Re: Guiding guidance for Mach2?


Terri Zittritsch
 

Hi Rolando, thanks for the response and sorry if my question wasn’t clear… but it’s not been very clear in my mind either so I’ll try again and then probably noodle on it some more. I’m trying to understand the benefits of absolute encoders to guiding and tracking for A-P. I did re-read your post.
I understand the mount accuracy benefits, and to get my 0.6 RMS or better tracking I typically can’t have 3” excursions on the Atlas (takes a long time to RMS that out) and can avoid them (mostly) 30 degrees around zenith but lower towards the horizon things get horrible. And almost always 3 or more hours into a session something goes horrible in my tracking and I never really figure out why.. But I know the mechanicals are not so precise, backlash is pretty high, stiction is high in the cold weather, i have cables all over the place getting stiff. I throw away sometimes 30% of frames due to mysterious events which is why I’m pursuing a better mount. But I do use the TEC140 at prime focus with your AP flattener which gives me right around 1000mm of focal length, and my stars are usually pretty darn good (again, around the zenith). I also use an 8” sct at 1300mm with the same mount successfully.. again, keep rate is lower. Given vt skies, I’d like to not loose 30% of my frames (life with clear skies is literally too short here). Some day after retiring I’ll move to somewhere with better skies and no freezing weather.

Where i see the benefits of the machining quality is not having so much backlash or periodic error, so less work for guiding (but many premium mounts can do this). I think the benefits of absolute encoders are only the total elimination of back lash and periodic error, since you're making sure with a closed loop system that the correction or movement resulting at the shaft is the correction or movement requested and not just a calculated movement of the worm. As long as your internal clock and encoder resolution is high enough to achieve the design goal, you have a system which only needs active guiding for polar alignment inaccuracy (and maybe wind if the reaction time is quick enough). I think pointing accuracy is not all that important in this era of plate solving. I can get 50 pixel pointing in 3 or 4 iterations in a couple of minutes which is good enough for my images. So is the net benefit of encoders, providing less work for the guiding system? And the question is how important is the encoder versus the wonderful machining and accuracy of a premium mount such as you make? So my net question is, do encoders improve guiding accuracy or is it just making the job easier? Can I expect unguided shots at 1000mm focal length of how long (1-2-3 minutes?) I guess this is my ultimate question. Not changing my mind at all, and my supplier has contacted me already so I am getting excited.. but as a 39 year plus employee of semiconductor biz, everything tech gets me excited.

Thanks,
Terri

On Sep 10, 2019, at 12:24 PM, chris1011@... [ap-gto] <ap-gto@...> wrote:


Hi Terri,

I'm not quite sure what your question is. If you have a mount that tracks at 0.1 arc sec rms like my first chart, then you have an excellent mount. If 0.6 arc sec rms meets your needs, then again you have the mount already for your application.

Where the Mach2 comes in is at the precision level where the system can be controlled to 0.1 arc sec pk via commands by external software. 0.6 rms sounds low but in reality the peak excursions will generally be 5 times that, around 3 arc sec pk-pk. That works ok for short focus instruments of around 500mm, but falls way short when using longer scopes that have inherent high resolution.

The original question was how to set up PHD2 for guiding with this mount. The simple answer is to use 2 sec or longer guide exposures so that a good guide star centroid can be calculated. Shorter exposures are not recommended, even in great seeing conditions. The rest I explained in my previous post.

Rolando



-----Original Message-----
From: Terri Zittritsch theresamarie11@... <mailto:theresamarie11@...> [ap-gto] <ap-gto@... <mailto:ap-gto@...>>
To: chris1011@... <mailto:chris1011@...> [ap-gto] <ap-gto@... <mailto:ap-gto@...>>
Sent: Tue, Sep 10, 2019 11:04 am
Subject: Re: [ap-gto] Guiding guidance for Mach2?



Hi Rolando,
I’ve thought about this a bit now, and I know chasing the seeing can be counter-productive when doing astrophotography. If you have a mount that should be tracking at .25 arc-sec or less, and if the seeing is poor and causing you to make mount corrections of arc-seconds, isn’t this being counter productive? Now of course I’m assuming perfect polar alignment which is hard to have especially if you’re always portable like me, and every night is different. But I do use a polar camera/solver that seems to do well, and even with my Atlas can get .6 arc sec tracking with PHD and a 400mm guide scope around the zenith (+-30 degrees) and have a chart like your first one (maybe not quite so pretty). As I get towards the horizons all bets seem to be off and the atmosphere takes over and my excursions get much bigger (like your second chart only larger excursions).

I’m just trying to wrap my head around making all of these large corrections (on your poor seeing night example) and what the encoders are doing for you in this case versus just very precise mechanicals (like A-P has always had). This looks very much like what I deal with constantly with my atlas and as most nights don’t have great seeing. All you really want to correct for, is any long term drift due to poor polar alignment or RA speed inaccuracy. I’m probably not understanding something here.


Terri






On Sep 9, 2019, at 7:50 PM, chris1011@... <mailto:chris1011@...> [ap-gto] <ap-gto@... <mailto:ap-gto@...>> wrote:


Trying again.

Rolando


-----Original Message-----
From: chris1011 <chris1011@... <mailto:chris1011@...>>
To: ap-gto <ap-gto@... <mailto:ap-gto@...>>
Sent: Mon, Sep 9, 2019 5:46 pm
Subject: Re: [ap-gto] Guiding guidance for Mach2?

I have been guiding with 2 to 5 second exposures using PHD2. 2 sec for guiding on a calm night, up to 5 sec on a night of unsteady air. If you expose for less than 2 seconds, especially on a poor seeing night, you should dial in at least a 2 second delay between exposures. Otherwise the guide star is going to be jumping around a lot because the guide program is going to try to chase the seeing. I have tried as fast as 0.2 sec exposures with no delays, but the guiding is not great. At that exposure level the star's centroid will never stay still even on a good seeing night..

I set my Min Move according to what the atmospheric motion is that night. On a good seeing night it can be as low as 0.1 arc sec.. On a poor night I have it set to around 0.35 arc sec. If you use the Guide assistance, it will give you some very good starting parameters for these settings because it first measures the atmospheric motion and then sets the Min Move accordingly. A really powerful part of the program, I have found..

Aggressiveness setting is usually around 60% +- 10. On a good night i can sometimes set it higher, but above 80% doesn't buy much.

As far as Algorithms, I have used all of them and find not much difference in performance.

On a very steady night I have consistently gotten below 0.15 arc sec rms on both axes. Sometimes below 0.1. On the poorest night my guiding is around 0.45 arc sec rms.

Here is a typical result on a good night (4 out of 5 seeing):



And here is a poor night when the seeing was 2 out of 5:




-----Original Message-----
From: wayneh9026@yahoo..com <mailto:wayneh9026@...> [ap-gto] <ap-gto@... <mailto:ap-gto@...>>
To: ap-gto <ap-gto@... <mailto:ap-gto@...>>
Sent: Mon, Sep 9, 2019 5:00 pm
Subject: [ap-gto] Guiding guidance for Mach2?

Rolando, any guidance on autoguiding for the Mach2, given it’s absolute encoders? Such as settings for PHD2?

Thanks,

Wayne

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Posted by: wayneh9026@... <mailto:wayneh9026@...>
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