Polar alignment

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Piet Le Roux
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Polar alignment

#1

Post by Piet Le Roux » Mon Dec 02, 2019 6:41 am

I have a fork mounted 8" Meade LX90ACF and although I am competent in setting it up as a azimuth goto mount I want to become competent in setting it up as a polar mount, using its wedge, because I want to do some AP . I live on the south side so no pole star to assist me. The way I see it : 2 points that you have to get 100% is true South and your latitude angle which basically means your polar home position has to be near perfect. The next step is to check it and it appears that the only way to do that is with a drift test, remember I am in the south, so I have to learn to do that for the southern hemisphere. After I have accomplished this I must learn to do PEC training. I would like to hear from the accomplished AP guys as to how they would go about setting up at a unfamiliar star party site. I was thinking about placing a marker about 100m away exactly south to help me and placing cement blocks under my tripod legs to mark them and keep the legs stable. Any recommendations would be welcome.
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Re: Polar alignment

#2

Post by OzEclipse » Mon Dec 02, 2019 2:02 pm

Hi Piet,
My method relies on using a bright green laser. This may not be appreciated at a star party although it the procedure only takes 4-5 mins and can be performed during late twilight before AP or serious dark sky observing begins. You also need a small pair of handheld binoculars - 25-35mm diameter. Most important thing is that they are small and light enough to be easily hand held in one hand while you adjust the laser in the other. It also relies on having a polar finder. You may be able to fit a polar finder to one of the fork arms. The finder can be very small because you only need to be able to see a bright laser once the laser is pointed at the pole.

The laser is attached to a fixed part of the mount not too far from the polar axis to minimise parallax. I attach mine to the top of the south leg.

To find the pole stars in the small binoculars, I usually start at the SMC and 47 Tuc. Jump to beta Hyd then to three stars, then to the sigma Oct trapezium see diagrams below. These stars are circumpolar for me and will also be for you at Bloemfontein. After you do it a few times, you'll find it gets easier each time. before long you can do it in your sleep. There are two stars, mags 6.8 and 7.8 that are very close to the pole. They are faint but visible in 25-35mm binoculars and if you place the beam between them and offset a little as shown in the diagram, you will be within a couple of minutes of the pole.
pole-stars.jpg
pole-stars-closeup.jpg
Once the laser is pointed, to just have to point the polar finder to the laser. I usually lower the polar axis elevation down below the pole, sweep the mount across until I see the beam in the finder then elevate the axis to the point of the beam.

The only problem for you is that you need to fit a polar finder. Once fitted, spend one night doing an accurate drift alignment and once aligned, set and lock the polar finder in position. If the finder is to be removable, you need to have a positive exact locating connection like a three pin registration plate.

Over 40 years in astronomy, I have used many methods of polar alignment, drift aligning, magnetic correction applied to compass and clinometer. This is by far the quickest and easiest. If the engineering & machining required to fit a polar finder is beyond your skills, you may wish to look at the polar master or sharp star computer assisted. But this method is quicker and at least as accurate.

Cheers

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Re: Polar alignment

#3

Post by Piet Le Roux » Mon Dec 02, 2019 3:12 pm

The use of green lasers is , at this stage, accepted at my club so no problems there. A local Chinese importer had a whole selection of these lasers and I bought a couple of them.
here is a photo of my wedge/tripod south facing side.
wedge 002s.jpg
My leg is facing North but I don't see a problem mounting a adjustable laser mount on the tripod. But a polar scope is not a option. What if I set the laser on the polar South, the scope in its home position and then use my East/West fine adjuster and my wedge angle adjustment to fine tune it while I look through the telescope with a illuminated Astrometric eyepiece? I have a right angle spotting scope that I can mount on the scope would that not work?
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Re: Polar alignment

#4

Post by Star Dad » Mon Dec 02, 2019 3:56 pm

You mentioned AP, but I'm not sure in what context. AP requires accurate polar alignment - the better it is the more round your stars - less streaking. The Polemaster has a set up routine for southern hemisphere folks. I'd suggest checking it out. It does, however, require (USB) power, which may not be available in the field. Also the Polemaster is made to fit inside a polar scope hole, which I don't think your mount has.

