Results 1 to 8 of 8
  1. #1
    tts's Avatar
    tts
    tts is offline White Dwarf
    Points: 800, Level: 15
    Level completed: 34%, Points required for next Level: 100
    Overall activity: 0%
    Achievements:
    365 Days+ Registered Achievement!2 Posts Achievement20 Posts Achievement!5 Threads Achievement!
    Join Date
    Sep 2011
    Location
    Seattle
    Posts
    23
    Points
    800
    Level
    15
    Thanks
    20
    Thanked 0x 0 Posts

    Default Understanding Eyepieces for My Scope



    I have a C6-SGT f/10 telescope and have a few questions about eyepieces.

    1. My understanding is that the longer the focal length of your telescope, the more the image is "zoomed in", so-to-speak. On EPs, however, this is the opposite. Short focal length EPs offer an extremely narrow FOV when compared to a longer focal length which is more suited for DSO viewing. Is that correct?

    2. Someone told me that the optimal EP for a telescope is that which resolved at about a 2mm exit pupil (where the eye can perceive the most detail). This can be roughly determined by multiplying the focal ratio of your scope by 2 (e.g. f/10 x 2 = 20mm). Is there validity to that?

    3. I know a Barlow lens is no good for this scope. Is that because it would effectively increase the focal length on a scope that has a large focal length already? In other words, you're adding zoom to something that's already super-zoomed.

    Thanks in advance for any help.
    Celestron C6-SGT, Plossl 1.25" EP, CG-5 GT EQ Mount, Nikon D3100, Nikon 18-55mm f/3.5-5.6, MacBook Air, Stellarium, Pixelmator

  2. #2
    Joe Lalumia's Avatar
    Joe Lalumia is offline HYPER GIANT
    Points: 61,001, Level: 100
    Level completed: 0%, Points required for next Level: 0
    Overall activity: 0%
    Achievements:
    200+ Posts Achievement!Ghost Achievement! Averaging 5+ posts a day!Got three FriendsFirst 1000 Experience Points20+ Friends Achievement!
    Join Date
    Jun 2010
    Location
    Quinlan, Texas
    Posts
    10,947
    Points
    61,001
    Level
    100
    Thanks
    6,772
    Thanked 5,156x 3,400 Posts
    Blog Entries
    1

    Default

    You are OVER- analyzing this stuff.

    Your F 10 scope is a slow scope vs say an F4.5. For all practical purposes this means you can use lower cost eyepieces and still get great views, edge to edge, in the SLOW scope.

    A 2x barlow simply means a 20mm eyepiece, for example, can be used as a 10mm eyepiece-- effectively doubling your eyepiece collection at low cost. While keeping the eye relief of the 20mm. Which is probably better than the 10mm single eyepiece you were planning to buy.

    Just divide the number of the eyepiece into the focal length of your scope-- say it's 1200mm.

    1200mm divided by 10mm = 120 power magnification.

    if we had a scope of say 600mm and did the same thing with the same 10mm eyepiece it would give us 60 power magnification.

    For visual observing try to use eyepieces with at LEAST a 55 degree field of view-- upgrade the eyepieces a little to a 60-65 degree field of view eyepiece would be ideal for observing.

    Personally I do NOT like eyepieces with over about 70 degrees field of view but that's my personal opinion.
    ETX 125PE, Stellarvue 80mm BV & Televue TelePod tripod,
    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.
    8" LNT, 10x50, 15x70mm binoculars, Stellarvue binoviewers, solar filters for all three
    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.
    ..... plus a bunch of ham radios... Ham radio call sign - W1XWX

    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.


