What is good enough guiding? - Forums

Topic starter

May 22, 2022 · Edited: May 22, 2022

I'm using an OAG through a 1000mm F5 scope. I'm currently getting an average of 0.55 arc secs RMS guidance on a slightly cloudy night in a suburban area. I think this is probably as good as I can expect given my seeing, but wondered if I should be striving for better.

Key things I did to try and achieve a good guiding:

* Balanced the scope carefully in 3 axes
* Two SharpCap polar alignment sessions (i.e. restart after the first) ti hit an "Excellent" polar alignment after the second session.
* Careful focussing using EAF
* PHD2 calibration close to the meridian

Given my pixel scale of 0.86 arc secs per pixel at bin 1x1 I think this guiding is OK, but would welcome other's advice.

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Robert Winslow

May 22, 2022

What you are seeing sounds about right. I know when I was very worried about getting perfect numbers, I was told that if you run the guiding assistant and run the calibration and they do not report issues you are good enough. Heck, I was so worried about it that I would use my ipolar to polar align, then run sharpcaps polar alignment. It is to the point where I no longer worry if my numbers in PHD2 are perfect, all that matters is the end result.

What is even more important is how often you are guiding, people think that more frequent is better, but as others have documented, I have found that 2 -2.5 is about all you need. It is also very important that one insures good focus with the guide camera.

The main thing is this, do your stars look correct at your selected exposure? Are they nice and round with no visible streaks. When you stack, do you have an acceptable amount of stacking artifacts around the edge.

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Ken Bates

May 22, 2022

Another check is to see if your RMS guiding error is less than your image scale (arc seconds per pixel). If it’s less, that implies that any “jitter” you see will be no more than one pixel, and therefore not noticeable in the image. Sharp spikes seen in the PHD2 guiding graph may fall outside the RMS error, of course, but those should be taken care of by pixel rejection during processing.

At least, this seems like a reasonable metric to me…

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andrea tasselli

Frequent contributor

May 23, 2022

Andy Wray:
Key things I did to try and achieve a good guiding:

* Balanced the scope carefully in 3 axes
* Two SharpCap polar alignment sessions (i.e. restart after the first) ti hit an "Excellent" polar alignment after the second session.
* Careful focussing using EAF
* PHD2 calibration close to the meridian

I'm not sure what is meant by the 3rd axis, but, yes, good balancing is useful although not a guarantee per se. Sometimes a little unbalancing helps guiding with a temperemental mount.

Ditto for PA. Good yes but beyond that there is no point. After all you're plate solving so who cares?

Good focus but not to loose sleep over it. After all you're guiding on the centroid and that doesn't change much with a little defocusing.

I'd say that is needed for scalability and maximum sensitivity, assuming you are above the seeing threshold (which would mean 10-20 degrees above the meridian in the UK case). You'd achieve even better guiding by calibrating "in situ".

GalacticRAVE

May 23, 2022

A good check is whether your error in RA and DEC is of the same magnitude. It does not take much to have a star look eggy.

Georg N. Nyman

May 23, 2022

Let me add my two pennies worth….
RA and DEC should show similar values - I think that is quite important, as it was stated before.
The guiding error should by all means be less than the image pixel scale.
Next - you should not see any drift into any direction over a rather long time period.
There are two options - either you use a rather short exposure time for the guide camera and correct rather fast or you let the system float and correct over a longer time - say - 0.5sec as short with quick (smaller usually) corrections or 1,5sec with less but maybe a bit larger corrections
Allow a very small minium correction - smaller than the preset value - it helps
See of one axis is tending to overcorrect - then adjust the aggressivness accordingly - meaning - overcorrecting-less aggressive as example

These are some of my personal findings over the past years - others may probably have other experiences… I think all that counts is the result - if your stars an nice, round and their size is within a proportional relation to the focal length and pixel scale of the camera, you did a good job

CS
Georg

Christopher Davenport

May 23, 2022

Let's see if I can explain this, as I am looking into what my limit on RMS should be, before I cancel an exposure.

Guiding is accurate down to (on average) 0.004 pixels or 1/250th of a pixel. With a low amount of noise, the accuracy goes to 0.018 pixels or 1/56th of a pixel and with high amounts of noise it goes to 0.18 pixels or 1/5.5th of a pixel. What this means is that on bad seeing night, your worst ratio between a pixel and a star is about 1:5 whereas a night with good seeing will be about 1:50. Therefore most people target having a imaging to guide ration of smaller than 1:5 to account for the really bad seeing nights.
http://www.stark-labs.com/help/blog/files/PHDSubpixelAccuracy.php

Now my resolution ratio is at 1:3.75, which means that my guiding setup is more accurate than my imaging setup even with poor seeing. On a good night, my guiding can get to 10 times more accurate than my imaging rig. Now based on the above calculator, it is important to check that your focal length and your pixel size of your guider is correct for a RMS calculation. Many people claim low RMS numbers, but have the wrong FL or they have the incorrect sensor size setup for the calculation.
https://astronomy.tools/calculators/guidescope_suitability
(Using Indie I can do a plate solve on my guiding system to get super accurate resolution details, same is true for imaging rig.)

Anyway my imaging resolution is 1.71" per pixel with perfect seeing, so with worse seeing this will degrade.
On a brilliant night of seeing, I will be around 2.5" per pixel. Indi does this calc based on guiding system.
On a bad night of seeing, this can drop to 3" per pixel.
https://skyandtelescope.org/wp-content/uploads/SeeingDispersion_m.jpg

So what does this all mean?
If my RMS for DEC and RA, both stay below 1" per pixel, my total will be below 2", which means that my seeing is the limiting factor, not guiding.

