Page 1 of 2 12 LastLast
Results 1 to 10 of 15
Like Tree8Likes

Thread: 9mm vs 25mm eyepiece

  1. #1
    mborger's Avatar
    mborger is offline Star Giants
    Points: 955, Level: 17
    Level completed: 55%, Points required for next Level: 45
    Overall activity: 0%
    Achievements:
    2 Posts Achievement365 Days+ Registered Achievement!750 Days+ Registered Achievement!
    Join Date
    Apr 2015
    Location
    San Diego, CA
    Posts
    9
    Points
    955
    Level
    17
    Thanks
    1
    Thanked 0x 0 Posts

    Default 9mm vs 25mm eyepiece



    Hi everyone. Admittedly a newbie questions here, but I'm still learning my way around my Celestron 127SLT and the planets lately have been more than I could have dreamed of. The other night not only did I see Venus in phase, Jupiter and the four Galilean moons, but I also saw Saturn for the first time and its rings -- absolutely amazing.

    One question I have is that the scope came with both 9mm and 25mm eyepieces. The salesman said that I would have no real use for one of them (though I can't remember which). When would each be appropriate? I don't want to waste viewing time with the wrong eyepiece.

    Thank you.

  2. #2
    KathyNS's Avatar
    KathyNS is offline Super Moderator
    Points: 173,398, Level: 100
    Level completed: 0%, Points required for next Level: 0
    Overall activity: 32.0%
    Achievements:
    200+ Posts Achievement!Ghost Achievement! Averaging 5+ posts a day!First 1000 Experience Points400+ Posts AchievementGot three Friends
    Awards:
    Reply Award
    Join Date
    Nov 2010
    Location
    Nova Scotia
    Posts
    25,188
    Points
    173,398
    Level
    100
    Thanks
    6,660
    Thanked 14,988x 9,544 Posts
    Blog Entries
    3

    Default Re: 9mm vs 25mm eyepiece

    The 9mm is a bit over-powered for that scope. On a night of particularly good seeing, it might get you some good views, but it's not something you can count on every night. Your normal minimum focal length will be 12mm.
    mborger likes this.

    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 scope; 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.

  3. #3
    gcisko's Avatar
    gcisko is offline SUPER GIANT
    Points: 7,520, Level: 60
    Level completed: 85%, Points required for next Level: 30
    Overall activity: 0%
    Achievements:
    2 Posts Achievement20 Posts Achievement!50 Posts Achievement!200+ Posts Achievement!5 Threads Achievement!
    Join Date
    Jan 2015
    Location
    Illinois
    Posts
    1,005
    Points
    7,520
    Level
    60
    Thanks
    9
    Thanked 168x 156 Posts

    Default Re: 9mm vs 25mm eyepiece

    Quote Originally Posted by mborger View Post
    Hi everyone. Admittedly a newbie questions here, but I'm still learning my way around my Celestron 127SLT and the planets lately have been more than I could have dreamed of. The other night not only did I see Venus in phase, Jupiter and the four Galilean moons, but I also saw Saturn for the first time and its rings -- absolutely amazing.

    One question I have is that the scope came with both 9mm and 25mm eyepieces. The salesman said that I would have no real use for one of them (though I can't remember which). When would each be appropriate? I don't want to waste viewing time with the wrong eyepiece.

    Thank you.
    The 25mm is a definite keeper for wide field views. The 9mm may be a bit too much magnification as Keith says. The reality though it there are times where both are appropriate. Experience will teach you which.
    mborger likes this.
    Celestron 8SE | StarSense & SkySync GPS | 2" Star Diagonal | Rigel Quickfinder
    F 6.3
    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.
    | Powertank |
    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.
    7x50 Binoculars
    Astrotech AT72ED | Star Adventurer by
    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.

    Eyepieces: Tele Vue 32mm WideField & 6mm Ethos | Nagler - 9mm & 16mm Type 2
    Baader Hyperion 21mm, 13mm, 5mm with tuning rings
    University Optics Plossl 12.5mm Illuminated Reticle
    Filters: Lumimcon & Baader OIII, Baader Sky &
    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.
    Glow, 13% Moon

  4. #4
    mborger's Avatar
    mborger is offline Star Giants
    Points: 955, Level: 17
    Level completed: 55%, Points required for next Level: 45
    Overall activity: 0%
    Achievements:
    2 Posts Achievement365 Days+ Registered Achievement!750 Days+ Registered Achievement!
    Join Date
    Apr 2015
    Location
    San Diego, CA
    Posts
    9
    Points
    955
    Level
    17
    Thanks
    1
    Thanked 0x 0 Posts

    Default Re: 9mm vs 25mm eyepiece

    Thanks - so basically, the mm rating is the opposite of a camera (higher mm number is a more magnified view).

