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Periodic Error and seeing - How much is too much? (Warning - long post)


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Guys and Gals - Really hoping for some help from the imaging gurus and mount doctors here.

Lucas has been helping me with my Mesu 200, which arrived with what I believe was an out of spec RA servo/encoder. It's one of the latest versions with magnetic motor encoders rather than optical. Historically, mag encoders have never been as accurate as optical (although they are normally more reliable), and typically they're not well suited to being on the end of an electromagnetic servomotor, but he re-assured me that no-one else had reported problems.

My initial tracking graphs (PHD into PECPrep, 400mm ST80 into QHY5) revealed a 'fast' periodic error, going from around +2 to -2 arcsecs every 21 seconds (this is twice the fundamental motor rotation period). Not what I hoped for.

post-24538-0-47690800-1376515623_thumb.j

Here it is again with a little Low Pass filtering:

post-24538-0-89713200-1376515767_thumb.j

Now Lucas sent me one of the original optical encoders, which I have fitted and I believe it has resolved this fault.

Here is the new raw tracking plot:

post-24538-0-00667100-1376516197_thumb.j

I do still see a repeating PE, but it now has a 42s period, which makes more sense. It is also pretty small, in the region of 0.7 arc seconds pk-pk, but it is definitely a repetitive mechanical product.

Here it is again with the same Low Pass filtering:

post-24538-0-35844600-1376516247_thumb.j

Most of you will probably say, "Well, the seeing will be worse than that, so why worry". And I agree to a point, but I would sooner just suffer the seeing, if possible, than the seeing combined with a PE which repeats every 42s. I will be guiding anyway of course.

Also, everyone who reviews the Mesu mentions that it only has a very small and very slow tracking error, with it going gently from -2 to +2 in around 300+ seconds. Even the tracking curve on the Mesu website shows this:

post-24538-0-64931200-1376516577_thumb.j

If my mount is the 'norm', why doesn't the Mesu PE plot show a 0.7 arc second ripple superimposed on the curve?

When Bisque and AP quote PE's of 7 arcsec or 3 arcsec, do they also have this faster ripple superimposed on top (I mean faster PE components - I know any real world test using star to track will show random seeing noise too)?

Does anyone's Mesu 200 produce a PE curve which, without massive low pass filtering, looks like their graph on the website?

If Olly or Yves see this - does your mount do this? Does it produce those fantastic images anyway?

Can anyone tell me if I'm going mad? Not many folk seem to post raw PE curves, some have a huge amount of filtering applied, which makes it hard to know what to expect. Or tolerate.

I know Lucas will replace my mount if I ask him - he has been great throughout, but he gives me the impression that they will all have some level of this 42-second Periodic Error. On a mechanical level, this makes sense, but I did a huge amount of research before I bought this mount, and no-one mentioned a 42s PE.

If you made it this far - thanks for reading! Any advice appreciated.

Jack

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I can think of 2 reasons why you don't see the ripple on the mesu provided graph ...

- it was made with the older encoder and servocat system (the first units had other encoders)

- the plot is the same as the plot being shown with the no flip mount wich has a complete different gearing with bigger wheels ...

i think it's option 2 ...

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Gents,

Thanks for your responses.

My Mesu, as supplied, had magnetic motor encoders with 4096 tics per rev. I now have an optical encoder on the RA axis (I think these were standard up until two months ago) which have 4000 tics.

Jeffrey, your Maxim guide plot shows your mount has a small 'fast' PE, but it is clearly bigger than the declination corrections. Is this a Mesu? I thought you had an ASA?

Steve Richards has supplied a raw tracking plot for his Mesu which doesn't look great, but he was clearly happy enough with it to keep the one he was sent for review (and pay for it).

I think not many suppliers provide genuine raw tracking plots. 10 Micron show one which looks 'honest' for the GM1000:

post-24538-0-52721000-1376654235_thumb.j

Obviously the GM1000's encoders mean any long period (full worm rotation) PE is corrected, but it still shows some unavoidable and very small 'fast errors' (the 'grass' in the plot), I presume from the constant movement of the worm against the ring gear (and maybe the seeing). The fast noise here is only around 0.2 arcsecs, so almost meaningless. I would expect to see this on any mount using a worm and ring gear, even AP and Bisque. The slower error only goes _+0.5 arcsecs, which I guess is as good as the encoders can correct to.

Therefore I assume most mount suppliers take this fast tracking noise as a 'given', and remove it from their PE plots - just drawing a smoothed line.

I just wonder how fat the trace can be, or how tall the grass gets, before we get concerned and it starts softening our images.

At least I now know I'm not the only one to have this issue.

Happy to take further comments and input.

Thanks again

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Steve Richards has supplied a raw tracking plot for his Mesu which doesn't look great, but he was clearly happy enough with it to keep the one he was sent for review (and pay for it).

The graph I sent Jack was completely unfiltered which is not the way the data would normally be considered for determining the true PE of a mount but it does fill in some of the 'blanks' but these blanks are considerably below the typical 'seeing' anyway.

If Olly or Yves see this - does your mount do this? Does it produce those fantastic images anyway?

I certainly wouldn't claim that my own images are fantastic but having tested the mount on the sky (which is what I think users will be most interested in) I certainly wanted to keep the review mount even though it did cost me rather deep in the purse! I think that perhaps this is the key - how does it perform on the night sky? If you are pleased with the results then perhaps the minutiae of embedded ripples is not really an issue. However, if you don't get satisfactory images then you do have a problem that needs to be addressed.

