Omega Centauri, PHD2 guiding

[jakob]

First run with PHD2 from Home.
9x5min plus darks, ISO 1250, 2xBarlow, APO 80, fl 550mm.
Nikon D7000 camera.

Polar alignment within 15min both axis.
Limited visibility (trees) made it hard to improve the alignment (and approaching clouds)

Need help understand the graph.

[glend]

Nice Jakob, no star tails that I can see. I have the same issue with the PHD graph, i don't understand those big peaks.

What did you use to stack and process?

[Eden]

Great picture, Jakob -- and a good first attempt at using PHD2. I'll explain what's happening in the graph there.

The lines on the graph show you a few things:

1. The amount of error in your guiding, both in Declination (Red) and Right Ascension (Blue). This is indicated by how far the blue and red lines deviate from the center line (0). The value given for the amount of error is in arcseconds. If you prefer to see the size of the guiding error expressed in pixels, you can change it via the Settings button the left.

2. The direction in which the error occurred. This is indicated by whether the red or blue line deviates up or down from the center line.

3. The amount of adjustment that PHD is making (via guiding pulse to your mount) to correct that error. These corrections are indicated by the red and blue vertical bars which extend away from the center line in the opposite direction from the error to which they correspond.

Ideal guiding with an accurate polar alignment, by contrast, would have the red and blue lines close to the center (0) and few correction bars. If corrections have to be made, the smaller those bars are, the better.

In the graph you've provided, your guiding is showing an RA Oscillation value (shown in red down the bottom) of 0.07. Generally speaking, when this number is showing as red it's not a good thing. You want to aim for a value as close to 0.50 as possible. But at short focal lengths you can get away with less-than-perfect values on your graph, especially if you're binning your images. It all comes down to how much you want to get out of your imaging.

To give you an idea of how the values on your graph translate to impact on your image quality, consider the following.

Your D7000 camera has a pixel size of 4.78µ, which on your scope translates to 1.792 arcseconds per pixel. As soon as your RMS Errors in PHD2 exceed a quarter of that value -- 0.448 arcseconds -- shift is introduced into your image. This is not including image artifacts caused by circumstances such as mechanical imperfections in your mount or atmospheric seeing conditions.

You might wish to increase the delay between guiding camera exposure times so that your mount is making fewer corrections. Depending on the brightness of your guide star, 3-4 seconds is a good value.

Hope this helps!

[jakob]

Eden,
Thanks a lot for your explanations. Next opportunity I will make the adjustments. My problem is, that I don't read the instructions! Thanks.

Glen,
I use DSS to stack.
Thanks.

[glend]

Eden, not to diminish in anyway your good advice to Jakob, I need to comment on your advice ..... "You might wish to increase the delay between guiding camera exposure times so that your mount is making fewer corrections. Depending on the brightness of your guide star, 3-4 seconds is a good value."

This may work for cameras such as the SSAG or SBIG ST-1 but this will not work for the ZWO ASI cameras, or potentially any other video streaming type camera.

For those of us using a camera like the ZWO ASI130MM or ASI 120, these are sending a continuous video stream over the USB link to the computer at a much higher rate than any of the control exposure type cameras. For people using the ASI cameras you have to set the PHD rate to between 0.5 and 0.05 seconds in order for the frame rate of the ASI camera to sync with the rate at which PHD can handle the frames. Using a slower rate, such as anything one second or longer causes the image frame rate to become out of sync and PHD will start generating false Lost Star errors, and if PHD gets too far out of Sync with the camera PHD will appear to freeze. (this is all from a post on Cloudy Nights by fmhill). I have found this is the only way I can get PHD2 to work (consistently) with my ASI camera.

[Allan_L]

Nice photo Jakob !
Your journey to "The Dark Side" has commenced.

[Eden]

@glend:



What you're saying here glend is correct and thanks for pointing that out.

The recommendation I made above doesn't apply to all cameras/scenarios. Factors which should be taken into consideration when determining preferred exposure time for the guiding camera include the sort of camera you're using, whether you're using an OAG/AO or externally mounted guide scope, your focal length, the brightness of the target guide star, accuracy of your polar alignment and a few others besides.

[jakob]

I managed to get more frames to add to the previous stack, 14 @ 5min.