Polemaster tip that works for me to make more accurate PAs

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ram United States of America
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Polemaster tip that works for me to make more accurate PAs

#1

Post by ram »


For those of you who use Polemaster (PM), I've found that for me at least the choice of which star to pick to determine the celestial axis centre (i.e.,the celestial pole) affects the quality of the PA at a fine level.

In PM, you start with rotating the template around Polaris and then picking a star (other than polaris) whose rotation is tracked. My experience has been that the outermost star I pick leads to an improved PA leading me to the conclusion that the step where you select the axis centre is important (everything else then is tailored to moving your RA axis/centre to be lined up with that).

The difference is very tiny IMO. I can easily go seven minutes unguided at f/5 up to about 740mm FL on my mount regardless. If I don't pick a more outer star, sometimes I will get tiny star trails in some frames but I've now gone one shell (defined as a close bright star to rotate/track - which I've been doing for years), two shells out (medium near bright star), three shells out (further out bright star) and each change I've made I've noticed both better pointing* and tracking so that at this point I'm getting really high quality tracking for the vast majority if not all my frames.

If you're doing guided imaging you probably don't need to worry about this but if you're doing unguided then this should help IMO.

--Ram

* - In a GEM, if your PA is perfect and your mount is horizontal (perpendicular to the celestial axis), then your pointing is spot on even without any star alignment.
Tubes: Celestron 9.25" 235mm f/10 XLT EdgeHD SCT; Meade ETX 80mm f/5 achromat; Coronado SolarMax II 60mm f/6.6 Hα <0.7Å BF10 solar; Stellarvue 70mm f/6 triplet apochromat; Obsession UC18 457mm f/4.2 with Argo Navis & ServoCAT; Takahashi FS128 5" f/8.1 and FC100DF 4" f/7.4 fluorite doublet apochromats. Mounts: AVX; LXD75; Paramount MyT. Eyepieces: 2" Tele Vue Ethos 4.7/13/21mm, Paracorr, 2,4x Powermate; Stellarvue 0.8x, Takahashi 0.7x, 0.66x reducer/corrector. Cameras: ZWO ASI120MC-S; Lodestar X2c; X2m; Canon T7i; QHY163M; QHY247C; QHY294M-Pro. Filters: 1.25" Astrodon 5nm Ha, 3nm O3 and S2; Chroma LRGB.
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Re: Polemaster tip that works for me to make more accurate PAs

#2

Post by carastro »


That seems very logical Ram, however, I find if I pick a star too far away from polaris , it moves out of the FOV when you rotate so it is impossible to pick for the 2nd iteration. I tend to use the outermost star of a triangle of 3 as that is also easy to remember if I take my eye of the screen to watch the mount to see how how I have rotated it. I try to start with Polaris in the middle of the screen to start with, this will involve moving the Alt/AZ bolts so they are central, and if not, move the tripod until it is. That way i have reasonable leverage.

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Re: Polemaster tip that works for me to make more accurate PAs

#3

Post by Star Dad »


I cannot say I've noticed the accuracy/star trails phenomenon - but then I haven't been looking for it either. However, being an engineer I intuitively knew to choose the furthest star out from Polaris that would give me the required 60 degree plus rotation needed. Having had the Polemaster for a year now, I can safely say that it is a god-send. It has cut down my polar alignment from 15-30 minutes to less than 10 and all I have to do is sit in my chairs and turn knobs instead of getting down on all fours trying to squint through the polar scope and balance while trying to judge the correct star line up. Yikes! That was not easy. And I agree I could go unguided... and I do when clouds are present.
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Re: Polemaster tip that works for me to make more accurate PAs

#4

Post by STEVE333 »


Just FYI -
My iOptron mount came with iOptron's version of "PoleMaster" which they call iPolar. It is a bit easier to use than PoleMaster because you only have to do the mount rotations the first time you use the system. The mount rotations are used to determine the Center-of-Rotation for the mount, and, the system remembers where that is located. Once the Center-of-Rotation is known the system puts a red dot (the location of the Center-of-Rotation) and a red + sign on the display. You use the telescope mount Az/El knobs to move the red + to the center of the red dot. When the red + is close to the red dot the screen magnifies to give a closeup view so that the alignment can be more accurate. Once the red + is centered on the red dot the Polar Alignment is complete.

The next time you do a Polar alignment the screen will open up with the red + and red dot displayed and you only need to adjust the mount Az/El to center the red + on the red dot.

