Reasonable level of accuracy.

[George Gearless]

So I've finaly managed to succesfully set up my mount with Stellarmate and all the 'fixings' that I've bought over the summer. I've made a lot of beginners mistakes, but I think I'm finaly ready.

I successfully guided my mount for the first time yesterday and was extremely delighted that it worked so well. Especialy since I'm realy pushing the weightlimits of my mounts capabilities.

It's a EQM-35 Pro and the datasheet says that it can handle 10kg of additional load. Needless to say, I was a bit worried when I piled on my 180mm Maksutov (7kg tubeweight), Orion mini guiderscope, two ZWO cameras, extension tube and heat bands. But it seemed to handle it allright. Or did it?

In the Ekos guiding program, there is a graph representation of how much your scope 'wobbles' off the targeted star. I stared at it for about 10 to 15 mins to see if there were any major fluctuations. But it remained relatively steady. As far as I could tell, it never exeeded 0.5 of an arcsecond. Is that a reasonable level of accuracy for AP?

I should mention that my polar alignment was very rough and very dirty. It was somewhat on purpose, because I wished to 'stress-test' the guiding utility and see how well it compensated for a sloppy setup.

Edited October 8, 2019 by George Gearless

[wimvb]

4 hours ago, George Gearless said:

As far as I could tell, it never exeeded 0.5 of an arcsecond.

That would be an excellent result for the mount. Normally you measure the rms ("average") error over a certain time period. How good your guiding really is, also depends on your imaging resolution (arcsecs/pixel). Multiply imaging pixelsize in micrometer by 206 and divide by focal length in millimeter to get imaging resolution. Your guiding error should be less than that value.

[vlaiv]

8 hours ago, George Gearless said:

As far as I could tell, it never exeeded 0.5 of an arcsecond. Is that a reasonable level of accuracy for AP?

That is superb result if true. I'm a bit doubtful if it is indeed true.

There can be number of reasons why this number is not correct.

- first check if you entered focal length of guide scope and pixel size of guide camera correctly into guide program. This is needed to properly calculate arc seconds per pixel.

- It can be the case of guide system inability to properly measure guide error. What guide camera and scope are you using? You need to have enough resolution in guide system to properly measure guide error. If system is lacking resolution it will report much smoother guide errors - a bit like estimating height of people with 1meter stick (without cm subdivisions), it will either be 1m or 2m and will look "smooth".

In any case, as wimvb mentioned, imaging resolution plays a part in it as well, but if you really guide at 0.5" RMS (or there about) - that is really excellent result for EQ35 mount (that is excellent result for even heavier more expensive and precise mounts).

[George Gearless]

@vlaiv, @wimvb

All my equipment is very much stock. Expecting anything better than a 'stock result' is propably being hopefull.

I did enter the the guidescope and camera info into Stellarmate. But considering I am using a Orion miniscope (9x50) as guide, and a ZWO 120 Mono camera, I think it is more likely that my system is unable to properly determine the error.

I'm a complete newbie at using a guidescope. And using Stellarmate overall, for that matter. So I was just trying to figure out what kind of error margin I should expect. I simply had no frame of reference.

While there is still a long way to go, your replies leads me to believe that I am at least on the right track.

Thanks both of you.

 

[david_taurus83]

Are you using the internal guider or PHD2? Make sure the reading is in arc seconds and not pixels.

 

Also, make sure in the guiding tab that Ekos has selected the correct camera and scope.

Edited October 8, 2019 by david_taurus83

[spillage]

I always thought that you guiding should be half of you imaging resolution. Be nice if I am wrong as I always thought my quattro 8 and 1600 are pushing the capabilities of the neq6 a bit too much.

[vlaiv]

4 hours ago, George Gearless said:

@vlaiv, @wimvb

All my equipment is very much stock. Expecting anything better than a 'stock result' is propably being hopefull.

I did enter the the guidescope and camera info into Stellarmate. But considering I am using a Orion miniscope (9x50) as guide, and a ZWO 120 Mono camera, I think it is more likely that my system is unable to properly determine the error.

I'm a complete newbie at using a guidescope. And using Stellarmate overall, for that matter. So I was just trying to figure out what kind of error margin I should expect. I simply had no frame of reference.

While there is still a long way to go, your replies leads me to believe that I am at least on the right track.

Thanks both of you.

 

Stock mounts vary quite a bit in guide performance - some are smoother than others, but I would expect on average EQ35 class mount - which is probably closer to EQ5 than EQ3 in guide performance, to have something like 1-1.5" RMS guide error.

