Results 1 to 8 of 8
  1. #1
    Dublin sky watch's Avatar
    Dublin sky watch is offline Moderator
    Points: 45,920, Level: 100
    Level completed: 0%, Points required for next Level: 0
    Overall activity: 0%
    Achievements:
    Got three Friends200+ Posts Achievement!20+ Friends Achievement!5+ Referrals Achievement!First 1000 Experience Points
    Join Date
    Dec 2009
    Location
    Dublin,Ireland
    Posts
    6,842
    Points
    45,920
    Level
    100
    Thanks
    1,409
    Thanked 2,537x 1,696 Posts

    Default Are wedges worth there money?



    Hello forum friends.
    Well my attempts at astrophotography are improving in my view. However even I have noticed that an exposure of more then25-30 seconds is pushing it for avoiding star trails. I am aware of the type of lens I can use to limit the effects of this. My question is about wedges for a SCT. Do they improve the ability to expose longer or is it just an expensive gimmick. I am gathering information from as wide as possible number of people as well as magazines and the net in general.

    Here in Ireland it will cost me just under 500 euro (not Russian Rubles!!) So far the impression seems pretty evenly split. Anybody out here using one and can give me some photo evidence of the increase or decrease of tracking capabilities. Might post this else were to get as much feed back as possible.
    Thanks guys
    Declan.
    Celestron 8"Edge HD SCT.
    Skywatcher ED80mm refractor.
    Explore Scientific - 127mm f/7.5 Air-Spaced Triplet ED.
    Lunt - 60mm H-alpha Solar-Telescope with Double Stacked 50 Etalon system, B600 blocking filter and 2 " Crayford Focuser
    orion star shoot solitaire autoguider.
    Canon 5D Mark II dslr.
    Two men sat behind prison bars,One saw mud, the other saw stars. It's not where you are, it's what you see!!!

  2. #2
    adeybaby's Avatar
    adeybaby is offline Main Sequence
    Points: 9,264, Level: 66
    Level completed: 72%, Points required for next Level: 86
    Overall activity: 0%
    Achievements:
    10 Days registered365 Days+ Registered Achievement!First 1000 Experience Points200+ Posts Achievement!5+ Referrals Achievement!
    Join Date
    Jun 2009
    Location
    Leicester, England 52deg North 1deg west
    Posts
    180
    Points
    9,264
    Level
    66
    Thanks
    127
    Thanked 54x 38 Posts

    Default

    Hi Dublin, I have only just started to image with my scope but even i have noticed that using the built in wedge on my Nexstar 5se has reduced the trailing and rotation. I had never wedge aligned my scope until recently, i just used it in standard alt/az. I was worth the effort, as an image of M45 in alt/az was trailed and a recent image of M42 in wedge mode had no trailing only blurred focus( my fault!!).
    Hope this helps.

    Clear skies

  3. #3
    Dublin sky watch's Avatar
    Dublin sky watch is offline Moderator
    Points: 45,920, Level: 100
    Level completed: 0%, Points required for next Level: 0
    Overall activity: 0%
    Achievements:
    Got three Friends200+ Posts Achievement!20+ Friends Achievement!5+ Referrals Achievement!First 1000 Experience Points
    Join Date
    Dec 2009
    Location
    Dublin,Ireland
    Posts
    6,842
    Points
    45,920
    Level
    100
    Thanks
    1,409
    Thanked 2,537x 1,696 Posts

    Default

    Quote Originally Posted by adeybaby View Post
    Hi Dublin, I have only just started to image with my scope but even i have noticed that using the built in wedge on my Nexstar 5se has reduced the trailing and rotation. I had never wedge aligned my scope until recently, i just used it in standard alt/az. I was worth the effort, as an image of M45 in alt/az was trailed and a recent image of M42 in wedge mode had no trailing only blurred focus( my fault!!).
    Hope this helps.

    Clear skies
    Thanks adeybaby. So you have noticed an improvement? In your opinion was it worth the extra cost? How long have your exposure gone out to? Need to build a picture on this bit of potential kit. Thanks for the input.
    Declan.
    Celestron 8"Edge HD SCT.
    Skywatcher ED80mm refractor.
    Explore Scientific - 127mm f/7.5 Air-Spaced Triplet ED.
    Lunt - 60mm H-alpha Solar-Telescope with Double Stacked 50 Etalon system, B600 blocking filter and 2 " Crayford Focuser
    orion star shoot solitaire autoguider.
    Canon 5D Mark II dslr.
    Two men sat behind prison bars,One saw mud, the other saw stars. It's not where you are, it's what you see!!!

