Help to complete my ep's set

I have a Nexstar 5i and these eyepieces:
6mm, 17mm, 25mm and 40mm (all Celestron Plossls), and also a Barlow Ultima 2x.
I'm thinking about a 9mm UO ortho or a Siebert standard 10mm (due to the 65º AFOV) to complete the high power range and a Siebert 21mm standard (because of the 75º AFOV, is this possible ???), a Meade's 4000 SP 32mm or a Celestron Ultima 30mm for the low power range.
I know the very good perfomance of OU ortho's, Meade's 4000 SP and Celestron Ultima's but really don't know about the real off axis performance of Siebert's standards (astigmatism, coma, false color)
I'm also thinking about a 0,63x FR and wish to know if the eyepieces above work well with this reducer.
I really would like to know your opinions and please feel free to make any other suggestion (but please not Naglers or very expensive ep's).

Regards,

Martin

Hey Martin:

I wouldn't buy a bunch more eyepieces yet if I were you. I think I would just fill in the gap between the 6mm and the 17mm a bit with another plossl. I would not buy a 32mm unless you want to get rid of the 40mm. One low power eyepiece is enough. If you get a 32mm, you will likely stop using one or the other. Fill in the gap, and stay with what you have. You will likely "know" after using it awhile what you are missing (if anything). You actually have a nicely balanced set now except for "that" gap. Don't be in a panic......

Clear Skies - Rob

I'll comment on the off-axis performance of the Siebert standard eyepieces (I own a 7mm Siebert standard). In my 80mm F8 apo refractor, the field size is about 40', the moon takes up 30' and is barley sharp at it's limb when centered in the eyepiece. I would estimate the critically sharp area at 27' or about 44 degrees of the 65 degree field of view. When combined with 2x shorty 3 element barlow, the sharpness increases a great deal, to about 58 degrees of the 65 degree field of view.
I use it in conjuction with the barlow most of the time, so it performs quite well. Eye placement is critical to avoid false color, this seems to be due to the large eye lens as compared to a plossl or ortho. The coatings on it can't compete with multicoating though and I have noticed ghosting on brighter objects, particulary when used without the barlow on a 10" newtonian.
Rather than a Focal Reducer, I would see about setting up the Nexstar for 2" viewing with a quality diagonal and a good widefield eyepice. The Proxima 31mm comes to mind, it's about $100 used on astromart. The view through a 70+ degree fov 2" eyepiece is amazing. I would skip the Siebert 21mm and go with an Apogee 20mm widescan (which will have slightly less fov, but it will be much sharper at the edges). For higher power, the U.O. 9mm ortho would be a great choice.

Clear Skies,

Joplin

Fernando, TREAD CAREFULLY. When selecting eyepieces it is very important to consider not only magnification, but true field of view as well. A higher power eyepiece with the same true field of view as a lower one will **usually** be perferable. You will see just as much sky but with more magnificaiton, translating to more detail and contrast.

Remember:

Mag = FL(telescope) / FL(eyepiece)
TFOV = AFOV / Mag

With this in mind, you should revamp your eyepiece collection. Replace your 40mm with either the 30 or 32mm plossl you mention. The 40mm has no significant advantage in a 1.25" eyepiece. The field of view is restricted by the barrel to an AFOV of 40-45 degrees. A 30 or 32mm will have the full 50 AFOV resulting in a similar true field of view as the 40mm. Since a 30 or 32mm eyepiece is close (magnification wise) to your 25mm and with a barlow comes close to your 17mm, you have also obsoleted two other eyepieces. You might consider replacing these with a single 22mm-ish eyepiece to fill the gap, giving 30, 22, 15 (30 w/Barlow), 11 (22 w/Barlow), and 6mm which is a pretty decent selection.

If you mix wide fields into the fold, be careful about mixing them with your plossls. You could easilly make several of them obsolete. For example, the 21mm Siebert you mentioned and a whole host of other wide field low power eyepieces will very probably provide as wide a true field of view as the 40 (or 30)mm and wider true field of view than the 25mm. There is then little advantage to keeping those eyepieces. Likewise, the 10mm Siebert might trump the 17mm Plossl. You'll just have to run the numbers and see.

The Siebert standards are generally speaking very good performers. I have his 15, 12.5, 10, and 7mm and have no problems recommending them. The only distortion visable in any of them is right near the field stop where some astigmatism is present. I have heard mixed opinions on his 21mm. All I can suggest is to try them. Siebert has a 30 day return policy that I did use once with no problems what-so-ever.