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Fine tuning PHD guide settings

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#1 jtrezzo

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Posted 19 June 2017 - 06:55 PM

I have been working on getting my guiding better, not to much avail really. My RMS is hovering around 1.25" total on my last session with my Atlas Pro and AT106LE. Ideally I'd like to get that under 1" at least. I have noticed that the noise is very bad on my guiding image. Is the 2x2 noise reduction worth using? I may try redoing the dark frames for it also. I have the gain set to about 80, and as I understand it shouldn't have to mess with offset for guiding. I believe I am balancing well and I have a Polemaster for polar alignment so it should be spot on. I have set tried varying settings with the aggressiveness and am looking into using the PEC on EQMOD next time out, I finally figured out how to get that to show up on the interface. Not entirely sure what guide rate to use on it either, I am guessing 0.5x sidereal should be fine?

 

Any good settings for the Atlas Pro would be appreciated - something to try and start on then fine tune the next time I get the chance to go out.



#2 xiando

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Posted 19 June 2017 - 07:05 PM

Filtering can help damp transient effects in a controller. I'd suggest you try it and see if it helps you.



#3 mistateo

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Posted 19 June 2017 - 07:18 PM

I have been working on getting my guiding better, not to much avail really. My RMS is hovering around 1.25" total on my last session with my Atlas Pro and AT106LE. Ideally I'd like to get that under 1" at least. I have noticed that the noise is very bad on my guiding image. Is the 2x2 noise reduction worth using? I may try redoing the dark frames for it also. I have the gain set to about 80, and as I understand it shouldn't have to mess with offset for guiding. I believe I am balancing well and I have a Polemaster for polar alignment so it should be spot on. I have set tried varying settings with the aggressiveness and am looking into using the PEC on EQMOD next time out, I finally figured out how to get that to show up on the interface. Not entirely sure what guide rate to use on it either, I am guessing 0.5x sidereal should be fine?

 

Any good settings for the Atlas Pro would be appreciated - something to try and start on then fine tune the next time I get the chance to go out.

I am in a similar boat with my Atlas Pro.  I acquired it about a month ago.  One night I achieved less than .5" RMS combined, with incredible seeing, but the transparency was HORRIBLE, so I question the validity of that session.  That was also with drift alignment at less than .2' error over 10 minutes.  With an approximate careful alignment with the (pretty darn good) polar scope, I usually get between 1.1 to 1.25ish combined RMS with my Eon120 + stuff (about 20 lbs).  When I do a drift alignment, I can usually get under 1" combined RMS, barely, with good dec balance and east heavy RA balance.  I have only replicated freakishly good (for the mount) sub .5" combined RMS once, and again, incredibly still air, with horrible transparency, making imaging pointless.  I think we both might benefit from a longer focal length guide scope.



#4 jtrezzo

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Posted 20 June 2017 - 12:53 PM

Filtering can help damp transient effects in a controller. I'd suggest you try it and see if it helps you.

At risk of sounding like an idiot, what does that entail?



#5 jtrezzo

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Posted 20 June 2017 - 12:56 PM

I am in a similar boat with my Atlas Pro.  I acquired it about a month ago.  One night I achieved less than .5" RMS combined, with incredible seeing, but the transparency was HORRIBLE, so I question the validity of that session.  

That may be part of the problem for me. The humidity at night gets up near fully saturated so I'm sure that is not helping. I am thinking about upping my guide scope to an Orion ST80 since they are nice and cheap. If I ever get an AT65EDQ, which I want at some point, I would probably just set that up as the guide scope when it's not being imaged with.



#6 xiando

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Posted 20 June 2017 - 01:06 PM

 

Filtering can help damp transient effects in a controller. I'd suggest you try it and see if it helps you.

At risk of sounding like an idiot, what does that entail?

 

Answering the question... "Is the 2x2 noise reduction worth using?"

 

Sorry I wasn't clearer. 

 

With digital control loops (in fact any, but digital control loops are more susceptible ime) , transient effects (discontinuities at transient events, can cause "hiccups" in the system, leading to overshoot and subsequent oscillation. filtering can help to ameliorate the discontinuities and lead to less erratic and/or oscillatory behavior.



#7 mistateo

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Posted 20 June 2017 - 01:25 PM

I just imaged 2 targets last night.  On the Trifid Nebula I was getting about 1.2" combined RMS.  Then I did the Cocoon Nebula, in a very different part of the sky, and got about .6 RMS.  I can't imagine the seeing was responsible for the discrepancy, as they were done back to back.  I am starting to think I need different balancing for different parts of the sky.



#8 xiando

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Posted 20 June 2017 - 01:40 PM

or possibly different settings in your guiding software depending on pointing location. It's one reason I have a slight envy of more sky-graced locations. The freedom to spend a few nights just tooling the system, rather than desperately doing as little on that front as "required" so I can squeeze out some data to process.... I have done my balance about as well as I can, but it still has areas of RA/DEC where the scope is naturally settled, and some where it swings of its own free volition (well gravity, but whatever). That's  direct implication that I have to pay attention to where I am pointing and tailor my guiding settings for that area. I'd like to get a mental map of where I need to be more aggressive and where I need to be less aggressive for "best' operation. Still so much to learn.



#9 happylimpet

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Posted 20 June 2017 - 01:43 PM

What exposure times are you using? Try 3-4secs at least. I use 2x2 binning but thats with an OAG; with a guidescaope im not sure of the benefits.  With my EQ8 (same right?) I get usually 0.4-0.6 rms.

