NGC5371 and surrounding galaxies


David Fischer
 

So would the ripple magnitude depend fairly directly upon the details of step rate, motor step size, belt characteristics, motor holding strength ?
Is this the sort of thing that mount design engineers analyze at design time ?

-- David F.


On Thu, Jun 4, 2020 at 3:09 PM uncarollo2 <chris1011@...> via groups.io <chris1011=aol.com@groups.io> wrote:

yes please! i would like to see

Ok, see below. Sorry for the poor quality screen shots. I took them with my camera aimed at the screen ;^)).

Top image is the raw periodic error of a typical microstepper driving a precision worm. You might think the worm is not great, but the reality is that the two pulleys have runout and produce the majority of the large scale periodic error. The worm itself actually has only about 2 arc sec error. The rest is caused by the commercial pulleys which have a fair amount of runout. This is basically what you get in almost all imported non-encoder mounts that use belts and microsteppers.

The plot is taken at 100msec sample rate so you can see what is really happening during a typical worm cycle. Normally when you see a PE curve, the sample rate is 10 times slower, so you don't see the fast moving errors on PE data sheets.

The real issue is not the large scale PE, which can be handled by guiding (as long as it is slow moving and not too large an excursion). It is the fast moving errors at the 1 arc sec level that cause problems for guiders. The second plot shows in detail that microsteppers with belts have two fast moving errors. The belt teeth cause a ripple every few seconds (in this case once every 12 seconds at approx 1 arc sec P-P). If your guiding cadence is fast enough you might be able to guide it out successfully. However, if your guide rate approaches once every 3 - 4 seconds, you will see a residual error that modulates with time.

The faster moving error repeats every 2 seconds with another 1 arc-sec P-P error. This one cannot be easily guided out because the guider is always a step behind and can actually amplify this motion. The error is caused by the stepper motor itself which has 5% angle variation for each winding step point. If your camera pixel scale is 2 - 4 arc sec per pixel, you may not see these kinds of errors. If your guider is exposing at 2 - 3 seconds, the guide graph will average out the tiny fast motions and will record perhaps very low rms values. However, the stars that you are recording on your imaging camera are doing this tiny back and forth dance in RA and slightly smearing the resolution of your image.

The third graph below shows how the absolute encoder tames all three errors.







Sébastien Doré
 

Interesting! Thanks for the 101 course on mount design tradeoffs Rolando.

Just wondering up to how much focal length would the Mach2 be suitable unguided. Do you think it would do it at say 1400mm FL (8in Edge+FR)? What about 2000mm (no reducer)?

Any feedback from the lucky folks that already got their hands on the Mach2?

Sebastien


Roland Christen
 

Ripple magnitude is pretty much fixed for a stepper motor in microstep mode. It has to do with the variation of position of the shaft during rotation. One way around the problem is to put a shaft encoder on the motor and use that to feed back the actual shaft rotation, similar to what is used on a DC servo. This adds large cost to the drive system and is used only on top end mounts.

Rolando

-----Original Message-----
From: David Fischer <dkn.fischer@...>
To: main@ap-gto.groups.io; David Fischer <dkn.fischer@...>
Sent: Thu, Jun 4, 2020 10:21 pm
Subject: Re: [ap-gto] NGC5371 and surrounding galaxies

So would the ripple magnitude depend fairly directly upon the details of step rate, motor step size, belt characteristics, motor holding strength ?
Is this the sort of thing that mount design engineers analyze at design time ?

-- David F.

On Thu, Jun 4, 2020 at 3:09 PM uncarollo2 <chris1011@...> via groups.io <chris1011=aol.com@groups.io> wrote:

yes please! i would like to see

Ok, see below. Sorry for the poor quality screen shots. I took them with my camera aimed at the screen ;^)).

