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When Push Here Dummy doesn't work in PHD2

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#26 nebulasaurus

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Posted 22 September 2021 - 01:24 PM

The arithmetic is quite clear - the most accurate calibration numbers will be obtained by calibrating always at the meridian and the equator and then using DEC compensation.

 

I've read Chris's book and the only thing that I recall is that he seems to think that if you decide to image something at DEC 80 or so, you might want to turn off DEC guiding altogether. I don't recall anything in the book that says it's inherntly better to calibrate at DEC 50 rather than DEC 0.

 

Thinking about it, though, if you were imaging from the Congo, DEC 0 is right on the horizon and that would make me vary my workflow. That's a good edge case! Some people are imaging these days from near the equator. 

 

Rgrds-Ross

Errm,

 

I may be dense here, but at the north pole polaris is directly overhead isn't it?  And Polaris is at dec 90?  Then wouldn't you have to be at the pole (N or S) to have dec zero at the horizon?

 

Wouldn't dec 0 pass from east to west through the zenith at the equator?  And wouldn't Dec 90 then be at the horizon?


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#27 rgsalinger

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Posted 22 September 2021 - 05:31 PM

Sorry for the typo.  At the equator DEC 90, not 0 would be on the horizon. My spell check is no substitute for accuracy apparently...................

 

Polaris is at 89 20 more or less. I think that I got that one right. 

 

Rgrds-Ross


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#28 bobzeq25

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Posted 23 September 2021 - 09:42 AM

The arithmetic is quite clear - the most accurate calibration numbers will be obtained by calibrating always at the meridian and the equator and then using DEC compensation.

 

I've read Chris's book and the only thing that I recall is that he seems to think that if you decide to image something at DEC 80 or so, you might want to turn off DEC guiding altogether. I don't recall anything in the book that says it's inherntly better to calibrate at DEC 50 rather than DEC 0.

 

Thinking about it, though, if you were imaging from the Congo, DEC 0 is right on the horizon and that would make me vary my workflow. That's a good edge case! Some people are imaging these days from near the equator. 

 

Rgrds-Ross

Classic CN thing.  Someone makes a theoretical argument in favor of position "A".  When their result is driven totally by what factors are included in the analysis, and which are ignored.  In real life the ignored factors can drive the train.  That "quite clear" arithmetic ignores what may be, in some circumstances, the crucial factor.

 

Woodhouse' analysis involves a factor ignored in the standard method.  And points out that, in some circumstances, it can drive the train.

 

That arithmetic is based solely on the geometry of RA and DEC, and how it changes across the sky.  Woodhouse points out that signal to noise ratio (which the equation ignores, there's no way to consider it without actual data) may be more important.  It varies less, but guiding is much more affected by it.  So, it may be more important.  Woodhouse describes a simple way to tell if, in the particular situation, it is.

 

A tweak.   Needless to say, I've only labeled it ("snr"), to get the full story you need to read the relevant section in the book.

 

The most important thing.  That tweak is just a very small part of Woodhouse' analysis of guiding.  Overall, it's the best thing I've ever seen on the subject.  By a mile.


Edited by bobzeq25, 23 September 2021 - 09:46 AM.


#29 rgsalinger

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Posted 23 September 2021 - 10:31 AM

In practice, over 11 years now, I've never had the need to do any of the tweaks that Chris describes. I've read the guiding section of the book. He's mostly writing about there about edge cases and it's all frankly just theory. I think that it's a CN thing to become enamored of personalities at the expense of both logic and experience. I personally rarely post theory unless it matches my experience.  

 

If anyone finds that turning off dec guiding when imaging near Polaris gets a better result, then go for it. If you find that turning off DEC compensation makes your guiding better because the seeing is a mess, then go for it.  Have fun. 

 

In the vast majority of use cases you're going to find that the standard method is standard for a reason. The reason is that the arithmetic is compelling. More people waste more time tweaking guiding to little or no purpose than probably anything else in the hobby of aesthetic imaging. I see it all the time when I walk around our dark sky site. 

 

Rgrds-Ross



#30 bobzeq25

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Posted 23 September 2021 - 01:49 PM

In practice, over 11 years now, I've never had the need to do any of the tweaks that Chris describes. I've read the guiding section of the book. He's mostly writing about there about edge cases and it's all frankly just theory. I think that it's a CN thing to become enamored of personalities at the expense of both logic and experience. I personally rarely post theory unless it matches my experience.  