Just for interest, the following is the latest image captured by me using the Mesu 200 mount - not a prize-winner but it is only 4 x 1000 seconds Ha with a WO FLT 98 refractor captured in a fairly short gap in the cloud cover - 5 second guiding. I am satisfied with the tracking but would you be too if it were yours? For me, this is the true test of the mount and the reason I wanted to buy one.

eastern_veil_140813_l.jpg

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Jeffrey, your Maxim guide plot shows your mount has a small 'fast' PE, but it is clearly bigger than the declination corrections. Is this a Mesu? I thought you had an ASA?

I also have a shared setup (with Yves). The mount is a Mesu with Sitech II controller. The fast PE is causing elongated stars at a focal length of 1680mm, 0.66"/px (10" ODK).

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Steve,

Your images fall into the excellent category as far as I'm concerned. I hope you didn't take my comment to mean that I believed you were less than critical in your assessment of mount performance or imaging. You are an acknowledged guru in this area, and well published. I completely agree that an unfiltered tracking plot has more noise than you'd normally want to see when assessing cyclic tracking errors, especially if it uses short exposures. You were kind enough to oblige my request for a raw PECprep plot, which I do greatly appreciate.

You also make a great point about the image produced being more important than any tracking plot. I'm talking about a cyclic error with an excursion of around 0.7 arcsecond pk-pk, which in many cases would be masked by seeing or sub-optimal focus. I'm pretty sure that I could use my mount with my wave 115 (800mm) refractor and it would produce great images and need minimal guiding corrections, but I bought the Mesu to use at 1600mm+. I'm concerned that if this error means half a pixel movement between guide corrections, that's something I was hoping to avoid.

Jeffrey, I had exactly a 21s error (2nd harmonic of the motor rotation period), which I believed was an encoder misalignment or encoder periodic error. This error went +_2 arc secs, so sometimes covered 4 arcseconds in 10s (-2 to +2 in 10s and then back to -2 in the next 10s), which I knew would not support 5s guide exposures at longer focal lengths. The new motor and encoder has reduced the amplitude by a factor of 4 (to just under 1arcsec), and increased the period to the motor fundamental (42s). So now the max rate is 1 arc second in 20 seconds.

If I hadn't experienced the faulty original servo/encoder, I might not have looked so closely, but now I'm left wondering "what is acceptable?", "what is realistic?". A pempro loaded AP1200 should have a PE around 1 arcsec pk-pk, but I'm guessing this too would have some higher frequency elements superimposed on this PE curve. How big would they be I wonder?

I've always analysed things too much, according to my wife

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I hope you didn't take my comment to mean that I believed you were less than critical in your assessment of mount performance or imaging.

Oh crikey, I really didn't think that for one moment :grin: !!

However, I am as guilty as anyone of over-analysing equipment performance and always hope for the perfect but the point I was trying to make (perhaps clumsily!) was that analysing the technical aspects of astro-imaging gear can often take you away from the main event - namely are you achieving images that give you pleasure?

You make a good point regarding the longer focal lengths that you are hoping to use - I am a 'nebula kind of guy' so relatively wide fields of view tend to float my boat - so I can appreciate your concern.

You ask "what is acceptable?", "what is realistic?". I guess 'acceptable' is less than typical seeing anomalies and, for no good technical reason - just a hunch - I'd be happy with 50% of the seeing. 'Realistic' is a little harder to define - we seek perfection but will never achieve that!

I hope that if nothing else, a reason for this wobble is found, especially if it is indeed a departure from the original Servo Cat/Argo Navis system and for the heck of it, I will try some shorter guiding exposures to see if they make any noticeable differences to my star shapes.

I hope you will shortly have an opportunity to use the mount on the night sky and that the results put your mind at rest.

I've always analysed things too much, according to my wife

You know what? My wife, Janie says the same thing about me too! :grin: :grin:

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We chose the Mesu because it is a friction system, and for that reason it has a slow periodic error. According to the graph on Mesu's website the periodic error is 4" peak-to-peak in 10 minutes. Because this PE is very slow, it can be guided very good with long guiding exposure times (5 seconds) to minimize seeing effects.

BUT, we have difficulties with the 'extra' faster periodic error on top of the slow 4" error. This faster PE of 2" peak-to-peak, that occurs every 21 seconds, is very difficult to guide. If we want to guide this fast PE, we should use guiding exposures of 1 or 2 seconds, and that means that we'll be guiding on seeing effects, causing a very unstable system with bloated stars.

We clearly see elongated stars when we try to guide the Mesu with 3 second guiding exposures. The PE is simply to fast.

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Does that mean that you should guide with about a 5s exposure, and expect an error of 2", with the knowledge that you can't use a fast guide exposure to eliminate the 2" error?

Although your maths are correct in that if the mount is going from -2 arc secs to +2, and back to -2 in 21 seconds, so it's moving at 4 arcsecs in 10s, autoguiding programs can't/don't typically pre-empt, or they'd guess wrong as often as they'd guess right.

So if it moved 2 arc sec during the first 5s guide sub, the guide star image would be smeared over 2 arc seconds, but its centroid would show a 1 arc second error. it would send 1 arcsecond correction and in the next 5 seconds the mount would move another 2 arc seconds. This then repeats endlessly.

So you either image at a scale where 2 arcseconds of 'smudging' aren't visible (400mm maybe), or guide every second. An EQ6 can achieve this.

Does that make sense?

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