Top image is the raw periodic error of a typical microstepper driving a precision worm. You might think the worm is not great, but the reality is that the two pulleys have runout and produce the majority of the large scale periodic error. The worm itself actually has only about 2 arc sec error. The rest is caused by the commercial pulleys which have a fair amount of runout. This is basically what you get in almost all imported non-encoder mounts that use belts and microsteppers.

The plot is taken at 100msec sample rate so you can see what is really happening during a typical worm cycle. Normally when you see a PE curve, the sample rate is 10 times slower, so you don't see the fast moving errors on PE data sheets.

The real issue is not the large scale PE, which can be handled by guiding (as long as it is slow moving and not too large an excursion). It is the fast moving errors at the 1 arc sec level that cause problems for guiders. The second plot shows in detail that microsteppers with belts have two fast moving errors. The belt teeth cause a ripple every few seconds (in this case once every 12 seconds at approx 1 arc sec P-P). If your guiding cadence is fast enough you might be able to guide it out successfully. However, if your guide rate approaches once every 3 - 4 seconds, you will see a residual error that modulates with time.

The faster moving error repeats every 2 seconds with another 1 arc-sec P-P error. This one cannot be easily guided out because the guider is always a step behind and can actually amplify this motion. The error is caused by the stepper motor itself which has 5% angle variation for each winding step point. If your camera pixel scale is 2 - 4 arc sec per pixel, you may not see these kinds of errors. If your guider is exposing at 2 - 3 seconds, the guide graph will average out the tiny fast motions and will record perhaps very low rms values. However, the stars that you are recording on your imaging camera are doing this tiny back and forth dance in RA and slightly smearing the resolution of your image.

The third graph below shows how the absolute encoder tames all three errors.







Bill Long
 

Unguided imaging with SCT's is a big challenge, unrelated to the mount. A similar sized RC would have no issues.


From: main@ap-gto.groups.io <main@ap-gto.groups.io> on behalf of Seb@stro <sebastiendore1@...>
Sent: Thursday, June 4, 2020 8:40 PM
To: main@ap-gto.groups.io <main@ap-gto.groups.io>
Subject: Re: [ap-gto] NGC5371 and surrounding galaxies
 

Interesting! Thanks for the 101 course on mount design tradeoffs Rolando.

Just wondering up to how much focal length would the Mach2 be suitable unguided. Do you think it would do it at say 1400mm FL (8in Edge+FR)? What about 2000mm (no reducer)?

Any feedback from the lucky folks that already got their hands on the Mach2?

Sebastien


Roland Christen
 


just wondering up to how much focal length would the Mach2 be suitable unguided.
Focal length has nothing to do with how long to do unguided. Unguided depends on how well your model is made and how much drift you are willing to accept in your image. In the image that I posted the drift rate was approximately 5 arc sec per hour. Each 10 minute exposure had approximately 0.67 arc sec of drift, more or less. After stacking, the resulting stars had a value of .065 for flatness in MaximDL (see attachment below). Some of the exposures had more, some had less, and I simply stacked them all with Median Combine.

5 arc sec per hour might seem high, but the setup I used was purposely offset from the pole and had very high drift rate. Before modeling the RA drift was measured at 48 arc-sec/hr and the Dec was 96 arc-sec/hour. I would expect that one could get modeling down to +- 1 arc-sec/hour with good polar alignment.

Roland





-----Original Message-----
From: Seb@stro <sebastiendore1@...>
To: main@ap-gto.groups.io
Sent: Thu, Jun 4, 2020 10:40 pm
Subject: Re: [ap-gto] NGC5371 and surrounding galaxies

Interesting! Thanks for the 101 course on mount design tradeoffs Rolando.

Just wondering up to how much focal length would the Mach2 be suitable unguided. Do you think it would do it at say 1400mm FL (8in Edge+FR)? What about 2000mm (no reducer)?
Any feedback from the lucky folks that already got their hands on the Mach2?

Sebastien


 

Any idea how much of that drift rate is due to polar misalignment vs other sources of RA drift (atmosphere, etc.)


