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Thread: Easiest way to select EPs for your first scope?

  1. #1
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    Default Easiest way to select EPs for your first scope?



    As everyone knows selecting EPs is arguably the most personal and complicated experience. 20 years ago it was quite easy: buy set of Plossls with your scopes and you are done. Now you have hundreds of EPs of dozens different designs, which make the process for beginners very intimidating.

    The best way and really the only way to go about this is to start observing: the more you observe the better you understand how optics works and make more educated decision in the process. But you have to start somewhere, right?

    Not pretending to be an optic guru, not by a long shot, I still like to propose a simple and easy 3 EP rule. Based on my experience if you select your first EPs this way you should get good starting experience no matter what telescope or EP design you use.

    The only thing you need to know is focal ratio of your scope, and to calculate EP focal length you use this formula:

    EP focal length = scope focal ratio x exit pupil. That's it

    So the first 3 EPs you need to start observing are the ones which produce 4mm, 2mm and 1mm exit pupil in your scope.

    Example 1: you bought 150mm F5 reflector - your EPs are

    5mm EP - 1mm exit pupil - good for planets, moon and small bright DSOs like PN
    10mm EP - 2mm exit pupil - best for galaxies and most of DSOs
    20mm EP - 4mm exit pupil - for large DSOs which would not fit in other EPs' FOV and wide field views.

    Example 2: you bought 90mm F14 Mak - your EPs are

    14mm EP - 1mm exit pupil - good for planets, moon and small bright DSOs like PN
    28mm EP - 2mm exit pupil - best for galaxies and most of DSOs
    56mm EP - 4mm exit pupil - for large DSOs which would not fit in other EPs' FOV and wide field views. You can't get 56mm in 1.25" barrel so you go for the next closest thing which is 42mm. 42mm will give you 3mm exit pupil and will be able to fit some of larger DSOs in the FOV.

    Example 3: 8" F10 SCT

    10mm, 20mm, 40mm

    Why 4mm exit pupil and not 5-7mm for the widest EP? From my experience 4mm exit pupil deliver good contrast more consistently under variety of observing conditions, while for 5-7mm you will need really dark sky. Also extra wide EPs with max or near max field stop are most prone to astigmatism and other FOV aberrations. But even 20mm Erfle should deliver in F5 reflector.

    Any way this is my take on the "What EPs to buy?" you see often at the beginners' section. This post is not to discuss intricacy and complexity of advanced EP selection process, I am well aware of that , but to come out with a simple and effective rule for beginners to select their first set and start the journey.
    DeanD, Leveye, bladekeeper and 9 others like this.

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    Default Re: Easiest way to select EPs for your first scope?

    I like to have 40x to 50x steps between lenses, up to your maximum which is determined by your atmospheric conditions and the limits of your scope.

    Start with a 24mm as your lowest, figure out your power: divide focal length scope by mm of eyepiece to get power. Say your result is 50x. So the next step up would be 100x. Divide 100 into scope focal length and you get the next mm eyepiece which in this case would be 12mm. Most scopes will goto 200x, so you will need 4 eyepieces. To save money you can use a barlow to reach higher powers.

    I usually just look at planets and by using 50x steps you can sneak up on the nights maximum power that the atmosphere is going to allow.
    Newtman but some refractors allowed.

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    Default Re: Easiest way to select EPs for your first scope?

    Like your avatar, Tom.

    Yes, we all end up with collection of EPs spaced at some intervals. However, same magnification will not work well in all scopes. The beauty of using exit pupil as a reference for selecting EPs is that the equation is already taking in to account telescope size and focal ratio. So 2mm exit pupil will give you the best ratio of contrast to magnification for galaxies in any scope. So if you are looking for EP best for galaxies it will be 20mm EP for 8" F10 scope -100x power, and 10mm for 80mm F5 scope - 40x power.

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    Default Re: Easiest way to select EPs for your first scope?

    Great topic! It couldn't possibly get contentious just because we sometimes have different opinions......... I would say that the most important factor that very few beginners and many experienced members of this site fail to consider is the matching of telescope and eyepiece characteristics especially for the typical beginner's scope. I admire Mental4astro for his patience in repeatedly posting very detailed information on this subject every time that he finds a member employing the "Try it and see if you like it" approach to eyepiece selection.

