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Guiding improvement suggestions - post-belt mod now


tooth_dr

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Hi all

Im looking some advice please. 

Im currently guiding with a finder guider and 120MM on an EQ6-pro, using PHD2.

I consistently get a guiding RMS of between 1.2 and 1.5, I’ve had it just below 1.0 on one night but normally above 1.2. The RA guiding has a higher RMS than the Dec.  My imaging camera has pixel size 5.4um, guiding camera pixel size is 3.75um.

This seems to be ok when I’m imaging with my ED80 at 2.13”/px, and I don’t lose any subs in a whole night of imaging.

Ive now put a bigger scope on the mount. I’m getting the same RMS, last night it was 1.1-1.3, but since I’m imaging at a pixel scale of 0.93”, I’m starting to see eggy stars and inconsistent subs.

What should I consider doing to help?

1) belt mod

2) longer guidescope

3) binning 2x2

Thanks in advance

Adam

Edited by tooth_dr
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On my belt modded HEQ5 I use the ASI120MM with both a 50mm finder and a dedicated 60/224 guidescope: usually the finder goes with the 72ED or the ED80 and the guider with the newton. 

Both options are fine, and I get RMS anywhere between 0,4" and 1,8", depending on seeing : I live in the centre of Rome, and that's my limiting factor.

So:

Belt modding yields a main improvement;

Bigger/longer scope helps a bit;

Binning is not, in my Experience, changing much, IF you have a good guidestar (i.e. SNR > 45 on PHD2) : it effectively halves the guidescope Focal Length

But, to fight the seeing, longer exposures really help, as well as SLIGHTLY defocusing: I assume the defocus improves thing because due to seeing the image doesn't pass THROUGH focus, and tends to be a tad steadier. 

Fabio

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41 minutes ago, FaDG said:

On my belt modded HEQ5 I use the ASI120MM with both a 50mm finder and a dedicated 60/224 guidescope: usually the finder goes with the 72ED or the ED80 and the guider with the newton. 

Both options are fine, and I get RMS anywhere between 0,4" and 1,8", depending on seeing : I live in the centre of Rome, and that's my limiting factor.

So:

Belt modding yields a main improvement;

Bigger/longer scope helps a bit;

Binning is not, in my Experience, changing much, IF you have a good guidestar (i.e. SNR > 45 on PHD2) : it effectively halves the guidescope Focal Length

But, to fight the seeing, longer exposures really help, as well as SLIGHTLY defocusing: I assume the defocus improves thing because due to seeing the image doesn't pass THROUGH focus, and tends to be a tad steadier. 

Fabio

Thanks Fabio. I meant binning my imaging camera, binning the guidescope would make things worse as you say.

The focal length of the imaging scope is 1200mm

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Start with the OAG. I suspect the eggy stars are the result of differential flexure with the finder guider. Your guiding RMS is not hugely different in RA and Dec so you would not expect eggy stars from that. The guiding RMS vaues could be down to poor seeing.

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From my experience of the NEQ6, I found it was quite happy to guide at around 1.4 arc seconds/pix producing subs with 30min duration without issue. However, after spending many hours adjusting various guiding parameters, I got the impression that I was on the limit of the mounts performance capabilities. 

At 0.93 arc seconds/pix I think you will struggle to obtain consistent results due to the intrinsic tracking limitations of the mount, even if you are well inside the imaging weight limits of the mount.

Alan

 

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Hi Adam,

I have a belt modded NEQ6 which carries either an ED80DS, WO-ZS71 or my Samyang 135mm along with my SkyWatcher EvoGuide with a ZWO ASI120MM. I take great care to level the mount and to polar align using only the built-in polarscope. I then do a two or three star alignment with CdC in the eastern sky only - can't see much of the western one!

Since carrying out the belt mod I don't recall my RMS ever exceeding 1.2" and typically I expect to achieve 0.5" - 0.8" RMS - the lowest I have ever achieved is 0.4" RMS. Prior to the belt mod I typically achieved 0.8" - 1.2" RMS.

Surprisingly if I do a manual median flip I don't bother recalibrating PHD2 the guiding usually improves!

The belt mod is definitely worth the expense and the effort.

HTH with your deliberations.

Adrian

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If you want to image at 0.93 arc seconds/pix then as as rough guide you need your rms guide corrections to be consistently less than half of this value eg better than about 0.46 arc seconds. If they are more then they will impact the shape and size and stars.  The information above suggests that even with a belt modification you are going to struggle to achieve satisfactory results. 

Alan  

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Have you analysed your guidelogs? As you, I had a RA rms larger than DEC rms, plus I had a 2 minutes period in the guide graph. I posted the log on the phd forum, and got the recommendation to use predictive pec guiding, with a fixed period. It improved my guideresults.

So, before you throw money at the problem, start with analysing your current setup.

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Thanks everyone for the input. @wimvb I’ve posted logs on the phd forum and the consensus is that my results are ok for the mount and there isn’t anything that can done to improve it with my current physical configuration. 

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11 hours ago, alan4908 said:

If you want to image at 0.93 arc seconds/pix then as as rough guide you need your rms guide corrections to be consistently less than half of this value eg better than about 0.46 arc seconds. If they are more then they will impact the shape and size and stars.  The information above suggests that even with a belt modification you are going to struggle to achieve satisfactory results. 