On Fri, Jun 5, 2020 at 9:43 AM uncarollo2 <chris1011@...> via groups.io <chris1011=aol.com@groups.io> wrote:

just wondering up to how much focal length would the Mach2 be suitable unguided.
Focal length has nothing to do with how long to do unguided. Unguided depends on how well your model is made and how much drift you are willing to accept in your image. In the image that I posted the drift rate was approximately 5 arc sec per hour. Each 10 minute exposure had approximately 0.67 arc sec of drift, more or less. After stacking, the resulting stars had a value of .065 for flatness in MaximDL (see attachment below). Some of the exposures had more, some had less, and I simply stacked them all with Median Combine.

5 arc sec per hour might seem high, but the setup I used was purposely offset from the pole and had very high drift rate. Before modeling the RA drift was measured at 48 arc-sec/hr and the Dec was 96 arc-sec/hour. I would expect that one could get modeling down to +- 1 arc-sec/hour with good polar alignment.

Roland





-----Original Message-----
From: Seb@stro <sebastiendore1@...>
To: main@ap-gto.groups.io
Sent: Thu, Jun 4, 2020 10:40 pm
Subject: Re: [ap-gto] NGC5371 and surrounding galaxies

Interesting! Thanks for the 101 course on mount design tradeoffs Rolando.

Just wondering up to how much focal length would the Mach2 be suitable unguided. Do you think it would do it at say 1400mm FL (8in Edge+FR)? What about 2000mm (no reducer)?
Any feedback from the lucky folks that already got their hands on the Mach2?

Sebastien



--
Brian 



Brian Valente


Sébastien Doré
 

Hi Bill,

Not sure how an RC would be that much easier than an SCT unguided. Could you elaborate ? Are you talking about focusing, mirror flop, thermal equilibrium issues, else ? Note that I'm not familiar at all with RCs. 

That said, Rolando is getting pretty impressive unguided results with its 160EDF F/7 which I suppose at 1120mm FL since its " just the 160 refractor straight to the 8300 chip". 

Which makes me wondering if a 1400mm scope would be within reach of the Mach2 unguided in the first place.

Sebastien



De : main@ap-gto.groups.io <main@ap-gto.groups.io> de la part de Bill Long <bill@...>
Envoyé : 5 juin 2020 11:24
À : main@ap-gto.groups.io <main@ap-gto.groups.io>
Objet : Re: [ap-gto] NGC5371 and surrounding galaxies
 
Unguided imaging with SCT's is a big challenge, unrelated to the mount. A similar sized RC would have no issues.


From: main@ap-gto.groups.io <main@ap-gto.groups.io> on behalf of Seb@stro <sebastiendore1@...>
Sent: Thursday, June 4, 2020 8:40 PM
To: main@ap-gto.groups.io <main@ap-gto.groups.io>
Subject: Re: [ap-gto] NGC5371 and surrounding galaxies
 

Interesting! Thanks for the 101 course on mount design tradeoffs Rolando.

Just wondering up to how much focal length would the Mach2 be suitable unguided. Do you think it would do it at say 1400mm FL (8in Edge+FR)? What about 2000mm (no reducer)?

Any feedback from the lucky folks that already got their hands on the Mach2?

Sebastien



Bill Long
 

The mirror on the SCT is usually the problem, or so I am told by those that have tested it out. I dont use SCT's. RC mirrors dont move though (neither do CDK mirrors) so those would be more suited to unguided imaging.

I shot unguided at 1720mm on the 1100 with AE and it worked wonderfully. I would not hesitate to do the same on the Mach 2 if I had one. That was on a iDK from AG Optical.