    A lot of these problems have been magnified by the recent increase in popularity of fast Newtonians as a first scope. Their desirability is understandable as they offer the most aperture and reasonable chance of seeing some DSO's compared to other similarly priced offerings. The older designs were all slower, which reduced field curvature and increased the chances of random selections producing decent results. The other trend has been the expansion of low cost wide field eyepieces which have inherently poorer edge performance, so we are confronted with a greater chance for a significant miss-match resulting in very curved and/or astigmatic field perimeters that are not the fault of just one or the other component, but due to a poor match of design characteristics in both items.

    Some folks simply accept this as the inevitable consequence of not jumping straight up to the very best wide eyepiece designs, but many excellent low to mid cost scope and eyepiece combinations can be assembled with a little attention to the many reviews posted here and on several forums. If you are pragmatic and have reasonable expectations, the choice of a low cost eyepiece usually comes down to whether you prefer a razor sharp view over a smaller field, or a slightly softer edge in a field which is often just as sharp over the same central field size as the narrow view eyepiece. This is a very subjective impression, so other people's opinions should be very carefully considered if you are making a selection without an option to return or exchange the eyepiece.

    I would toss one "fly" into your above suggested ointment, by pointing out that your formula results in exact doubling of your focal length ratios for your first three suggested eyepiece purchases. Many beginners expand their focal length combinations by using Barlow lenses, with the 2x models generally being the most popular first purchase as they offer a significant increment without being too extreme. I prefer a 5 to 4 to 3 ratio selection for a "first three" eyepiece kit with a single 2x Barlow. Using your above suggested exit pupil of 4mm for the longest eyepiece, this would give you six different focal lengths with a 20mm, 16mm, 12mm, and 10mm, 8mm, and 6mm using the Barlow. For your longer scope cited in example 3, you would get a 40mm, 32mm, 24mm combo which coincidently is very close to the 40, 32, 26, and 2x Barlow kit that Meade and Celestron sell with a 2 inch diagonal and a selection of filters as their upgrade kit for SCT users. This gives more gradual increments and helps with greater eye relief at the shorter focal lengths than you usually get with individual low cost eyepieces at these short focal ratios.
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    Default Re: Easiest way to select EPs for your first scope?

    My most used eyepieces for planetary are 4,5,6mm's. My 1200mm f.l. scopes, refractor and newt, get these powers:
    6=200x 5=245 4=300 and if the atmosphere is really steady 3.5mm=350x and 3mm=400x.
    Figuring powers are so much more straight forward to what you need to know than exit pupils.
    Newtman but some refractors allowed.

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    Default Re: Easiest way to select EPs for your first scope?

    Quote Originally Posted by Trombatissimo View Post
    I would toss one "fly" into your above suggested ointment, by pointing out that your formula results in exact doubling of your focal length ratios for your first three suggested eyepiece purchases. Many beginners expand their focal length combinations by using Barlow lenses, with the 2x models generally being the most popular first purchase as they offer a significant increment without being too extreme. I prefer a 5 to 4 to 3 ratio selection for a "first three" eyepiece kit with a single 2x Barlow. Using your above suggested exit pupil of 4mm for the longest eyepiece, this would give you six different focal lengths with a 20mm, 16mm, 12mm, and 10mm, 8mm, and 6mm using the Barlow. For your longer scope cited in example 3, you would get a 40mm, 32mm, 24mm combo which coincidently is very close to the 40, 32, 26, and 2x Barlow kit that Meade and Celestron sell with a 2 inch diagonal and a selection of filters as their upgrade kit for SCT users. This gives more gradual increments and helps with greater eye relief at the shorter focal lengths than you usually get with individual low cost eyepieces at these short focal ratios.
    There are a few good reasons for doubling focal length

    1) After your acquire first 3 EPs and play around, you can simply buy a couple or more extra EPs to drop in between to have 1.3-1.5x steps in your EP collection.

    2) Been avid faint stuff hunter I strive to minimize amount of glass in the light path, so I don't use barlows or focal reducers when hunting DSOs.

    3) I got burned by cheap barlows at the beginning. They reduce contrast, introduce light scatter and degrade image. On the other hand I don't want to suggest to beginners buying an expensive barlow before they figure out if they need one in the first place.

    I still appreciate and use good quality barlows, but for bright stuff (moon, planets, doubles). Basically to take my 1mm exit pupil EP and barlow to 0.5mm when condition allows.

    Saying all that, I can't argue that having a descent quality barlow at the beginning is a great way to effectively double your EP collection.