Alan  

Thanks Alan. If I bin 2x2 does that mean I’m imaging at 1.86”/px? I think a belt mod is on the cards over the summer, I may not need an OAG then?

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59 minutes ago, tooth_dr said:

Thanks Alan. If I bin 2x2 does that mean I’m imaging at 1.86”/px? I think a belt mod is on the cards over the summer, I may not need an OAG then?

If you bin 2x2 then your effective image scale will be 1.86 arc seconds per pixel.

However, I'd be careful of using software binning as a methodology of improving your guiding performance. This is because the original guiding error will still be present in the pre-binned image, all you have done is to reduce the overall resolution in an attempt to hide the error. 

This technique is equivalent to taking an image, measuring the average star aspect ratio, binning the image 2x2 and then re-measuring the aspect ratio.  If I do this for one of my own images, imaged at 0.7 arc seconds/pixel, then CCD Inspector gives me an aspect ratio of 8 for both images - eg it does not help.

I don't believe an OAG would help.

Alan 

 

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16 minutes ago, alan4908 said:

If you bin 2x2 then your effective image scale will be 1.86 arc seconds per pixel.

However, I'd be careful of using software binning as a methodology of improving your guiding performance. This is because the original guiding error will still be present in the pre-binned image, all you have done is to reduce the overall resolution in an attempt to hide the error. 

This technique is equivalent to taking an image, measuring the average star aspect ratio, binning the image 2x2 and then re-measuring the aspect ratio.  If I do this for one of my own images, imaged at 0.7 arc seconds/pixel, then CCD Inspector gives me an aspect ratio of 8 for both images - eg it does not help.

I don't believe an OAG would help.

Alan 

 

Thanks again Alan.  I've still new to CCD imaging, but I was referring to binning the camera during capture, not afterwards.  Does the same apply?  It does make sense that I'm just hiding the error though.

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I saw a pretty large improvement when I switched guide cameras. I am now using my old imaging cam, imx183 chip with it's tiny 2.4um pixels to guide through a zwo 60mm guide scope.

This means I nearly have identical resolution on my imaging as my guiding.

Was getting 0.5 to 0.6 regularly on an unmodded EQ6 over 3 days during Easter, using the predictive pec algorithm. Seeing was good though..

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9 hours ago, tooth_dr said:

Thanks again Alan.  I've still new to CCD imaging, but I was referring to binning the camera during capture, not afterwards.  Does the same apply?  It does make sense that I'm just hiding the error though.

Software and camera hardware binning are different, however, from the perspective of trying to improve guiding errors they are equivalent. Binning does have its place in imaging eg increasing the signal to noise ratio for faint objects or for rapid image downloads in real time plate solving but using binning for fixing guiding issues isn't one of them.  From an image perspective, indiscriminate binning means that you will be throwing away resolution and you will also be making the stars squarer. Your camera will also start to clip bright objects (eg stars) more frequently which is information that can never be recovered. 

If you do go ahead with the belt modification, I think you will be see improvements to your guiding and general operation of your mount. If you still end up with non-ideal aspect ratios for your stars, then I'd suggest you simply try to reduce their impact in post processing.  

Alan

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18 minutes ago, tooth_dr said:

Thanks for all the input everyone.  I've placed an order for a Rowan Belt kit, I think this is the next logical step forward.  I'll be sure to update this thread with my results.

I look forward to hear how the installation goes.. It looks pretty complicated on the EQ6 vs the HEQ5. :)

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7 minutes ago, jjosefsen said:

I look forward to hear how the installation goes.. It looks pretty complicated on the EQ6 vs the HEQ5. :)

Yes there is a lot to strip out.  This isnt the first time I've had my mount apart, but keep an eye out for a cheap mount for 'parts or repair' in case it doesnt work out ?

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The first time I did mine I think it took about 2 hours. It was surprisingly straight forward and after not being happy with a few things managed to refit in just under an hour.  It's the setting up of the worm gears/backlash that really takes the time. There was a member on here that put his videos of fitting the mod on youtube. I found it hand to watch first but Its also handy if you can have a laptop to hand in case you get a bit stuck.

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On 24/04/2019 at 14:23, spillage said:

The first time I did mine I think it took about 2 hours. It was surprisingly straight forward and after not being happy with a few things managed to refit in just under an hour.  It's the setting up of the worm gears/backlash that really takes the time. There was a member on here that put his videos of fitting the mod on youtube. I found it hand to watch first but Its also handy if you can have a laptop to hand in case you get a bit stuck.

I thought I’d share this from last time I stripped it lol Every time I always swear it will be the last time ?

 

C9E18352-7790-471C-ACFA-CA2C48A0F1BB.jpeg

EBDAAEE3-3D8F-43B5-A504-24CEE3BF9F0D.jpeg

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I thought I’d just continue to run with this thread.

Last night was my first evening with with belt modded EQ6

These are the best figures I achieved last night. Average around 1.2” total 

The mount seems to be correcting too much. It’ll be really flat for a few seconds then suddenly move off a bit and take a few cycles to get back.

One odd thing - it couldn’t do south movement to measure backlash. The graph was curved then went off scale. Prior to mod my return direction was straight albeit 2500ms backlash.

 

6AC84219-3C01-431B-BEBF-DADAB52BB06F.jpeg

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