I use "water-proof" duct tape to mark where my tripod legs go on my asphalt driveway. I've begun to notice that the legs (which have a nub on the bottom) are beginning to make an indentation in the asphalt - so my accuracy in placing the mount on the tape is really good, I guess. I have to replace the tape ~ every 4 months or so, but it is red and glossy and shows up well using a red flashlight. Also the tape leaves a white residue when it is worn out which makes placing replacement tape easy to do.

Hope this helps!
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Re: Polar alignment

#5

Post by Piet Le Roux » Mon Dec 02, 2019 5:38 pm

Thanks for the feedback guys. I think Joe's idea is going to work for me but I am going to mount the laser on the wedge plate, that is 90 degrees to polar south, not on the tripod base. The laser I am going to use is designed for a gun sight and has internal adjustment for x and y. It will be able to shine on the polar south when the scope is in polar home position. The tilt of the wedge is adjustable and so is the east/west fine adjuster. So I would set the tripod up do a ruff adjustment on polar with the laser then mount the scope check it again using the fine adjustments, lock the plate and wedge nuts. One of the problems with a fork mount on a wedge is to get the OTA perfectly 90 degrees with its base plate, checking the polar setup through the spotting scope or the scope itself at the end should solve that problem. It will, take some time to get the laser properly aligned but I think its doable.
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Re: Polar alignment

#6

Post by SkyHiker » Mon Dec 02, 2019 6:07 pm

Here's another suggestion that requires a DSLR with an LCD display and a piece of clear tape with a pinhole through it.

The idea is to first find the scope's RA axis on the LCD using an RA slew with the DSLR at 6400 ISO/30seconds, then marking it on the LCD display by putting the pinhole at the center of the circular RA traces. Then, adjust the Alt/Az controls of the wedge to align the pinhole with the SCP (that is actually easier than with the NCP because we don't have as many bright stars near the NCP) taking 3 second pictures iteratively. We assume that you have identified a asterism near the SCP that will help you eyeball the SCP location - should not be too hard. It's not much different than what the electronic polar alignment gadgets do, just without a computer.

If you try this you will probably find that the hardest part is to get the center of the circular tracks of the RA slew within the LCD screen. This will take a bit of cursing and trying but I can be done and practice helps.
... Henk. :D Telescopes: 6" Mak-Newt (Comet Hunter), ES ED127CF, ES ED80, Zhumell Z12, Coulter Odyssey 10, AT6RC, Venture RX-7, Celestron Skymaster 20x80, Mounts and tripod: Losmandy G11S, AVX, LXD55, Tiltall, Cameras: Fuji X-a1, Canon SX40, Xt, XSi, T6, ELPH 100HS, DIY: Dob and camera barndoor trackers, afocal adapter, Dob with foldable base and Az/Alt setting circles, Accessories: SSAG, Plossls, Barlows, Telrad, laser collimators (Seben LK1, Z12, Howie Glatter), Cheshire, 2 Orion RACIs 8x50, Software: DSS, ImageMagick, PHD, Nebulosity, Photo Gallery, Gimp, CHDK
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Re: Polar alignment

#7

Post by Gulf Coast Guy » Mon Dec 02, 2019 6:31 pm

I use SharpCap's Polar alignment routine.
https://www.sharpcap.co.uk/sharpcap/fea ... -alignment
Works North or South. You just need to get within ~5° of either pole and follow the on-screen instructions.

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Re: Polar alignment

#8

Post by Piet Le Roux » Mon Dec 02, 2019 8:57 pm

Ok mounting the laser is not going to be a problem...The scope is in polar home and the laser has a clear view of the celestial pole.
laser 001s.jpg
hope the weather clears so I can start testing!
Main Equipment : 15" Obsession F4.5 Classic, Tele Vue 1.25" 7&13mm Nagler, Tele Vue 2" 27mm Panoptic, Tele Vue Big Barlow, Tele Vue Paracorr II : 8" Meade LX90ACF with Meade 2.0" Enhanced Diagonal, Baarder Hyperion MK III 24-8mm zoom
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Re: Polar alignment

