Results 1 to 7 of 7
Like Tree2Likes
  • 1 Post By Pitter
  • 1 Post By Astronautilus

Thread: Polar alignment

  1. #1
    Astronautilus's Avatar
    Astronautilus is offline White Dwarf
    Points: 3,889, Level: 41
    Level completed: 60%, Points required for next Level: 61
    Overall activity: 0%
    Achievements:
    First 1000 Experience Points365 Days+ Registered Achievement!750 Days+ Registered Achievement!2 Posts Achievement1000 Days+ Registered Achievement!
    Join Date
    Jan 2011
    Location
    Spain
    Posts
    7
    Points
    3,889
    Level
    41
    Thanks
    0
    Thanked 0x 0 Posts

    Default Polar alignment



    Reading about polar alignment is almost enough to make me give up with astrophotography. How many arc minutes is Polaris from the NCP this year? Where am I in my time zone? What time of year is it? Etc...

    I know that Polaris and the NCP are in different places, but at what point does it become critical to have the scope aligned with the celestial pole?

    I have a Skywatcher telescope on an EQ3 mount, and I've just bought myself the R.A and Dec motor kit, and I'm thinking of piggy-backing my camera on the scope, probably for 50mm Milky Way shots, and maybe 300mm for (say) the Orion Nebula.

    I imagine 50mm wouldn't be too much of a problem, but at 300mm, how long can an exposure be before I get star trails?

  2. #2
    KenGS's Avatar
    KenGS is offline HYPER GIANT
    Points: 14,405, Level: 82
    Level completed: 73%, Points required for next Level: 95
    Overall activity: 0%
    Achievements:
    2 Posts Achievement20 Posts Achievement!50 Posts Achievement!Ghost Achievement! Averaging 5+ posts a day!200+ Posts Achievement!
    Join Date
    Jan 2015
    Location
    Nowhere
    Posts
    2,001
    Points
    14,405
    Level
    82
    Thanks
    170
    Thanked 527x 478 Posts

    Default Re: Polar alignment

    There are many factors that lead to star trails. Assuming that your scope tracks at a accurate speed then the next things would be declination drift from poor polar alignment and RA movement from periodic error.
    Considering the declination drift, if you are say 6 arc minutes out on your alignment then over a 6 hour period you would drift those 6 arc-minutes. That's 1 arc-minute per hour or 1 arc-second per minute. But as the amount of drift is a sine function, the peak drift rate is about 1.6 times the average so 1.6 arc-seconds per minute. With a bigger PA error the rate would be correspondingly larger.
    Periodic error in mass-produced mounts varies from mount to mount but can be in the order of 30 arc-seconds over the period of the worm drive which is typically 5 to 10 minutes. So lets say the periodic error is 32 arc-seconds over an 8 minute period but that means that half the time the error is in one direction and the other half in the other direction so the rate becomes 32 arc-seconds over 4 minutes or 8 arc-seconds per minute. This is also a sine function but with a shortish period we can ignore that. The thing with RA is that the drift you see on your image depends on the declination you are imaging at since near the pole an arc-second of RA is much smaller than an arc-second at the equator.
    Add the two together (or rather take the root of the sum of the squares) and you can work out your overall drift then depending on your focal length and pixel size determine if it is significant or not.

    Another way is to just take some images at various exposure times to see how it works out. What have you got to lose? Use the above info to understand the cause of trails when you see them.
    Unguided at 300mm I'd guess a minute or maybe two.

    If you are piggy backing you could go old school and manually guide through the scope at high magnification using the handset (really old school used the slow-mo controls) but it takes a keen eye and a steady hand.
    I've moved to https://theskysearchers.com/index.php

  3. #3
    Pitter's Avatar
    Pitter is offline HYPER GIANT
    Points: 24,236, Level: 100
    Level completed: 0%, Points required for next Level: 0
    Overall activity: 0%
    Achievements:
    2 Posts Achievement20 Posts Achievement!Gallery Achievement!50 Posts Achievement!Album Achievement!
    Join Date
    Dec 2012
    Location
    Hamilton, Ontario, Canada
    Posts
    4,833
    Points
    24,236
    Level
    100
    Thanks
    438
    Thanked 1,281x 1,155 Posts
    Blog Entries
    2

