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The
REAL question is WHY??
?
PHD2
guides to a fraction of a pixel *and* most everyone has seeing that is not good
enough to realize such an improvement in encoder resolution.. Add in the size
of the imaging chip, Nyquist /proper sampling)and..
?
WHY??
It¡¯s a semi rhetorical question, but there is also ¡°Why should I
not be happy with *my* 0.7¡± rms guiding?¡± i.e what am I missing
(aka teach me) that makes people want more.. What imaging theory points to
needing 0.2¡± rms guiding?? I mean I understand inventing, tinkering,
etc..
?
I
see people ¡°struggling¡± with the adjustment of spring loaded
mounts, changing encoder resolution, or swapping out 25:1 for 50:1 gearboxes..
Frankly I am *GLAD* I have my 2 piece worms.. My rms values are exceedingly
stable, my imaging scale is at 0.5¡±.. Even if I switch out my perfect
130mm refractor for my less than perfect 12¡± SCT to increase resolution,
why would my 0.7¡± guiding not be ¡°pretty darn good¡±??
?
I
don¡¯t mean to disparage anyone, I want to know what I am missing.. Whether
we are talking guiding or pointing, why isn¡¯t a 1/2 arcsecond sufficient,
especially considering seeing (environmental, as well as local)..
?
Derek
?
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From: [email protected] [mailto:[email protected]] On Behalf Of Michael Herman
Sent: Saturday, June 12, 2021 6:47
AM
To: [email protected]
Subject: Re: [Losmandy_users_io]
Changing the servo motor encoders from 256 to 512 or 1024. Any reason not to do
this?
?
Page 31 of the Gemini-1 users manual has a table. It says the G11
and? GM8 have 6400 steps per worm revolution (25 gearbox x 256 motor encoder
revolution).? There are 360 degrees per revolution and the G11 ring gear
has 360 teeth, so that's one degree per worm revolution.? So each degree
having 3600 arcsec per degree is divided by the steps per worm revolution that
is 6400.? ? So that's 3600/6400 arcsec per step = 0.56 arcsec per
step.??
As for reasons not to go with a higher motor encoder: the Gemini-1
clock is 1.5 MHz (per the G-1 manual).? I don't know the clock speed of a
G-2.? You might find the encoder detector chip unable to keep up with the
codes at a high slew rate....but that's only a factor of 2...so it might be
just fine.? It's worth a test.? Getting the motor encoder out is a
major soldering challenge.? Replacing the fragile encoder disk is easier
but really...these motors are hard to get apart and repairs of them are
rare.? (Brendan Smith and David Partridge are expert in this
subject.? I tried and had no success.)??
To double the step accuracy, you can also replace the 25:1 gearbox with
a 50:1 gearbox.? [I'm only familiar with the McLennan versions.? The
McLennan company didn't recommend using their 50:1 gearbox with a HiTorque
motor because it's shaft is ~3.2 mm and the small 50:1 gearbox pinion gear is
3.0 mm...they thought enlarging the pinion ID would get too close to the teeth.?
Alternatively, Stuart Hutchins recommended using the motor rotation and a file
to reduce the shaft OD to 3.0 mm to fit the pinion ID.? Stuart thinks this
is the best way to keep everything I have not tried that idea...the question is
what is best to modify. ].?
?
Here is my math:?(360*3600)/(256*25*360) = 0.5625 arc sec per
step.
This seems rather coarse. We have 2.3 million steps per rotation of the Ra
shaft. From talking with Sideral Technologies high end mounts have a greater
number of steps. If it is possible to change the encoder used in the motor this
could be increased to 4.6 or even 9.2 million steps. So 0.28 or 0.14 arc sec
per step. This I think should have a significant impact on improving guiding
performance, especially in the Dec axis, as right now the minimum correction is
0.56 arc sec. My imaging system is 0.76 arc sec per pixel. I would welcome any
thoughts. Perhaps Losmandy can comment on wether the controller will work with
a higher resolution encoder. They cost around $65 per axis to do the upgrade,
so well worth running a test I feel.
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Hi Derek,
You raise some good points. It is important to remember that RMS guiding error while important, is just one measurement that tells only part of the story. The ultimate goal is the best possible resolution of the image, and that depends on much more than just guiding.
Regards,
? ? -Paul
On Sun, Jun 13, 2021 at 01:52 PM, Derek C Breit wrote:
The REAL question is WHY??
?
PHD2 guides to a fraction of a pixel *and* most everyone has seeing that is not good enough to realize such an improvement in encoder resolution.. Add in the size of the imaging chip, Nyquist /proper sampling)and..
?
