Declination effect of guiding

[Guest]

Am I right in saying that the further you are from declination zero the better the guiding appears to be?

Steve

 

[michael8554]

Probably one for vlaiv to answer, but here goes:

A RA guide error of 1 arcsec at Dec = 0, and a guide error of 1 arcsec at Dec  = 70, will have the same magnitude of effect on star shape.

Are you asking whether guiding is EASIER at higher Dec ?

Probably, as the stars appear to "move more slowly" in RA as Dec increases.

Michael

[Guest]

 

On 06/09/2021 at 13:26, michael8554 said:

Probably one for vlaiv to answer, but here goes:

A RA guide error of 1 arcsec at Dec = 0, and a guide error of 1 arcsec at Dec  = 70, will have the same magnitude of effect on star shape.

Are you asking whether guiding is EASIER at higher Dec ?

Probably, as the stars appear to "move more slowly" in RA as Dec increases.

Michael

Thanks Michael, I'm just learning the ins and outs of guiding. It was just that it appeared that the phd2 guide graph at dec=+9 looked more erratic and noisy than the graph at dec=60.  Probably an illusion.  It was something someone said about recording a PE curve that it's better to do it at DEC=0.

[michael8554]

4 hours ago, woodblock said:

It was just that it appeared that the phd2 guide graph at dec=+9 looked more erratic and noisy than the graph at dec=60

Probably, because the stars appear to "move faster" at Dec 9 than at Dec 60, so they are harder to guide, so a more erratic guide graph.

Also Dec 9 is closer to the terrestrial horizon, down where the atmosphere is more turbulent.

Michael

[vlaiv]

It has to do with "diagonal" in polar coordinates.

Look at these two triangles:

In first we have small angle (left vertex) and hypotenuse is almost as long as bottom edge. In right triangle - we have large angle and this time hypotenuse is much longer than bottom edge.

Ra revolves always at the same rate - but it traces larger or smaller arc on the celestial sphere.

Try to picture small FOV of camera and circle that is traced at DEC 0 - you'll probably end up with masses of FOVs stacked next to each other like this:

(I draw arrow because I got tired of drawing little rectangles).

Now look what happens at North celestial pole:

"Whole" circle can fit in single FOV.

Both circles are traced out by RA of mount in 24h - but "distance" traveled for each in terms of number of FOVs is different.  At DEC 90° - only one FOV is traced out in 24h but at DEC 0° - many FOVs are traced out.

This means that "speed" in pixels is different as each FOV contains the same number of pixels.

Error of the mount is difference of two speeds - sidereal and speed the mount tracks. If speed in pixels depends on DEC - so does difference of speeds. Although mount error in arc seconds remains the same - it is much less expressed in pixels. For this reason RA error graph "calms" down when you are high in DEC - but also - your stars get tighter in RA as well for same effect - as stars are projected onto pixels.

That is also the reason why we should calibrate our guider at DEC 0° - because that way we have highest precision of calibration as we have best ratio of error - to pixel.

[Guest]

Excellent, thank you Vlaiv

Steve