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  1. #1
    TonyBegg's Avatar
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    Default Rich Field and Mars - eyepiece dilemma



    Am trying to put together some dream optics and running out of money. I have decided on an Explore Scientific 127 mm APO CF and a Celestron CG5GT mount. Now need eyepieces. I wear specs so 20 mm eye relief is in my specifications. I want both Rich Field (wide FOV) and to get a good look at Mars this March while it is close. Words like contrast and immersion excite me reading about eyepieces and am considering the 82 degree Explore Scientific 2 inch 30 mm eyepiece which gets me fairly close to richest field (that and the more expensive Nagler 5 are the only "immersive" EP I know of that will get me the required low magnification). But for Mars I need magnification and I was wondering whether to go for the Televue Delos 72 degree FOV 6 mm (about 159X magnification on the APO) or to get a Televue 2 inch 4 X Powermate (a sort of parfocal Barlow) to make the 30 mm effectively 7.5 mm (about 127X on the APO). Would the 82 degree apparent FOV be maintained with the Powermate? Worried about making decision before end Feb ES deadline. Will fill in the eyepieces as time and money permit.

  2. #2
    randyroy's Avatar
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    Rich field means little when we are talking about planetary viewing. Even when Mars makes its closest approach, you need all the magnification York scope and seeing conditions can allow. I have viewed both Olumpus Mons and Syrtis Major (the shark's fin) at 303X with my C8 this season. If all you care to see is an orange disk and perhaps the south pole, powers well under 200x may do. For more detailed views with your scope, I would get up to 250x or slightly higher with an eyepiece design favoring your requirment for 20mm ER and fewer elements over FOV. Add too much glass and you will just have a larger orange ball. Good luck. Mars is real showpiece every 2 years.

    Randy

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  4. #3
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    Default

    Sideline note:

    As you focus the scope yu are effectively manipulating the amount of spherical correction of the image. So if your eyeglass prescription is mostly spherical, you can adjust this out at the focuser. {And that is why Televue invented Dipotrix to deal with residual astigmatism.}

    Thus, you might be able to observe with your glasses off (whichobviates the need for long eye relief EPs.

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  6. #4
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    Default

    am impressed randy with what your C8 can do. so it looks like I should try to push it on magnification (hear that 50 x aperture in inches is pushing it so your 250 x suggestion is spot on) and not be finicky on FOV for the Mars view. I have ordered the 127 mm APO this evening with the 82 degree 30 mm EP so now will look wider for a short focal length EP. Useful perspective. From you too Mitch. Thanks to both.

  7. #5
    randyroy's Avatar
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    Maximum magnification has a lot to do with your seeing. You will see a lot of posts warning not to use too much magnification. I generally agree with this, but some objects really require magnification to get the most out of them. Mars is one good example. I rarely go above 300X with the C8, but 400x on 10" Dob is a useful magnification for Mars when seeing is exceptional. Fortunately for me, the seeing while often obscured a bit by humidity, is very steady many nights each month. Bright objects such as Mars during its close approaches respond beautifully to high mags where I live.

    If the seeing is good 250X is certainly within the capability of your scope.

    Randy

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  9. #6
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    Default 4 mm radian looks good

    Thanks again Randy. I happened to have the Televue eyepiece calculator printed out for the ES 127 so I looked on that. The Televue Radian series has 20 mm eye relief at even the shortest EP focal lengths. I believe I can go to 4 mm (238X - a bit short of the 250X you suggest for Mars) which has 0.53 mm exit pupil. The 3mm gets 317.3X but is flagged because they suggest no more than 60 x aperture in inches (= 300 with my scope). It has an exit pupil of 0.40 mm, and I am a bit wary of that because I have read that too low an exit pupil (Televue say less than 0.40, others less than 0.50) can cause trouble with floaters in the eye. When the gel next to my retina in my best eye (right) liquified - people with short sight like me this happens sooner - I had a virtual aquarium in my eye of dark green pigmentation. The biggest inhabitant I called the jellyfish because it looked like one is still there but somewhat emaciated. So I have a stringy floater rather than a transit of Mercury style one.

  10. #7
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    Hey, I'm a believer in buying an eyepiece that is the highest power you can use regularly. For higher powers that you may only use every year or two, I say get a less expensive Barlow. I don't use exit pupils smaller than about .5mms with my 61 year old eyes and with my C8 that is about 400X. But that is my once every year or two magnification. I use 300x or an exit pupil of .67 much more often. You will find the 4mm great for Saturn, Uranus and Neptune as well as most of the globular clusters. M13 and M92 look good at 150X, but literally sparkle at about 230-240X. Some very tight doubles aren't doable at 150X, but split cleanly at about 230X. For your scope, I would much prefer the 4mm over the 3mm.

    Randy

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  12. #8
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    Default

    Randy, thanks again. This has been a really useful discussion for me. My eyes are 63 years old so I appreciate you have a similar perspective on how far to push it. I shall order the 4 mm tonight and when I recover from all the expense (telescope, mount, two eyepieces in 2 days) I shall fill in the gap between my 32X and 238X with maybe one more EP and one or two Powermates/Barlows.

  13. #9
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    Default

    Randy. This is a first light report and I figure if I put it here then you will be notified. I have had the CG5GT mount since Monday, and have been been carefully getting to know it. It came with only one counterweight so I improvised extra weights with barbell weights and some rubber from tubular metal chair feet while I am waiting for a second counterweight to come. My friend's birthday took up Wednesday and Thursday evenings. Today I powered up the CG5GT (I had read to move the tripod lug to a safer position) and at one point the set-up program believed I was in South China (telescope pointed to the floor). Switching off and on reset things. So I took the ES 127 mm APO off and put the mount on my South facing patio and re-attached the telescope. I have an ES 82 degree 30mm EP giving me 32x mag, and the Televue Radian 4mm giving me 238X mag, and an ES 82 degree 6.7mm on order (fingers crossed on eye relief). After aligning (my horizon is very restricted but I could see Sirius, Rigel and Procyon) - I had used the Sun to give me true North at local noon - I went straight to Mars and after a little trouble with the hand control (need to invert one direction I think) managed to center Mars for the Televue Radian 4mm. I could see structure but there was too much light! I think it is because I am taking Bactrim for an infection - a side effect is photosensitivity. I then looked at a few Messier objects and the Hyades with the 30 mm EP. It is astounding how point-like the stars look. I had no trouble with eye-relief - a bit of trouble with finding the exit pupil (remember I am a complete beginner after a 40 year break) which I think is just because there are so many sources of glare on my patio. Anyway I spent over 2 hours outside in what I think was pretty good seeing. Now I know a few other things I need! An observing chair, perhaps extend the tripod legs but that might make it difficult to get outside without dismantling the mount, may be a Bactrim-compensation filter! And an observing hood might be the only solution to the glare, since I noticed shading myself with a Sky and Telescope magazine helped. Anyway I am totally blissed out - thank you Randy you were exactly right to push the magnification - the telescope and seeing would take it but there was just too much light from Mars!

  14. #10
    MG1962's Avatar
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    Something to remember, when you are after a specific point source like say a planet or double star etc the FOV becomes far less important. Personally I have wide FOV for my 25mm 21mm and 17mm - for up close and personal I use a zoom lens with only a 40 degree FOV
    Celestron SE8 - 25mm and 15mm
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    Epic II ED. Baader Hyperion 21 mm 17mm plus 14 and 28mm tunning ring

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