WHY?? It¡¯s a semi rhetorical question, but there is also ¡°Why should I not be happy with *my* 0.7¡± rms guiding?¡± i.e what am I missing (aka teach me) that makes people want more.. What imaging theory points to needing 0.2¡± rms guiding?? I mean I understand inventing, tinkering, etc..
?
I see people ¡°struggling¡± with the adjustment of spring loaded mounts, changing encoder resolution, or swapping out 25:1 for 50:1 gearboxes.. Frankly I am *GLAD* I have my 2 piece worms.. My rms values are exceedingly stable, my imaging scale is at 0.5¡±.. Even if I switch out my perfect 130mm refractor for my less than perfect 12¡± SCT to increase resolution, why would my 0.7¡± guiding not be ¡°pretty darn good¡±??
?
I don¡¯t mean to disparage anyone, I want to know what I am missing.. Whether we are talking guiding or pointing, why isn¡¯t a 1/2 arcsecond sufficient, especially considering seeing (environmental, as well as local)..
?
Derek
?
?
Page 31 of the Gemini-1 users manual has a table. It says the G11 and? GM8 have 6400 steps per worm revolution (25 gearbox x 256 motor encoder revolution).? There are 360 degrees per revolution and the G11 ring gear has 360 teeth, so that's one degree per worm revolution.? So each degree having 3600 arcsec per degree is divided by the steps per worm revolution that is 6400.? ? So that's 3600/6400 arcsec per step = 0.56 arcsec per step.??
As for reasons not to go with a higher motor encoder: the Gemini-1 clock is 1.5 MHz (per the G-1 manual).? I don't know the clock speed of a G-2.? You might find the encoder detector chip unable to keep up with the codes at a high slew rate....but that's only a factor of 2...so it might be just fine.? It's worth a test.? Getting the motor encoder out is a major soldering challenge.? Replacing the fragile encoder disk is easier but really...these motors are hard to get apart and repairs of them are rare.? (Brendan Smith and David Partridge are expert in this subject.? I tried and had no success.)??
To double the step accuracy, you can also replace the 25:1 gearbox with a 50:1 gearbox.? [I'm only familiar with the McLennan versions.? The McLennan company didn't recommend using their 50:1 gearbox with a HiTorque motor because it's shaft is ~3.2 mm and the small 50:1 gearbox pinion gear is 3.0 mm...they thought enlarging the pinion ID would get too close to the teeth.? Alternatively, Stuart Hutchins recommended using the motor rotation and a file to reduce the shaft OD to 3.0 mm to fit the pinion ID.? Stuart thinks this is the best way to keep everything I have not tried that idea...the question is what is best to modify. ].?
?
Here is my math:?(360*3600)/(256*25*360) = 0.5625 arc sec per step.
This seems rather coarse. We have 2.3 million steps per rotation of the Ra shaft. From talking with Sideral Technologies high end mounts have a greater number of steps. If it is possible to change the encoder used in the motor this could be increased to 4.6 or even 9.2 million steps. So 0.28 or 0.14 arc sec per step. This I think should have a significant impact on improving guiding performance, especially in the Dec axis, as right now the minimum correction is 0.56 arc sec. My imaging system is 0.76 arc sec per pixel. I would welcome any thoughts. Perhaps Losmandy can comment on wether the controller will work with a higher resolution encoder. They cost around $65 per axis to do the upgrade, so well worth running a test I feel.
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The Dawes limit for a 130 mm telescope at say, 6000 Angstrom is 6e-7/130e-3=4e-6 radians or 4e-6*180/pi=240e-6 degrees or 240e-6*3600=1".? An RMS of 0.7" would be fine for that.? A 12"=300 mm SCT has a Dawes limit of 130/300=0.25 of the above which is 0.25" so 0.7 is not good enough.
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Derek et al
If you live in anywhere but space, your viewing with air turbulence is about 1" at best. I'd say of all the parameters, the air turbulence limits is far more than mount accuracy. What say you?
Chuck
On Sunday, June 13, 2021, 12:30:09 PM PDT, Paul Kanevsky <yh@...> wrote:
Hi Derek,
You raise some good points. It is important to remember that RMS guiding error while important, is just one measurement that tells only part of the story. The ultimate goal is the best possible resolution of the image, and that depends on much more than just guiding.
