 

 

stars.JPG

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Where do I see those figures?  I watched a video last night on PHD2 and the developer was saying guide rate should be somewhere between 0.5 and 1, mine is currently 0.1 so I will try increasing that to 0.5 first

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7 hours ago, michael8554 said:

Is this RA or Dec elongation? 

Unfortunately it's both.  Here's the OP's image overlaid with an equatorial grid:

498048085_trailing3.jpg.9a245ba22586bacd84a560222ce3609c.jpg

 

 

37 minutes ago, blinky said:

Where do I see those figures? 

PHD2 Log Viewer is a handy tool, written by one of the current developers, for analysing guiding performance.  If you don't have a copy there are some details here and a download page here 

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Do you guide with the newt/ oag setup or guide with the w/o?

Would you have a screen shot of the guide graph?

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I would think that the EQ6 should be able to guide that ok, provided its well balanced etc.  I have imaged at 0.91”/arcsec and definitely lost a few subs but didn’t have consistent trailing. 

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I would have thought so as well. When I had my AVX and was imaging at 1" pp my guiding would routinely be around and sometimes over. The result for me was bigger stars, not elongated if the guide error was approximately equal across the axes.

I assume your guiding is therefore struggling on one axis more than the other? 

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Ok, so had another look tonight - discovered that 'use dec compensation' was switched off which would not be helping!  I also redid my calibration near the equator as suggested and upped my guide rate to 0.5x(from 0.1x) and exposure to 3s (from 2s)

here's a 5min exposure, just taking a 6min one now as that was what the previous sample at the top of this post was....

 

5min.JPG

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Also enabled the Predictive PEC, its just finished training, or at least should just have finished training, an indication of this would be good.

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This is the guiding, the axis is 3 arc-secs (I believe)

PHD3.JPG

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On 12/09/2019 at 22:57, blinky said:

6min - looks a lot better to me!

 

6min100%.JPG

Is that from the newt, from the centre of the frame Craig?  Asking as I don't see any difraction spikes.

There's some elongation but it's not terrible.

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7 minutes ago, Starflyer said:

Is that from the newt, from the centre of the frame Craig?  Asking as I don't see any difraction spikes.

There's some elongation but it's not terrible.

Yeah it was but it was with the Ha filter in place

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I normally use the phd target as a guide to my star shapes.  I think your circle of stars on the target looks a little taller than wider. This would certainly support the stars even still being slightly elongated.  With those figures you’d expect round stars given that RA and Dec are equal.

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Try setting your history to a lot more than 50 data points and your target to more than 100 data points. It won't improve your guiding but will give a better idea of what's happening over the full six minutes or more.

As tooth_dr says, the shape of your target point spread should mirror what you're seeing in your stars. But over the 150 seconds in your history the RA and DEC RMS errors are equal, this should mean round stars.

It is possible to get flex in an OAG, my SX OAG was useless to begin with, the stalk would rock back and forth. I fitted a tiny grub screw into the stalk holder and now it's solid. I'm not saying yours is the same, but something to check.

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1 minute ago, Starflyer said:

Try setting your history to a lot more than 50 data points and your target to more than 100 data points. It won't improve your guiding but will give a better idea of what's happening over the full six minutes or more.

As tooth_dr says, the shape of your target point spread should mirror what you're seeing in your stars. But over the 150 seconds in your history the RA and DEC RMS errors are equal, this should mean round stars.

It is possible to get flex in an OAG, my SX OAG was useless to begin with, the stalk would rock back and forth. I fitted a tiny grub screw into the stalk holder and now it's solid. I'm not saying yours is the same, but something to check.

Will give that a go and have a wee look at the OAG, it’s very thin and never was the best

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Ok so there was lots of high cloud so guiding was terrible! But.... My OAG was a little loose, I could wiggle it slightly back and forth!  So I tightened the screw up a tiny bit and its rock solid.  As I said, guiding was pretty terrible and in fact, unguided was actually better than guided it was that bad.  Turns out my polar alignment must be good though as 3 min subs at 800mm focal/1arcs pixel showed pretty much no trailing.

Hopefully will be better next time Im out but feeling even more confident I have sorted the guiding issues now.

