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    Default Eyepiece Focal Length Question



    Hi,

    I managed to see objects during day time. We used 20mm and 10 mm lenses both. Still did not mount red dot finder due to busy schedule but should be able to do. Or will ask you all for further help. BTW, to see Space objects, what is the ideal focal length for lens? I mean if I want to see Moon Craters or even venus/Saturn etc., shall I use lower focal length lenses like 5mm or even 4mm? Which size will give good result while having clarity too.

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    Default Re: Red dot finder

    Normally your focal ratio determines the eyepieces you can use so for a 70mm with a focal length of 900 will make it a F 12.8 so a 12 mm eyepiece in theory should be your lowest eyepiece however with long refractors I find one can easily lower this by at least 20% so in your case a 9 or 10mm
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    Default Re: Eyepiece Focal Length Question

    Here's a great explanation of eyepieces in relation to your telescope.

    Your Complete Telescope Eyepiece Guide from Sky & Telescope


    Also keep in mind that the sky's seeing and transparency also becomes a deciding factor when you can use your higher powered eyepieces.
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    Default Re: Red dot finder

    I already have 10mm and 20mm Lenses (came with the Telescope). So I guess anything lower than 10mm will not be useful. Right?

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    Default Re: Red dot finder

    Quote Originally Posted by AkashS04 View Post
    I already have 10mm and 20mm Lenses (came with the Telescope). So I guess anything lower than 10mm will not be useful. Right?
    It depends on the scope and the seeing and transparency conditions at the time. What scope do you have? You can go over slightly suggested shorter mm eyepieces on exceptionally clear nights I have found. So far for me 5mm has been the limit in my 1200mm doublet refractor.
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    Default Re: Red dot finder

    Ok. What is the benefit of smaller size lenses? It will zoom more? So view of Saturn or Jupiter will be better? But blur?

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    Default Re: Eyepiece Focal Length Question

    For members to accurately provide information and help, we need to know what scope you have... or at least the focal ratio (f/?)
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    Default Re: Eyepiece Focal Length Question

    Quote Originally Posted by Lowjiber View Post
    For members to accurately provide information and help, we need to know what scope you have... or at least the focal ratio (f/?)
    I have Celestron AstroMaster 70AZ (Celestron 21061). Focal Length is 900mm. The link to this product is: https://www.bhphotovideo.com/c/produ...Refractor.html
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    Default Re: Eyepiece Focal Length Question

    Thank you for the information regarding your scope. Now, we can get down to addressing your question.

    Quote Originally Posted by AkashS04 View Post
    ...BTW, to see Space objects, what is the ideal focal length for lens? I mean if I want to see Moon Craters or even venus/Saturn etc., shall I use lower focal length lenses like 5mm or even 4mm? Which size will give good result while having clarity too.
    Quote Originally Posted by AkashS04 View Post
    I have Celestron AstroMaster 70AZ (Celestron 21061). Focal Length is 900mm...
    The first thing to get out of the way is the focal ratio of your scope. It's the Focal Length/aperture = 900/70 = f/13. Remember that number (13). It can be your guide to eyepiece focal length for as long as you use the scope.

    Here's a simple relationship between magnification and surface brightness... As magnification increases (ie... eyepiece focal length decreases), the surface brightness of your target decreases. With solar system objects (not the Sun!), that is seldom a problem. Planets like Jupiter, Saturn, etc. are bright enough that the above doesn't have much effect. The outer, dim planets (spell that Pluto, Neptune, etc.) will be tough enough to view with a 70mm scope and you'll need as much surface brightness as you can get just to catch a glimmer of them (spell that a long focal length eyepiece).

    Let's talk about atmosphere for a moment... There are three factors that must be considered in eyepiece selection: Seeing, Transparency, and Altitude of the target above the horizon.

    Seeing is a somewhat arbitrary method of describing the turbulence present in the air between you and the target.

    Transparency is an arbitrary method of describing the "junk" (moisture, dirt, pollen, etc.) that is between you and the target.

    We often sorta lob those two things together when talking about how good the seeing is. A good night will find the upper air mass fairly steady and the transparency very "clear" due to a lack of "junk" suspended in the air. That seems difficult to assess, however if you walk outside and the stars are twinkling, that's not good because it indicates a lack of transparency in the air that is causing photons to be deflected on the way to your eye.

    The altitude of the target is another consideration. If you look straight up (zenith), you are looking through the least amount of atmosphere that is possible. As your target's altitude decreases toward the horizon, the amount of atmosphere between you and the target increases. There is a point below which most astronomers simply will not attack a target, called the 2x point. The 2x point is where the altitude of the target is 30 degrees above the horizon. It's called the 2x point because that's where you have twice the amount of air between you and the target that you have when looking at zenith (straight up).

    Now, back to your scope and that magic number 13...

    With the exception of the moon, your maximum useful magnification will be obtained with an eyepiece whose Focal Length is approximately equal to your scope's focal ratio. By "useful", I mean the target will likely focus just fine. The caveat to that is that the seeing/transparency/target altitude must all be "excellent"... something that will happen to you perhaps three sessions a year. (sorry)

    The moon because it's so close and so bright can take a ton of magnification, even on an "average" seeing night. Many astronomers carry an eyepiece with a Focal Length that is around 50-60% of their scope's focal ratio, just for viewing details on the moon. In your case, a 6mm eyepiece will likely fill the bill. If the seeing is not great, just back the eyepiece off a a bit and continue moon watching.

    Here's the last thing I want you to remember... Generally speaking, in good seeing conditions the optimum blend of magnification and surface brightness is attained with an eyepiece whose Focal Length is approximately equal to twice the scope's focal ratio expressed in millimeters. In your case, a 25mm eyepiece will fit the bill.

