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How often do you need to polar align a permanent mount?
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Daniel Marcus
Just curious, about polar alignment. How often do you need to repolar align a mount if it is permanent - assuming no frost heaving or ground moving? How many arc sec does the NCP move each year, and how often do we need to correct for it?? Am I correct NCP moves about 5arc sec per year, or is it more like 18 arc sec per year?? So what is "good enough" for 10 min unguided in Dec with a 2000mm focal length??
Dan Marcus |
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Really depends on your local conditions. I would normally check polar alignment once a year. If you have an autoguider, autoguiding logs will tell you if your alignment is off. If you are running unguided then the quality if your images will likely tell you when you need to re-check your polar alignment, assuming everything else in your mount and optics is rigid and stable. On Tue, Nov 3, 2020, 3:52 PM Daniel Marcus <DMa3141551@...> wrote: Just curious, about polar alignment. How often do you need to repolar align a mount if it is permanent - assuming no frost heaving or ground moving? How many arc sec does the NCP move each year, and how often do we need to correct for it?? Am I correct NCP moves about 5arc sec per year, or is it more like 18 arc sec per year?? So what is "good enough" for 10 min unguided in Dec with a 2000mm focal length?? |
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Daniel Marcus
Now you have my curiosity piqued. How much does plate tectonics affect scopes on the Big Island? We have frost heaving, so yah late spring and after a major drought I normally check especially now that polemaster and sharpcap are easily used.
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david w pearson
Not a clear cut answer to your question, as too many options that can be used to minimize polar alignment error affects. I had a scope permanently mounted scope working at 1700mm with no encoders, but using PEC for 3 years and never had to realign after initially aligning to slightly less than 2 arc minutes (Even had a small earthquake). >Using PemPro or any other drift alignment process makes it fairly easy to achieve less than 2 arc-minutes. But i could never do a 10 minute unguided exposure using Maxim DL, more like a max of 5 minutes. I couldn't achieve 10min unguided until i did a pointing model with APCC. And that doesn't last forever, meaning you have to periodically redo a model more than once a year. i normally auto guide, so I am not as sensitive to using an old pointing model, so that is the reason i don't update the pointing model very frequently. I have had one of the authors of PHD say that they prefer that the polar alignment is offset by like 8 arc-minutes. Because if makes it easier to measure and compensate for a DEC polar error. Hope that helps dave |
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Plate tectonics don't effect us in Hawaii but our local volcanoes sure do. Earthquakes, swelling, etc. The professional Observatories all actively monitor seismic activity under their telescopes. The biggest scopes here (Keck 1&2, Gemini North & Subaru) are all Alt-Az so aren't concerned with polar alignments. UH88, CFHT & IRTF monitor their polar alignments constantly. Most of the rest are Alt-Az. They monitor pointing changes that result from any source, mechanical or geological. On Tue, Nov 3, 2020, 5:12 PM Daniel Marcus <DMa3141551@...> wrote:
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vk3cjk
Here in Australia we had a Mag 5.4 earthquake that very slightly altered my polar alignment. Apart from that, it's not something that I need to worry about too much. Cheers, Chris |
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>>>" I have had one of the authors of PHD say that they prefer that the polar alignment is offset by like 8 arc-minutes. Because if makes it easier to measure and compensate for a DEC polar error." if you were experiencing significant dec backlash and their recommendation was to guide in only one direction, this offset PA advice makes sense. otherwise the more precise the PA the better for guiding. (I was a little surprised by this comment so i checked with the authors, and they confirmed my response)
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I just chatted with them just now, and they wanted to clarify they don't necessarily agree with a blanket statement "the more precise the PA the better" Polar alignment error is only a problem when it creates image rotation and that’s pretty rare except for very wide field imaging. That’s not really the same as saying “the more precise the PA the better” unless high accuracy just falls out from using PoleMaster or something. Or unless you’re trying to run unguided, in which case it matters a lot. Brian
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I have my 1200 on a concrete pier with 2 foot footings, rebar. Polar alignment periodically changes for reasons I am not sure off...it even could be small earth quakes here. I use Sharpcap Pro to polar align. I can tell its off when stars start moving with longer exposures. I probably need to adjust every few weeks.
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davidcfinch9
I had a concrete pier with eight foot footings. Polar alignment changed due to ground shifting (finally settled after about a year) and concrete differential shrinkage (continued for about four years). The concrete shrinkage was the same effect observed in many observatories-including the Hale 200”. David Finch
From: main@ap-gto.groups.io <main@ap-gto.groups.io> On Behalf Of Peter Bresler via groups.io
Sent: Wednesday, November 4, 2020 11:58 AM To: main@ap-gto.groups.io Subject: Re: [ap-gto] How often do you need to polar align a permanent mount?