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9 hours ago, Starflyer said:

Try setting your history to a lot more than 50 data points and your target to more than 100 data points. It won't improve your guiding but will give a better idea of what's happening over the full six minutes or more.

As tooth_dr says, the shape of your target point spread should mirror what you're seeing in your stars. But over the 150 seconds in your history the RA and DEC RMS errors are equal, this should mean round stars.

It is possible to get flex in an OAG, my SX OAG was useless to begin with, the stalk would rock back and forth. I fitted a tiny grub screw into the stalk holder and now it's solid. I'm not saying yours is the same, but something to check.

Was thinking this morning, if my unguided is basically as good as my guided (will post that image later on) then I wonder if I need to up my guiding speed?  It was 0.1x but last time I increased it to 0.5, but wonder if something like 0.7 would allow it  to correct fast enough?  The other thing is the guider has bigger pixels than the ASI1600 and is binned 2 as that’s what’s suggested.  I wonder if this is outdated and due to the CMOS cameras themselves having small pixels I should do away with the binning of the Lodestar, so it will ‘see’ guiding errors quicker?

I wont change anything yet though, want to see how guiding goes next time after I tightened up the OAG last night first, then if I’ve still problems I will increase the guide speed and recalibrate then try dropping to Bin 1 and redoing calibration.  I fell I’m getting there though, its all about small incremental changes.

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Have you got the figures entered correctly into phd for the equipment profile so that the graph etc is accurate?

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Yeah scope is 800mm focal length and a I use an OAG so is the guide cam. I believe PHD2 gets the pixel size from the guide cam and I assume knows to double it since I am in Bin2

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Just realised in my above screenshot the total error is 0.94 arc secs, I'm imaging at 0.95 so it should... be ok

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I use 0.3x with my 10" newt, my reasoning is that I wanted 'gentle' corrections as it's a big scope to move around quickly and it overshoots easily.  0.5 sounds good for your newt, maybe 0.7 for your frac.

I believe the reason the lodestar should be binned is that it's an interlaced sensor. Each download at 1x1 only downloads every other pixel row, so if you have a star with the centroid over a number of pixels it will appear to PHD to jump around.  At 2x2 you download the whole sensor image every frame, you're also getting less latency between what your camera is seeing and what your mount is doing.

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22 hours ago, blinky said:

Just realised in my above screenshot the total error is 0.94 arc secs, I'm imaging at 0.95 so it should... be ok

You really want RMS to be under half of your resolution because the peaks will always be larger than the RMS, your setup seems to have peaks around 2"
There's generally 3 things that limits guiding, firstly the mount of course, how well will it guide?
There's also the seeing and of course how high in the sky you image, there's big differences through the night and how high your scope is pointing if you are seeing limited.
 

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2 hours ago, Starflyer said:

I use 0.3x with my 10" newt, my reasoning is that I wanted 'gentle' corrections as it's a big scope to move around quickly and it overshoots easily.  0.5 sounds good for your newt, maybe 0.7 for your frac.

I believe the reason the lodestar should be binned is that it's an interlaced sensor. Each download at 1x1 only downloads every other pixel row, so if you have a star with the centroid over a number of pixels it will appear to PHD to jump around.  At 2x2 you download the whole sensor image every frame, you're also getting less latency between what your camera is seeing and what your mount is doing.

That was it Ian, I knew there was a reason to do bin2, what’s strange is in the other thread somebody has said they always use bin 1 and have no issues! Regardless, I found last night during testing that my polar alignment is very slightly off in azimuth and that my guide star was not in focus. With regards to the focus, this explains why when at bin1 my stars all looked like doubles, a combination of the OAG placing cutting off some light and the star being a slight donut rather than round.  I’m going to look at moving the OAG prism further into the light path tonight to see if I can improve star shapes as even when properly focused the stars at 1x were not great (although this might be a limitation of the sensor as you point out!)

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I wonder if it’s time to sell the Lodestar..... if I need to use bin2 to get round interlacing, realistically it having a guide camera with 16um pixels guiding a camera with 3um pixels going to work? Might be better to sell and buy the ZWO guider instead. 

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