Telescope Reviews: Maximum Field w/ Fast Dob

I'm assuming your scope has an aperture of 305mm (12"). I used to own an Orion 12" f/4.9 Dob as well. One of the reasons I owned a 12-incher was the increased light grasp over smaller scopes I'd owned. It was that increased light grasp that made lugging the 50+ pound OTA worth the effort.

Some, including one or two in the group of folks I routinely observe with, will disagree with this first point (*cough* hoof *cough*

), but in my opinion exit pupil is one factor that *should* limit your choice of eyepiece focal length.

I'll be 44 in October. My pupil's maximum dilation is just about 6mm. Accordingly, if the eyepiece I use throws up a larger exit pupil much than 6mm, the photons contained in the portion of the light cone exiting the eyepiece that doesn't hit my pupil are lost, effectively reducing the aperture of the scope I'm using and defeating the point of having that aperture in the first place.

You can calculate exit pupil in a number of ways, but the simplest is to divide the focal length of the eyepiece by the f-ratio of the scope. A 50mm eyepiece in an f/4.9 scope puts up a 10+mm eyepiece. No human eye has a 10mm pupil diameter. A young healthy person has a maximum pupil diameter of about 7mm. Such a person, were they being mindful of exit pupil in selection an eyepiece, would be looking at eyepieces with a focal length of less than 35mm.

When I had my 12" f/4.9, I used a Siebert Observatory Series 34mm 2" eyepiece as my lowest power, dark site finder eyepiece. For my eyes that was a little large in the exit pupil department, but not massively so. That eyepiece had a 70 degree AFOV, produced 44x and an approximate TFOV of just over 1.5 degrees. In my opinion, this is the largest logical TFOV, being mindful of exit pupil, one can obtain effectively in such a scope. Siebert Observatory Series go for about $200 new. I really enjoyed mine.

My replacement for the 12" is a 10" f/4.7 Dob. I chose a Baader Hyperion Aspheric 31mm as the widest true field eyepiece for that scope. The 6.6mm exit pupil is a better match for my eyes. The TFOV of that eyepiece in the 10" scope is just over 1.8 degrees. I'm happy with this one as well but the Siebert had less astigmatism at the edge of the field than does the Baader. To its credit, the Baader has better coatings and less internal reflection than did the Siebert. I think I paid about $190 for the Baader which has a 72 degree AFOV.

Affordable 2" wide field's I would consider in an f/4.9 instrument would include the 35mm Baader Scopos Extreme, 36mm Baader Hyperion Aspheric, 31mm Baader Hyperion Aspheric, 36mm Siebert Observatory Series, and 34mm Siebert Observatory Series. The Scopos Extreme is a beast, however. It might mess up the balance of your scope.

There are less costly 2" wide fields in these focal lengths, but those that I've used in fast instruments were very poor performers in faster scopes.

The "ultimate" in this class would be a 31mm Nagler Type 5. In a 12" f/4.9 it would produce a TFOV of over 1.6 degrees, but is awfully expensive.

Regards,

Jim

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