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

    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.
    search for W1XWX to see my amateur radio web site

    "If you can't explain it to a six year old, you don't understand it yourself.” - Albert Einstein

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

  3. The Following User Says Thank You to Joe Lalumia For This Useful Post:

    dmbryan (09-23-2011)

  4. #3
    Bob327's Avatar
    Bob327 is offline HYPER GIANT
    Points: 41,366, Level: 100
    Level completed: 0%, Points required for next Level: 0
    Overall activity: 0%
    Achievements:
    10 Days registered200+ Posts Achievement!First 1000 Experience PointsGot three Friends365 Days+ Registered Achievement!
    Join Date
    Sep 2009
    Location
    Frederick Maryland
    Posts
    9,247
    Points
    41,366
    Level
    100
    Thanks
    19
    Thanked 4,039x 3,068 Posts

    Default

    I have to agree COMPLETELY with Joe Lalumia yet again... lol

    But You are over thinking "eyepeices".. and their uses... You definitely can make good use of a 2x Barlow as Joe mentioned it basically doubles your eyepiece collection ...nothing wrong with having 20 25 30 and 40 mm eyepieces ans a barlow...that basically gives you a 10, 12.5 , 15, 20 , 25, 30 and a 40 mm and all of them will have good eye relief. and be more comfortable to use...

    I believe you telescope is a 1500 mm scope so doing the math your 7 eyepieces become 7 (the 40 barlowed is the same as your 20mm)

    BTW....I'm more then willing to bet that you will be viewing using lower powered eyepieces much more then you will using high powered eyepieces...astronomy is all about gathering light and not about magnification... I also agree with Joe in that I also avoid using eyepieces with a wider FOV then about 68-70 degrees ... BUT we both are in the minority about that...

    CLEAR SKIES

    Bob G
    CPC1100 housed in a slotted domed observatory (Exploradome) 4 and 5 inch refractors for use from the lawn, a 8" Sct (NS 8i) for star parties...
    I Hate the winter so I use heated Motorcycle clothing to stay warm while observing in winter
    Retired, also have 2 other hobbies
    1. tinker with older Corvettes (6 in garage)
    2. make a heck of a lot of sawdust in my wood shop.

  5. The Following 2 Users Say Thank You to Bob327 For This Useful Post:

    dmbryan (09-23-2011),Joe Lalumia (09-24-2011)

  6. #4
    MitchAlsup's Avatar
    MitchAlsup is offline SUPER GIANT
    Points: 5,847, Level: 52
    Level completed: 49%, Points required for next Level: 103
    Overall activity: 0%
    Achievements:
    200+ Posts Achievement!First 1000 Experience Points400+ Posts Achievement50 Posts Achievement!365 Days+ Registered Achievement!
    Join Date
    Sep 2011
    Location
    Austin Texas
    Posts
    1,066
    Points
    5,847
    Level
    52
    Thanks
    1
    Thanked 505x 381 Posts

    Default

    1. My understanding is that the longer the focal length of your telescope, the more the image is "zoomed in", so-to-speak.

    You should think of it more like:: a longer focal length creates an image at a bigger scale.

    On EPs, however, this is the opposite. Short focal length EPs offer an extremely narrow FOV when compared to a longer focal length which is more suited for DSO viewing. Is that correct?

    The focal length of the scope divided by the focal length of the eyepiece is the magnification::

    Mag = FLscope/FLep

    2. Someone told me that the optimal EP for a telescope is that which resolved at about a 2mm exit pupil (where the eye can perceive the most detail). This can be roughly determined by multiplying the focal ratio of your scope by 2 (e.g. f/10 x 2 = 20mm). Is there validity to that?

    Somewhere in the 1mm-2mm range of exit pupils is where the human eye can resolve maximum details over a significant area. You can push higher 0.5mm-1mm exit pupils to darken the sky to look at things that are just on the edge of detection.

    3. I know a Barlow lens is no good for this scope. Is that because it would effectively increase the focal length on a scope that has a large focal length already? In other words, you're adding zoom to something that's already super-zoomed.

    There is no reason that a barlow would be of any harm whatsoever. A barlow is an EyePiece magnification device. A barlow can double the effective number of eyepieces you own and operate (given that you choose EPs wisely).

    Given a 2X barlow: and 30mm, 20mm, 13mm eyepieces; the barlow makes the 30mm into a 15mm, makes the 20mm into a 10mm, and makes the 13mm into a 6.5mm. Notice that there is no duplication in available focal lengths. So, in effect, you have access to 30mm, 20mm, 15mm, 13mm, 10mm, and 6.5mm.