#9

Post by Piet Le Roux » Mon Dec 02, 2019 10:04 pm

I had a look at my old equatorial mount and the polar scope is mounted at your latitude angle and when it faces the celestial pole your mount is aligned. The polar scope is independent of any Ra and Dec movement. I have mounted my laser in the same way. So if my laser is aligned with my mount and is pointing exactly at the celestial pole, and my OTA is also pointing to the celestial pole, I can begin my two star alignment and I should be good to go? So Joe my conclusion is if you replace your polar scope with a laser you don't need a polar scope ?
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Re: Polar alignment

#10

Post by OzEclipse » Tue Dec 03, 2019 3:03 am

Piet Le Roux wrote:
Mon Dec 02, 2019 10:04 pm
I had a look at my old equatorial mount and the polar scope is mounted at your latitude angle and when it faces the celestial pole your mount is aligned. The polar scope is independent of any Ra and Dec movement. I have mounted my laser in the same way. So if my laser is aligned with my mount and is pointing exactly at the celestial pole, and my OTA is also pointing to the celestial pole, I can begin my two star alignment and I should be good to go? So Joe my conclusion is if you replace your polar scope with a laser you don't need a polar scope ?
Hi Piet,

I am glad you seem to have solved your problem. The method you are using is different to mine but yes, if the laser moves in ALT AZ with the polar axis and is precisely aligned then no you don't need a polar scope.

In my method, it is essential that the laser be attached to a stationary part of the mount. The laser is trained on the pole and then the mount pointed at the bright laser spot on the sky. I did it this way because I already have a precisely aligned polar scope but because of the mount weight, it's difficult to scan the loaded mount around and work out where I am pointing. The laser gives me a beam and an end point that are easy to fine with the mount

It's great that your scope has polar home position. This is effectively a polar scope so you don't need a separate one as I suggested. My technique does not work if the laser moves as you adjust the polar axis direction.

By attaching the laser to the wedge, the artificial pole star moves with the mount. Does it only move in azimuth or in elevation as well? I cannot tell from your picture.

If the laser moves in ALT and AZ with the polar axis then this is a defacto polar scope.
1 set the scope to polar home position
2 adjust the laser to the centre of the field of the scope
3 use handheld binoculars in one hand or the telescope finder to find the pole stars.
4 Point the laser at the 7th mag pole star and fine tune through the main scope

This method relies on the scope being able to reproducibly and accurately point at the mounts polar axis position. You can check this by pointing the scope polar axis at a distant object(even a close one would be ok since there is zero parallax), put the mount into polar home and slew the RA and see if the mount is pointing at the centre of rotation.

Joe
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Re: Polar alignment

#11

Post by Piet Le Roux » Tue Dec 03, 2019 8:50 am

The binoculars in hand while pointing a laser routine is something I am use to : My Dobson is a treat to use since I fitted a green laser on it but when looking for dim objects I first get it in view with binoculars and then get the laser in view.
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Re: Polar alignment

#12

Post by SkyHiker » Tue Dec 03, 2019 10:56 am

With all respect, I doubt if this will work Joe. Your step 2 is difficult to implement and depends on uncertain assumptions. First, can we assume that the mechanical fork axis is exactly perpendicular to its base? Secondly, if that is the case, can we assume that the optical axis is aligned with the fork's mechanical axis? Third, can you align the laser exactly to the center of the scope's FOV? Each of these three requirements must be accurate to about 1'. I think everything combined with your method this can easily be off by a full degree, and it is a lot of unnecessary alignment work.

This is why I advocate methods that use an RA slew to determine where the RA axis is pointing relative to the Az axis, then adjust the Az axis using the AltAz controls so this point coincides with the SCP. By doing that, none of your assumptions are needed and any inaccuracies resulting from implementing these assumptions, are avoided. By locating the center of RA rotation on a digital camera then moving that point to the SCP with the AltAz controls, you can achieve 1' accuracy directly. The Polemaster, Sharpcap and the computer-less method that I proposed will all do that. The use of a camera makes all of the intermediate alignment steps that you require, obsolete, and makes it possible to do PA to within 1' quickly.
... Henk. :D Telescopes: 6" Mak-Newt (Comet Hunter), ES ED127CF, ES ED80, Zhumell Z12, Coulter Odyssey 10, AT6RC, Venture RX-7, Celestron Skymaster 20x80, Mounts and tripod: Losmandy G11S, AVX, LXD55, Tiltall, Cameras: Fuji X-a1, Canon SX40, Xt, XSi, T6, ELPH 100HS, DIY: Dob and camera barndoor trackers, afocal adapter, Dob with foldable base and Az/Alt setting circles, Accessories: SSAG, Plossls, Barlows, Telrad, laser collimators (Seben LK1, Z12, Howie Glatter), Cheshire, 2 Orion RACIs 8x50, Software: DSS, ImageMagick, PHD, Nebulosity, Photo Gallery, Gimp, CHDK
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Re: Polar alignment