From: main@ap-gto.groups.io <main@ap-gto.groups.io> on behalf of Seb@stro <sebastiendore1@...>
Sent: Friday, June 5, 2020 9:45 AM
To: main@ap-gto.groups.io <main@ap-gto.groups.io>
Subject: Re: [ap-gto] NGC5371 and surrounding galaxies
 
Hi Bill,

Not sure how an RC would be that much easier than an SCT unguided. Could you elaborate ? Are you talking about focusing, mirror flop, thermal equilibrium issues, else ? Note that I'm not familiar at all with RCs. 

That said, Rolando is getting pretty impressive unguided results with its 160EDF F/7 which I suppose at 1120mm FL since its " just the 160 refractor straight to the 8300 chip". 

Which makes me wondering if a 1400mm scope would be within reach of the Mach2 unguided in the first place.

Sebastien



De : main@ap-gto.groups.io <main@ap-gto.groups.io> de la part de Bill Long <bill@...>
Envoyé : 5 juin 2020 11:24
À : main@ap-gto.groups.io <main@ap-gto.groups.io>
Objet : Re: [ap-gto] NGC5371 and surrounding galaxies
 
Unguided imaging with SCT's is a big challenge, unrelated to the mount. A similar sized RC would have no issues.


From: main@ap-gto.groups.io <main@ap-gto.groups.io> on behalf of Seb@stro <sebastiendore1@...>
Sent: Thursday, June 4, 2020 8:40 PM
To: main@ap-gto.groups.io <main@ap-gto.groups.io>
Subject: Re: [ap-gto] NGC5371 and surrounding galaxies
 

Interesting! Thanks for the 101 course on mount design tradeoffs Rolando.

Just wondering up to how much focal length would the Mach2 be suitable unguided. Do you think it would do it at say 1400mm FL (8in Edge+FR)? What about 2000mm (no reducer)?

Any feedback from the lucky folks that already got their hands on the Mach2?

Sebastien



Roland Christen
 

About 90% of that drift rate is due to polar misalignment. Normally one would want to polar align a mount to have a Dec drift of 1 arc-sec per 5 minutes, or better.
The whole idea was to see in the extreme what the tracking accuracy could be with a simple model. Obviously you would not want to set up a mount this poorly to do imaging. A good polar scope alone could get you 10 times closer to the pole.

Rolando



-----Original Message-----
From: Brian Valente <bvalente@...>
To: main@ap-gto.groups.io
Sent: Fri, Jun 5, 2020 11:47 am
Subject: Re: [ap-gto] NGC5371 and surrounding galaxies

Any idea how much of that drift rate is due to polar misalignment vs other sources of RA drift (atmosphere, etc.)

On Fri, Jun 5, 2020 at 9:43 AM uncarollo2 <chris1011@...> via groups.io <chris1011=aol.com@groups.io> wrote:

just wondering up to how much focal length would the Mach2 be suitable unguided.
Focal length has nothing to do with how long to do unguided. Unguided depends on how well your model is made and how much drift you are willing to accept in your image. In the image that I posted the drift rate was approximately 5 arc sec per hour. Each 10 minute exposure had approximately 0.67 arc sec of drift, more or less. After stacking, the resulting stars had a value of .065 for flatness in MaximDL (see attachment below). Some of the exposures had more, some had less, and I simply stacked them all with Median Combine.

5 arc sec per hour might seem high, but the setup I used was purposely offset from the pole and had very high drift rate. Before modeling the RA drift was measured at 48 arc-sec/hr and the Dec was 96 arc-sec/hour. I would expect that one could get modeling down to +- 1 arc-sec/hour with good polar alignment.

Roland





-----Original Message-----
From: Seb@stro <sebastiendore1@...>
To: main@ap-gto.groups.io
Sent: Thu, Jun 4, 2020 10:40 pm
Subject: Re: [ap-gto] NGC5371 and surrounding galaxies

Interesting! Thanks for the 101 course on mount design tradeoffs Rolando.

Just wondering up to how much focal length would the Mach2 be suitable unguided. Do you think it would do it at say 1400mm FL (8in Edge+FR)? What about 2000mm (no reducer)?
Any feedback from the lucky folks that already got their hands on the Mach2?

