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Hi all,
Just a couple of things on long focal length eyepieces I think that need to be kept in mind before buying or dismissing that include optical principles and the physiology of our eyes:
* In a fast scope, eg f/5 and faster, even f/6, the shape of the light cone is somewhat 'stumpy' compared to a slower scope. This means that the eyepiece will need to deal with light falling on it that will challenge its capabilities a lot more than a 'skinnier' cone. This in cheap eyepieces shows up as a distortion and blur along the outer edge of the field of view as it cannot focus the whole field at once. The faster the scope the greater the distortion.
* The wider the apparent FOV that the eyepiece provides, the greater the likely hood of distortion. Even the finest 100degree eyepieces WILL show this distortion in a fast scope. Here then it becomes necessary to use a 'coma corrector' to improve the image.
* Short focal length eyepieces show less distortion as they recieve light at a finer point from the cone than long focal length eyepieces.
* Using too long a focal length eyepiece for the focal ratio of the scope will result in lossing precious light coming out of the eyepiece. You will get a wider view, but the image will not be brighter as the 'exit pupil' will also illuminate your iris, not falling exclusively into you pupil.
Wavytone showed me a neat rule of thumb to show the maximum eyepiece focal length:
6mm X f/ratio of scope = max focal length of eyepiece
So for your 8" f/6 scope, the longest focal length would be 36mm. The 42mm Superview would 'waste' some light, but probably not too much.
I also think that the wider the FOV the better is overrated. Our eyes are only able to SEE a set amount in the one direction before we need to move our head. This for me wastes light, time and makes for more effort for our eyes. 68 degrees is the most our eyes can see. I looked through a 100degree EP in my dob, too much work. My opinion.
Then it is up to you to decide on the best balance of price vs. quality.
I own a GSO Superview 30mm. I've also tried eyepieces twice its price of the same focal length. The Superview showed a much smaller distortion than this more expensive EP! Not a bad alternative if a $300+ EP is not an option. I'm happy with it while I save my pennies to first remodel my dob and then to get a TMB Paragon 30mm, that is a good EP.
Shorter focal length eyepieces, like 5mm, here I think that the wide FOV of 60degrees + needs to be considered if using a dob. It makes a big difference. I have a Meade series 5000 5.5mm plossl. A good EP, though the amount of times I can use it due to atmospheric conditions I am comfortable with its price of $130 new from Bintel. If the TMB Planetary gives a generous FOV, it would be a good option for your scope.
But before you buy, try out different ones first.
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