Regards,
? ? -Paul
On Sun, Jun 13, 2021 at 01:52 PM, Derek C Breit wrote: The REAL question is WHY?? ? PHD2 guides to a fraction of a pixel *and* most everyone has seeing that is not good enough to realize such an improvement in encoder resolution.. Add in the size of the imaging chip, Nyquist /proper sampling)and.. ? WHY?? It¡¯s a semi rhetorical question, but there is also ¡°Why should I not be happy with *my* 0.7¡± rms guiding?¡± i.e what am I missing (aka teach me) that makes people want more.. What imaging theory points to needing 0.2¡± rms guiding?? I mean I understand inventing, tinkering, etc.. ? I see people ¡°struggling¡± with the adjustment of spring loaded mounts, changing encoder resolution, or swapping out 25:1 for 50:1 gearboxes.. Frankly I am *GLAD* I have my 2 piece worms.. My rms values are exceedingly stable, my imaging scale is at 0.5¡±.. Even if I switch out my perfect 130mm refractor for my less than perfect 12¡± SCT to increase resolution, why would my 0.7¡± guiding not be ¡°pretty darn good¡±?? ? I don¡¯t mean to disparage anyone, I want to know what I am missing.. Whether we are talking guiding or pointing, why isn¡¯t a 1/2 arcsecond sufficient, especially considering seeing (environmental, as well as local).. ? Derek ? ? Page 31 of the Gemini-1 users manual has a table. It says the G11 and? GM8 have 6400 steps per worm revolution (25 gearbox x 256 motor encoder revolution).? There are 360 degrees per revolution and the G11 ring gear has 360 teeth, so that's one degree per worm revolution.? So each degree having 3600 arcsec per degree is divided by the steps per worm revolution that is 6400.? ? So that's 3600/6400 arcsec per step = 0.56 arcsec per step.?? As for reasons not to go with a higher motor encoder: the Gemini-1 clock is 1.5 MHz (per the G-1 manual).? I don't know the clock speed of a G-2.? You might find the encoder detector chip unable to keep up with the codes at a high slew rate....but that's only a factor of 2...so it might be just fine.? It's worth a test.? Getting the motor encoder out is a major soldering challenge.? Replacing the fragile encoder disk is easier but really...these motors are hard to get apart and repairs of them are rare.? (Brendan Smith and David Partridge are expert in this subject.? I tried and had no success.)?? To double the step accuracy, you can also replace the 25:1 gearbox with a 50:1 gearbox.? [I'm only familiar with the McLennan versions.? The McLennan company didn't recommend using their 50:1 gearbox with a HiTorque motor because it's shaft is ~3.2 mm and the small 50:1 gearbox pinion gear is 3.0 mm...they thought enlarging the pinion ID would get too close to the teeth.? Alternatively, Stuart Hutchins recommended using the motor rotation and a file to reduce the shaft OD to 3.0 mm to fit the pinion ID.? Stuart thinks this is the best way to keep everything I have not tried that idea...the question is what is best to modify. ].? ? Here is my math:?(360*3600)/(256*25*360) = 0.5625 arc sec per step.
This seems rather coarse. We have 2.3 million steps per rotation of the Ra shaft. From talking with Sideral Technologies high end mounts have a greater number of steps. If it is possible to change the encoder used in the motor this could be increased to 4.6 or even 9.2 million steps. So 0.28 or 0.14 arc sec per step. This I think should have a significant impact on improving guiding performance, especially in the Dec axis, as right now the minimum correction is 0.56 arc sec. My imaging system is 0.76 arc sec per pixel. I would welcome any thoughts. Perhaps Losmandy can comment on wether the controller will work with a higher resolution encoder. They cost around $65 per axis to do the upgrade, so well worth running a test I feel.
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As a rule of thumb, without adaptive optics you will not get better resolution for apertures larger than 8".? So for visual, a 6" frac will be as good as a 12" SCT for almost every day of the year.? Of course resolution is not everything, light flux matters too.? For planetary observing imaging at 120+ Hz we depend on lucky seeing so the stacker picks the frames for which the Dawes limit matters.? So yes, it matters.
PS I made a mistake, 130/30 is not 0.25 but more like 0.4 but the conclusion remains that 0.7 is not good enough.
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Still a beginner, but I thought I'd give my take
To verify the quality of my guiding, I like to do the following test.? I try to get the polar alignment as accurate as possible and disable guiding (i.e. guiding assistant in PHD2).? I minimize dec drift as much as possible to simulate what a "perfect" guided sub would look like.? If I take that dec RMS error, and assume I got the same error in RA, I can calculate a reasonable limit on guiding performance based on seeing conditions.
With my skies, I often see numbers like 0.5" on dec.? This would result in a total RMS error of 0.7" if I assumed the RA error was the same.? The RA axis is never going to be that accurately guided, so for more realistic numbers I assume my RA is about 1.5x worse (based on experience) resulting in a total RMS error of about 0.9" in this example.
I am guessing that a lot of people are hitting their seeing limits like me.? One day I'll get a chance to verify what guiding performance looks like with a mount with absolute encoders, but my guess is that I will only get closer to approaching RA=dec error.