    To that end, wouldn't a higher-powered eyepiece, like the 9mm, make objects (ex. Saturn and its rings) appear closer than they do in my 25mm?

    Perhaps I'm missing a key idea here.

  5. #5
    arsene37's Avatar
    arsene37 is offline Bronze Supporter
    Points: 8,238, Level: 63
    Level completed: 30%, Points required for next Level: 212
    Overall activity: 0%
    Achievements:
    2 Posts Achievement20 Posts Achievement!50 Posts Achievement!200+ Posts Achievement!Gallery Achievement!
    Join Date
    Jun 2013
    Location
    Amboise, France
    Posts
    407
    Points
    8,238
    Level
    63
    Thanks
    248
    Thanked 112x 94 Posts

    Default Re: 9mm vs 25mm eyepiece

    Hi !
    Quote Originally Posted by mborger View Post
    To that end, wouldn't a higher-powered eyepiece, like the 9mm, make objects (ex. Saturn and its rings) appear closer than they do in my 25mm?
    You are right : short eyepiece focal length gives greater magnification and smaller true field of view (with usual limits), and long one gives smaller magnification and greater true field of view.
    Limitation are : bad definition if your EP has too short focal length, and if your EP has too long focal length, you will not use the extern part of your mirror (light rays will not go inside your eye).

    I use a 12 inches dobson, and only in my yard, with no real steps, so I move it from house with homemade wheelbarrow and inflatable wheels ; I put it on pieces of wood to avoid waddles.
    Have a good day (or night for you ? )
    Arsène
    mborger likes this.
    Petzval quadruplet 152/760 — XX12
    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.
    Skyquest — 5.5" Celestron Comet Catcher (30 years old) — Perl-Vixen 130 mm/720 mm EQ — 8 mmm, 17mm and 31 mm aspheric Hyperion — BST Explorer ED 5 mm — Baader MPCC II — Leica EP 20x-60x — 2x
    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.
    — TS Optics Binoculars 20x80 Triplet.

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

  6. #6
    jerryTheC's Avatar
    jerryTheC is offline Super Moderator
    Points: 26,972, Level: 100
    Level completed: 0%, Points required for next Level: 0
    Overall activity: 11.0%
    Achievements:
    365 Days+ Registered Achievement!50 Posts Achievement!5 Threads Achievement!200+ Posts Achievement!First 1000 Experience Points
    Join Date
    Apr 2011
    Location
    Harrow, UK
    Posts
    6,031
    Points
    26,972
    Level
    100
    Thanks
    335
    Thanked 1,915x 1,623 Posts

    Default

    Magnification is scope focal length divided by eyepiece focal length, so a shorter focal length eyepiece gives you higher magnification but less brightness (you're spreading the same amount of light out more).

    Planets are nice and bright, so high magnifications work well, but there's also a limit to how much magnification you can usefully use. As a ballpark guide, the everyday max magnification is around your scope's aperture in mm - and you get that with an eyepiece with focal length equal to your scopes f-ratio - so 12mm for an f12 scope.

    Given perfect conditions, you can go to around twice that magnification (half the eyepiece focal length) which is what the manufacturers usually quote as the max magnification - but most of the time atmospheric turbulence ("seeing") tends to mush things up, hence the lower everyday guideline.
    mborger likes this.

  7. #7
    TCampbell's Avatar
    TCampbell is offline HYPER GIANT
    Points: 13,047, Level: 78
    Level completed: 85%, Points required for next Level: 53
    Overall activity: 0%
    Achievements:
    2 Posts Achievement20 Posts Achievement!50 Posts Achievement!200+ Posts Achievement!365 Days+ Registered Achievement!
    Join Date
    Apr 2012
    Location
    Dearborn, MI
    Posts
    1,797
    Points
    13,047
    Level
    78
    Thanks
    4
    Thanked 759x 569 Posts

    Default Re: 9mm vs 25mm eyepiece

    Your scope has a focal ratio of f/12. As a general guideline, you typically will not want to use any eyepiece whose focal length is shorter than the focal ratio of the telescope. In other words, since you have an f/12 scope, you won't want to use any eyepiece with a focal length shorter than 12mm (so the 9mm is out).