 

If anyone finds that turning off dec guiding when imaging near Polaris gets a better result, then go for it. If you find that turning off DEC compensation makes your guiding better because the seeing is a mess, then go for it.  Have fun. 

 

In the vast majority of use cases you're going to find that the standard method is standard for a reason. The reason is that the arithmetic is compelling. More people waste more time tweaking guiding to little or no purpose than probably anything else in the hobby of aesthetic imaging. I see it all the time when I walk around our dark sky site. 

 

Rgrds-Ross

You misunderstand what Woodhouse is saying.  Oversimplify. 

 

He's talking about both how to calibrate and DEC compensation.

 

I go back to.  If you really want the very best guiding, paying attention to Woodhouse can help.  I quite agree that, on this issue, he's talking about edge cases.  But that talk is not just theoretical, it's solidly based on his actual experience.  These things actually happen.

 

The "clear arithmetic" you're talking about is what's just theoretical.

 

And, taken as a whole.  Just about anyone could benefit from Woodhouse's discussion about guiding.  The majority of it has nothing at all to do with edge cases, it's universally applicable.


Edited by bobzeq25, 23 September 2021 - 01:53 PM.


#31 rgsalinger

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Posted 23 September 2021 - 06:33 PM

As I said before, I've found that the theory behind DEC compensation matches the results that I get and that's why I recommend it as a general best practice. You must have missed that part of my post.

 

To be clear, my guiding does not get worse as declination increases as a general rule. I do not need longer exposure either. There are two glaring mistakes buried in that chapter. The first is that seeing is a function of target declination. It is not. If anything it's a function of target altitude. Second, if you want to smooth out the guiding you use a longer cadence, not a longer exposure. All that a longer exposure will do (particularly with an F4 guide scope) is to give you a big saturated star. 

 

So, as I said,  my experience is that DEC compensation works and that the best approach is to calibrate your guiding at DEC 0. If you then want to image Polaris from the Congo, then his suggestions make a great deal of sense. If you want to image from more general locations think about cadence, not exposure and altitude, not declination. 

 

Rgrds-Ross 



#32 pemongillo

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Posted 24 September 2021 - 11:36 AM

Well, I figured out the answer to the original question I posted here. If you remember I could not get my ASIAIR PRO to get beyond the calibration phase so never could get to guiding. I had guiding working weeks earlier. Also I set up guiding through PHD2 and it worked. Very puzzling. A fellow told me if you use Sky Safari with an iOptron mount and occasionally an AVX it interferes with the guiding calibration if you connect it to your mount. Just disconnecting it from your mount doesn't fix it. You have to turn off Sky Safari, and reset your ASIAIR and wala, my calibrations are going smoothly and I'm guiding again. I got Sky Safari, because you can not search for named object in Asiair. I will continue to use it for only that an never connect it to my mount again. Fun Stuff :-)



#33 bobzeq25

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Posted 24 September 2021 - 02:34 PM

Well, I figured out the answer to the original question I posted here. If you remember I could not get my ASIAIR PRO to get beyond the calibration phase so never could get to guiding. I had guiding working weeks earlier. Also I set up guiding through PHD2 and it worked. Very puzzling. A fellow told me if you use Sky Safari with an iOptron mount and occasionally an AVX it interferes with the guiding calibration if you connect it to your mount. Just disconnecting it from your mount doesn't fix it. You have to turn off Sky Safari, and reset your ASIAIR and wala, my calibrations are going smoothly and I'm guiding again. I got Sky Safari, because you can not search for named object in Asiair. I will continue to use it for only that an never connect it to my mount again. Fun Stuff :-)

Things like this are why I prefer a general purpose computer.  I use a NUC.  It runs a very wide variety of software simultaneously (including industrial strength PhD2 <smile> ) with ease.

 

I note you're trying PixInsight.  One of the programs I run simultaneously while imaging is PixInsight.  That lets me do detailed analysis of the subs I'm collecting.  Great for evaluating subexposure, focus...  You'll not be doing that with an ASIAir.  <smile>

 

Pretty low end i3 NUC.  Similar to this.

 

https://www.amazon.c.../dp/B07525HWWH/

 

Wouldn't be great for serious processing in PI, works fine for evaluating a sub.


Edited by bobzeq25, 24 September 2021 - 02:47 PM.

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