Page 2 of 2 FirstFirst 12
Results 11 to 14 of 14
Like Tree15Likes

Thread: Maximum focal length

  1. #11
    Bigzmey's Avatar
    Bigzmey is offline HYPER GIANT
    Points: 40,753, Level: 100
    Level completed: 0%, Points required for next Level: 0
    Overall activity: 59.0%
    Achievements:
    2 Posts Achievement20 Posts Achievement!5 Threads Achievement!50 Posts Achievement!200+ Posts Achievement!
    Join Date
    Sep 2015
    Location
    Planet Earth
    Posts
    7,545
    Points
    40,753
    Level
    100
    Thanks
    1,801
    Thanked 3,185x 2,291 Posts

    Default Re: Maximum focal length



    Quote Originally Posted by not_Fritz_Argelander View Post
    No. It depends on the eye AND whether the scope is obstructed. I've noticed that my ability to accommodate long focal length eyepieces without a doughnut shadow effect has declined over the years. When I was younger a 7mm exit pupil was not a problem in an obstructed scope. Now it's iffy. If you don't believe me check this article from Gary Seronik.

    Exploring Low-Power Limits – Gary Seronik

    He has no problems considering 7mm exit pupils for obstructed scopes. The shadow in the center in obstructed scopes is because the exit pupil is larger than the pupil of the eye. That's just what optics says, it's not an opinion.
    Got it! The truth is in the eye of the beholder.

  2. #12
    Nimitz's Avatar
    Nimitz is offline Bright Giants
    Points: 4,881, Level: 47
    Level completed: 66%, Points required for next Level: 69
    Overall activity: 2.0%
    Achievements:
    2 Posts Achievement20 Posts Achievement!5 Threads Achievement!50 Posts Achievement!200+ Posts Achievement!
    Join Date
    Mar 2017
    Location
    Florida
    Posts
    513
    Points
    4,881
    Level
    47
    Thanks
    5
    Thanked 24x 21 Posts

    Default Re: Maximum focal length

    Quote Originally Posted by Bigzmey View Post
    I got the same EP and like it. Just remember it has a field stop of ~40mm, so it does not give you the max possible FOV which EP with 46mm field stop would provide.

    Russell Optics 56mm Super Plossl - FOV 41 degree (AFV in my 8" SCT 1.13 deg)
    TV 55mm Plossl - FOV 48 deg (AFV in my 8" SCT 1.3 deg)
    Not trying to hyjack the thread, just trying to understand this better .....
    Isn't Max TFOV of the SCT limited by the size of scope's baffle tube and for something like my 9.25" I think it's around ~1.1 degree, not sure if or how different it is for an 8"? I already achieve this max TFOV with say my GSO SV 42mm EP so using 50mm EP or FR would not increase TFOV, it will just result in a smaller image. True?
    Intercepting & temporarily inconveniencing light photons as they travel through the universe since 1989
    What do I have? A M.S. degree in AstroPhysics w/minor in planetary geology
    XX14g SkyQuest
    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.
    Dob; WO 81GT APO triplet;
    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.
    EQ-6R Pro
    UO 4mm Ortho, KK 9mm & 12.5MM Orthos, Delite 7mm, Delos 10mm, 14mm, Panoptic 19mm, 24mm, Baader 8-24mm Mark IV Zoom, Nikon D5300 & a bunch of NV stuff

  3. #13
    not_Fritz_Argelander's Avatar
    not_Fritz_Argelander is offline HYPER GIANT
    Points: 183,440, Level: 100
    Level completed: 0%, Points required for next Level: 0
    Overall activity: 29.0%
    Achievements:
    365 Days+ Registered Achievement!2 Posts Achievement20 Posts Achievement!750 Days+ Registered Achievement!First 1000 Experience Points
    Awards:
    Thread Award
    Join Date
    Jul 2011
    Location
    US
    Posts
    22,039
    Points
    183,440
    Level
    100
    Thanks
    7,254
    Thanked 46,396x 14,953 Posts
    Blog Entries
    59

    Default Re: Maximum focal length

    Quote Originally Posted by Nimitz View Post
    Not trying to hyjack the thread, just trying to understand this better .....
    Isn't Max TFOV of the SCT limited by the size of scope's baffle tube and for something like my 9.25" I think it's around ~1.1 degree, not sure if or how different it is for an 8"? I already achieve this max TFOV with say my GSO SV 42mm EP so using 50mm EP or FR would not increase TFOV, it will just result in a smaller image. True?
    Since the baffle tube is larger than the field stop of the eyepiece that will overestimate the TFOV. Here's a formula.