    This rule isn't absolute... it's just a good guideline.

    You can put any eyepiece into the telescope (as long as the barrel size fits in the eyepiece tube.) But the more you magnify, the less sharp the image will be (you are magnifying all of the flaws). The larger the diameter of the telescope, the more it can resolve finer detail. A scope's theoretical useful magnification is 2.5x the diameter of the scope when measured in millimeters or 50x the diameter if measured in inches. Your 127mm scope would have a limit of 254x. It turns out that the useful limit is really based on IDEAL viewing circumstances -- so ideal that they aren't very realistic. You need very steady skies usually only possible if you're on a small island in the middle of a large flat body of water with no warm fronts, cold fronts, or jet streams within 200 miles of you (I basically never get these "ideal" viewing conditions. If you do, it may just be a few nights per year. So using about half of that maximum useful limit tends to be the more practical guideline.

    When you do the math... anytime the focal length of the eyepiece is EQUAL to the focal length of the telescope, then the magnification will be equal to 1x the diameter of the scope when measured in millimeters. And that lower end limit tends to be more useful. Every so often I get a night of below-average (even for my location) seeing conditions and I have to back off on the magnification or I won't be happy with the quality of what I can see in the scope.

    If you get a night with PARTICULARLY good seeing conditions (really steady skies) then the detail can really pop for you and you'll see features that were never visible to you before. Don't pitch the 9mm in the waste bin... if you get one of those rare better-than-average nights then the 9mm will be useful to you. The lower theoretical limit for your scope is 6mm.

    If you wanted an eyepiece with more magnification than your 25mm... but likely to be useful on most nights (unlike the 9mm which requires above-average seeing conditions) then I might suggest looking for something around the 15mm focal length. That's not right down to the 12mm limit so it gives you a little bit of a buffer.

    The atmosphere itself works a bit like a lens. Objects near the horizon will be more distorted than objects up high near the zenith. But overall stability will distort views too. The analogy I like to use is to consider a coin in the bottom of a pool. If nobody is making any waves and the water is perfectly still, then you can probably get a clear view of the coin in the bottom of the pool. But now imaging that someone is making some waves in your pool. You'll have difficulty seeing the coin. You may be able to tell that the coin is there, but you'll probably never really get a good view of it. Atmospheric "seeing" conditions are a bit like this.

    The scope can only do so much... but then you're at the mercy of the sky quality.
    bladekeeper and mborger like this.
    Tim Campbell

    Scopes: PlaneWave 12.5” CDK - Meade 14" LX-200 ACF - TeleVue NP101is - Lunt LS80T Ha/PT/DSII
    Mounts: Losmandy G11 &
    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.
    2 - Losmandy GM8 & Gemini 2 - Meade LX80
    Cameras: ZWO ASI128MC-Pro - ZWO ASI174MM-Cool - Canon 60Da

  8. #8
    mwillis321's Avatar
    mwillis321 is offline Bright Giants
    Points: 5,388, Level: 50
    Level completed: 19%, Points required for next Level: 162
    Overall activity: 1.0%
    Achievements:
    2 Posts Achievement20 Posts Achievement!Got three Friends50 Posts Achievement!20+ Friends Achievement!
    Join Date
    Apr 2015
    Location
    State College, Pennsylvania
    Posts
    309
    Points
    5,388
    Level
    50
    Thanks
    491
    Thanked 309x 130 Posts

    Default Re: 9mm vs 25mm eyepiece

    I'd also add that it's never really wasting time using any of your EPs on different objects. Although there are some limits and rough guidelines, EPs are highly personal to the specific person using them and the scope they're using them in. I try as many possible magnifications as I reasonably can (obviously if I can't find or bring an object into good focus at a lower magnification I won't try anything smaller) on every object I view from planets to DSOs. So in other words, you won't know what works best for you or what you like most about different EPs and magnifications if you don't spend/waste time using different combinations of them on objects you're viewing.