One could do sub 1" RMS on such mount if mount is smooth and conditions are particularly favorable on a given night - meaning 0 wind and very good seeing conditions.

Orion miniscope has 162mm focal length (if I'm not mistaken) and ZWO 120 mono has 3.75um pixel size. Together that gives 4.77"/px of guide resolution. Centroid calculations are able to determine star position to about 1/16 to 1/20 of single pixel (depending on SNR) - which means ~0.25", and RMS calculation should be precise if it is larger than about x3 of that, so if RMS figure is larger than 0.75" then it can be "trusted" (it is about right) - smaller than that, I'm simply thinking that you won't be able to get accurate reading.

Mind you, that is enough precision to guide such mount on stock values (meaning about 1"-1.5" RMS), but does limit your imaging resolution to about 2"/px.

Another way to tell if your guiding was really that good is to examine subs you've taken while guiding. What sort of star shapes did you get? Are stars distorted in any way - for example not round, maybe elongated in one direction or egg shaped? If they are round - which means that guide error was uniform in every direction (good thing), next what you want to look at is FWHM of stars in arcseconds. This value is true measure of achieved resolution on the image, and if that number is comparatively small (depends on guiding, scope size and seeing on that night) - then yes, you were indeed guiding really well.

4 hours ago, spillage said:

I always thought that you guiding should be half of you imaging resolution. Be nice if I am wrong as I always thought my quattro 8 and 1600 are pushing the capabilities of the neq6 a bit too much.

Yes, that is quite good general rule of thumb to have guide RMS at least half of imaging resolution or smaller (smaller is of course better, and larger is worse, but it will not "ruin" image - it will just look a bit blurrier).

[George Gearless]

5 hours ago, david_taurus83 said:

Are you using the internal guider or PHD2? Make sure the reading is in arc seconds and not pixels.

 

Also, make sure in the guiding tab that Ekos has selected the correct camera and scope.

Since I don't know the difference between the internal guider and PHD2, I'm guessing I'm using the internal guider :). I pretty much went with whatever default settings Ekos offered. The graph explicitly said "arcseconds" on the Y-axis and time on the X-axis. There is also a circular graphic representation of the error (kind of like a bulls-eye where it marks little dots at each measurement). But that is hard to read accurately. Although it gives a good visual representation to check if everything is working fine.

Yes, I am quite sure that I selected the right scope and camera in the guiding tab. Just for fun, I tried changing the camera's in the menu, and it warned me that I was now trying to guide with my main scope.

[George Gearless]

1 hour ago, vlaiv said:

Stock mounts vary quite a bit in guide performance - some are smoother than others, but I would expect on average EQ35 class mount - which is probably closer to EQ5 than EQ3 in guide performance, to have something like 1-1.5" RMS guide error.

One could do sub 1" RMS on such mount if mount is smooth and conditions are particularly favorable on a given night - meaning 0 wind and very good seeing conditions.

Orion miniscope has 162mm focal length (if I'm not mistaken) and ZWO 120 mono has 3.75um pixel size. Together that gives 4.77"/px of guide resolution. Centroid calculations are able to determine star position to about 1/16 to 1/20 of single pixel (depending on SNR) - which means ~0.25", and RMS calculation should be precise if it is larger than about x3 of that, so if RMS figure is larger than 0.75" then it can be "trusted" (it is about right) - smaller than that, I'm simply thinking that you won't be able to get accurate reading.

Mind you, that is enough precision to guide such mount on stock values (meaning about 1"-1.5" RMS), but does limit your imaging resolution to about 2"/px.

Another way to tell if your guiding was really that good is to examine subs you've taken while guiding. What sort of star shapes did you get? Are stars distorted in any way - for example not round, maybe elongated in one direction or egg shaped? If they are round - which means that guide error was uniform in every direction (good thing), next what you want to look at is FWHM of stars in arcseconds. This value is true measure of achieved resolution on the image, and if that number is comparatively small (depends on guiding, scope size and seeing on that night) - then yes, you were indeed guiding really well.

Yes, that is quite good general rule of thumb to have guide RMS at least half of imaging resolution or smaller (smaller is of course better, and larger is worse, but it will not "ruin" image - it will just look a bit blurrier).

Some good info there Vlaiv. Thanks.

