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PHD 2 graph, Pixel or arc second

red dwalf

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been getting to grips with PHD 2 and as i`ve had a few troubles with funny shaped stars and poor tracking but i think i`ve got it almost as good as it`s going to get having made a few "brain" and EQ mod changes to the settings, also getting the right driver for the ASI camera makes a big differance, there`s 3 of them to choose from !

anyway i was wondering when people use the graph to check there tracking is ok do you use the pixel setting or arc second settings on the graph as these two give totally different views of the tracking corrections.


top image shows pixel view tracking.




bottom view shows arc second tracking.



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The Arc-second isn't an option on v1.4 is it? I use pixels on 1.4 anyway!  :)


(Just checked both, not on v1.4 only on v2.0!)




Edited by Ron Clarke
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Guest Kheldar

I thought my OCD was bad :P The two graphs actually show EXACTLY the same thing (at the same time, your examples are taken 20 minutes apart or so)  :)


It all depends on your image scale (or FOV.)


Example: Let's imagine a camera that's 1024 pixels square with two different FOVs


FOV 1.5 degrees (DSLR/GT51 ish?) : 1 pixel = 1024 divided by (1.5 * 60 * 60 arcseconds) = 0.2 arcseconds (graph is scaled 5:1, arcsecond will look wild, pixels nominal)


FOV 0.5 degrees (314/200P ish?) : 1 pixel = 1024 divided by (0.5 * 60 * 60 arcseconds) = 0.7 arcseconds (graph is scaled 1.4:1, arcsecond will look approximate to pixels, nominal)


Unless my maths is off or I'm rambling ...

Edited by Kheldar
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Thanks Steve, it makes sense , I have been doing my best to get the pixel graph as flat or as good as could get it which has been a pain due to the combination of bad drivers for the camera using PhD2 and horriable poor seeing conditions lately, and the fact that some of the subs I've got I've had to throw away due to the poor guiding.

So I've been making adjustments to the brain settings in PhD2 to try and get the best all round. But making suttle changes seems to do very little to the graph although I suppose it looks worse when viewing on arc seconds.

But I must say it worked very well last night and out of two hours worth of 5 minute subs only a couple had to be thrown away.

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looking at the guiding graph sometimes can make you think that your subs will be poor.


My acid test is if the subs look OK I don't care what the guide graph says.


If you saw my guide graphs at the moment it would put you off they look a bit like a profile of the Himalayas.


As far as I can tell you are not doing what I used to and chase the seeing by using very short guide exposures

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Last night I was doing 3 second exposures as it was the clearest night we have had for a while and it worked a treat but I still got a few bad subs , I suppose it looks worse due to the smallish chip and field of view, if i was using the canon size chip then I guess you wouldn't be able to tell.

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