    Although the 9mm is slightly over your normal maximum magnification, the manufacturers usually provide EPs that will work with the scope they've come with, and since you already own the 9mm EP I'd use it, especially on the planets, but a good bit of DSOs will take some magnification well too. Typically you'll want the 25mm EP for locating objects and viewing DSOs. Again try both on everything you view, along with any other combination of EPs you currently have.

    Best of luck, and clear, dark skies.
    mborger likes this.
    Matt

    Telescope:
    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.
    f/4.9 Dob, ED80CF f/6 APO Triplet
    Mount:
    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.
    EQ-G
    Camera: Canon EOS Rebel T6i

  9. #9
    smeyer8015's Avatar
    smeyer8015 is offline HYPER GIANT
    Points: 29,208, Level: 100
    Level completed: 0%, Points required for next Level: 0
    Overall activity: 4.0%
    Achievements:
    2 Posts Achievement20 Posts Achievement!5 Threads Achievement!50 Posts Achievement!200+ Posts Achievement!
    Join Date
    Jul 2014
    Location
    Independence, MO
    Posts
    4,394
    Points
    29,208
    Level
    100
    Thanks
    5
    Thanked 1,375x 960 Posts

    Default Re: 9mm vs 25mm eyepiece

    You might want to get a 3rd EP around 18mm. This would give you about 83x.
    I highly recommend the Celestron X-Cel LX series. Just under $60. Very good EP for the price.
    Then down the road a 30mm and 2x barlow would give you a nice range of magnification.

    Scott
    mborger likes this.
    Celestron Classic 8, Old Sears 60mm f/15 refractor, Old Edmund Scientific 6 inch F/8 newt, GSO 2" Diag, ES 30mm/70, ES 25mm/70,
    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.
    32 Super Plossl, Mead 24mm MA,
    X-Cel LX 18mm, Luminos 15mm, F/6.3 reducer, Luminos 2.5x
    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.
    , Telrad, Celestron 9x50 RACI, DIY Baader Solar Filter, Celestron Skymaster 20x80, Bresser EXOS-2
    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.
    , Revolution Imager R1

    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.
    No trees were killed in the sending of this post. However, a large number of electrons were terribly inconvenienced
    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.

  10. #10
    mborger's Avatar
    mborger is offline Star Giants
    Points: 955, Level: 17
    Level completed: 55%, Points required for next Level: 45
    Overall activity: 0%
    Achievements:
    2 Posts Achievement365 Days+ Registered Achievement!750 Days+ Registered Achievement!
    Join Date
    Apr 2015
    Location
    San Diego, CA
    Posts
    9
    Points
    955
    Level
    17
    Thanks
    1
    Thanked 0x 0 Posts

    Default Re: 9mm vs 25mm eyepiece

    This is fantastic. I appreciate everyone of you for providing your opinions and insight. I've read every response and will bookmark this page because I think I'll need to refer to it repeatedly. For now, I'm going to experiment with both EPs and develop some comfort with both and then likely pick up a medium length as recommended a couple times. I'm in San Diego, so we normally have decent skies.

    On a related note, is there a page on here with diagrams that explains the math of scopes in general? I'm a visual learner and I think that will dramatically shorten my learning curve.

 

 
Page 1 of 2 12 LastLast

Similar Threads

  1. 25mm eyepiece is just a tad too wide to fit...
    By NBertrand in forum Telescope Eyepieces Forum
    Replies: 4
    Last Post: 12-27-2014, 05:15 AM
  2. First Light on a Celestron X-Cel 25mm eyepiece
    By wgbeatty in forum Astronomy Reports Forum
    Replies: 6
    Last Post: 05-18-2013, 09:44 PM
  3. 25mm vs 32mm eyepiece
    By svc52 in forum Telescope Eyepieces Forum
    Replies: 14
    Last Post: 03-08-2011, 01:32 PM
  4. Antares W70 25mm eyepiece
    By Jim Easterbrook in forum UK Astronomy Forum
    Replies: 0
    Last Post: 11-12-2003, 08:38 PM
  5. Antares W70 25mm eyepiece
    By Jim Easterbrook in forum Amateur Astronomy Forum
    Replies: 0
    Last Post: 11-12-2003, 08:38 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 08:48 AM.