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Question about Guiding Accuracy vs Camera ability

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#1 ClownFish

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Posted 11 January 2019 - 09:57 AM

My camera and scope combination is able to resolve 2.25 arc seconds per pixel.

 

Does this mean I should set my imaging system up to toss out images where the guiding error exceeded 2.5 arc seconds?
What's a good level to try and maintain without being very aggressive?  I want god images, with no blurred stars, but I also do not want to toss out images that wouldn't have shown that error anyway.

Second - Is there an advantage to getting my guiding error well below 2.5?  For example, I adjust my polar alignment to the pint where the error was below 1 arc second.  Was that a waste of time, if my system could only see 2.5 arc sec?

 

CF 



#2 ks__observer

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Posted 11 January 2019 - 10:13 AM

Does this mean I should set my imaging system up to toss out images where the guiding error exceeded 2.5 arc seconds?
What's a good level to try and maintain without being very aggressive?  I want god images, with no blurred stars, but I also do not want to toss out images that wouldn't have shown that error anyway.

Second - Is there an advantage to getting my guiding error well below 2.5?  For example, I adjust my polar alignment to the pint where the error was below 1 arc second.  Was that a waste of time, if my system could only see 2.5 arc sec?

We all want "god" like pics :) (I know typo....)

I believe the guiding error accounts for seeing error, periodic error, PA drift error, and other mechanical errors all at the same time.  The error blurs the size of your Airy Disk (theoretical best resolution).  The smaller the image scale the more noticeable the blur will be.  If you can get it smaller all the better.

Re polar alignment tolerance: Using Sharpcap is so easy it should not be a problem.  But I recommend this site: http://celestialwond...xErrorCalc.html



#3 Tapio

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Posted 11 January 2019 - 10:18 AM

Polar alignment error affects field rotation and it shows on longer exposures.
Somehow I doubt that you get below 1 arc second accuracy in polar alignment.
Anyhow I thing too much speculating/calculating is just waste of time.
Local seeing conditions are often the biggest factor how tight stars you get.
Just start taking images and if your stars are not tight/round enough then start tweaking your process.

#4 ClownFish

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Posted 11 January 2019 - 10:41 AM

Well, maybe not 1 arc second, but seemed to be a lot less than 2.25.

Maybe I'm not reading this screen correctly.  Total RMS is 0.64.   This is all new to me.

 

49300507_10156294060192858_2220620889441959936_n.jpg

 

As far as using polar alignment tools, I am unable to see either pole, as I live at 00 degrees, 19 minutes N, and my often-used dark-sky site is at 00 degrees, 05 seconds South.  I have to use the Polar Iteration alignment procedure that the iOptron mounts have.  Takes about 10 minutes but does a decent job.

Here's the last image I took, that each used 180 second subs.

 

Final3-CN.jpg
 


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#5 Tapio

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Posted 11 January 2019 - 11:19 AM

Looks good to me.

#6 rgsalinger

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Posted 11 January 2019 - 11:49 AM

You should check the eccentricity of your output and discard those with elongated stars - particularly if the elongation is in the direction of the RA axis. Then you should keep track of what your guiding RMS is and correlate it to your eccentricity. That will give you and idea of how good your guiding needs to be before you are unhappy with the output. Polar alignment is really not that important at your image scale. However, a good drift alignment never hurts!

Rgrds-Ross



#7 ClownFish

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Posted 11 January 2019 - 12:48 PM

Well I just went out and started another imaging session, as the sky was clear, which is a rare treat. I polar aligned and my total RMS is 0.34. As long as I keep getting it that smooth, I am not worried. It’s when it jumps above 2.25 is what I am concerned about. If my camera/scope resolution is 2.25 arc seconds per pixel, is there a fast rule as to what my guiding RMS needs to stay under?

#8 Stelios

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Posted 11 January 2019 - 01:06 PM

Well I just went out and started another imaging session, as the sky was clear, which is a rare treat. I polar aligned and my total RMS is 0.34. As long as I keep getting it that smooth, I am not worried. It’s when it jumps above 2.25 is what I am concerned about. If my camera/scope resolution is 2.25 arc seconds per pixel, is there a fast rule as to what my guiding RMS needs to stay under?

Your story is somewhat unusual. If your RMS is indeed 0.34 in arcsec (which would be remarkable even in a premium mount with good conditions), then you should never be even close to 2.25 arcsec unless you kick the mount. Perhaps you can tell us more about what mount, camera and guidescope/guide camera (apparently a Lodestar) you are using? 

 

As Ross said, the important thing is the roundness and size of your stars. This is a function of your seeing and your guiding. I don't know any "fast rule" for RMS with respect to camera image scale, and it's a pain to be watching guiding in order to decide whether to keep the image. Software can measure the FWHM (a measure of star size) and eccentricity of your stars, and you should use *that* to decide whether to keep images--the advantage is that this can be done the next day so you don't have to hover over your setup like a hawk :)



#9 bobzeq25

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Posted 11 January 2019 - 03:21 PM

Well I just went out and started another imaging session, as the sky was clear, which is a rare treat. I polar aligned and my total RMS is 0.34. As long as I keep getting it that smooth, I am not worried. It’s when it jumps above 2.25 is what I am concerned about. If my camera/scope resolution is 2.25 arc seconds per pixel, is there a fast rule as to what my guiding RMS needs to stay under?

Fast rule.  You guiding RMS needs to be less than your seeing.  Camera/scope resolution is not the major thing.  Half the seeing is better.

 

But this is all very complicated.  Your guiding numbers are suspiciously good.  Do you have the proper guidescope focal length and guide camera pixel size in your software?


Edited by bobzeq25, 11 January 2019 - 03:23 PM.


#10 Alex McConahay

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Posted 11 January 2019 - 03:34 PM

In general, do not set a "Anything over XXX.X. is thrown out" rule. Instead, evaluate all your pics and rank them. Then, depending on the curve of the results, see what you can afford to throw out. If you have a dozen at less than 1.5, and three at 2.5, and however many at 3.0 and above, go ahead and throw out everything above 1.5---a dozen good ones means you don't need the rest.

 

On the other hand, if you have a three at 1.5,  a dozen at 2.5, and however many above 2.5, throw out anything above 2.5, keeping the rest. At least you have something to practice processing on. 

 

In other words, use the relative ranking to decide and not the absolute. At least you have something to work with. 

 

Alex


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