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Thread: When should I use a filter/ how large should my eye piece be?

  1. #1
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    Default When should I use a filter/ how large should my eye piece be?



    Hi! I'm in the process of constructing an 8" dob for DSOs. Should I buy a large eye piece such as a 40mm, a 27mm, or a 15mm? I'm a bit confused on if I should buy a large mm eye piece for DSOs. In addition, I live in a somewhat light polluted area in NY (~40 min from NYC) and I'm wondering if I should purchase a filter to make DSO's more vibrant. Planetary observations are not really my priority, but I'm still interested so should I purchase a planetary filter as well? Any help is much appreciated

    PS. Also, do you guys recommend purchasing a barlow eye piece? I'm a bit confused as to what they do. Thanks!
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    Default Re: When should I use a filter/ how large should my eye piece be?

    Have you settled on the focal length of the scope? That will play a part in recommending a selection of eyepieces
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    Default Re: When should I use a filter/ how large should my eye piece be?

    A barlow basically changes the magnification of an eyepiece.... so for example a 20mm EP in a 2x barlow effectively becomes a 10mm EP.... but it also changes the eye relief and can possibly vignette....

    In contrast a telecentric focal extender (as sold by Explore Scientific as their Focal Extenders, or by Televue as their Powermates) avoid all those drawbacks by not changing anything except magnification...the only downside is they cost more.


    As to filters- read this link:
    Useful Filters For Viewing Deep-Sky Objects | The Prairie Astronomy Club

    Short version- a narrow band like the Lumicon UHC (not any other brand of UHC) or the Orion Ultrablock will be your best choice if you only buy 1 filter- but it's going to be for things like emission nebula- it's not gonna really help with galaxies or clusters.... Nothing other than dark skies will majorly help with those things.... a wideband filter can help a LITTLE, but the improvement is pretty tiny and just going to a higher magnification might do the same thing.



    As to what size EP to get.... really depends on a few things.... your budget for one.... the F/ratio of your scope for another... What kind of DSOs you're wanting to view for another.... some respond better to higher mags than others.

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    Default Re: When should I use a filter/ how large should my eye piece be?

    Hello d42,

    as you may have a considerable level of light pollution, your working horse eyepiece may need a focal length in the range 10mm up to about 15mm (exit pupil 2mm up to about 3mm).
    On your place, I'd start with an ultrawide (82°) f=16mm, 1.25" filter thread eyepiece, like my WO UWAN, or similar. This eyepiece has got the nickname: the Sweet Sixteen. The WO UWAN is a light weight ultrawide, and not so much behind the ultrawide TeleVues, as the price difference would let you expect.

    Regarding the filters, all depends on the DSOs you want to start with.
    Should these be the Messiers nebulae (diffuse and planetary), than a quality OIII filter (Lumicon or better), will do the best job under the light polluted skies.
    The UHC filters will probably be not aggressive enough, even if the Orion Ultrablock is also worth considering.
    On the galaxies, no filter helps much against the high levels of the light pollution. Here you must magnify more to darken the lighted sky background. So, another eyepiece, like f=7mm, may become necessary on most of the galaxies within your reach.

    Best,

    JG
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    Default Re: When should I use a filter/ how large should my eye piece be?

    Eyepiece selection depends on the optical characteristics of the scope. A handy guide is that the maximum magnification is obtained with an eyepiece whose focal length is equal to your focal ratio, and the minimum magnification is obtained with an eyepiece whose focal length is equal to 7x your focal ratio. So, for example, an f/6 scope would use eyepieces from 6mm to 42mm. An f/8 scope would use eyepieces from 8mm to 56mm.

    In general, the longer focal length eyepiece (lower magnification) are better for finding DSOs, since they give higher surface brightness to the objects you are looking at. That would be especially true if you plan to use filters to cut out light pollution. Filters only remove light; they do not add anything. So you would want to start with the objects as bright as possible.
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    Default Re: When should I use a filter/ how large should my eye piece be?

    38mm SWA is very popular (my first I bought), you can see all of The Pleiades. Take it slow and make sure you get the feel of what your looking at and what you want to achieve.
    ES24mm is a great EP to start with for a 8" Dob, depends on how much money you want to spend you can use a Barlow as a shortcut or buy the actual EP.
    Neil
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    Default Re: When should I use a filter/ how large should my eye piece be?

    It's an 8" f/4 scope! Thanks for your response though!

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    Default Re: When should I use a filter/ how large should my eye piece be?

    If you're young then your maximum exit pupil size is about 7mm, which would mean your lowest power eyepiece should be no longer than (for f/4): 4x7mm=28mm.

    For an older person I would use 6mm as the maximum exit pupil size, 4x6mm=24mm. That will give you your lowest power eyepiece. From there you will want to go up in power. A ~12mm eyepiece will probably be ideal for many deep sky objects. And then you'll want a high power eyepiece ~7mm. A wide apparent field of view (80 deg or larger) will also help with finding and tracking objects.

    A narrow bandpass filter like the Orion UltraBlock is great for viewing faint nebula.
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