    While a 70mm f/13 scope is primarily a solar system device, you can still see some Deep Space Objects (M42, the Andromeda Galaxy (M31), and some brighter star clusters) pretty well on a good seeing night. Agreed, the fine details won't pop out at you, but the views can still be stunning. What eyepiece are you gonna use? Why the 25mm, of course.

    Most beginning astronomers are under the misconception that this hobby is all about magnification. It's not. It's all about gathering as many photons at a given moment as possible. Your beginner 70mm is great for getting a feel for the hobby and what is possible. Keep in mind that it is a decent sized planetary/moon scope that will serve you well for years to come in that role. Don't be discouraged when you can't find all 110 Messier objects with it... it's simply not designed to do that.

    There's an old saying that aperture rules the deep space arena. A 10" Dob (pretty much the norm in DSO viewing) delivers over 400% more photons than your 70mm... opening up those very dim deep space targets. I encourage you to learn as much as you can with your present scope, and when "aperture fever" strikes... buy as much aperture as you can afford.

    I hope that helped some.

    Clear, Dark Skies
    Last edited by Lowjiber; 02-28-2017 at 03:15 PM.
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    Default Re: Eyepiece Focal Length Question

    Quote Originally Posted by Lowjiber View Post
    Thank you for the information regarding your scope. Now, we can get down to addressing your question.





    The first thing to get out of the way is the focal ratio of your scope. It's the Focal Length/aperture = 900/70 = f/13. Remember that number (13). It can be your guide to eyepiece focal length for as long as you use the scope.

    Here's a simple relationship between magnification and surface brightness... As magnification increases (ie... eyepiece focal length decreases), the surface brightness of your target decreases. With solar system objects (not the Sun!), that is seldom a problem. Planets like Jupiter, Saturn, etc. are bright enough that the above doesn't have much effect. The outer, dim planets (spell that Pluto, Neptune, etc.) will be tough enough to view with a 70mm scope and you'll need as much surface brightness as you can get just to catch a glimmer of them (spell that a long focal length eyepiece).

    Let's talk about atmosphere for a moment... There are three factors that must be considered in eyepiece selection: Seeing, Transparency, and Altitude of the target above the horizon.

    Seeing is a somewhat arbitrary method of describing the turbulence present in the air between you and the target.

    Transparency is an arbitrary method of describing the "junk" (moisture, dirt, pollen, etc.) that is between you and the target.

    We often sorta lob those two things together when talking about how good the seeing is. A good night will find the upper air mass fairly steady and the transparency very "clear" due to a lack of "junk" suspended in the air. That seems difficult to assess, however if you walk outside and the stars are twinkling, that's not good because it indicates a lack of transparency in the air that is causing photons to be deflected on the way to your eye.

    The altitude of the target is another consideration. If you look straight up (zenith), you are looking through the least amount of atmosphere that is possible. As your target's altitude decreases toward the horizon, the amount of atmosphere between you and the target increases. There is a point below which most astronomers simply will not attack a target, called the 2x point. The 2x point is where the altitude of the target is 30 degrees above the horizon. It's called the 2x point because that's where you have twice the amount of air between you and the target that you have when looking at zenith (straight up).

    Now, back to your scope and that magic number 13...

    With the exception of the moon, your maximum useful magnification will be obtained with an eyepiece whose Focal Length is approximately equal to your scope's focal ratio. By "useful", I mean the target will likely focus just fine. The caveat to that is that the seeing/transparency/target altitude must all be "excellent"... something that will happen to you perhaps three sessions a year. (sorry)

    The moon because it's so close and so bright can take a ton of magnification, even on an "average" seeing night. Many astronomers carry an eyepiece with a Focal Length that is around 50-60% of their scope's focal ratio, just for viewing details on the moon. In your case, a 6mm eyepiece will likely fill the bill. If the seeing is not great, just back the eyepiece off a a bit and continue moon watching.

    Here's the last thing I want you to remember... Generally speaking, in good seeing conditions the optimum blend of magnification and surface brightness is attained with an eyepiece whose Focal Length is approximately equal to twice the scope's focal ratio expressed in millimeters. In your case, a 25mm eyepiece will fit the bill.

    While a 70mm f/13 scope is primarily a solar system device, you can still see some Deep Space Objects (M42, the Andromeda Galaxy (M31), and some brighter star clusters) pretty well on a good seeing night. Agreed, the fine details won't pop out at you, but the views can still be stunning. What eyepiece are you gonna use? Why the 25mm, of course.

    Most beginning astronomers are under the misconception that this hobby is all about magnification. It's not. It's all about gathering as many photons at a given moment as possible. Your beginner 70mm is great for getting a feel for the hobby and what is possible. Keep in mind that it is a decent sized planetary/moon scope that will serve you well for years to come in that role. Don't be discouraged when you can't find all 110 Messier objects with it... it's simply not designed to do that.

    There's an old saying that aperture rules the deep space arena. A 10" Dob (pretty much the norm in DSO viewing) delivers over 400% more photons than your 70mm... opening up those very dim deep space targets. I encourage you to learn as much as you can with your present scope, and when "aperture fever" strikes... buy as much aperture as you can afford.

    I hope that helped some.

    Clear, Dark Skies
    This is very good information and things are getting cleared to me now. I really appreciate your post above. From your post above, if I understand correct then I should but 25mm and 6mm eye pieces. 25mm for better view of deep objects. Correct? I do have 20mm eye piece which came with the Telescope but I will keep this in my mind that 25mm will be better.

 

 
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