Lately, I am finding my subs get ruined most by wind...
Thanks,
Dwight
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The answer might be location, location, location....
With east coast light pollution and the generally poor seeing from suburbia, the best I am getting is about 0.5" guiding around the zenith. Down at 30 degrees altitude about 1.0 guiding is average on a good night.? My 9.25 inch SCT is probably the most aperture local conditions will support. An 11" might be useable some of the time, but a 14" would probably be a fruitless effort for deep sky imaging. So even if I had super accurate mount, the air currents would probably bloat and distort the longer focal lengths and the cost of equipment wouldn't be worth it.?
Now if I had some Bortle 3 or less skies at some desert location with stable air, then the extra precision and investment in bigger and better equipment might be justified. And nearest dark sky site is about 4 hours away, so not a road trip that can be done on a regular basis with the larger equipment.
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If you¡¯re imaging at 120 Hz, or even at 10 Hz, isn¡¯t the guiding immaterial? ?Guiding doesn¡¯t happen anywhere near that fast; any drift should be handled by the registration before stacking. ?For that matter, at 120 Hz it¡¯s hardly necessary to even track, except to keep the target from drifting out of the frame. ?
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On 13 Jun 2021, at 13:50, Henk Aling < haling@...> wrote:
As a rule of thumb, without adaptive optics you will not get better resolution for apertures larger than 8".? So for visual, a 6" frac will be as good as a 12" SCT for almost every day of the year.? Of course resolution is not everything, light flux matters too.? For planetary observing imaging at 120+ Hz we depend on lucky seeing so the stacker picks the frames for which the Dawes limit matters.? So yes, it matters.
PS I made a mistake, 130/30 is not 0.25 but more like 0.4 but the conclusion remains that 0.7 is not good enough.
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I assume that most who are trying to fine-tune guiding are not using AO. As you say, there's little need to improve guiding if AO is making the corrections. Re-centering the star when it's getting out of frame doesn't have to be very precise, just close enough. The AO will take care of making precise corrections after the guider bump.
Regards,
? ? ?-Paul
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On Mon, Jun 14, 2021 at 02:40 AM, Les Niles wrote:
If you¡¯re imaging at 120 Hz, or even at 10 Hz, isn¡¯t the guiding immaterial? ?Guiding doesn¡¯t happen anywhere near that fast; any drift should be handled by the registration before stacking. ?For that matter, at 120 Hz it¡¯s hardly necessary to even track, except to keep the target from drifting out of the frame. ?
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At some point if I decide to up my game, I may try an AO device. That seems to be somewhat of a silver bullet, and maybe cheaper than getting a higher end mount. For now I'm have to say that I'm really happy with my GM811G. I've had it for a couple of years, and it's taken me a while to learn how to adjust it, but I tend to get very reliable results over long periods without doing any adjustments. I just take it outside, setup and start imaging. It seems that the worm might need adjusting once in spring and once in fall. I tend to get results that range from 1.1 to 0.4 RMS, and that suits me just fine. When multi-star came out I saw results below 0.4 RMS for the first time. I was worried that it was just a fluke. I'm guessing that it only happens when seeing is good, but I've seen it fairly regularly over the last few months. The last few nights of imaging I was getting guiding around 0.5 RMS with it dipping into the low 0.4's. I only get obsessed with tweaking things for performance when I'm imaging small things like the cat's eye which I was doing last night. -- and I didn't have to tweak a single thing, it just worked!?
An AO device may be what I do for a future upgrade if I want to get better images with the small objects. I'm also wondering how sensitive cameras will get over time. Maybe subs will get shorter and shorter making all this work tweaking the mechanics obsolete?
Jamie
?
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I always wonder about AO devices.? I see them for the old CCD cameras for sale at around $1000.? I thought by now someone would have figured a way to integrate them for any modern camera setup. Writing an ascom driver or something.? And why hasn't some manufacture come up with a mainstream AO device??
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Starlight Xpress has a 'mainstream' AO unit
Adaptive Optics is still complicated because it's essentially guiding PLUS a bunch of other stuff. and you have to worry about backfocus?distance, parfocal for the guide camera, etc.
It would be great if there was an image stabilization type thing built into the camera like we see in mirrorless with IBIS
On Mon, Jun 14, 2021 at 12:24 PM David Malanick < malanick@...> wrote: I always wonder about AO devices.? I see them for the old CCD cameras for sale at around $1000.? I thought by now someone would have figured a way to integrate them for any modern camera setup. Writing an ascom driver or something.? And why hasn't some manufacture come up with a mainstream AO device??
-- Brian?
Brian Valente portfolio
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Paul Kanevsky
Jamie Amendolagine
David Malanick