Rather than requiring the user to center any stars the iPolar system uses plate solves to determine the star positions.

Not saying it is better or worse, just sharing the differences for comparison.

Cheers,
Steve
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Re: Polemaster tip that works for me to make more accurate PAs

#5

Post by ram »


Thank you Steve but does that work even if the mount moves? I mean if you place the mount in another location (or if you adjust the height, etc.) would the centre of rotation hold? PoleMaster also gives me the option of reusing the axis centre from the night before but I see my mount shifting every so slightly so I don't do it. Also my PoleMaster itself is taken out and put back each time (it's mounted on the dovetail).

Using plate solves is smart...

--Ram
Tubes: Celestron 9.25" 235mm f/10 XLT EdgeHD SCT; Meade ETX 80mm f/5 achromat; Coronado SolarMax II 60mm f/6.6 Hα <0.7Å BF10 solar; Stellarvue 70mm f/6 triplet apochromat; Obsession UC18 457mm f/4.2 with Argo Navis & ServoCAT; Takahashi FS128 5" f/8.1 and FC100DF 4" f/7.4 fluorite doublet apochromats. Mounts: AVX; LXD75; Paramount MyT. Eyepieces: 2" Tele Vue Ethos 4.7/13/21mm, Paracorr, 2,4x Powermate; Stellarvue 0.8x, Takahashi 0.7x, 0.66x reducer/corrector. Cameras: ZWO ASI120MC-S; Lodestar X2c; X2m; Canon T7i; QHY163M; QHY247C; QHY294M-Pro. Filters: 1.25" Astrodon 5nm Ha, 3nm O3 and S2; Chroma LRGB.
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Re: Polemaster tip that works for me to make more accurate PAs

#6

Post by STEVE333 »


ram wrote: Fri Sep 18, 2020 5:05 am Thank you Steve but does that work even if the mount moves? I mean if you place the mount in another location (or if you adjust the height, etc.) would the centre of rotation hold? PoleMaster also gives me the option of reusing the axis centre from the night before but I see my mount shifting every so slightly so I don't do it. Also my PoleMaster itself is taken out and put back each time (it's mounted on the dovetail).

Using plate solves is smart...

--Ram

My iPolar unit is built into the mount, so, once I find the center of rotation it remains fixed. Moving the mount has no effect nor does adjusting the mount height.

Because your PoleMaster is taken out and then put back each time I would expect that you would need to perform the rotations each time.

Steve
Steve King: Light Pollution (Bortle 5)
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Re: Polemaster tip that works for me to make more accurate PAs

#7

Post by ram »


Okay, that's interesting then - that might mean it's worth investing in the Polemaster adapter for the MyT so I can affix it to the mount's RA axis?

Then if it is affixed to the RA axis, it shouldn't matter, right? It's just relative to the camera and the centre of rotation of the RA axis?

--Ram
Tubes: Celestron 9.25" 235mm f/10 XLT EdgeHD SCT; Meade ETX 80mm f/5 achromat; Coronado SolarMax II 60mm f/6.6 Hα <0.7Å BF10 solar; Stellarvue 70mm f/6 triplet apochromat; Obsession UC18 457mm f/4.2 with Argo Navis & ServoCAT; Takahashi FS128 5" f/8.1 and FC100DF 4" f/7.4 fluorite doublet apochromats. Mounts: AVX; LXD75; Paramount MyT. Eyepieces: 2" Tele Vue Ethos 4.7/13/21mm, Paracorr, 2,4x Powermate; Stellarvue 0.8x, Takahashi 0.7x, 0.66x reducer/corrector. Cameras: ZWO ASI120MC-S; Lodestar X2c; X2m; Canon T7i; QHY163M; QHY247C; QHY294M-Pro. Filters: 1.25" Astrodon 5nm Ha, 3nm O3 and S2; Chroma LRGB.
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Re: Polemaster tip that works for me to make more accurate PAs

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Post by STEVE333 »


ram wrote: Fri Sep 18, 2020 7:00 am Okay, that's interesting then - that might mean it's worth investing in the Polemaster adapter for the MyT so I can affix it to the mount's RA axis?

Then if it is affixed to the RA axis, it shouldn't matter, right? It's just relative to the camera and the centre of rotation of the RA axis?