If you live on top of a mountain and a high pressure settles over you, maybe you can get down to 1" seeing, well then you should have better kit than me

(Oh and invite me to come visit please)

Hope this helps, also any errors, please let me know.

Christopher.

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Scott Lockwood

May 23, 2022

Might I add, for me, longer guiding exposures are better. My default time is 4 seconds. Any faster, I feel, will just be chasing bad seeing. Occasionally I will go to as much as 6 or 8 seconds if needed to get a very dim star to guide on.
Scott

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andrea tasselli

Frequent contributor

May 23, 2022

Christopher Davenport:
Anyway my imaging resolution is 1.71" per pixel with perfect seeing, so with worse seeing this will degrade.
On a brilliant night of seeing, I will be around 2.5" per pixel. Indi does this calc based on guiding system.
On a bad night of seeing, this can drop to 3" per pixel.

This doesn't make much sense. By imaging resolution I imagine you mean image scale which has got nothing to do with actual resolution which is a way more involved matter. The rest I frankly do not grasp. Are you trying to quote your PSF in some form?

Christopher Davenport

May 23, 2022 · Edited: May 23, 2022

andrea tasselli:
Christopher Davenport:
Anyway my imaging resolution is 1.71" per pixel with perfect seeing, so with worse seeing this will degrade.
On a brilliant night of seeing, I will be around 2.5" per pixel. Indi does this calc based on guiding system.
On a bad night of seeing, this can drop to 3" per pixel.

This doesn't make much sense. By imaging resolution I imagine you mean image scale which has got nothing to do with actual resolution which is a way more involved matter. The rest I frankly do not grasp. Are you trying to quote your PSF in some form?

Apologies if post did not make sense, was me working through it to understand it myself, this was just proving out what Ken is saying above.
My RMS is usually about 0.6 arcseconds in DEC and 0.8 arcseconds in RA, with a total RMS which hovers around 1 arcsecond.

Trying to tune this any further makes no sense as the imaging resolution of my setup is limited to 1.71 arcseconds.
So movements due to guiding that are kept under this limit and will not (in theory) affect any pixels.

If you go into Astronomy tools, they use the following formula for imaging resolution:
Resolution Formula: ( Pixel Size / Telescope Focal Length ) X 206.265 with a result in arcsecond per pixel on your camera.
So the resolution of my setup is 1.71 arcseconds per pixel, it cannot resolve any fainter detail than this resolution.

However as my seeing limits me to 2 to 3 arcseconds anyway, my RMS error can be larger, so long as I stay under the seeing limit.

Also I agree with Scott post above about using a 4 to 7 second exposure as it stops you from chasing the seeing and you get smoother guiding.

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andrea tasselli

Frequent contributor

May 23, 2022

Christopher Davenport:
f you go into Astronomy tools, they use the following formula for imaging resolution:
Resolution Formula: ( Pixel Size / Telescope Focal Length ) X 206.265 with a result in arcsecond per pixel on your camera.
So the resolution of my setup is 1.71 arcseconds per pixel, it cannot resolve any fainter detail than this resolution.

That is the image scale, it isn't the image resolution, which mostly depends on the seeing. At that image scale you'd probably be able to resolve details (with catches along the way) around twice as much. Think two stars apart separated by 1.71". They would form one single luminous blob two pixels wide by 1 pixel long but you'd be hard-pushed to tell which one is which. You could and should drizzle to regain some of the lost resolution but even that isn't a free meal either.

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Matthew Proulx

May 23, 2022

Good enough is when the stars are round with no noticable trailing on the small stars. (once collimation and tilt are accounted for).
1/2 of your image scale which should be 1/2 to 1/3 of your seeing is a good place.

Kevin Morefield

IOTD Staff

May 23, 2022

andrea tasselli:
I'm not sure what is meant by the 3rd axis

3rd axis is balancing the payload side to side after Dec and RA. You point the scope vertically at the zenith and loosen the Dec. If the scope moves at all one side is heavier. A little off balance is OK for most GEMs but a lot of movement can be a problem. For the direct drive mounts like the Planewaves, no movement should be accepted.

There is actually a fourth axis which is the rotational axis. After doing the RA, DEC, and side-to-side balancing, and with the scope still pointing up, I begin rotating the camera to see if that produces any movement. I use counterweights on the FW or camera, etc to balance. While I do this out of habit on my GEM, it's really only needed for long FL work on the direct drive. But it is helpful for unguided imaging on all mounts.

Kevin

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andrea tasselli

Frequent contributor

May 23, 2022

My original point is that there is no third axis on any mount (altough there have been proposal of alt-az mounts which rotate the optical tube around its axis). Obviously you would try to make the tube as well balanced as possible, one of the way is the one you suggested. I follow another one as my degrees of freedom are different. I also balance aft and fore on a workbench by sitting the OTA on a tube and fixing the balance point reference mark. DD mount are not the only ones sensitive to balancing, the same applies to friction mounts too.

Andy Wray

Topic starter

May 24, 2022

Kevin Morefield:
andrea tasselli:
I'm not sure what is meant by the 3rd axis

3rd axis is balancing the payload side to side after Dec and RA. You point the scope vertically at the zenith and loosen the Dec. If the scope moves at all one side is heavier. A little off balance is OK for most GEMs but a lot of movement can be a problem. For the direct drive mounts like the Planewaves, no movement should be accepted.

There is actually a fourth axis which is the rotational axis. After doing the RA, DEC, and side-to-side balancing, and with the scope still pointing up, I begin rotating the camera to see if that produces any movement. I use counterweights on the FW or camera, etc to balance. While I do this out of habit on my GEM, it's really only needed for long FL work on the direct drive. But it is helpful for unguided imaging on all mounts.

Kevin

This is what I was referring to when I said a third axis.