  4. #4
    adeybaby's Avatar
    adeybaby is offline Main Sequence
    Points: 9,264, Level: 66
    Level completed: 72%, Points required for next Level: 86
    Overall activity: 0%
    Achievements:
    10 Days registered365 Days+ Registered Achievement!First 1000 Experience Points200+ Posts Achievement!5+ Referrals Achievement!
    Join Date
    Jun 2009
    Location
    Leicester, England 52deg North 1deg west
    Posts
    180
    Points
    9,264
    Level
    66
    Thanks
    127
    Thanked 54x 38 Posts

    Default

    There was no extra cost as the nexstar 5se has a wedge built into the tripod. To compare the shots of M45 were 30 secs of exposure due to not having a release cable and there was definate trailing but on M45 i had a release cable so i shot for 1 min and there was a definite improvement. I'm only just starting to image so please get more advice as there are people here who are more experienced.
    I will find and post the 2 shots so you can compare.

  5. The Following User Says Thank You to adeybaby For This Useful Post:

    Dublin sky watch (01-21-2010)

  6. #5
    adeybaby's Avatar
    adeybaby is offline Main Sequence
    Points: 9,264, Level: 66
    Level completed: 72%, Points required for next Level: 86
    Overall activity: 0%
    Achievements:
    10 Days registered365 Days+ Registered Achievement!First 1000 Experience Points200+ Posts Achievement!5+ Referrals Achievement!
    Join Date
    Jun 2009
    Location
    Leicester, England 52deg North 1deg west
    Posts
    180
    Points
    9,264
    Level
    66
    Thanks
    127
    Thanked 54x 38 Posts

    Default

    Here are the 2 shots.....
    One shot was in alt/az at 30 secs exposure and the other was with the wedge at 1 min. Focus could be better but like i said i'm new to this.
    You can see the first shot has trailed a bit but the second it was the focus that was out. I'll get it next time.
    Hope this helps

    Clear skies
    Attached Images Attached Images
    Last edited by adeybaby; 01-21-2010 at 08:24 PM.

  7. The Following User Says Thank You to adeybaby For This Useful Post:

    Dublin sky watch (01-21-2010)

  8. #6
    AustinPSD's Avatar
    AustinPSD is offline Super Moderator
    Points: 56,828, Level: 100
    Level completed: 0%, Points required for next Level: 0
    Overall activity: 2.0%
    Achievements:
    200+ Posts Achievement!First 1000 Experience PointsGot three Friends20+ Friends Achievement!5+ Referrals Achievement!
    Join Date
    Sep 2009
    Location
    McDonald Observatory, Mt. Locke
    Posts
    6,923
    Points
    56,828
    Level
    100
    Thanks
    743
    Thanked 5,865x 3,115 Posts
    Blog Entries
    1

    Default

    Trailing and rotation are two different motion-related exposure defects.

    Trailing is more related to tracking error. Rotation, or field rotation is related to lack of, or poor polar alignment.

    Trailing defects happen within the frame of a single exposure, and happen for the most part because the mount's RA motion is either too fast, too slow, or otherwise erratic - for example periodic stalling.

    With an alt-az mount, on a wedge, even with perfect, or near perfect polar alignment, you can still experience trailing defects.

    Field rotation is a frame-to-frame defect. This occurs either due to a complete lack of polar alignment, or poor polar alignment.

    Let's assume you are not polar aligned, i.e. just an alt-az mount on a tripod. Each imaged frame will have the stars in the field rotating in the field to the left (west) of the prior image frame. Independent of any trailing (tracking) defect, these frames can be used if they are relatively few in number, and either manually aligned by frame rotation, or in some cases aligned by software processes (rarely).

    In the next case, consider a wedge-adapted alt-az mount that is poorly aligned. The same problem will happen, along with exaggerated DEC positioning error. Any trailing (tracking related) defects will be exacerbated by the alignment problem.

    To eliminate trailing defects, there are a number of steps that one takes, including attention to polar alignment (requires a wedge), tuning the mount's mechanical system, and use of auto-guiding to correct tracking error.

    Your alt-az mount, even if on a wedge and properly drift/polar aligned will have a tracking error of 60 - 90 arc-seconds without auto-guiding. This translates into an exposure duration ranging from 1 to 2 minutes, without trailing defect.

    With guiding, this error can be reduced to 40 - 50 arc-seconds, and with mount tuning, probably reduced to the 20 - 30 arc-second range (along with guiding). In turn, that translates into a maximum exposure duration of probably 4 - 5 minutes.
    CGEM 800 HD, NexGuide,
    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.
    XT8 Limited Edition, Oberwerk BT-100, Canon 20D/20Da/T3i/60D/5D Mk III, various eyepieces, adapters, geegaws, widgets, and tiddlybits

  9. The Following 3 Users Say Thank You to AustinPSD For This Useful Post:

    adeybaby (01-21-2010),Dublin sky watch (01-21-2010),Tombstone17 (01-22-2010)

  10. #7
    Dublin sky watch's Avatar
    Dublin sky watch is offline Moderator
    Points: 45,920, Level: 100
    Level completed: 0%, Points required for next Level: 0
    Overall activity: 0%
    Achievements:
    Got three Friends200+ Posts Achievement!20+ Friends Achievement!5+ Referrals Achievement!First 1000 Experience Points
    Join Date
    Dec 2009
    Location
    Dublin,Ireland
    Posts
    6,842
    Points
    45,920
    Level
    100
    Thanks
    1,409
    Thanked 2,537x 1,696 Posts

    Default

    Quote Originally Posted by AustinPSD View Post
    Trailing and rotation are two different motion-related exposure defects.

