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Thread: Calculating EP focal lengths to build a balanced set

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    Default Re: Calculating EP focal lengths to build a balanced set



    So utilizing Gabby’s formulae, and using a 1.6x magnification decrease my list was nearly identical to her “set 2”

    3.5mm - 0.5ep - 204x
    5.6mm - 0.8ep - 127.5x
    9mm - 1.29ep - 80x
    14mm - 2.0ep - 50x
    23mm - 3.29ep - 31.25x
    36mm - 5.14ep - 19.5x

    So this brings me to the question, should I opt for the 9mm, 14mm and 23mm and utilize a Barlow, or should I go with the 5.6mm - 36mm and leave the lower FL out for now?
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  2. #12
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    Default Re: Calculating EP focal lengths to build a balanced set

    Depending on what you targets are, the 9, 14, 23 is a good mix to start with.
    The 14mm gives you the 2mm exit pupil which is ideal.
    You could slip a 7mm in there later to get a 1mm exit pupil and barlowed a 0.5mm for high power.
    A 35mm such as a Orion Deep View or a 35mm TV Panoptic would give you a 5mm exit pupil.
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    Default Re: Calculating EP focal lengths to build a balanced set

    Thanks Gabby! I understand my EP collection will grow massively over time, especially when I get a feel for what I like. I am looking simply for a decent set to build upon without breaking the bank. In the future, I will have no issues dropping $300+ on a great EP, but I’m just not at that stage. My targets will be mostly DSOs with some occasional lunar and planetary. I’m loving the Barlow+7mm idea and I think I’m going with your suggestions on the spacing as well. The hard part now, figuring out which EPs to got with. I’ll keep my eyes open for the LVWs you suggested as well. At over $300 a piece new, that’s beyond my budget right now
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    Default Re: Calculating EP focal lengths to build a balanced set

    The TV was just an example
    There are quite a few good eyepieces in that range that are less and finding them used are considerably cheaper.
    If you have a price per eyepiece you are looking at the members can usually point you in the right direction
    Refractors: Antares 105 f/15, Celestron 150 f/8, Stellarvue NHNG 80 f/6.9, TAL 100RS f/10, TS 102 f/11, UR 70 f/10, Vixen SD115s f/7.7
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    Default Re: Calculating EP focal lengths to build a balanced set

    Quote Originally Posted by JustinG View Post
    Thanks Gabby! I understand my EP collection will grow massively over time, especially when I get a feel for what I like. I am looking simply for a decent set to build upon without breaking the bank. In the future, I will have no issues dropping $300+ on a great EP, but I’m just not at that stage. My targets will be mostly DSOs with some occasional lunar and planetary. I’m loving the Barlow+7mm idea and I think I’m going with your suggestions on the spacing as well. The hard part now, figuring out which EPs to got with. I’ll keep my eyes open for the LVWs you suggested as well. At over $300 a piece new, that’s beyond my budget right now
    With an f7 refractor you could do well for much less with Plossls or Orthoscopics. The KK Orthoscopics are very fine. The BCOs are almost as good on contrast but have a wider AFOV. Orthoscopics have better eye relief than Plossls too.

    For DSO contrast as well as planetary and lunar contrast my KK and BCO are just a bit better than TV Panoptics and Delos that I have. Much less expensive too.

    Even lower cost and quite good are the Vixen NPLs.

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    Default Re: Calculating EP focal lengths to build a balanced set

    Quote Originally Posted by not_Fritz_Argelander View Post
    With an f7 refractor you could do well for much less with Plossls or Orthoscopics. The KK Orthoscopics are very fine. The BCOs are almost as good on contrast but have a wider AFOV. Orthoscopics have better eye relief than Plossls too.

    For DSO contrast as well as planetary and lunar contrast my KK and BCO are just a bit better than TV Panoptics and Delos that I have. Much less expensive too.

    Even lower cost and quite good are the Vixen NPLs.
    Hello Justin,

    that's exatly what I would do on your place:
    - The Vixen NPL Plssls for the longer focus lengths
    - The Abbe orthos (KK Fujiyama or BCO, or both) for the shorter focus lengths
    These are keepers, even if later you may add some expensive wide field eyepieces for the large nebulae.

    You may also consider the Baader triplet Barlow from the Q-Set, it works fine with the Plssls, but less fine with the Abbe orthos.

    The sequence of the focus lengths has been already suggested to you by Gabby.

    Best,

    JG
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    Default Re: Calculating EP focal lengths to build a balanced set

    Spacing thoughts.... Gabby's idea of using a factor to apply to eyepiece focal length is quite sound as a starting point. I also find that filling in more frequently at exit pupils smaller than 2mm is helpful to get the most out of the available seeing conditions.

    Take the KK sequence as an example:
    The 25, 18, 12.5, 9, and 7 mm units show a roughly 1.5 factor in ascending focal lengths. Shorter going from 6mm to 7 doesn't fit the pattern. Going from 4 to 6 does but I think that is an accident. The previous incarnations of this line had 6, 5, and 4mm. I'm missing the 5mm and hope that when this line goes into production again that there is a 5mm.......

    But still as a starting point picking a scale factor is excellent to start. In addition Orthoscopics and Plossls are scalable designs, so performance is pretty uniform throughout the series.
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    Default Re: Calculating EP focal lengths to build a balanced set

    Hello not_Fritz,

    two KK 5mm have been available at Lacerta in Vienna (Austria), one of them is now in my arsenal.
    You might eventually drop a line to Lacerta, if there is still one left.

    Some manufacturers, like Pentax, have been scaling the EPs focus lengths with the factor sqrt(2)=1.4, which has some good reason, like doubling the surface of the pencil of light at the exit pupil.
    This corresponds with doubling the surface of the DSO across the light sensitive rods in the retina in eye.

    At the short focus lengths, I also prefer a step of 1mm in the focus length.

    Best,

    JG
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    Default Re: Calculating EP focal lengths to build a balanced set

    Quote Originally Posted by j.gardavsky View Post
    Hello not_Fritz,

    two KK 5mm have been available at Lacerta in Vienna (Austria), one of them is now in my arsenal.
    You might eventually drop a line to Lacerta, if there is still one left.

    Some manufacturers, like Pentax, have been scaling the EPs focus lengths with the factor sqrt(2)=1.4, which has some good reason, like doubling the surface of the pencil of light at the exit pupil.
    This corresponds with doubling the surface of the DSO across the light sensitive rods in the retina in eye.

    At the short focus lengths, I also prefer a step of 1mm in the focus length.

    Best,

    JG
    Many thanks for the tip. I NEED a 5mm KK!

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    Default Re: Calculating EP focal lengths to build a balanced set

    A lot of what people like or hate in an eyepiece is subjective to that persons tastes and tolerances. I like wide field eyepieces, but I have had people look through some of mine and say that makes me nauseous. I hate when stars out near the edge are distorted or have halos or strange colors, yet some people will take no notice of them.
    I would go to a star party or a local clubs outreach or observing session and look through other people scopes and eyepieces. Ask some people if they can bring a couple of their eyepieces over to your scope to try. I think you will find most people accommodating. This is way better way of selecting eyepieces or at least getting an ideal of what you like and what you don't.
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