    So having 3 EPs spaced at 1.5x interval plus barlow is another good way to start. In this case I would go 4.5mm, 3mm and 2mm exit pupil plus 2x barlow.
    DeanD and ProfEclipse like this.

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    Default Re: Easiest way to select EPs for your first scope?

    Quote Originally Posted by Trombatissimo View Post
    I would say that the most important factor that very few beginners and many experienced members of this site fail to consider is the matching of telescope and eyepiece characteristics especially for the typical beginner's scope.
    Agree, but I would consider 'EP matching the type of the scope' to be advanced EP collecting strategy, to be consistent you need to include all optical chain into consideration including observers eye (glasses to in my case) and even type of diagonal (star vs prism).

    We don't want to overwhelm beginner with info and too many options to result in paralysis by analysis.

    If you conservative with EP selection (i.e. don't buy extra wide EP with field stop approaching max value) even "wrong" type EP will perform reasonably well in your scope.
    DeanD likes this.

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    Default Re: Easiest way to select EPs for your first scope?

    I wish that I had seen this thread when I had started getting eyepieces but then again I was rapidly acquiring Telescopes at the time. The first scope I acquired was given to me after my father in law passed away. A 114mm 910mm f/l reflector wit just an SMA 25mm eyepiece. I ordered a Powerseeker kit with a 15 and 9mm Kellner set and a couple of filters. That kit is actually best on Planets and was very inexpensive. Next up I bought a Nexstar 8 SE SCT and it came with a 25mm Plossl. Shortly afterwards I ordered a Meade 2x Barlow and a Sirius 10mm Plossl. I had by then started in the forums and was reading about 2 inch eyepieces. So I bought a Celestron 2 inch diagonal for the Nexstar 8SE the original 1.25 Diagonal sits in a box unused now. I was given a Meade QX 26mm as my first 2 inch eyepiece. During the next few months I bought new and sometimes used a Televue Panoptic 27mm a Panoptic 35mm (great for the 8 inch SCT) a GSO 42mm. also acquired were a ES 68 24mm AT Paradigm 18 15 8 and 5mm ES 82 11mm. I also had acquired an AT 115EDT which I sold to a friend a few months later and finally an AT 130EDT. if you count the Vixen ED 80sf that I am holding for the friend that bought the AT 115 from me I have 4 telescopes here to use.

    This is the chart I made up for the telescopes and eyepieces I have now.
    Telescope: AT 115EDT 805mm
    Nexstar 8SE 2032mm
    114 Reflector/AT 130EDT 910mm
    Vixen ED80sf 600mm
    42mm GSO (2) 35mm Panoptic (2) 27mm Panoptic (2) 26mm QX (2) 25mm Plossl 24mm ES
    3.13 19.17X 2.96 23X 2.28 29.81X 2.26 30.96 1.86 32.2 2.03 33.54X
    1.24 48.38X 1.17 58.06X 0.9 75.26X 0.9 78.15X 0.74 81.28X 0.8 84.67X
    2.78 21.55 2.62 26X 2.02 34X 2.01 34.81X 1.65 36.4X 1.79 37.92X
    4.2 14.29X 3.97 17.14X 3.06 22.22X 3.03 23.08 2.5 24X 2.72 25X

    18mm Paradigm 15mm Paradigm 11mm ES 82 10mm Plossl 8mm Paradigm
    1.34 44.72X 1.12 53.67X 1.12 73.18X 0.65 80.5X 0.6 100.63X
    0.53 112.89X 0.44 135.47X 0.44 184.73X 0.26 203.2X 0.24 254X
    1.19 50.56X 0.99 60.67X 0.99 82.73X 0.55 91X 0.53 113.75X
    1.85 33.33X 1.55 40X 1.5 54.55X 0.87 60X 0.83 75X

    5mm Paradigm
    .37 161X
    .15 406X
    .33 181X
    .52 120X
    2X Barlow Doubles the 1.25 Eyepiece numbers.
    Astro-Tech AT 152EDT CGEM II Mount AKA FracZilla
    Celestron CPC 1100 GPS AKA Cloudzilla
    Home Made 8 inch Newtonian Reflector on Rocker Box AKA Scopezilla
    Celestron 4 1/2 114 mm Newtonian Telescope 910 F/L GT Mount AKA Frankenscope.
    TS Supercharge Bino Viewer.

    David

 

 

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