    Trailing is more related to tracking error. Rotation, or field rotation is related to lack of, or poor polar alignment.

    Trailing defects happen within the frame of a single exposure, and happen for the most part because the mount's RA motion is either too fast, too slow, or otherwise erratic - for example periodic stalling.

    With an alt-az mount, on a wedge, even with perfect, or near perfect polar alignment, you can still experience trailing defects.

    Field rotation is a frame-to-frame defect. This occurs either due to a complete lack of polar alignment, or poor polar alignment.

    Let's assume you are not polar aligned, i.e. just an alt-az mount on a tripod. Each imaged frame will have the stars in the field rotating in the field to the left (west) of the prior image frame. Independent of any trailing (tracking) defect, these frames can be used if they are relatively few in number, and either manually aligned by frame rotation, or in some cases aligned by software processes (rarely).

    In the next case, consider a wedge-adapted alt-az mount that is poorly aligned. The same problem will happen, along with exaggerated DEC positioning error. Any trailing (tracking related) defects will be exacerbated by the alignment problem.

    To eliminate trailing defects, there are a number of steps that one takes, including attention to polar alignment (requires a wedge), tuning the mount's mechanical system, and use of auto-guiding to correct tracking error.

    Your alt-az mount, even if on a wedge and properly drift/polar aligned will have a tracking error of 60 - 90 arc-seconds without auto-guiding. This translates into an exposure duration ranging from 1 to 2 minutes, without trailing defect.

    With guiding, this error can be reduced to 40 - 50 arc-seconds, and with mount tuning, probably reduced to the 20 - 30 arc-second range (along with guiding).
    Code:
     In turn, that translates into a maximum exposure duration of probably 4 - 5 minutes.
    OK. I follow that. So no matter how accurate my alignment is to begin with,exposure is still precise for only about 5 mins using a wedge and auto guider? Hmmm it still seems like a solution to my exposure dilemma.
    Thank you both very much for your help.
    Declan.
    Celestron 8"Edge HD SCT.
    Skywatcher ED80mm refractor.
    Explore Scientific - 127mm f/7.5 Air-Spaced Triplet ED.
    Lunt - 60mm H-alpha Solar-Telescope with Double Stacked 50 Etalon system, B600 blocking filter and 2 " Crayford Focuser
    orion star shoot solitaire autoguider.
    Canon 5D Mark II dslr.
    Two men sat behind prison bars,One saw mud, the other saw stars. It's not where you are, it's what you see!!!

  11. #8
    AustinPSD's Avatar
    AustinPSD is offline Super Moderator
    Points: 56,828, Level: 100
    Level completed: 0%, Points required for next Level: 0
    Overall activity: 2.0%
    Achievements:
    200+ Posts Achievement!First 1000 Experience PointsGot three Friends20+ Friends Achievement!5+ Referrals Achievement!
    Join Date
    Sep 2009
    Location
    McDonald Observatory, Mt. Locke
    Posts
    6,923
    Points
    56,828
    Level
    100
    Thanks
    743
    Thanked 5,865x 3,115 Posts
    Blog Entries
    1

    Default

    That's pretty much it - for any given mount, exposure durations longer than the mount's inherent tracking precision will trail - to go beyond it to the next threshold, you have to guide.

    Here's the in-depth explanation in this thread:

    Mount Tracking Error / Mount Charateristics
    CGEM 800 HD, NexGuide,
    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.
    XT8 Limited Edition, Oberwerk BT-100, Canon 20D/20Da/T3i/60D/5D Mk III, various eyepieces, adapters, geegaws, widgets, and tiddlybits

  12. The Following 3 Users Say Thank You to AustinPSD For This Useful Post:

    adeybaby (01-21-2010),Dublin sky watch (01-21-2010),Tombstone17 (01-22-2010)

 

 

Similar Threads

  1. dse 12.5, worth the money?
    By lankeeyankee in forum Orion Telescopes Forum
    Replies: 12
    Last Post: 12-28-2010, 09:56 PM
  2. Are 2" Eyepieces Worth the Money?
    By jerryholcombe in forum Telescope Eyepieces Forum
    Replies: 12
    Last Post: 10-10-2009, 10:31 PM
  3. Telescope Wedges
    By rocket in forum Telescope Accessories Forum
    Replies: 1
    Last Post: 05-20-2008, 10:12 PM
  4. Quick money, doesn't everyone want that?? MAKE MONEY FAST!
    By T.J. in forum General Astronomy Forum
    Replies: 0
    Last Post: 03-03-2005, 12:04 PM

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 02:00 AM.