Sebastien


--
Brian 



Brian Valente


J. Belden
 

Exactly what I expected.  Like the difference between a real Rolex watch vs a knockoff, you can see the micro stepping if you look close enough.

Joe


Sébastien Doré
 

Focal length has nothing to do with how long to do unguided. Unguided depends on how well your model is made and how much drift you are willing to accept in your image. 

Rolando, I'm not sure I'm getting this right. I'm probably missing something about the modeling part here... Please enlighten me.

I thought the "amount of drift I'm willing to accept" was the amount of recorded signal (pixels) that is "offset" in the frame due to the mount's (in)ability to accurately track an object,(isn't it why we usually guide ?) which is also related to the image scale, which in turn relates to the focal length of the telescope...

So I'm not sure how two scopes with different focal lengths would lead to the same amount of drift in an unguided sub of the same exposure time (all other parameters being equal). Seems to me that the more focal length, the more drift (pixel offsets over time) I get, wether it's caused by PE (which I understand is not an issue with AE) or bad PA.

Sorry if my question seems silly, I'm a kind of "advanced newbie in astrophotography" as I like to call myself... 😉


Regards,

Sebastien


Roland Christen
 

I'm not into pixels, I'm more looking at allowable arc seconds. The skies some nights allow you to capture 3 arc sec FWHM, but on good nights you can get perhaps 1.5 arc sec FWHM. With a shorter scope you might only be able to get 4 arc sec FWHM on even the best of nights. So, the amount of allowable drift changes with the resolution capabilities of the scope and your local seeing conditions.

Rolando



-----Original Message-----
From: Seb@stro <sebastiendore1@...>
To: main@ap-gto.groups.io <main@ap-gto.groups.io>
Sent: Sat, Jun 6, 2020 10:30 am
Subject: Re: [ap-gto] NGC5371 and surrounding galaxies

Focal length has nothing to do with how long to do unguided. Unguided depends on how well your model is made and how much drift you are willing to accept in your image. 

Rolando, I'm not sure I'm getting this right. I'm probably missing something about the modeling part here... Please enlighten me.

I thought the "amount of drift I'm willing to accept" was the amount of recorded signal (pixels) that is "offset" in the frame due to the mount's (in)ability to accurately track an object,(isn't it why we usually guide ?) which is also related to the image scale, which in turn relates to the focal length of the telescope...

So I'm not sure how two scopes with different focal lengths would lead to the same amount of drift in an unguided sub of the same exposure time (all other parameters being equal). Seems to me that the more focal length, the more drift (pixel offsets over time) I get, wether it's caused by PE (which I understand is not an issue with AE) or bad PA.

Sorry if my question seems silly, I'm a kind of "advanced newbie in astrophotography" as I like to call myself... 😉


Regards,

Sebastien


Worsel
 

Sebastien

The amount of drift, measured in arc-seconds per unit time, will depend only on the mount. How that drift appears in an image will be a function of the scope and camera.

Bryn


REDIGER-LIZLOV Didier
 

Link works ans vert nice field.

Didier REDIGER-LIZLOV

Le 2 juin 2020 23:14, "uncarollo2 <chris1011@...> via groups.io" <chris1011@...> a écrit :
Hi Astronuts,

I joined AstroBin and posted my first image. Hope the link works:
https://www.astrobin.com/full/vsz056/0/?nc=user

This is an image taken with the Mach2 and 160EDF refractor, unguided over a period of 2 hours. I modeled a path that the object would take with 3 data points and let MaximDL take a set of images until the scope ran out of room and stopped at the limit. I'm shooting in very light polluted skies, the Moon was up and bright, so i was pleased that i was able to record some very faint stuff.

No guide scope was used, no field flattener, just the 160 refractor straight to the 8300 chip. Larger chips would need a flattener of course.

Rolando