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PHD2 Guiding Assistant - Seeing Indication?

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#1 KTAZ

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Posted 15 July 2020 - 12:36 PM

I am pretty new to PHD2 and am in the process of learning how to effectively guide.

 

The PHD2 tutorials include a comment about using GA to assess seeing, "It may help to run the Guiding Assistant periodically to get a sense of the typical seeing conditions at your site...".

 

After looking at the logs, there does not seem to be any specific FWHM type of seeing measurement. Does this exist? Does this info need to be extropolated and, if so, how is it done?



#2 SilverLitz

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Posted 15 July 2020 - 02:40 PM

I expect that looking at the High Frequency statistic (I believe in the upper right) gets you a handle on seeing related conditions.

 

My newbie intuition on this is that is a measure of the best you can achieve (for the given night), even with perfect PA and PEC.  It would be what is left and uncorrectable, and is basically seeing related.  This will vary from night to night.   I have seen this as high as 0.7" on some hot, steamy steamy nights, and my location has better seeing (typically ~1.5") than most locations, and those nights my actual guiding was > 1.0" RMS.   In nice conditions, I have recently had realized total RMS below 0.5".


Edited by SilverLitz, 15 July 2020 - 02:41 PM.

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#3 StephenW

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Posted 15 July 2020 - 02:53 PM

>I expect that looking at the High Frequency statistic (I believe in the upper right) gets you a handle on seeing related conditions.

 

Yep, it's PHD2's measurement of star movement and it uses it to try set MinMove recommendations that will prevent or minimize "chasing seeing".



#4 KTAZ

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Posted 15 July 2020 - 02:58 PM

I’ll have to look for that next time I run the GA. I guess it does not get captured in the log (at least, not that I can see).

 

It would be helpful if that is the case; knowing that my seeing is less than optimal could drive me to increase my loop exposure time to, hopefully, help the mount compensate less trying to chase the seeing.



#5 PhotonHunter1

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Posted 16 July 2020 - 07:23 PM

I’ve run guiding assistant in the past. Two questions: 1) is it recommended to run this each night you are imaging? 2) Once you let it run and then move on to see the GA assessment it takes FOREVER - well actually like 10 minutes which seems like forever. I forget the exact PHD2 terminology but it runs through 1 of say 120 steps and it does this 4 times (if I am remembering correctly). All that context to get to my question - do I need to let it run through all those “steps”?



#6 KTAZ

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Posted 16 July 2020 - 08:22 PM

My understanding is it is unneccesary to recalibrate every time you use your scope since you can automatically have it load the last calibration.

 

I also understand that you SHOULD re-calibrate if you remove or rotate your imaging train.



#7 StephenW

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Posted 16 July 2020 - 08:48 PM

To clarify:  GA and calibration are two different things.

 

GA characterizes your mount (PE, PAE and backlash) and your local seeing conditions.  In general I would say that you do not need to run GA that often, unless your current MinMove settings were determined on a night of exceptional seeing and so may be too aggressive on "regular" nights, or vice versa: your minmove settings were determined on a night of really bad seeing and aren't aggressive enough for your regular seeing....  I probably run GA once every couple weeks or maybe once a month, just as a sanity check...

 

And all those "steps" at the end occur during DEC backlash calibration - you used to be able to disable that - haven't checked if that is still easily done.


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#8 imtl

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Posted 16 July 2020 - 08:53 PM

I’ve run guiding assistant in the past. Two questions: 1) is it recommended to run this each night you are imaging? 2) Once you let it run and then move on to see the GA assessment it takes FOREVER - well actually like 10 minutes which seems like forever. I forget the exact PHD2 terminology but it runs through 1 of say 120 steps and it does this 4 times (if I am remembering correctly). All that context to get to my question - do I need to let it run through all those “steps”?

You don't HAVE to do anything. Guiding assistance is there to do exactly that, assist. Usually to beginners that do not know how to configure the guiding parameters to match different conditions (which some of them change through the course of the night).

 

Nothing in GA is mandatory, but it does provide a decent set of initial parameters for a novice to start guiding and unless something is very wrong then 95% of the time it will do a good job predicting what are good parameters to use.

 

The more time you give it the better estimation of the different parameters. Of course going more than a few worm cycles starts to have diminishing returns. I would say going through a couple is good. On average that means 10 minutes.

 

After gaining experience I rarely run it since I got a good understanding of things and what needs to be tweaked as conditions change.

 

What I do see a lot is that people get stressed if they got a minute or two of "bad guiding" and starts changing their parameters like crazy with no direction. It usually ends up with things going wacky. Guiding is really about long range stabilization of your system and not about fixing an occasional burp of your mount or local seeing. It you start to see constant burps then something is wrong and you should try and fix it.

 

Oh yeah and what Stephen wrote.

 

Eyal


Edited by imtl, 16 July 2020 - 09:19 PM.

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#9 Stelios

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Posted 17 July 2020 - 01:08 AM

For people who image a couple of hours because they haven't got their routine automated or need to hurry back in/home, 10 minutes can seem like an eternity. But if you routinely image all night, it's, well, only 10 minutes :)

 

Personally I rarely run GA. I kind of have some idea of the seeing, both from clear sky chart, the HFR autofocus measurements *and* eyeballing the sky for twinkliness, and I have a "regular" setting and an "exceptional" setting in mind, and switch between the two as needed. I don't bother optimizing for "lousy" seeing, as few keepers lurk there.

 

My advice would be that *initially*, you run GA reasonably frequently (say once every two or three imaging sessions) to get a feel of the differences, and try to associate them with information about the seeing. After a while, you may find, like I did, that you have an approximation "good enough for Army work."  

 

It also depends on what your RMS is relative to your focal length. Improving RMS from 1" to 0.8" for a focal length of 420mm will not make much if any difference, but for a focal length of 2000mm it would be worth doing. 


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