Understanding what is acceptable RMS in PHD2

[sagramore]

Hi everyone. Thanks for coming to read my question.

I have been running PHD2 to guide with an ASI120MC-S on a Startravel 80. My imaging train is a Skywatcher 130PDS newt with 0.9x coma corrector and a Canon EOS 550D.

While looking at my PHD2 statistics tonight, I started to wonder "Are these good? Are these good enough? What is good enough?" and it got me thinking. I started googling "what are acceptable PHD2 guiding statistics" and getting nowhere when I realised that I could probably work out what "good enough" looks like, however I wanted to check that I was thinking about it the right way, so here I am.

 

Using the tool at https://astronomy.tools/calculators/ccd I entered my focal length and pixel size, which (with the 0.9x CC) are 585 mm and 4.29 microns respectively. This then says that my pixel resolution is 1.51 arcseconds per pixel.

So, my question is this: does this mean that as long as my total RMS error in PHD2 is *below* 1.51 arcseconds, my guiding is "good enough" that it shouldn't have a negative effect on my imaging? If not, what assumptions have I made that are incorrect?

 

I wanted to post a picture of my PHD2 stats but the laptop is still imaging right now - I will try to add one later.

 

Thanks!

[wimvb]

With 1.5"/p, your guiding rms should "in theory" be 0.75" or better. But as sky conditions (seeing) usually limit the resolution that we can achieve, I think you're safe if you can keep the rms around 1".

[sagramore]

Thanks, Wim! That makes sense. I was starting to be a bit worried as (at various positions) my guiding total RMS was hovering around 0.8-1.2" but I know for a fact the seeing is pretty poor tonight so I imagine that's affecting my resolution far more. 

I can at least be glad that the seeing is (mostly) out of my control!

[wimvb]

My experience is that even with guide exposures of 3-4 seconds, rms varies with seeing. I hope that with multi-star guiding, there will be somewhat less variation. Every 0.1” counts.

[vlaiv]

6 hours ago, sagramore said:

Thanks, Wim! That makes sense. I was starting to be a bit worried as (at various positions) my guiding total RMS was hovering around 0.8-1.2" but I know for a fact the seeing is pretty poor tonight so I imagine that's affecting my resolution far more. 

I can at least be glad that the seeing is (mostly) out of my control!

If you have stock EQ6 that is about what you will get at best - 0.8" RMS.

Its not down to seeing, it's down to mount performance. If you suspect that seeing is problem - try longer guide exposure to see if it changes anything.

Btw - acceptable guide RMS is the lowest one you can get with your setup . It always impacts final result in negative way. It needs to be quite a bit smaller than other factors to be "masked" by them.  Guiding error half the size of seeing error will act as if seeing error was increased by about 12% and guiding error was 0. We can say that when guiding error is less than half that of seeing it starts being "masked". Only problem is that you must match "units" for this calculation. Guiding error is in RMS while seeing error is in FWHM. There is conversion factor of ~2.355. This means that when seeing is 4" FWHM you actually need 4" / 2.355 and then half of that in guide error - ~0.85" RMS guide error.

You always want the best guiding you can get really.

[sagramore]

3 hours ago, vlaiv said:

If you have stock EQ6 that is about what you will get at best - 0.8" RMS.

Its not down to seeing, it's down to mount performance. If you suspect that seeing is problem - try longer guide exposure to see if it changes anything.

Btw - acceptable guide RMS is the lowest one you can get with your setup . It always impacts final result in negative way. It needs to be quite a bit smaller than other factors to be "masked" by them.  Guiding error half the size of seeing error will act as if seeing error was increased by about 12% and guiding error was 0. We can say that when guiding error is less than half that of seeing it starts being "masked". Only problem is that you must match "units" for this calculation. Guiding error is in RMS while seeing error is in FWHM. There is conversion factor of ~2.355. This means that when seeing is 4" FWHM you actually need 4" / 2.355 and then half of that in guide error - ~0.85" RMS guide error.

You always want the best guiding you can get really.

Hi vlaiv, thanks for the detailed reply. It is comforting to know that a total RMS of ~0.8" is considered OK/good. Your other points make sense and are helpful too. As you say - get it as good as you can!

I am still tempted by the belt mod as I have heard good things about them and, living in a pretty populated area, it would be nice to have a quieter scope. I felt like I was keeping the neighbours up last night slewing....