 

On the EQ8 many people have tried PEC and while it can help for unguided work, theres no point if you're guiding.

 

Assume youve run the guiding assistant in PHD2?



#10 Goofi

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Posted 20 June 2017 - 03:23 PM

My setup is similar to yours (and mistateo's) in most ways, but different in one particular way; my guiding RMS is usually around 0.6".  I'm using an AtlasPro mount, with similar guide camera but a very different guide scope. I use an OAG and so my guiding is done through the main scope at 540mm focal length.  It helps.

 

There's two reason for the OAG helping. First, the longer focal length means a better image scale and more precision in guiding. But second, no flexure in the guide scope.  External guidescopes are not bad/wrong, but you have to pay a lot of attention to your setup to eliminate flexure. Often the little ring mounts are flimsy and allow a bit of wiggle; that shifting throughout the night is subtle, but plays havoc with your guiding.

 

So, some things I'd check: Is your camera securely attached to the guidescope? Is your guidescope locked down tight (no play/movement)? Does your mounting assembly for the guidescope have any play in it? Finally, while checking things over, review your cables and make sure there's no chance of snags.

 

The Trifid and Cocoon are in very different parts of the sky. It's possible you had good balance for one location but lousy balance for the other.  Plus, the Trifid is pretty low .. if you're imaging over the roof of a house you can pick up heat currents, etc that make guiding harder.


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#11 Alex McConahay

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Posted 20 June 2017 - 06:27 PM

Goofi,

 

I do not think flexure will change what PhD reports as rms guide error. You may have a point about focal length, although I would have to think about that (after I finish mudding the  dry-wall on my garage, or maybe during....it is probably more pleasant to think about than the mud caking in this heat).

 

But flexure would have nothing to do with reported guide error. PhD could happily guide away on that one little guide star, and everything would be fine, as far as it goes. But, the imaging scope could become progressively more mismatched with the guide scope (because of  flexure). It would show as elongated stars in the imaging sensor, but not be reflected in the guide star or rms error reported by PhD.

 

Alex


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#12 jtrezzo

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Posted 20 June 2017 - 06:56 PM

 

 

Filtering can help damp transient effects in a controller. I'd suggest you try it and see if it helps you.

At risk of sounding like an idiot, what does that entail?

 

Answering the question... "Is the 2x2 noise reduction worth using?"

 

Sorry I wasn't clearer. 

 

With digital control loops (in fact any, but digital control loops are more susceptible ime) , transient effects (discontinuities at transient events, can cause "hiccups" in the system, leading to overshoot and subsequent oscillation. filtering can help to ameliorate the discontinuities and lead to less erratic and/or oscillatory behavior.

 

OK that is what I was thinking, but I don't usually see NR referred to as filtering so it threw me off. I will give it a try next time to see if it helps. 

 

 

I just imaged 2 targets last night.  On the Trifid Nebula I was getting about 1.2" combined RMS.  Then I did the Cocoon Nebula, in a very different part of the sky, and got about .6 RMS.  I can't imagine the seeing was responsible for the discrepancy, as they were done back to back.  I am starting to think I need different balancing for different parts of the sky.

I have also noticed better performance in certain parts of the sky. The other night when I was out I was getting the Lagoon as it went across the meridian, which of course changed the balance a bit about half way through. I knew this was going to happen so I purposely tried to get an even balance from the start instead of slightly east heavy so that when the counterweights were up it wouldn't go absolutely insane. This resulted in guiding between 1.25" early and 1.5" later. Then I switched to the Bubble Nebula which was rising in the northeast, and performance was closer to 1" and much smoother. I still think there is plenty of room for improvement though, especially based on people's commonly reported 0.6" with this mount.

 

 

My setup is similar to yours (and mistateo's) in most ways, but different in one particular way; my guiding RMS is usually around 0.6".  I'm using an AtlasPro mount, with similar guide camera but a very different guide scope. I use an OAG and so my guiding is done through the main scope at 540mm focal length.  It helps.

 

There's two reason for the OAG helping. First, the longer focal length means a better image scale and more precision in guiding. But second, no flexure in the guide scope.  External guidescopes are not bad/wrong, but you have to pay a lot of attention to your setup to eliminate flexure. Often the little ring mounts are flimsy and allow a bit of wiggle; that shifting throughout the night is subtle, but plays havoc with your guiding.

 

So, some things I'd check: Is your camera securely attached to the guidescope? Is your guidescope locked down tight (no play/movement)? Does your mounting assembly for the guidescope have any play in it? Finally, while checking things over, review your cables and make sure there's no chance of snags.

 

The Trifid and Cocoon are in very different parts of the sky. It's possible you had good balance for one location but lousy balance for the other.  Plus, the Trifid is pretty low .. if you're imaging over the roof of a house you can pick up heat currents, etc that make guiding harder.

Good points, I will double check that everything is tightened down. It sounds like OAG is where it's at but I am assuming they don't work with a DSLR? I think OAG would render my field flatteners unusable due to the backspacing requirement of 55mm?



#13 mistateo

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Posted 20 June 2017 - 07:04 PM

The 55mm backfocus on DSLRs is a killer.  I have a guide scope for this reason.



#14 xiando

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Posted 20 June 2017 - 07:57 PM

 

OK that is what I was thinking, but I don't usually see NR referred to as filtering so it threw me off. I will give it a try next time to see if it helps.

My background is in part in control systems, electronic engineering and other engineering pursuits....Noise reduction has two fundamental parts, first, when possible, reducing the sources of noise so it's not there in the first place, and second filtering, to ameliorate that which you can't prevent in the first place.




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