#13

Post by Piet Le Roux » Tue Dec 03, 2019 11:54 am

SkyHiker wrote:
Tue Dec 03, 2019 10:56 am
With all respect, I doubt if this will work Joe. Your step 2 is difficult to implement and depends on uncertain assumptions. First, can we assume that the mechanical fork axis is exactly perpendicular to its base? Secondly, if that is the case, can we assume that the optical axis is aligned with the fork's mechanical axis? Third, can you align the laser exactly to the center of the scope's FOV? Each of these three requirements must be accurate to about 1'. I think everything combined with your method this can easily be off by a full degree, and it is a lot of unnecessary alignment work.

This is why I advocate methods that use an RA slew to determine where the RA axis is pointing relative to the Az axis, then adjust the Az axis using the AltAz controls so this point coincides with the SCP. By doing that, none of your assumptions are needed and any inaccuracies resulting from implementing these assumptions, are avoided. By locating the center of RA rotation on a digital camera then moving that point to the SCP with the AltAz controls, you can achieve 1' accuracy directly. The Polemaster, Sharpcap and the computer-less method that I proposed will all do that. The use of a camera makes all of the intermediate alignment steps that you require, obsolete, and makes it possible to do PA to within 1' quickly.
I don't understand the point you trying to make: The fork mount is a polar mount and works like any other. The fork's rotation is your RA rotation and RA has to be perpendicular to your base that has been polar aligned!? the rotation of the OTA between the forks is your Dec movement, again just like any other polar mount. The rest becomes confusing to me because you talking about Az, Az axis and AltAz controls?? lets keep to Ra And Dec because as I said this is a polar mount. The way Joe does his: He sets his mount up like any polar mount that uses a polar scope, the only difference is he is using an artificial Pole star by pointing a laser to where the celestial south pole is. I don't have a polar scope so I have mounted a laser the same way as you would mount a polar scope (getting this laser aligned with my mount will be the hard part) after that I would mount the scope put it in polar home position: forks horizontal (I have a spirit level mounted for that and at 90 degrees Dec. I would use the wedge and east/west fine adjustments to get the laser on the pole(checking with my binoculars) check that the laser beam is in the centre of my spotting scope and eyepiece (not difficult), then do the normal 2 star aliment and I should be close.
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Re: Polar alignment

#14

Post by Piet Le Roux » Tue Dec 03, 2019 12:02 pm

SkyHiker wrote:
Tue Dec 03, 2019 10:56 am
With all respect, I doubt if this will work Joe. Your step 2 is difficult to implement and depends on uncertain assumptions. First, can we assume that the mechanical fork axis is exactly perpendicular to its base? Secondly, if that is the case, can we assume that the optical axis is aligned with the fork's mechanical axis? Third, can you align the laser exactly to the center of the scope's FOV? Each of these three requirements must be accurate to about 1'. I think everything combined with your method this can easily be off by a full degree, and it is a lot of unnecessary alignment work.