I have my 1200 on a concrete pier with 2 foot footings, rebar. Polar alignment periodically changes for reasons I am not sure off...it even could be small earth quakes here. I use Sharpcap Pro to polar align. I can tell its off when stars start moving with longer exposures. I probably need to adjust every few weeks. |
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CurtisC <calypte@...>
Picking up from Mr. Erickson's post -- Palomar Mountain is bounded by major faults on both the northeastern and southwestern sides of the block, and it is, of course, pretty close to the San Andreas and San Jacinto Fault Zones. The foundation of the 200-inch has adjustments for polar alignment, but -- to my knowledge -- it has never been readjusted due to effects from an earthquake. Nobody currently on staff remembers any such realignment. The mm 7.2 Cucapah Earthquake in Baja Calif. in 2010 was strong enough to slosh water in the observatory's water tanks, but it didn't affect any of the telescopes.
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Dale Ghent
I think for the typical backyard observatory, it's going to come down to local soil type, local soil hydraulic nature, and weather more than earthquakes and faults. Even if the footer extends below the frost line, the ground swells and shrinks with seasonal changes in the water table and ground saturation.
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On Nov 4, 2020, at 11:14 PM, CurtisC via groups.io <calypte@...> wrote: |
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Mike Shade
In SE Arizona at 5000' elevation, where we can have a pretty good heat cycle from day to night, I check and adjust polar alignment if needed once a year. If there are any adjustments needed, they are small. But remember metal can sag, fatigue, heat cycle, bolts can loosen up, fasteners can back off and so on. I also go over all the fasteners on the OTA and make sure they are snug. Generally in September after monsoon is over and prior to starting the new season.
Mike J. Shade Mike J. Shade Photography:
In War: Resolution In Defeat: Defiance In Victory: Magnanimity In Peace: Goodwill Sir Winston Churchill Already, in the gathering dusk, a few of the stars are turning on their lights. Vega, the brightest one, is now dropping towards the west. Can it be half a year since I watched her April rising in the east? Low in the southwest Antares blinks a sad farwell to fall... Leslie Peltier, Starlight Nights
International Dark Sky Association: www.darksky.org
From:
main@ap-gto.groups.io [mailto:main@ap-gto.groups.io] On Behalf Of CurtisC
via groups.io
Picking up from Mr. Erickson's post -- Palomar Mountain is bounded by major faults on both the northeastern and southwestern sides of the block, and it is, of course, pretty close to the San Andreas and San Jacinto Fault Zones. The foundation of the 200-inch has adjustments for polar alignment, but -- to my knowledge -- it has never been readjusted due to effects from an earthquake. Nobody currently on staff remembers any such realignment. The mm 7.2 Cucapah Earthquake in Baja Calif. in 2010 was strong enough to slosh water in the observatory's water tanks, but it didn't affect any of the telescopes. |
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Bruce Donzanti
I have a permanent setup in my Florida observatory- lots of sand base. Cement pier goes from 4 feet below ground up 10 ft above ground where an adjustable steel pier is on top of it. Then my AP1100 mount where my C11" EdgeHD sits. Checking my polar alignment over the past 2 years, I've only had to adjust it 4 times using SharpCap. Even at that, it has never been way off. |
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My 900GTO 10" Newtonian is on a permanent AP 8" portable pier on three
14?" square stepping stones in vegetable garden with a 365 Telegizmos cover.
I attribute my changing polar alignment to moisture in soil.
If your soil has a high clay content, when dried out it will contract and leave cracks so that soil moves around.
But I'm lucky to be a maintenance man and enjoy minimizing polar alignment and orthogonality error.
Jimmy
Inside my head I have a good wolf and a bad wolf.
I was brought up to feed and nourish the good wolf.
I think everyone has a choice
as long as they realize they have a choice.
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George
To stabilize a concrete pier you need to run the concrete pier down below the worst case frost line and attach it to a footing. The pier is not a footing, it is a pier and it will move all around. A footing is an enlarged slab of concrete with rebar running into the pier. It is the only thing that stabilizes a pier.
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My concrete pier runs down 5' to a 4' x 4' footing that is 10" thick (with rebar). There are some single pour pier footings that are heavy gauge plastic to which you add a sonotube. Regards, George George Whitney Astro-Physics, Inc. Phone: 815-222-6538 (direct line) Phone: 815-282-1513 (office) Email: george@... -----Original Message-----
From: main@ap-gto.groups.io <main@ap-gto.groups.io> On Behalf Of Dale Ghent Sent: Thursday, November 5, 2020 6:57 AM To: main@ap-gto.groups.io Subject: Re: [ap-gto] How often do you need to polar align a permanent mount? I think for the typical backyard observatory, it's going to come down to local soil type, local soil hydraulic nature, and weather more than earthquakes and faults. Even if the footer extends below the frost line, the ground swells and shrinks with seasonal changes in the water table and ground saturation. On Nov 4, 2020, at 11:14 PM, CurtisC via groups.io <calypte@...> wrote: |
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I have a losmandy 3 legged stand in my dome with Mach1 on it. I might check PA maybe once a year.  On Thu, Nov 5, 2020 at 8:36 AM Bruce Donzanti <donza2735@...> wrote:
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Ron Kramer |
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