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Guiding, focal length and seeing distortion.

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#1 telfish

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Posted 22 May 2019 - 09:55 PM

I  am luck enough to live in an area with really dark skies. Unfortunately  we also have terrible seeing with the jet stream overhead more often than not.  Average seeing is in 2 to 2.5 range and good times are in the 1.3 to 1.6 range.

 

So we get a lot of squishy stars to guide on. Having changed from a guide scope at 240 mm to using an OAG on my newt at 812 mm I am seeing far more stars that are hard to guide on. just a hot pulsating mess. So the question is does the focal length and magnification of the star on the chip make OAG guiding worthwhile in areas of bad seeing?

 

Last night for instance was impossible to guide as the stars were so spread out and pulsating there was no discernible centroid.



#2 DmitriNet

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Posted 22 May 2019 - 11:18 PM

One thing to try is to use filter for guide camera --- something like Long Pass filter >=600nm



#3 John Tucker

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Posted 23 May 2019 - 08:32 AM

One thing to try is to use filter for guide camera --- something like Long Pass filter >=600nm

Could you expand on this?



#4 OldManSky

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Posted 23 May 2019 - 08:39 AM

Are you sure your Newt was sufficiently cooled-down, and the "bad seeing" wasn't tube currents?

Just asking...

 

The usual strategy for guiding with less-than-ideal seeing is to increase the guide camera exposure time, so that the seeing wobbles "average out" during the longer exposure.  So if you've been using 1-sec exposures, go up to 2-5 seconds.  The stars will be "fat," but PHD can figure out the centroid just fine.



#5 telfish

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Posted 23 May 2019 - 08:47 AM

Are you sure your Newt was sufficiently cooled-down, and the "bad seeing" wasn't tube currents?

Just asking...

 

The usual strategy for guiding with less-than-ideal seeing is to increase the guide camera exposure time, so that the seeing wobbles "average out" during the longer exposure.  So if you've been using 1-sec exposures, go up to 2-5 seconds.  The stars will be "fat," but PHD can figure out the centroid just fine.

My newt is housed in an observatory all the time so it should be pretty much at ambient. I always open the roof as soon as the sun gets off it. I always use 3 second plus exposures.



#6 telfish

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Posted 23 May 2019 - 08:48 AM

One thing to try is to use filter for guide camera --- something like Long Pass filter >=600nm

I do have an IR long pass I could try.



#7 dakinemaui

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Posted 23 May 2019 - 11:19 AM

Could you expand on [use of a filter passing > 600 nm]?

Seeing improves slightly as the wavelength increases. Goes inversely as wavelength^1.2, which improves faster than diffraction blur increases, leading to a more stable & smaller guide star blur.

 

Interestingly, there is also a basis for the exact opposite recommendation -- i.e., use a violet filter. The idea is that the seeing decorrelates faster at shorter wavelengths. This gives more realizations combining within a given exposure time and therefore a more consistent image. Stars will be fatter than at 0.6 um (since seeing is dominating diffraction), but consistency is the important part.

 

In short, you might experiment to see which end of the spectrum is more advantageous (if at all!) in your particular case.


Edited by dakinemaui, 23 May 2019 - 11:21 AM.

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