--Ram

That's exactly correct. If the PoleMaster is permanently affixed to the mount, then, you only need to find the center of rotation one time!

Cheers
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Re: Polemaster tip that works for me to make more accurate PAs

#9

Post by Star Dad »


Oh wow! I have wondered what that requester was really meaning when it asks (I think) if you want to use the previous axis of rotation. I will definitely try that out tomorrow night (clear skies). I keep my polemaster always mounted. That would save a good couple of minutes (maybe 5) on set up. Time is precious so every little savings is important.

Learned something new today! :)
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Re: Polemaster tip that works for me to make more accurate PAs

#10

Post by John999R »


I remove my Pole Master from my CEM60EC after the PA process just so it's out of the way and I don't bump it accidentally, especially while transporting the mount back into the house. Are you saying I should leave it connected even if my set up is mobile?
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Re: Polemaster tip that works for me to make more accurate PAs

#11

Post by STEVE333 »


John999R wrote: Sat Sep 19, 2020 5:44 am I remove my Pole Master from my CEM60EC after the PA process just so it's out of the way and I don't bump it accidentally, especially while transporting the mount back into the house. Are you saying I should leave it connected even if my set up is mobile?

Hi John - The advantage of leaving it connected is that you can use the same center of Rotation over and over (assuming you don't bump the PoleMaster). However, if you "bump" the PoleMaster it might change its aiming which would require determining the Center of Rotation again. The fact that you move your setup won't affect the Center of Rotation determined by the PoleMaster.

My iPolar is built into the mount, so, no chance of "bumping" it. Thus, I can use the same Center of Rotation over and over and over. I haven't redone it in months and I have no star trails at all.

Steve
Steve King: Light Pollution (Bortle 5)
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Re: Polemaster tip that works for me to make more accurate PAs

#12

Post by John999R »


STEVE333 wrote: Sat Sep 19, 2020 7:10 am
John999R wrote: Sat Sep 19, 2020 5:44 am I remove my Pole Master from my CEM60EC after the PA process just so it's out of the way and I don't bump it accidentally, especially while transporting the mount back into the house. Are you saying I should leave it connected even if my set up is mobile?

Hi John - The advantage of leaving it connected is that you can use the same center of Rotation over and over (assuming you don't bump the PoleMaster). However, if you "bump" the PoleMaster it might change its aiming which would require determining the Center of Rotation again. The fact that you move your setup won't affect the Center of Rotation determined by the PoleMaster.

My iPolar is built into the mount, so, no chance of "bumping" it. Thus, I can use the same Center of Rotation over and over and over. I haven't redone it in months and I have no star trails at all.

Steve
Another reason why I disconnect and remove the PM after calibration is to free up a USB port. I could use one of the four cable management USB ports (non-powered) that are built into the DEC axis of the CEM60 mounts, they can handle up to five amps combined. I use two of the ports now, my DSLR and I believe the USB2COM converter and this post reminds me to find out how many amps I'm using and maybe I could also plug the Pole Master into the hub. Used efficiently, the cable management ports enabled me to cut down on length of some dangling and messy looking cables. I haven't researched the best way to use the hub, it's time that I do that. Like many iOptron CEM60/EC owners, I wish the USB hub was powered. Speaking of which, I recall a forum member on the old Astronomy.net? website working very hard at designing a powered hub just for the CEM60, fitting it into the space where the existing non powered hub is located. I have no idea where that went, but from what I remember he was very close to a final product when the website fell off the map. I'm assuming many of the members migrated here (as I did), I thank KathyNS for bringing this place to my attention.

The idea of using the third star during PM calibration will be part of my routine and I like the idea of leaving the PM attached now, I'll try that as well and hopefully the non-powered hub is up to the task because that will clean up cable clutter even more. :clap:

John
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Re: Polemaster tip that works for me to make more accurate PAs

#13

Post by SkyHiker »


I don't use the PM but rotate in the home position using a DSLR, mark the LCD at the center of rotation then move the NCP to it using the Alt/Az controls iterating over 3 second shots. I find that the larger circles (outer stars) suffer less from inaccuracies due to play in the RA axis. Especially when going back and forth you can tell the difference, the smaller circles are much jumpier than the large ones. So I always look at the outer circles, making sure I rotate Westward (Northern hemisphere) to make sure the axis is in as similar as possible a position compared to tracking. The axis has a few arc seconds play so it is never in the exact mechanical position we would like it to be.
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