  7. #5
    j.gardavsky's Avatar
    j.gardavsky is offline Super Moderator
    Points: 111,430, Level: 100
    Level completed: 0%, Points required for next Level: 0
    Overall activity: 53.0%
    Achievements:
    Ghost Achievement! Averaging 5+ posts a day!200+ Posts Achievement!400+ Posts AchievementFirst 1000 Experience Points365 Days+ Registered Achievement!
    Join Date
    Feb 2011
    Location
    Germany
    Posts
    14,418
    Points
    111,430
    Level
    100
    Thanks
    14,420
    Thanked 8,798x 5,726 Posts

    Default

    Hello tts, hello Mitch Alsup,

    I am using a very simple rule of the thumb: 1-2-3, and eventually 4.

    1. You'll simply take the f number of your scope, like 10, and your maximum reasonable magnifying EP will be f=10mm. Herewith your scope with such an eyepiece choice gives you its exit pupil equal to 1mm. Good for the Moon, planets, globular clusters, binary stars. Herewith you'll experience a crisp view. With a 6incher at f/10 this will be a magnification of 150x. Magnifying any large is just magnifying, not necessarilly giving more details, but loosing brightness and some contrast.

    2. You'll multiply the f number of your scope, like 10, with the number 2, and get in this case the EP focal length of 20mm. This is the case of the exit pupil of 2mm, frequently recommended. Lots of the tiny DSOs may reveal some detail, the Ring Nebula will show a donut, the Dumbbell Nebula will show the "cork" inside. With a 6incher at f/10 this will be a magnification of 75x.

    3. You'll multiply the f number of your scope, like 10, with the number 3, and get in this case the EP focal length of 30mm. This is the case of the exit pupil of 3mm - my working horse EP under the suburban skies, excellent for hunting the faint galaxies, and most of the open clusters and large emission nebulas. With a 6incher at f/10 this will be a magnification of 50x.

    Check, if such EPs are in your set, or if the EPs you already have, are close the values calculated this way. A 20% difference (plus or minus) in the EP focal length to the values as calculated above, does not matter much or even at all.

    Lets take all simple and try it first out before spending money

    JG
    Last edited by j.gardavsky; 09-24-2011 at 06:11 PM.
    Binoculars: Leica Ultravid 7x42, 8x42HD; Swarovski EL 8.5x42 Swarovision; Nikon 10x70 Astroluxe; Docter Nobilem 7x50 Porro; Jenoptem 7x50W, 10x50W; BA8: 10.5x70, 15x85; 25x100FB, AsahiPentax 8x40, Refractors: Sky-Watcher 150mm/750mm; Leica APO Televid 82mm (25x-50x WW ASPH); EPs:Baader Classic Orthos; Fujiyama ortho, Leica B WW, ultrawide zoom ASPH, Periplan GF, HC Plan S, L; DOCTER UWA; Wild UW mil; Tele Vue Delos, Nagler Zoom, Plössls; Swarovski SW; Pentax XW; ZEISS diascope B WW T*, Carl Zeiss E-Pl; Hensoldt mil; Filters: Astrodon, Astronomik, Baader (CCD), TS;
    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.
    : AstroTrac; Leica R7: Leica 2/50, 2/90mm, 2.8/180mm lenses
    Astronomy Forum Rules and Terms of Service
    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.

  8. The Following 2 Users Say Thank You to j.gardavsky For This Useful Post:

    Joe Lalumia (09-24-2011),Johann von Hesse (09-27-2011)

  9. #6
    tts's Avatar
    tts
    tts is offline White Dwarf
    Points: 800, Level: 15
    Level completed: 34%, Points required for next Level: 100
    Overall activity: 0%
    Achievements:
    365 Days+ Registered Achievement!2 Posts Achievement20 Posts Achievement!5 Threads Achievement!
    Join Date
    Sep 2011
    Location
    Seattle
    Posts
    23
    Points
    800
    Level
    15
    Thanks
    20
    Thanked 0x 0 Posts

    Default

    Interesting... thanks! I'll pick up a few of those to start and see where it takes me.
    Celestron C6-SGT, Plossl 1.25" EP, CG-5 GT EQ Mount, Nikon D3100, Nikon 18-55mm f/3.5-5.6, MacBook Air, Stellarium, Pixelmator

