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Originally Posted by astronut
 I need advice on eyepieces that work well on an F5.
I'm prepared to pay the premium price to get good E/P's. |
John,
I know you mentioned you want premium eyepieces, but are you prepared to go all the way price wise ? IMO the best low power widefield eyepiece for use in your 12"/F5 scope would be the 26mm Nagler T5. This costs just over $900, thats a lot of hoot for a lot of eyepiece.
With over 30 yrs experience you're obviously an old bloke like me (47), which means your pupil dillation is less than that of a younger person. Consequently I would avoid using eyepieces which create an exit pupil larger than 6mm. This causes the views to lose contrast and the sky background becomes a little "milky" and washed out. In your F5 scope this means a maximum focal length of about 30mm. Under even marginally light polluted skies this effect becomes noticeably worse.
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Originally Posted by astronut
At the moment I have a Ultima 24mm excellent even on an F5, second, a Sirius optics widescan 111, 16mm another excellent e/p but it only just reaches focus ( inward ) with the F5 and with a barlow forget it!!
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The Widescan III is made by Kokasai Kohki in Japan. They are a high quality eyepiece but being based on a modified Erfle design are not well suited in an F5 scope, regardless of whether they reach focus or not. I would be selling it as it's a good eyepiece and someone will pay a good price for it to use in a slow SCT or MCT, then putting the proceeds towards something better suited to an F5 light cone. The 24mm Ultima is an excellent eyepiece with lovely sharp views to the EOF but it has a narrow FOV compared to the premium widefield designs. If you were to buy the 26mm Nagler T5 or the 27mm TV Panoptic as your low power widefield you may as well sell the 24mm Ultima as it would be superfluos.
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Originally Posted by astronut
The other e/p is the 2" 26mm that came with the LB, 2/3 of view is good, last third affected by mild coma.
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That eyepiece is the Meade 26mm QX. As you have found out, it is not great in an F5 scope or at least I dont think its great. A 27mm TV Panoptic it is not !!! The 26mm Meade QX does a good job in slower scopes and again has a reasonable resale value for someone that owns a slower scope.
Additionally that aberration you are seeing, which worsens as you go further off axis, is likely to be "off axis astigmatism" and "field curvature" caused by the eyepieces' inability to cope with the steep angle of incidence of the light cone, created by the fast (F5) primary optic as it approaches the EOF. These aberrations will effectively "overpower" any coma that you would otherwise see eminating from the parabolic primary mirror. Normally with cheaper widefield eyepieces used in conjunction with fast scopes, the dominant aberration is off axis astigmatism with secondary field curvature. Using good quality properly corrected eyepieces at F5 you will normally not see much coma at the EOF and the premium eyepieces partially correct for it anyway.
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Originally Posted by astronut
I would like to have a good range from 40mm to about 7mm, taking into account I have a Meade 2X barlow- 1 1/4".
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Forget about 40mm eyepieces in your F5 scope, that creates an 8mm exit pupil and you and I are both wayyyyyyyyyyy too old for that, regardless of how dark the sky is. As I mentioned above, stick to a maximum focal length of about 30mm to 32mm and I would recommend 25mm to 30mm as being the best option. Although the FOV is slightly narrower using these focal lengths the views are improved due to better contrast resulting from the smaller exit pupil.
If you care to tell me the maximum amount (ballpark) you're prepared to spend, I will give you my thoughts having regard to your budget and what you may be able to raise as a "cash back" for selling some of those eyepieces not ideally suited to your new F5 scope.
CS-John B