    Tele Vue Optics: Choosing Eyepieces (Basic)

    So, although it makes no sense to have a 2" visual back on a scope with a smaller baffle, if you rigged that up somehow the baffle becomes the field stop of the eyepiece. It'd be fuzzy and out of focus.

    There are throughput reasons why a smaller image and larger exit pupil would be useful though. Exit pupil determines what faint fuzzy things are detectable.
    Bigzmey likes this.

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

    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.
    ST80 (mods for white light solar), SV ED80 f7 Newtonians: Z12 f5 Catadioptrics: VMC110L, Intes MK66 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binocular Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker III, Vixen Porta II, Celestron CG5, Orion
    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.
    EQG

  4. #14
    Bigzmey's Avatar
    Bigzmey is offline HYPER GIANT
    Points: 40,753, Level: 100
    Level completed: 0%, Points required for next Level: 0
    Overall activity: 59.0%
    Achievements:
    2 Posts Achievement20 Posts Achievement!5 Threads Achievement!50 Posts Achievement!200+ Posts Achievement!
    Join Date
    Sep 2015
    Location
    Planet Earth
    Posts
    7,545
    Points
    40,753
    Level
    100
    Thanks
    1,801
    Thanked 3,185x 2,291 Posts

    Default Re: Maximum focal length

    Quote Originally Posted by Nimitz View Post
    Not trying to hyjack the thread, just trying to understand this better .....
    Isn't Max TFOV of the SCT limited by the size of scope's baffle tube and for something like my 9.25" I think it's around ~1.1 degree, not sure if or how different it is for an 8"? I already achieve this max TFOV with say my GSO SV 42mm EP so using 50mm EP or FR would not increase TFOV, it will just result in a smaller image. True?
    Unfortunately, many manufacturers overestimate (some quite generously) their EP FOV. GSO SV 42mm does not provide max FOV available in 2" because it has field stop of 41mm. To achieve max FOV you need EP with field stop close to 46mm. Does not need to be 50mm EP. For example Baader Aspheric 36mm has 45mm field stop. Orion Q70 38mm and William Optics Swan 40mm are two other EPs with max FOV.
    Coincidentally, your 9.25" has baffle tube diameter of 46mm, so it can utilize 2" EPs to the max. Max AFV for 9.25" SCT is indeed ~1.1 deg. GSO 42mm however gives ~1.0 deg in your scope. Thus, EP with 46mm field stop will show you ~20% more field than GSO.

    But, as nFA indicated there are other advantages of 50-56mm EPs for our F10 scopes. They will produce 5-5.6mm exit pupil which is beneficial for observing faint large DSO and using nebula filters.

 

 
Page 2 of 2 FirstFirst 12

Similar Threads

  1. Poll: ISO? Focal Length? Subframe Length?
    By Stephanisk in forum Astronomy Digital Cameras Forum
    Replies: 4
    Last Post: 02-08-2015, 06:13 AM
  2. Replies: 11
    Last Post: 04-24-2014, 02:42 AM
  3. Confusion about maximum focal length eyepiece
    By Albireo in forum Astronomy Beginners Forum
    Replies: 4
    Last Post: 06-05-2012, 05:36 PM
  4. Replies: 3
    Last Post: 12-04-2009, 05:36 PM
  5. Scope focal length vs Eyepiece focal length
    By PS Surfer in forum General Astronomy Forum
    Replies: 1
    Last Post: 09-22-2003, 11:57 PM

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 05:09 PM.