Whats the lowest magnification I should buy?

Still getting my feet wet when it comes to eyepieces.
I plan on buying a Tele Vue 2" 27mm Panoptic and a 12mm Radian. Should I shoot for a nice 40+mm ep as well? I know this is kind of a vague question but feedback would be appreciated thanks a bunch!

Casey Whalen said:

Still getting my feet wet when it comes to eyepieces.
I plan on buying a Tele Vue 2" 27mm Panoptic and a 12mm Radian. Should I shoot for a nice 40+mm ep as well? I know this is kind of a vague question but feedback would be appreciated thanks a bunch!

Hi Casey,

You have to be conscious of exit pupil. I have a 10" XTi and my 26mm is the longest I use with it. I had the choice between the Nagler 26mm and the 31mm hand grenade and chose the 26 due to its smaller exit pupil. A young eye will dilate to about 7mm. My vintage '57 eyes will dilate to a little over 5mm, so the 26mm was a better match for me. YMWV

I think your choice of a 27Pan will be a good match for your scope. You might pick up a longer EP to use as a finder EP, but the sky background will be lighter and you will waste a few photons.

Go to the TeleVue website and play with their eyepiece calculator. Plug in your scopes parameters and it will give you magnifications and eye relief among other things.

>>>Still getting my feet wet when it comes to eyepieces.
I plan on buying a Tele Vue 2" 27mm Panoptic and a 12mm Radian. Should I shoot for a nice 40+mm ep as well? I know this is kind of a vague question but feedback would be appreciated thanks a bunch!
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Hi Casey:

I agree with what Dave said, in an F/5 scope, I don't have much use for a 42mm eyepiece. For one thing it is up in the 8mm exit pupil range which means that you will not get the full use of your 12 inches of mirror. In general a 7mm exit pupil is consider the maximum useable but that varies with the observer.

There are other reasons to avoid large exit pupils, they make full use of your eye's lens so any small amount of astigmatism you may have shows up as flares and "spikey stars."

The 27mm is probably a reasonable maximum in an F/5 scope though I probably would suggest skipping the 27mm and going with something in the 30mm range as your widest widefield.

jon

Casey,

It depends on what kind of scope you are using and your entrance pupil.

If you have a scope with an obstructed view (like an SCT or Newt) you can have a hole in the middle of the field of view (shadow of the secondary) if you have too much of an exit pupil. This does not happen with a refractor.

Also, if you are young, and still can get your pupil open to 7 mm, then you can tolerate an exit pupil of 7 mm. If not, you are "wasting light" using a five mm eyeball on a 7 mm exit pupil.

Alex

Casey Whalen said:
Still getting my feet wet when it comes to eyepieces. I plan on buying a Tele Vue 2" 27mm Panoptic and a 12mm Radian. Should I shoot for a nice 40+mm ep as well?

Casey,

In your XT12i the 27 Pan will give you about 1.23 deg TFOV with a 5.5mm exit pupil. If you go with a 40mm Paragon your true field will increase by about a little more than 6/10th deg but the exit pupil become over 8mm. You will notice dramatic light loss on Nebula! I have the 10XTi and moving from a 32mm to a 40mm (6.8mm to 8.5mm exit pupil) shows a dramatic loss in nebulocity. However, star fields are still very pleasant so I keep this 40mm for largest TFOV scanning.

So if all you are looking for is the widest possible TFOV then the compromise with the 40mm will be light losss making DSO viewing less than optimim.

Although expensive, the 31mm Nagler will get you close with a 5/10th deg increase TFOV over your 27 Pan and still keep the exit pupil managable at 6.3mm. So this would be your best choice.

If it were me, and it wouldn't be a stretch to afford it, I'd compliment your intended 27Pan and 12Radian with a 31Nagler and 19Pan. With these you'd have a nice rannge of varying TFOVs out of your XT12i of 1.70deg, 1.23deg, .88deg, and .48deg. A nice spread.

-Bill

Casey,
I have a 40mm GSO that I use in my F6 Dobsonian. I use it basically to scan the sky and zero in on my target. I'll then switch to a 27mm 2" eyepiece for a better view. It's no Nagler, but it's much lower in cost and lighter in weight so it works well for that purpose.
David E