Unfortunately I did not have time to take actual photos, so I cannot comment on the shapes of the images.  I was simply playing around with the guiding setup for the first time to see how I fared. I struggled for a long time to get the focus right. I am still having problems with the Stellarmate App, so I had to run inside to my desktop computer with WiFi to check the picture, then run back out to make adjustments, then back inside to check, and so on. The point of the exercise was primarily to check out the guiding function and to see if my mount could handle the weight. Keep in mind that I will be using my Mak 180mm primarily for lunar and planetary observations/photographs. In that situation I will not be needing guiding. For DS I will be using my trusty EvoStar 80ED. The experiment was purely to stress/weight test the mount with guidance.  After one hour of tracking, I could not detect any movement of the star that my main was pointed at. Combined with the apparently excellent small margin of error in the tracking graph, I feel confident that I will be attempting long exposure photographs in the very near future.

And that will be the true test. 

[Ouroboros]

I have recently started to use EKOS and internal guider to control my AZ-EQ6 mount.  I got guiding values closer to 0.75" RMS with good polar alignment. I too am slightly suspiscious of the <0.5" guiding you reported @George Gearless. 

Sorry if I missed it, George, but did you input the value of your guidescope's focal length into the set up in EKOS?  It needs that to calculate the guiding error correctly. 

[vlaiv]

34 minutes ago, George Gearless said:

Some good info there Vlaiv. Thanks.

Unfortunately I did not have time to take actual photos, so I cannot comment on the shapes of the images.  I was simply playing around with the guiding setup for the first time to see how I fared. I struggled for a long time to get the focus right. I am still having problems with the Stellarmate App, so I had to run inside to my desktop computer with WiFi to check the picture, then run back out to make adjustments, then back inside to check, and so on. The point of the exercise was primarily to check out the guiding function and to see if my mount could handle the weight. Keep in mind that I will be using my Mak 180mm primarily for lunar and planetary observations/photographs. In that situation I will not be needing guiding. For DS I will be using my trusty EvoStar 80ED. The experiment was purely to stress/weight test the mount with guidance.  After one hour of tracking, I could not detect any movement of the star that my main was pointed at. Combined with the apparently excellent small margin of error in the tracking graph, I feel confident that I will be attempting long exposure photographs in the very near future.

And that will be the true test. 

With 80ED it is totally different story - I would not worry about guide result being poorly measured - it is certainly within limits that you should aim for with ED80. It has more than x5 less focal length (with field flattener / reducer) compared to Mak180 and hence x5 less sampling resolution - with most cameras it is around 2"/px or higher and you need guiding of about 1" RMS for that - and I think you will be able to do it without problem if your reported values are correct (focal length and pixel size correctly entered) regardless of any lack of precision to measure that value, we can safely say that you were guiding at at least 1" RMS.

It is also lighter setup and that can only help.

[George Gearless]

32 minutes ago, Ouroboros said:

I have recently started to use EKOS and internal guider to control my AZ-EQ6 mount.  I got guiding values closer to 0.75" RMS with good polar alignment. I too am slightly suspiscious of the <0.5" guiding you reported @George Gearless. 

Sorry if I missed it, George, but did you input the value of your guidescope's focal length into the set up in EKOS?  It needs that to calculate the guiding error correctly. 

I have been pretty dilligent in checking all the boxes and 'dotting the I's'. Without remembering specifically, I'd say that I did enter it correctly. But I'll check again next time I get the chance to set up on a clear night. 

[George Gearless]

28 minutes ago, vlaiv said:

 

It is also lighter setup and that can only help.

Exactly why I piled pretty much everything I had on the mount. If it could handle this with usable results, then I wouldn't have problems with the 80. And as reported, I think it performed rather well. I'm quite happy with my little experiment. Even if the error reporting may turn out to be flawed.

[wimvb]

8 hours ago, spillage said:

I always thought that you guiding should be half of you imaging resolution. Be nice if I am wrong as I always thought my quattro 8 and 1600 are pushing the capabilities of the neq6 a bit too much.

Don't get hung up on the numbers too much. The best (most detailed) images of dsos I've seen were taken with oversampled setups having an imaging resolution of 0.5 "/pixel or thereabout. I don't think that guiding error was half that value for those setups. Star blurryness depends on many things, and in Western/Northern Europe seeing is most likely the main culprit. As long as guiding error is 'considerably less' than any other cause, there's really no need to worry.

To evaluate the tracking quality of your mount, you should analyse the guide graph and look for things like short period oscillations, backlash, etc, as well as possible differential flexure. If a guide graph doesn't show unexpected events, and images look good, the rig is very likely purring along nicely. 

@George Gearless if you used StellarMate defaults, you probably used the internal guider. StellarMate does have phd2 as an alternative. While the internal guider is simpler to use, phd2 is more powerfull and has tools to help you optimise your setup. Otoh, if it ain't broke, don't try to fix it.