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RMS vs photos

 

So I had an absolutely awesome night last night photographing R for the Iris nebula. My guiding was looking real nice, usually in the 0.7s, but dipping into the mid? .6's. not a lot of corrections so I was expecting great photos. And they were great, but as I went through my green photos from the nights before trying to choose the best one for pixinsight , most of the green shots from the night before had sharper rounder stars. And the guiding was good, but more like 0.9 0.8 RMS, but not as good. From memory it also had more jagged corrections. So RMS is a great tool, but not everything. I'll have to look at the logs to see what might account for the better results. My guide exposures may have been longer...?
Arun Hegde
 
Edited

Jamie,?

The Point Spread Function, which is the way a point source of light such as a star is rendered, depends on the Airy disc size of your scope, the guiding, and the seeing. Mathematically, it is a convolution of these three things. The guiding is affected by seeing of course, but also by mechanical issues. When you run Guiding Assistant, PHD2 will give you an estimate of high frequency star motion; that is an estimate of the extent to which things such as seeing and high frequency vibrations in the mechanics of your mount affect your overall RMS. When I ran Guiding Assistant, I got high frequency star motion RMS values of about 0.27", while my overall guiding was about 1". So in my case, the rest of the impact on guiding RMS came from mechanical properties of my mount and set up - PHD2 cannot correct for high frequency motion since its pulses are on time scales much larger than these motions.

The most likely thing is that your seeing was better. A seeing number of 2" has the same impact on your star size as a guiding RMS of about 0.85" in a long exposure sub. So if the seeing improves significantly (say to around 1" on a great night), then your overall star size will be reduced even if the guiding RMS is slightly worse.

Arun
 

Thanks for that explanation, that is the clearest one I've heard about this topic yet. Seeing condition measurement still feels like a bit of a mystery for me. I'll have to take some time reading up on it using some of those terms for my google searches. My understanding right now is that the guiding assistant RMS values are the best to measure of seeing conditions, but it sounds like it is also measuring imperfections in the mount. I didn't run the assistant for either of the two nights since my guiding was so good, I didn't want to mess with anything. I just wanted to do some captures, and now I've got an excellent set for G, and a good set for R.?

I've been meaning to get pempro going so that I could look into some of the mounts contribution to the noise, but the only windows machine is my wife's laptop, and I'm not going to abscond that for astronomy. I'm maybe half way there with getting it running under wine...
 

>>> Seeing condition measurement still feels like a bit of a mystery for me.?

it's a mystery for a lot of people! it's very difficult to quantify

>>>My understanding right now is that the guiding assistant RMS values are the best to measure of seeing conditions, but it sounds like it is also measuring imperfections in the mount.?

RMS values are a combination of all components. PHD can distinguish between higher frequency movements (i.e., seeing) and lower frequency movements (i..e, mount periodic error) but it's a rough approximation

the best way to measure your mount's performance is to use PEMPro and gather about a hour's worth of data. Ray's algorithms can filter non-mount related errors (seeing, polar misalignment, etc.) and give you the best approximation of your mount's performance



On Tue, Jun 23, 2020 at 10:24 AM Jamie Amendolagine <jamie.amendolagine@...> wrote:
Thanks for that explanation, that is the clearest one I've heard about this topic yet. Seeing condition measurement still feels like a bit of a mystery for me. I'll have to take some time reading up on it using some of those terms for my google searches. My understanding right now is that the guiding assistant RMS values are the best to measure of seeing conditions, but it sounds like it is also measuring imperfections in the mount. I didn't run the assistant for either of the two nights since my guiding was so good, I didn't want to mess with anything. I just wanted to do some captures, and now I've got an excellent set for G, and a good set for R.?

I've been meaning to get pempro going so that I could look into some of the mounts contribution to the noise, but the only windows machine is my wife's laptop, and I'm not going to abscond that for astronomy. I'm maybe half way there with getting it running under wine...



--
Brian?



