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Thread: Testing an eyepiece

  1. #1
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    Default Testing an eyepiece



    Well... It would be great to be able to test the incoming TMB Planetary II 4mm eyepiece to ensure that it's a good eyepiece.

    Given that the forecast is not looking too brilliant for the near future - Are there some good tests in any way on terrestrial objects that would give me a good idea if the eyepiece is a good one or not? Obviously the real proof is once it's used in the field on the Moon or Jupiter or perhaps its ability to split a difficult Double star - all of these are dependant on seeing conditions though! I have a mix of objects to go at from TV aerials to cranes in the distance with red lamps on them so low flying aircraft don't bump in to them... A few buildings have detail / artwork, trees etc.
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    Scopes etc: Skywatcher 12" Dob; Skywatcher 80ED & 120ST; EQ5; Grab and Go Alt/Az; 60mm RACI & Rigel Quickfinder.
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    Default Re: Testing an eyepiece

    A distant brick wall or mesh fence will allow you to test for symmetry, curvature, and sharpness. Any building viewed at an angle with full sun on one face, will allow a comparative test for contrast. Compare the shadow details seen in the shaded areas against your other eyepieces. You can also print out and tack the attached targets to a wall if you want to log all your eyepieces to record their capabilities:

    Distortion Analysis Grid.jpg
    fuji resolution target.jpg
    Koren 2003 lens resolution bars.jpg
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    Default Re: Testing an eyepiece

    That's great! Many thanks.
    Scopes etc: Skywatcher 12" Dob; Skywatcher 80ED & 120ST; EQ5; Grab and Go Alt/Az; 60mm RACI & Rigel Quickfinder.
    Eyepieces: Tele Vue Panoptic 24mm, Delos 10/14/17.3mm; Pentax XW 5/7mm; ES Maxvision 28mm; Baader Classic 10/32mm
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    Default Re: Testing an eyepiece

    Test it not just with a distant grid pattern, but also with as many different scopes as you can. Newt, frac, AND different focal ratios. To test with just one instrument is not fair on you or your purchase.

    Better than a distant grid pattern is plain old star testing. Looking for distortions at the edge of the field of view. Look for seagulls that signify astigmatism, progressive bloating that suggests field curvature, pincushion, eye placement, blackouts and kidney beaning, lateral colour control. All of these are better markers than a grid test. Also you can compare for quality of transmission with another eyepiece of similar focal length. Biggest challenge you will face with short focal length EPs is having good seeing conditions, Can prove VERY frustrating...

    Some people also look for tinting or any colouration of stars due to the EP. I'm always hesitant of this as colour perception is entirely subjective, and as the percentage of males being colour blind to some degree is around 30%, in the dark astro environment colour perception I take with not a pinch of salt but a handful.

    While you are testing your TMB, test ALL your eyepieces too. You NEED to come to understand what you are looking at and looking for. Again, it's not fair on you or your purchase to make a judgement call based on just one experience.

    To better understand what aberrations look like so to recognise them, read through at least the following article on eyepiece aberrations. This is just one part of testing as you also NEED to also understand the aberrations inherent to the scopes you are using. Otherwise you are judging the a car based on how soft the seat is instead of understanding the way the whole of the car works. But at least familiarise yourself with eyepieces, it will go a long way to best informing you on what you are looking at/for:

    Eyepiece aberrations

    The above article is just one of a whole series of articles to do with telescope optics. You don't need to follow the mathematics involved. Just understanding the concepts is plenty.

    Alex.
    Last edited by mental4astro; 05-19-2017 at 09:18 AM.

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    Default Re: Testing an eyepiece

    Thanks Alex.

    I'll be able to compare to some extent the colour rendition (at least to my eyes) with the 6mm BCO ortho which should be as neutral as it gets. Perhaps a good test is against a typical yellow star, white, red/orange and blue. Although colour is not the most important aspect it can help with planetary like Jupiter where this is reasonable amount of colour available so the transmission % of colour in particular bandwidth may make a large difference to viewing detail and the contrast perhaps?

    Sharpness is going to be a tricky one needing good seeing at least. I wondered about resolving doubles that perhaps are on the edge of being resolved for the particular scope perhaps.

    Perhaps the most tough tests I have would be using it on my Heritage 100p reflector - at f/4 it can show a truly horrible view with an eyepiece that is not suited for fast scopes. At least the f/4.92 dob is rather easier... although I have the option of plugging in the Paracorr to flatten the field which may be suitable for particular applications (although I'm not particularly keen on the idea of sticking more glass in the way...)
    Scopes etc: Skywatcher 12" Dob; Skywatcher 80ED & 120ST; EQ5; Grab and Go Alt/Az; 60mm RACI & Rigel Quickfinder.
    Eyepieces: Tele Vue Panoptic 24mm, Delos 10/14/17.3mm; Pentax XW 5/7mm; ES Maxvision 28mm; Baader Classic 10/32mm
    Filters: Astronomik UHC and OIII; Baader OIII; ES H-Beta
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    Default Re: Testing an eyepiece

    Hello Dave,

    the test strategies depend on the intended use of the telescope + eyepiece.

    You can print out a test target, like that from Trombatissimo, or the USAF line chart https://de.wikipedia.org/wiki/USAF-Chart , put it on a barn door, and check out the ultimate lines pairs resolution - this gives you the cutoff of the modulation transfer.
    You can also generate bright grey squares on black background with decreasing contrast, to check the contrast performance of your scope + EP. This test is important when you want to observe the low contrast nebulae and low contrast galaxies.
    Regarding the point spread function (point resolution), the designers of the binoculars take 1 arcminute in the center field, and a few arcminutes at the edge of the FOV for the artificial star blur test. That 1 arcminute is the typical resolution of the unaided human vision. So, all aberrations should be within 1 arcminute in the center field, and within several arcminutes towards the edge of the FOV.
    Finlay, you'll test the optics on the close double stars having about the same brightness.
    The light transmission and contrast is tested counting the stars in the clusters - makes sense when you compare two EPs of the same focal length.

    One can spend quite a lot of time with testing the optics, and some of the tests can be documented when mounting a compact camera behind the eyepiece.

    Best,

    JG
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    Default Re: Testing an eyepiece

    Not sure what you expect from TMB Planetary having much better performers like Delos, Pentax XW, ES82 and BCO. I'd rather get a good telecentric Barlow like 2x ES Focal Extender, Revelation or TV Powermate etc. depending on your budget. I don't have direct experience with the 4mm TMB, but have the 6mm TMB Planetary. I strongly suspect that, for example BCO+ Barlow will easily outperform the TMB in sharpness department. The others, probably, too. As usual, your mileage may vary.
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