    Default Re: Polar alignment

    Quote Originally Posted by Astronautilus View Post
    I know that Polaris and the NCP are in different places, but at what point does it become critical to have the scope aligned with the celestial pole?
    The NCP is always in the same place which is approximately .8 degrees from Polaris. Using the Kochab Clock you will always know where to find the NCP in relation to Polaris. For visual observing aligning on Polaris is fine. For any AP you must properly polar align on the NCP.
    bladekeeper likes this.
    Mark
    Celestron: C11 ~ C8 Orange ~ Celestar 8 ~ CR150HD6 ~ C5 Orange ~ Comet Catcher ~ Firstscope 114
    Meade: Starfinder 10eq ~ AS80
    Imaging: ZWO ASI120MC ~ ZWO ASI224MC ~ ZWO ADC ~ Canon T3i ~ Meade OAG
    Mounts: CGEM ~ Vixen GPEQ5 Synscan ~ Wedged Forks ~ Starfinder GEM

  4. #4
    Astronautilus's Avatar
    Astronautilus is offline White Dwarf
    Points: 3,889, Level: 41
    Level completed: 60%, Points required for next Level: 61
    Overall activity: 0%
    Achievements:
    First 1000 Experience Points365 Days+ Registered Achievement!750 Days+ Registered Achievement!2 Posts Achievement1000 Days+ Registered Achievement!
    Join Date
    Jan 2011
    Location
    Spain
    Posts
    7
    Points
    3,889
    Level
    41
    Thanks
    0
    Thanked 0x 0 Posts

    Default Re: Polar alignment

    Quote Originally Posted by Pitter View Post
    The NCP is always in the same place which is approximately .8 degrees from Polaris. Using the Kochab Clock you will always know where to find the NCP in relation to Polaris. For visual observing aligning on Polaris is fine. For any AP you must properly polar align on the NCP.
    I've read that the NCP was 42.5 arc minutes from Polaris in 2005, and will be 40 arc minutes in 2017...

    Anyway, thanks for the Kochab clock reference.
    Pitter likes this.

  5. #5
    Pitter's Avatar
    Pitter is offline HYPER GIANT
    Points: 24,236, Level: 100
    Level completed: 0%, Points required for next Level: 0
    Overall activity: 0%
    Achievements:
    2 Posts Achievement20 Posts Achievement!Gallery Achievement!50 Posts Achievement!Album Achievement!
    Join Date
    Dec 2012
    Location
    Hamilton, Ontario, Canada
    Posts
    4,833
    Points
    24,236
    Level
    100
    Thanks
    438
    Thanked 1,281x 1,155 Posts
    Blog Entries
    2

    Default Re: Polar alignment

    Quote Originally Posted by Astronautilus View Post
    I've read that the NCP was 42.5 arc minutes from Polaris in 2005, and will be 40 arc minutes in 2017...

    Anyway, thanks for the Kochab clock reference.
    Reverse that statement and you are correct. Polaris will be further from the NCP. Polaris was not always the pole star , nor will it remain the pole star, but we have about 1000 years before we have to worry about that.
    Mark
    Celestron: C11 ~ C8 Orange ~ Celestar 8 ~ CR150HD6 ~ C5 Orange ~ Comet Catcher ~ Firstscope 114
    Meade: Starfinder 10eq ~ AS80
    Imaging: ZWO ASI120MC ~ ZWO ASI224MC ~ ZWO ADC ~ Canon T3i ~ Meade OAG
    Mounts: CGEM ~ Vixen GPEQ5 Synscan ~ Wedged Forks ~ Starfinder GEM

  6. #6
    KathyNS's Avatar
    KathyNS is offline Super Moderator
    Points: 174,700, Level: 100
    Level completed: 0%, Points required for next Level: 0
    Overall activity: 26.0%
    Achievements:
    200+ Posts Achievement!Ghost Achievement! Averaging 5+ posts a day!First 1000 Experience Points400+ Posts AchievementGot three Friends
    Awards:
    Activity & Post & Thread Master
    Join Date
    Nov 2010
    Location
    Nova Scotia
    Posts
    25,315
    Points
    174,700
    Level
    100
    Thanks
    6,661
    Thanked 14,997x 9,551 Posts
    Blog Entries
    3

