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  1. #1
    starman345's Avatar
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    Default Which Barlow or Powermate?



    I've been thinking of getting a barlow mostly to use with my 13mm Ethos. Antares has a 2" 1.6x model that would nicely give me 8mm when coupled with my Ethos. But I'm not sure if I should go with a 2.0x Powermate instead. I read great things about these powermates. At the moment I really only use the Ethos, I have an 18mm meade and a 3x Antares barlow but honestly its nothing like looking through the Ethos so doesn't get used much.
    In the near future I would like to start filling out the holes in my eyepiece collection, and a quality barlow would probably help fill some of those spaces...so I'm a little confused about which one to get and which power 2x 2.5x, 4x, 5x. I've noticed the pricing on the powermates doesn't go up with magnification, actually I think the 2x is the most expensive of the lot, does that mean you lose something in the views with the higher magnification models?
    I'll probably get a low power eyepiece later, something in the 27-35mm range, maybe a panoptic.
    The lady of the house has told me to ready my Christmas list so the pressure is on to decide what I want.....course I may end up getting a lump of coal
    My scope is a 10" skywatcher, anyone have any input on powermates? good or bad? or would the Antares 1.6x do me just as well?
    !6" F/3.8 under construction
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    -Brian



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  2. #2
    MitchAlsup's Avatar
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    Default

    I can report that the 2X PowerMate is very good and sice it it telecentric it will work perfectly with yout 13E giving 6.5E should you desire that effective focal length.

    With a 2X barlow, one can approach the EP collection with a simple rule of thumb FL[N] = 2/3 * FL[N+1]. So with your 13E * 3/2 = 20ES or 21E * 3/2 = 30ES or 31NT5. When barlowed you get no duplication: 31NT5, 21E, 15.5NT5, 13E, 10.5E, 6.5E leaving only the 8mm band empty.

    There is some ratio for a 1.6 barlow similar to the above.

    The message here is that the selection of a Barlow/PowerMate needs to go hand in hand with the planning of the rest of your EP collection.

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    starman345 (11-02-2011)

  4. #3
    starman345's Avatar
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    Default

    That is useful information. I'd never seen that calculation before. I guess I could go with the Antares 1.6x on the Ethos 13mm to fill the 8mm length and a 2x Powermate for the rest. Thanks!
    !6" F/3.8 under construction
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    -Brian



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  5. #4
    cmeg1's Avatar
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    Default

    I love my 4X powermate,that is what worked best for me.Very impressed with the powermate.

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    w/Telrad
    Ep's: Explore Scientific 82*30,24,18 and 100*14,9 and Televue 4X Powermate

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  7. #5
    MitchAlsup's Avatar
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    Default

    Over on another astronomy fora, today, I ended up having to draw out the <approximate) light path in order to explain the difference between a barlow and a powermate. I though it only fair to provide the same information over here.

    The top figure shows light from the primary (way off to the right) converging towards focus. The important thing to see, here, is that the center point (axis centerline) of the converging light cones is <essentially> parallel.

    The EP admits the now expanding light cones, collimates the cones, and makes them pass through an exit pupil. The optical prescription of the PE is HIGHLY dependent on the parallel nature of the axis centerlines.

    The middle figure shows what a Barlow does to the light cones. The Barlow is a negative lens that intercepts the light cones prior to focus, slows the rate of convergence and causes the axis centerlines to diverge. This divergence is what can cause an EP to vignette inthe outer field and sometimes to increase EP aberations in the outer field.

    The bottom figure shows what a PowerMate does to the light cones. There is a negative lens in the PowerMate that intercepts the converging light cones, and other than being somewhat stronger than the Barlow version, noting particularly interesting is happeingin with the optical prescription of the diverging lens. Following the diverging lens, there is a positive powered lens that cause the diverging axis centerlines to becomes parallel again. Thus, the formed image is twice as big, but there is no divergence of the axis centerlines.
    Attached Images Attached Images

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