Telescope Reviews: Eyepieces for the Oberwerk BT100 & BT80

I wrote some on this topic in the articles forum and based on a discussion with NW, I tried some new things tonight so I'm bringing the discussion over here.

The BT100 is a straight thru 100mm binocular with a focal length of 620mm. The standard issue eyepieces both have an Afov of 62°. One is 24.5mm for 25x with 2.5° and the other is 10mm for 62x with 1.0°.

I've owned a 30mm Ultima for 5 or 6 years and it is a favorite in my various scopes. In the BT100, it would give 20.7x with an exit pupil of 4.8mm and a field of view of 2.4°. I've tried my single 30mm in the BT and it performs pretty well. It's edge performance is not as well corrected as the Televue plossls, but it's not bad either. the view is very bright. Seems to me it would make a great large exit pupil eyepiece. So, a few months ago, and again this week, I purchased three other 30mm Celestron Ultimas. Well the 30mm Ultima is notorious for having a tight barrel. And I can now say the BT100 has extremely tight tolerance for a 1.25" eyepiece. None of the three new purchases will fit in the barrel. I measured the barrels on all the 30s with my calipers. The three I recently purchased are all one half of 1/10th of a mm wider. I'm getting ready to give up on the 30mm Ultima.

So far my favorite low powered eyepiece is an unmatched pair of older 26mm Televue plossls. These give 23.8x with a 2.2° filed of view and a 4.2mm exit pupil. The view of nebula is stunning.

My most used eyepiece is a pair of 20mm Televue plossls. At 31x the little extra power is quite noticable. I think it helps finding faint galaxies. Field of view is 1.6°, and exit pupil is 3.2mm.

I have a pair of 17mm Orion Sirius plossls that I like for sharpness of image. magnification is 36x, fov is just under 1.5°. The 20mm TV actually sees more stars, but these 17mm are pretty good if I want a little magnification boost.

I tried my single 18mm Meade SWA in one barrel, but the edge sharpness is much better in the 17mm Sirius plossl and the 20mm TV plossl, so the 18SWA got passed over as a choice. I tried my single 15mm TV plossl in one side, but the eye relief is so short I would need to use these without my glasses, so I decided not to purchase another one. I have astigmatism in one eye that becomes noticable with exit pupils larger than 2mm.

I have a pair of 12.5mm UO orthos that, although very narrow about just under 1°, provide the most incredible crystal clear sharpest view you could imagine. At 50x, it's a great power. With these I have seen resolution in several globulars. Far better image than the 10mm WA. One of the best high-powered views ever in the BT100 was with a 12.5mm UO ortho at 50 power when I resolved some 30 to 40 stars in the very dense open cluster M11.

I also have a pair of 9mm UO orthos (69x), but I found the eye relief to be much to short. In most of my scopes, I don't use my glasses with the 9mm, in the BT100 I do. To get the best use out of the 12.5mm and 9mm orhto, I have to take my glasses off. In addition, it is extremely difficult getting the tiny orthos in and out of the tight BT eyepiece holder. I still have the pair of 9mm, but I haven't used them in a while. I find it easy enough to just not stick the 12.5 UOs all the way into the barrel. There is almost 28mm of focus travel in the BT100, so there is plenty of room for focusing.

The stock 10mm WA Oberwerk eyepiece gives 62x with a 1° field of view. However, the 12.5mm and 9mm UO orhtos seem to pass more light. I can see fainter stars in the 12.5mm UO than I can in the 10mm WA. In addition, 62x accentuates some slight astigmatism in one barrel and a little bit of miscollimation. I must read the instructions and get this taken care of to really get the best out of these high powered eyepieces.

The TV20, Orion17 and UO12.5 provide such an incredibly sharp image right out to the very edge of the field that I have observed doubles of less than 7" with all of these right up to the field stop. The standard issue 25x WA eyepiece is good to 70% out and the 62x WA maybe to 80-85% out.

Recently I purchased a pair of Meade 13.8mm SWAs on sale. At $99 each I thought these would be a great choice. After using these several times, I am extremely disappointed in the performance of this eyepiece. Beyond 60% out from the center of the view, the image is deplorable. The eyepiece has extremely short eyerelief and I can only see about 75%-80% of the field if wearing glasses. Without glasses, the eye relief is not much better than my 12.5mm UO orhto, in fact I'd say it's more difficult to use because it has a wider filed to try and see. The 12.5 UO ortho, even though eye relief is not long, is pretty easy to see the 44° Afov. The 13.8mm SWA is definitely not made to be used in an f6.2 instrument. In fact I tried one of these 13.8s in my f11 5" SCT and I was still not at all impressed with the performance. I will be passing this pair of 13.8 SWAs on to a new home.

I'm considering purchasing another 14mm Radian to go with my existing. I think these might work well at 44x with a 1.4° fov.

Because I prefer to use my glasses while binocular observing, I would prefer to use all long eye relief eyepieces. The 14mm would be a good choice. Tonight I tried using a barlow. The TV 2x barlow fits with plenty of room to spare without hitting the prisms and works well with my 30mm Ultimas and my Televue eyepieces. This is encouraging. And the performance of the 30mm Ultima was much improved in the 2x barlow. With the addition of another 2x TV barlow I could use my 30mm Ultimas, my 26mm TV plossls and my 20mm TV plossls to also provide 15mm, 13mm and 10mm combos, although fields would be narrow, but better than the high powered orthos.

I tried my Celestron Ultima 2x barlow. Interesting was that it worked with the TV20mm, but would not work with the 30mm Ultima eyepiece. Needed more in travel.

Here's what else I tried tonight. I tried to use a diagonal. Wouldn't it be nice if a diagonal could be used with the BT100 to give 90° eyepiece viewing! I didn't expect it to work and it didn't. That's because the focal point is inside the focus tube and the diagonal is like trying to extend the tube by maybe 50 to 60mm. You can't do it. But, if you put a barlow in front of the diagonal it changes the position of the focal point. So I tried out barlows in front of diagonals with several combinations. One thing to keep in mind, a 2x barlow placed in front of the diagonal is acting more like a 3x barlow. That makes for a very long focal length binocular with a narrow filed of view. But it's worth a shot.

With my TV 2x barlow, I needed less than a mm more of in travel in the focuser to bring my 30mm Ultima and 20mm TV into focus. I mean, it was so close that less than a half turn of the focuser dial would have brought these eyepieces into focus. I used two different 90°diagonals, a Celestron prism and an standard issue mirror diagonal. I also tried my Celestron Ultima 2x barlow in front of the diagonals, but it needed even more in travel in the focuser. The 2x TV barlow seems like it may hold some promise. I have at least three other diagonals around the house that I'm going to try. I have a TV standard diagonal and a TV Everbright. If the length of the light path is just a bit different in one of these, It might just work.

Keep in mind, with the barlow in front of the diagonal and a 30mm eyepiece, If it works I'm going to get the equivalent of a 10mm eyepiece or 62x. But man you should see the Orion nebula at 62x in the BT100. Wow! And just imagine if I can get this to work with a 90° eyepiece setup!

edz

Edited by EdZ (01/15/06 05:13 PM)