Is my guiding rational correct?

[libraryman]

I recently upgraded my guide camera to a QHY5-L2 Mono....it happens that my Guiding ratio is 2.3 arc sec per pixel...almost exactly the same as my imaging resolution @ 2.1 arc sec Per pixel.

am i correct in assuming that my guiding  RMS as it is reported in PHD or Maxim  is what i am getting in my image?

I believe it must be but i just need to confirm it.

Ray

[JRWASTRO]

Greetings Ray,

Re:   " ... am I correct in assuming that my guiding  RMS as it is reported in PHD or Maxim  is what i am getting in my image? ... "

WRT the information that you provided the answer is No.

Why? Because the information provided is incomplete.

You have given us the image scale of the guide and imaging cameras  and this is not an indication of the quality of the guiding.

What the numbers tell me is that you may drop the focal length of the guide telescope by a factor of 4 and still get the same guiding quality!!

Re:  " ... guiding  RMS as it is reported in PHD or Maxim   ..."

We need to know what is this guiding quality metric as defined by the designers of Maxim and PHD. 

The term "  guiding  RMS " does not mean much . Let me explain:

RMS stands for Root Mean Square. 

A more complete description of RMS would be to say " Root Mean Square Deviation from the Mean " .

Here we have two quantities :

Positions of the star image at time #t1

and

Position of the star image at time #2 before the guide correction is made.

These positions are computed from the centroids of star images  in dimensions x and y.

If we take one dimension (say x)  we can find the error in the star image by

subtracting the positions of the centroids (the answer may be negative )

and after many samples find the average or mean error in the x dimension.

Ideally this number should be zero.

Otherwise there is a bias (in x) in the system that will need to be corrected!!

And will indicate the "roundness" of the star images.

We now subtract this mean error from the x sample values and this the deviation from the mean.

We now average these numbers and take the square root NOW this number is the RMS error in the x direction.

The same is done for the y dimension so, now, we have errors in x and y , Ex and Ey.

These numbers may be combined to give an overall figure as follows :

Erms  = Sqrt( Ex^2 + Ey^) . This is an excellent quality metric.

Bit of a long winded explanation but there you are,

Jeremy.

[libraryman]

He he Jeremy, many thanks for the excellent explanation, i accept the fact that the info is incomplete, and your good enough to explain why.

I have understood that aspect and the point of my question has been answered, an interesting aspect of your reply was that I might change my guide scope relationship and still achieve the same guiding result, I'm sure that is info others might find useful.

The maths are somewhat complicated but I get the jist!

Ray

[JRWASTRO]

Hello again Ray,

To tell about the guiding quality you need two numbers. As the star image on the CCD is

two dimensional we need to specify errors in two dimensions viz. In x and y.

With these two numbers one can draw an ellipse ( a circle is a special case of an ellipse).

It is the size and aspect of this ellipse that tells us about guiding quality.

This type of analysis is very common in source geo-location problems (eg. gps) and , dare I say, in the analysis of weapon systems

where the error can be an ellipsoid (ie. 3D). If you are interested in this sought of stuff I'll point you to the "buzz words"  :

Error Ellipse and CEP (Circular error probability).

The selection of the guide telescope  focal length is an interesting question. There is very little analysis done on this issue.

Suffice to say the guide scope focal length is a function of the image scales of the imaging and guide scope and , most importantly, a "gain term"

that is estimated from the method used to calculate the centroid of the star image on the guide CCD and the sensitivity of this method to changes in

position of this star image on the guide scope CCD.

I know that, under ideal conditions, one may detect changes of better than 1/10 of a pixel length but I am conservative and allow 1/4 pixel movement detection.

This is why I made the  statement about your guide scope focal length.

Jeremy.

[sharkmelley]

Hi Ray,

Returning to your original question, since the arcsec/pixel on the guider more or less matches the arcsec/pixel on the imager then the RMS of the guiding errors does translate into errors of the same magnitude in your image.  However be aware that the RMS reported by PHD is only one component of the guiding errors. 

Also, it may actually be the FWHM (i.e. blurriness) of the stars each sub (or in the final image) that really interests you, then guiding errors are only one factor of many that drive the total FWHM.

Mark

[Physicist13]

Far more important is how far away your guide star is from your imaging point!

P