  10. #7
    MitchAlsup's Avatar
    MitchAlsup is offline SUPER GIANT
    Points: 5,847, Level: 52
    Level completed: 49%, Points required for next Level: 103
    Overall activity: 0%
    Achievements:
    200+ Posts Achievement!First 1000 Experience Points400+ Posts Achievement50 Posts Achievement!365 Days+ Registered Achievement!
    Join Date
    Sep 2011
    Location
    Austin Texas
    Posts
    1,066
    Points
    5,847
    Level
    52
    Thanks
    1
    Thanked 505x 381 Posts

    Default

    Quote Originally Posted by j.gardavsky View Post
    I am using a very simple rule of the thumb: 1-2-3, and eventually 4.
    The 2 and 3 part of your synopsis is the 2/3 or 3/2 that I use to space my EPs. Starting with a 30mm nagler-width FoV:: 30mm * 2/3 = 20mm * 2/3 = 13mm. Then throw in a 2X barlow and get 15mm, 10mm and 6.5mm for free. Leaving only a gap in the 8mm range.

    Thus, your 1,2,3 rule of thumb is highly related to my 2/3 or 3/2 geometric multiplier.

  11. The Following User Says Thank You to MitchAlsup For This Useful Post:

    j.gardavsky (09-27-2011)

  12. #8
    j.gardavsky's Avatar
    j.gardavsky is offline Super Moderator
    Points: 111,430, Level: 100
    Level completed: 0%, Points required for next Level: 0
    Overall activity: 53.0%
    Achievements:
    Ghost Achievement! Averaging 5+ posts a day!200+ Posts Achievement!400+ Posts AchievementFirst 1000 Experience Points365 Days+ Registered Achievement!
    Join Date
    Feb 2011
    Location
    Germany
    Posts
    14,418
    Points
    111,430
    Level
    100
    Thanks
    14,420
    Thanked 8,798x 5,726 Posts

    Default

    Hello MitchAlsup,

    there are lots of ways how tooptimize the EP set.

    Back to the 1-2-3 rule, I am mostly using the EPs ad3) and ad1).

    Thanks for your post

    JG
    Binoculars: Leica Ultravid 7x42, 8x42HD; Swarovski EL 8.5x42 Swarovision; Nikon 10x70 Astroluxe; Docter Nobilem 7x50 Porro; Jenoptem 7x50W, 10x50W; BA8: 10.5x70, 15x85; 25x100FB, AsahiPentax 8x40, Refractors: Sky-Watcher 150mm/750mm; Leica APO Televid 82mm (25x-50x WW ASPH); EPs:Baader Classic Orthos; Fujiyama ortho, Leica B WW, ultrawide zoom ASPH, Periplan GF, HC Plan S, L; DOCTER UWA; Wild UW mil; Tele Vue Delos, Nagler Zoom, Plössls; Swarovski SW; Pentax XW; ZEISS diascope B WW T*, Carl Zeiss E-Pl; Hensoldt mil; Filters: Astrodon, Astronomik, Baader (CCD), TS;
    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.
    : AstroTrac; Leica R7: Leica 2/50, 2/90mm, 2.8/180mm lenses
    Astronomy Forum Rules and Terms of Service
    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.

 

 

Similar Threads

  1. 10i build' a scope eyepieces??
    By Heirwalker in forum Telescope Eyepieces Forum
    Replies: 5
    Last Post: 05-19-2010, 02:45 AM
  2. Got my new scope, now need to find some eyepieces
    By Orof in forum Telescope Eyepieces Forum
    Replies: 4
    Last Post: 04-23-2009, 05:11 PM
  3. gifted a very old scope - need eyepieces!
    By sc in forum UK Astronomy Forum
    Replies: 4
    Last Post: 08-21-2008, 05:45 PM
  4. fast scope eyepieces
    By ULB in forum Amateur Astronomy Forum
    Replies: 3
    Last Post: 02-20-2004, 02:35 AM
  5. What eyepieces should I get for my scope?
    By Dave in forum Amateur Astronomy Forum
    Replies: 0
    Last Post: 07-22-2003, 06:03 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 10:44 AM.