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View Full Version here: : 8" sct/low power eyepiece vignetting?


casstony
26-11-2006, 02:23 PM
After finally being able to see the Helix nebula with a Televue nebustar filter on a 27 panoptic, I'm considering getting a 50 to 55mm eyepiece to get greater image brightness for large dim DSO's. Some of these eyepieces will vignette and some will not. Am I correct in thinking that the view through a vignetted eyepiece will be just as good as that through an eyepiece that exactly matches the scope, assuming identical parameters other than AFOV? In other words, I just don't get to utilise the extra potential field of view of the vignetted eyepiece?

Gama
27-11-2006, 08:30 PM
Youre going to start seeing the secondary shadow in the eyepiece with longer focal length eyepieces.
You need to experiment with how much you can tolerate.
You may get some vignetting, but again you may not notice, or may tolerate it for the image.

ausastronomer
28-11-2006, 09:12 AM
Gama,

Not with an F10 scope he isn't. A 55mm Plossl still only creates a 5.5mm exit pupil which shouldn't show the secondary shadow unless he is very very old, like over 100. If he is that old he should have the cue in the rack :). Any exit pupil under 6mm should be fine even under slightly light polluted skies. Of course the smaller the exit pupil the better the contrast and the darker the sky background, to a certain point.



Tony,

Thats basically what happens. Whilst some might be happy with it, I don't find such views aesthetically pleasing and I wouldn't recommend you go there. If you are looking for a mid priced eyepiece that works very well as a low power wide-field eyepiece in an 8" SCT then the University Optics 30mm MK-80 is a great choice. This will give about the maximum FOV in an 8" SCT without vignetting and in the slow F10 SCT this eyepiece is very sharp.

CS-John B

casstony
28-11-2006, 11:02 AM
Thanks for the replies guys. I decided to risk $60 and ordered a 50mm superview this morning. I'm chasing maximum image brightness rather than maximum field of view, and I'm thinking the nebula filter will take care of the increased skyglow. Of course I could just use my new Dob to get the extra brightness, but my undeveloped finding skills make chasing invisible nebulae a little difficult. With the LX90 I can know that the object is in the FOV, then try a filter to see if it appears. It's all fun :-)

Gama
28-11-2006, 11:56 PM
With my 10" F10 LX200 and using Meades 4000 series 55mm. i could see the secondary shadow. But i am fussy, some may not even notice it, unless you look for it.
The exit pupil is approximatly the same, but like i said, some people can notice it easier than others.

ausastronomer
29-11-2006, 06:41 AM
Gama,

I don't doubt that you see it, everyone is different. How light polluted are your skies, or does it only happen when you look at the Moon with it ?

Meade actually designed that eyepiece for use in their own F10 Schmidt Cassegrain Telescope, so you have to assume it works fine for most people, in most situations. The main issue with using it in an 8" SCT, is that the 8" scope has a narrower baffle tube and will vignette, which as I said before, is a look I don't really like. But again that is a matter of personal preference.

CS-John B

Don Pensack
06-12-2006, 03:46 PM
An 8" SCT only illuminates 38mm of field, so having an eyepiece with a 46mm field stop doesn't really make sense. There is a high likelihood you will see vignetting (light loss) at/near the edge of the field. If you stick with eyepieces closer to the widest field stop, like a 31 Nagler (42mm field stop) or even better a 35 Panoptic (38.7mm field stop), vignetting is less likely, and the higher powers will give a darker field of view and better contrast.

As for exit pupil, most people can tolerate up to 6mm of exit pupil, though it probably stops the scope down for most folks older than 50. My own pupils only open to 4.5mm at night, so, though I can use an eyepiece that produces a 6mm exit pupil, I know my eye is essentially stopping down the scope. The good news is that the light loss at such low power is usually not noticeable because I am not seeking the faintest objects at that low a magnification.