Brian Valente
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Arun Hegde
 
Edited

Seeing is actually a very interesting phenomenon. Very simplistically, the movement of air causes what should be an Airy pattern determined primarily by the aperture of your scope and scatters it over a larger area. Over a long exposure sub, the effect of this is to result in a larger star size than what you might expect purely from scope optics. The movement of your scope is superimposed on this.

One way within PHD2 to separate out the effect of seeing versus mount mechanics is to look at the RA and DEC components of high frequency star motion separately. The DEC component will always be lower than RA - because only the RA motor is running when you run GA, so DEC isn't or should not be affected by mount mechanics. You would expect that seeing equally influences RA and DEC whereas RA is also influenced by mount mechanics. For example, suppose the DEC component is 0.10" and RA is 0.16", then the contribution of mount mechanics is SQRT(0.16^2-0.10^2) or about 0.12".

A couple of other interesting things:
  1. Seeing quality can vary significantly over short periods of time whereas it is averaged over long exposures. Planetary imagers take advantage of this by taking many short exposures and selecting the best ones to stack. Works for bright planets, less so for dim DSOs.
  2. If you use PixInsight, you've probably done or heard of deconvolution. These algorithms attempt to recover the distortion caused by all these factors to some degree. This is why the deconvolution algorithm? asks you to select stars - it is trying to determine how what should theoretically be a point (a star with near zero angular size) is rendered, which is an estimate of the Point Spread Function of your setup. When you run deconvolution successfully, you will see your star size reduce and detail in structures enhanced.?

Arun
 

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Hi!

Thanks for your clarity! However, could you explain the logic here:

"For example, suppose the DEC component is 0.10" and RA is 0.16", then the contribution of mount mechanics is SQRT(0.16^2-0.11^2) or about 0.12"."

Thanks :)

Magnus
 

Great stuff, this looks like a very deep rabbit hole, too bad I've got work to do!

I thought that Airy was just a way of describing the way it looked, but no that's the person that figured out why they look like that! I think that I kind of understand where the rings are coming from, and it has to do with the wavelength of the light that comes through the aperture acting kind of like a diffraction grating, but circular.?
Arun Hegde
 

Magnus -?

Noise adds in quadrature. The reported RMS numbers are standard deviations, comprised of different effects (eg. noise, mount mechanics) each with their own individual RMS values or standard deviations. If the effects are independent (eg. seeing and mount mechanics) then the overall effect can be approximated by squaring the standard deviations of the individual effects and adding them, then taking the square root. That is what I've done here.

I think also that it is important to differentiate between high frequency noise - which is things like vibrations and seeing - and the longer period oscillation from worm movement, which is typically what we talk about when we discuss periodic error correctable by PEC or PHD2. Bruce Waddington covers this very nicely in his video that was shared here recently. Things like vibrations and high frequency oscillations from seeing are not corrected by guiding software. I like to think of the high frequency star motion RMS as the absolute best guiding you can get on any given night. It is basically everything that PHD2 cannot correct!

Arun
 

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Thanks! Adds in quadrature, yes, I tend to forget that.

RMS is an interesting and useful number, but as you have shown, it really helps to understand it more in depth.

Best,

Magnus


Den 2020-06-24 kl. 03:45, skrev Arun Hegde:

Magnus -?

Noise adds in quadrature. The reported RMS numbers are standard deviations, comprised of different effects (eg. noise, mount mechanics) each with their own individual RMS values or standard deviations. If the effects are independent (eg. seeing and mount mechanics) then the overall effect can be approximated by squaring the standard deviations of the individual effects and adding them, then taking the square root. That is what I've done here.

I think also that it is important to differentiate between high frequency noise - which is things like vibrations and seeing - and the longer period oscillation from worm movement, which is typically what we talk about when we discuss periodic error correctable by PEC or PHD2. Bruce Waddington covers this very nicely in his video that was shared here recently. Things like vibrations and high frequency oscillations from seeing are not corrected by guiding software. I like to think of the high frequency star motion RMS as the absolute best guiding you can get on any given night. It is basically everything that PHD2 cannot correct!

Arun

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