This is why I advocate methods that use an RA slew to determine where the RA axis is pointing relative to the Az axis, then adjust the Az axis using the AltAz controls so this point coincides with the SCP. By doing that, none of your assumptions are needed and any inaccuracies resulting from implementing these assumptions, are avoided. By locating the center of RA rotation on a digital camera then moving that point to the SCP with the AltAz controls, you can achieve 1' accuracy directly. The Polemaster, Sharpcap and the computer-less method that I proposed will all do that. The use of a camera makes all of the intermediate alignment steps that you require, obsolete, and makes it possible to do PA to within 1' quickly.
I don't understand the point you trying to make: The fork mount is a polar mount and works like any other. The fork's rotation is your RA rotation and RA has to be perpendicular to your base that has been polar aligned!? the rotation of the OTA between the forks is your Dec movement, again just like any other polar mount. The rest becomes confusing to me because you talking about Az, Az axis and AltAz controls?? lets keep to Ra And Dec because as I said this is a polar mount. The way Joe does his: He sets his mount up like any polar mount that uses a polar scope, the only difference is he is using an artificial Pole star by pointing a laser to where the celestial south pole is. I don't have a polar scope so I have mounted a laser the same way as you would mount a polar scope (getting this laser aligned with my mount will be the hard part) after that I would mount the scope put it in polar home position: forks horizontal (I have a spirit level mounted for that) and at 90 degrees Dec. I would use the wedge and east/west fine adjustments to get the laser on the pole(checking with my binoculars) check that the laser beam is in the centre of my spotting scope and eyepiece (not difficult), then do the normal 2 star aliment and I should be close.
Polar 001s.jpg
here is a conventional polar mount compared to a fork mount in polar configuration...
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Re: Polar alignment

#15

Post by SkyHiker » Wed Dec 04, 2019 1:55 am

To answer the confusion about the AltAz, when I refer to AltAz in your configuration I mean the wedge's AltAz adjustments.

Yes I understand exactly what you are doing and what Oz is doing. I just want to point out that IMHO you will never get about 1' or 2' accuracy, which is what is needed for good astrophotography. I suspect that yours will be 10' or more. This results in DEC corrections which are detrimental.

This is why I pointed out that there are better methods using a camera that will give you better alignment. Many have failed making AP work with a wedge. But no pressure, do what you like and good luck.
... Henk. :D Telescopes: 6" Mak-Newt (Comet Hunter), ES ED127CF, ES ED80, Zhumell Z12, Coulter Odyssey 10, AT6RC, Venture RX-7, Celestron Skymaster 20x80, Mounts and tripod: Losmandy G11S, AVX, LXD55, Tiltall, Cameras: Fuji X-a1, Canon SX40, Xt, XSi, T6, ELPH 100HS, DIY: Dob and camera barndoor trackers, afocal adapter, Dob with foldable base and Az/Alt setting circles, Accessories: SSAG, Plossls, Barlows, Telrad, laser collimators (Seben LK1, Z12, Howie Glatter), Cheshire, 2 Orion RACIs 8x50, Software: DSS, ImageMagick, PHD, Nebulosity, Photo Gallery, Gimp, CHDK
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Re: Polar alignment

#16

Post by JayTee » Wed Dec 04, 2019 2:44 am

Hi Piet,

As another alternative, I recently adapted my ZWO planetary webcam to fit my Canon lenses. If you get a "V" or "D" bar for the top of your OTA (they're relatively inexpensive) and the adapter (also relatively inexpensive) to affix this setup to that bar, you can then use either SharpCap Pro or PHD2 to do your static PA. It will require a computer (laptop, tablet, etc) and it will take more time than the laser, but you won't be burdened with installation error that may be inherent in your laser setup. Plus it could also be used to autoguide your scope when doing long exposure AP. In the attached image, I have it mounted directly on the saddle of my mount (for doing a static PA) but it would work equally well attached to the top of your OTA. Hope this helps.
ZWO Lens3.jpg
VPA Pole.jpg
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Re: Polar alignment

#17

Post by Piet Le Roux » Wed Dec 04, 2019 6:35 am

Thanks again for all the inputs. But at this stage I am not looking for perfection I just want to learn the basics. So lets keep to the KISS principle.
step one: I want to find a practical way to do polar alignment(in the southern hemisphere) that does not need a lot of gear and is a pain to do.
Step Two: I want to learn to test my polar alignment with the drift method for the southern hemisphere.
Step three: I want to learn to do PEC training.
Once I am competent doing the above I will attach a camera.
Main Equipment : 15" Obsession F4.5 Classic, Tele Vue 1.25" 7&13mm Nagler, Tele Vue 2" 27mm Panoptic, Tele Vue Big Barlow, Tele Vue Paracorr II : 8" Meade LX90ACF with Meade 2.0" Enhanced Diagonal, Baarder Hyperion MK III 24-8mm zoom
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