Quote:
Originally Posted by Liz
Would a 2" 40mm let in much more light than a 1 1/2"?
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Yes and no. But, the answer is a little more complicated. What a 2" EP will do in the longer focal lengths, say 25mm and above, is give a wider field of view than a 1.25". This is because of what is called the "field stop". In a nutshell, the wider the field stop, the wider the possible field of view. The kicker is that the longer focal length EPs require a wider cone of light to enter the EP.
Get two EPs, both the same focal length of 40mm, but one 2" and the other 1.25". What you will see is that the aparent FOV of the 2" will typically be greater than 65 deg., while the 1.25" will not exceed 45 deg. This is an enormous difference in field size, but both allow roughtly the same amount of light to pass into the eye. A 2" EP should give a brighter image, but I doubt weather our eyes are sensitive enough to notice the percentage difference.
This is due to the size of the "cone of light" that is able to enter through the 2" EP being wider.
Eyepieces upto and less than 25mm in the 1.25" size require a smaller cone size to enter the EP to give wider FOV.
I've got a 30mm 2" GSO superview. It shows both coma and astigmatism aroung the edge of the FOV. When I get around to it, one-day, I'll be happy to upgrade to a mid range EP like the Paragon or Titan Type II 30mm. 68deg. FOV, no astigmatism, tiny, tiny amount of coma, and better contrast in the image. I don't need anything wider, nor bigger price tag. I just don't see the need. You really don't do serious observing at the very edge of the FOV which only shows more coma, and then more money to correct for. Again, that's me.
The rest of the money I then spend on flowers for brownie points to be allowed to go out into the dark night and play with strangers in remote and lonely places,
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Adrian, I'm just waiting to the time that animal-to-human transplants becomes safe so I can have leopard eyes transplanted to replace my current whimppie human eyes. Leopard eyes are about 5 times as sensitive to light as ours,
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