    Default Re: Polar alignment

    Quote Originally Posted by Astronautilus View Post
    How many arc minutes is Polaris from the NCP this year? Where am I in my time zone? What time of year is it? Etc...
    All of that calculation stuff is easily taken care of by asking your favourite software (Stellarium, for example), "What is the current hour angle of Polaris?"

    [rant]
    It doesn't help that most mount manuals are written incompetently. My HEQ5 manual goes on for pages of incomprehensible instructions about using fiddly little dials to compute longitude offsets, using Polaris transit times that are snatched out of thin air with no explanation, blabity-blah. It is all BS, because the only thing you need to know is that the handset tells you the Polaris hour angle when you switch it on!
    [/rant]

    I know that Polaris and the NCP are in different places, but at what point does it become critical to have the scope aligned with the celestial pole?
    If you are doing long exposure AP at focal lengths more than about 200mm, you want to be aligned to within 1 arc minute of the actual pole.

    You have to know not only how far Polaris is from the NCP, but also in what direction it is currently. Like all stars, it rotates around the pole a full 360 degrees in 23 hours 56 minutes. THat is why you need to know its current hour angle.

    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.


    DSO AP:
    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.
    Newtonian Astrograph; ATIK 383L+; EFW2 filter wheel; Astrodon Ha,LRGB filters; KWIQ/QHY5 guide
    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.
    ; Planetary AP: Celestron C-11; ZWO ASI120MC; Portable: Celestron C-8 on
    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.
    pro; C-90 on wedge; 20x80 binos; Etc: Canon 350D; Various EPs, etc. Obs: 8' Exploradome;
    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.
    (pier);
    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.
    .

    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.

  7. #7
    SonnyE's Avatar
    SonnyE is offline Banned
    Points: 6,026, Level: 53
    Level completed: 38%, Points required for next Level: 124
    Overall activity: 0%
    Achievements:
    365 Days+ Registered Achievement!2 Posts Achievement20 Posts Achievement!50 Posts Achievement!5 Threads Achievement!
    Join Date
    Apr 2015
    Location
    Santa Clarita, California
    Posts
    446
    Points
    6,026
    Level
    53
    Thanks
    180
    Thanked 80x 71 Posts

    Default Re: Polar alignment

    Beats the crap out of me.
    I just take pictures. Very long time frame single shots. *

    I set my tripod up as close to perfectly level as I can with a digital level. I settle for 0.0 degrees on two planes.
    Then do my Alt-Az and elevations with my Polar Scope.
    Then forge ahead with Alignment, and run a polar alignment.
    After which, I just get out there.
    Then I bring home my tiny treasures.
    People WAY over think a lot of this.


    *= Like this 1400 second image.
    1454614438875-1400s_ha7nm_2w_pp_colored_ps.jpg

    Or the other night, where I just poked Polaris in the cross hairs of my Illuminated EP, and forged ahead. I think this was 600 seconds.
    1455211325307-horsehead_nebula_1.jpg

    It just is not worth getting a headache over.

 

 

Similar Threads

  1. Polar alignment w/ polar scope question
    By Phoenix1583 in forum Astronomy Beginners Forum
    Replies: 12
    Last Post: 09-27-2015, 09:32 AM
  2. Hints and tricks on polar alignment i polar areas
    By oddabrov in forum Astronomy Forum
    Replies: 2
    Last Post: 03-11-2015, 12:19 PM
  3. Replies: 3
    Last Post: 08-28-2014, 04:19 PM
  4. Drift Alignment versus Synscan Polar Alignment
    By davidjbillo in forum Astro Imaging Forum
    Replies: 20
    Last Post: 03-19-2014, 04:13 AM

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Powered by vBulletin« Version 4.2.0
Powered by vBulletin«
All times are GMT. The time now is 02:13 AM.