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PHD high RMS and FL?

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#1 Footbag

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Posted 09 January 2013 - 09:07 AM

I had the Mach 1 out the other night and did some wide field imaging. I actually guided using the SSAG through the Edge 800 HD. The first time I've guided through this scope.

I noticed on the PHD graph, RA and DEC seemed to bounce around a lot. I changed the min motion and that cleaned up the graph a bit. But my RMS was still in the .4~ range. This is higher then I was expecting.

I'm wondering. What should be a good RMS when guiding at a long FL?

#2 orlyandico

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Posted 09 January 2013 - 09:35 AM

On my Mach1 I get about 0.03 - 0.05 when guiding at 200mm focal length. This equates to about 0.3" RMS.

A SSAG on a 2000mm FL would have a pixel scale of 0.53" per pixel. So an RMS of 0.4 is about 0.21" RMS. That's pretty good considering that the seeing normally won't let you get much below 0.5" RMS.

Are you using long enough exposures that seeing is not a factor? I would say at least 4 seconds long..

Well Ray will say that RMS is not a good measure of guiding. He also said 0.05" RMS should be possible. Maybe try longer guide exposures to smooth out the seeing..

#3 Peter in Reno

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Posted 09 January 2013 - 09:39 AM

That's darn good for 2000mm focal length.

Peter

#4 orlyandico

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Posted 09 January 2013 - 09:45 AM

Peter, had a look at your gallery and love those galaxy images. Very helpful of you to also put your PHD settings in the description.

Are those C8EDGE images cropped? I don't have an EDGE and am wondering how much I'll have to cut off unless I use a flattener of some sort..

#5 Peter in Reno

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Posted 09 January 2013 - 10:33 AM

Some images are cropped because I was using SXVR-M25C and the camera went out of collimation due to collimation screws getting loosed. There were star elongations at either side of the images so I cropped to remove them.

The EdgeHD is very flat. Before the camera went out of collimation, the stars were pretty good from edge to edge. I believe my NGC891 is barely cropped and my very first with EdgeHD. After that, the camera went out of collimation. Notice the star elongations at left side of NGC4565, this was the worse camera collimation I did. I have been constantly tweaking the camera and the images were slowly getting better but I end up buying Atik 460EX mono camera.

My latest Bubble Nebula was with Atik 460EX mono and that's barely cropped to remove black borders due to dithering and multiple nights imaging sessions.

Here are my typical PHD settings:

RA Aggressiveness: 60
RA Hysteresis: 10
Max Dec Duration: 75
Min Motion: 0.70
Calibration Steps: 125msec (larger if imaging close to pole)
Auto/Resist Switching
Extreme dithering and Settled at < 0.5
2 - 5 sec guiding exposure.

Adam,

Your SSAG has smaller pixels than my Lodestar so the Min Motion should be a little smaller, maybe 0.4. The RA Aggressive is low due to less than ideal seeing conditions. Even with good seeing, this is fine because we both have smooth tracking mount.

Peter

#6 korborh

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Posted 09 January 2013 - 12:27 PM

He also said 0.05" RMS should be possible.


I doubt that. 0.05" is incredibly small. The centroid calculation is perhaps even not that accurate.

#7 orlyandico

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Posted 09 January 2013 - 12:33 PM

in this thread...
http://www.cloudynig...5596389/page...

he sez (toward the bottom)

In image sets when using 10-second autoguider exposures RMS is usually about 0.05 arc-secs on good nights. If you want a minimum RMS number to shoot for that is probably close to it. And it is obtainable without too much effort on a good mount with a good PEC curve and tracking rate modeling.


i don't know enough to know if that is doable or not. in any case it seems to be overkill for most sensible pixel scales.

#8 Footbag

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Posted 09 January 2013 - 01:31 PM

OK. I was using 2s exposures. I wasn't thinking it was as good as it was. I used to get .02 when guiding through the WO66. But I didn't know how the FL effected it.

I think I may try some of Peters settings for this. I was slowly edging my settings towards his, but was afraid of going too far. By the looks of it, I had a ways to go.

All in all, my stars were shaped nicely, but my concern is more when I'm imaging and guiding at that FL through a new OAG. For that,I want to make sure my PHD settings are as good as can be.

#9 Peter in Reno

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Posted 09 January 2013 - 01:39 PM

It's normal to see a guide star wobbling at such long focal length when viewing PHD screen. Don't let that worry you. Also, don't worry about PHD graph bouncing up and down since the unit is in pixels instead of arcseconds. If the result look good, then you must be doing something right.

Peter

#10 Peter in Reno

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Posted 09 January 2013 - 01:42 PM

This is an image of PHD graph using Mach1 for the first time. Did yours look like this?

PHD Graph Mach1GTO

Peter

#11 Footbag

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Posted 09 January 2013 - 01:51 PM

This is an image of PHD graph using Mach1 for the first time. Did yours look like this?

PHD Graph Mach1GTO

Peter


Exactly like that. I guess even a cue-ball is bumpy if you look close enough.

#12 Peter in Reno

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Posted 09 January 2013 - 02:01 PM

You should see the graph on my old CPC0800. The declination axis had excessive backlash. It was driving me nuts.

When the seeing is bad, the graph will get worse. Nothing you can do about it.

Peter

#13 Footbag

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Posted 09 January 2013 - 02:09 PM

I came from a wedge mounted CPC800 as well. The difference between it and the Mach 1 is night and day. My WO66 guiding looked like your graph, now it nearly flatlines.

I still miss the CPC, but not for imaging.






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