ONAG at work:

Guiding with a long focal

The images below are 1 minute (bin 1x1) unprocessed luminance frames taken with an Apogee U8300 (5.4x5.4 microns) at prime focus of a Hyperion (f =2.54m @ f/8).
Both have been cropped the same way.
Field of view: 224x224 arcsec near NGC 2683.

There are 38 minutes apart.
credit: Frank Colosimo
Blue Mtn Vista Observatory

Mount: Paramount ME
Guider: ONAG+SBIG ST402
Seeing: Average

The reference star is marked with a cross (not the guide star).


 demo

Initial image (t=0'), reference star:
FWHM=2.44 arcsec
Centroid X=80.71 Y=171.83 pixel

 

Star 3D profile (Maxim DL) 

 


 

2nd image (t=38'), reference star:
FWHM=2.36 arcsec
Centroid X=81.38 Y=171.40 pixel

The reference star is offset by 0.79 pixels.
With the U8300 pixels (0.43x0.43 arcsec) this translates to 0.34 arcsec.
This is a total offset including all sources of error.

Below both images have been combined, without any registration and alignment, to provide an easy estimation and visualization of the total tracking performance.



Average image, reference star:
FWHM=2.43 arcsec
Centroid X=81.08 Y=171.54 pixel


Star 3D profile (Maxim DL) 

The result shows no visible guiding effect.

To know more about guiding error
and near infrared (NIR) visit:

How much guiding error is too much?

Guiding with NIR

   

Heavy duty focuser:

Full body compressing ring

The ONAG features a low profile 1.25" guider focuser (GF) associated with a T-thread (M42 x0.75mm) and integrated with the ONAG's X/Y stage.

It has been designed to remove any possible flexure even with a heavy camera. The focuser uses a full length compressing ring mechanism made of high grade 6061 aluminum alloy. It applies a considerable pressure (on 360 degrees) all the long the focuser drawtube insuring a constant and efficient grip.

As a matter of fact when the focuser screw is hand tighten the all system is as rigid as an unique solid piece of aluminum.

The next image shows the full body compression ring element and its screw.

GF compressing ring alone

Unlike thumb screws used with low cost systems, the stain less steel focuser screw compresses the all focuser body against the drawtube. The mechanism is lubricated with an extended temperature range anti-seize grease. It is designed with 2 groves and set screws to insure the drawtube will not leave the focuser by accident.

Should you want, or need, to use any 1.25" nosepiece just remove the set screws and replace the focuser drawtube by your  piece of equipment. A very handy solution if your guider camera nosepiece does not come off.

Below the compressing ring with the focuser drawtube in place.

GF compressing ring and drawtube


 

 

 

 

 


How much guiding error is too much?


Guiding errors are inevitable, however they can be reduced to a minimum with the correct guiding hardware and strategy. 

Although our ONAG goes a long way for achieving very low guiding error, most people do not recognize how paramount is to correctly set-up and use an auto guiding software, you can find further information and useful tips on this matter in the ONAG's user manual.

One fundamental question one should ask, and eventually answer, for astro-photography is how much guiding error is too much?

From the answer to this question depends grandly the quality of the images. Each star on your frames shows the point spread function (PSF) of your system as well as the performance of the tracking and guiding.

Image inspection can reveal naked eye if the stars look right, tight and round enough. However using numbers to back-up this feeling is necessary to improve the performances, and eventually the image quality.

There are two good figures of merit for this task:


The FWHM (Full Width at Half Maximum):

The FWHM of star describes how much the star's profile extends from its center. Most of the time, in order to compare between optical set-ups, this value is given in arc-second (arc").



FWHM plot     FWHM 3D plot



The two plots above show the FWHM concept, on the left as a cross section of the star, on the right its 3D version (credit: Maxim-DL,Cyanogen).

The minimum FWHM is always bounded by either:


-1- From space, diffraction limited (airy disk)

Airy Disk FHWM

Where l is the light wavelength [m] and D the scope aperture [m].

In such a case we could expect to make the FWHM as small as we would like by choosing a large enough scope aperture D.

-2- From Earth, seeing limited (Gaussian like shape).

Seeing limited FWHM

Average seeing is around 2 arc". Smaller the FWHM is tighter the star will look.

We are seldom diffraction limited, unless you have an exceptional site and some expensive piece of equipment (adaptive optics with an artificial guide star). Most of the time only professional astronomers can afford and expect seeing as low as 0.5 arc", or less.


The flatness (SFLT) of a star:

If the FWHM describes how a star is spread, or "fat", its flatness relates to how circular the star profile (shape) is, which is also very important for image appearance and quality.

If we consider, in first approximation, a star shape is like an ellipse (worst case). A classical definition for the flatness is simply given by how much longer the major axis is relative to the minor axis of the ellipse.

A SFLT of zero means the star is a circle indeed. It is expected that a SFLT lower than 0.1 (or 10% of elongation) is not perceptible by most people.

                        
SFLT = (Major axis - Minor axis) / (Minor axis)

In order to keep the SFLT < 0.1 we need to make the guiding error small enough. Most of the time the error is much more on Right Ascension (RA) axis then Declination (DEC).

This is because the mount RA drive motor is always moving at sidereal speed (15 arc" per second) for compensating Earth rotation, while with a good polar alignment the mount DEC drive motor seldom moves. The result is that guiding errors are more prone in the RA direction (East - West), as it can be seen on the image below, a typical situation and symptom of a guiding problem.


                                           Guiding error AR axis

 

 

Guiding error is most of the time given and reported from most software by its RMS (Root Mean Square), the average deviation from nominal.

There is a link between the FWHM and the level of RMS guiding error we can afford to keep the SFLT below the 0.1 (10%) threshold.
The parameter of interest for driving this being the local seeing FWHM assumed without any guiding error.

 

It seems logical to expect a larger RMS guiding error would be more acceptable when the local seeing is worse, and the other way around. If you do the calculations you find a good rule of thumb* (to achieve SFLT < 0.1) is:

                                           

RMS tracking error < 1/4 FWHMseeing


Guiding error table



For instance for a target SFLT < 0.1 with a local seeing of 2 arc" (average), a 2m scope focal length, and a 8 microns imager pixel size, we need to keep the RMS guiding error at, or below:

 
FWHMseeing = 0.83 pixel, leading to a RMS error < 0.21 pixel

You do not need much to experience an error of this magnitude. It is why all aspects of the system must be considered, such a flexure, part motions (mirror, focuser play, ...), mount tracking quality, auto-guiding software and set-up.

Our product offers a unique solution for a flexure and part motion effects free system without any compromise on the image quality and guider field of view.


* We assume a normal RMS error probability density function in the calculation.

 

 

 

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