Oberwerk 20x90's at TSP
Posted 25 May 2004 - 08:55 AM
Tuesday, I arrived at Prude Ranch (where TSP is held) later than planned because of a tire failure on the way. The vender area was closed and I had hoped to pick up a Unimount for the 20x90's there. I was able to jury rig them onto an older parallelogram mount I had for some smaller Nikon 10x42 SE's I have.
I was a bit disappointed with my first views under the stars. I was able to merge the images fine, but they did not seem as crisp as I hoped. I suspected the collimation was not as good as I first believed. There was also substantial flare on bright starts as I moved my head around in front of the eyepiece. It was hard to impossible to find a position that did not have any problem. This is a problem I've had with other binoculars but the 20x magnification seems to make it more noticable. Other than that, vews were pretty nice and the added aperture and magnification was great on DSO's.
The next day I picked up a Unimount Light Basic and went to the local library for internet access in order to print out the collimation instructions from BigBinoculars.com. With instructions in hand and the binocs on the new mount, I was able to see that there was significant vertical miscollimation. I adjusted the right prism slightly (maybe 1/10th of a turn) and brought the vertical alignment where it should be. The left eyepiece image was slightly to the left of the right one (by maybe 1/20th the FOV), but the instructions indicated this was OK and preferable to them being crossed.
That night I was able to try them again and while there was improvement, things still did not seem as sharp as hoped. They did feel more comfortable to look through. The edge performance was a bit worse that I'm used to with the image starting to degrade about 60-70% from the center.
I was able to complete the TSP Binocular Observing Program, the Binocular Challenge Program, and the Binocular Program from *bleep* with these binoculars over the next two nights.
I compared the OB 20x90's with some Fugi 16x20's and while the edge performance was better in the Fugi's and they were a bit sharper, I probably slightly preferred the view in the 20x90's (higher mag and larger aperture). The eye relief in the 20x90's was just about perfect for me while wearing my glasses. There was no problem seeing the entire field stop. With the Fugi's, I had to move my head around a bit to see the entire field. If the Fugi's had a couple of mm longer relief I probably would have preferred those.
Other issues I had with the OB 20x90's
1. The binocular rocked too easily on the center mounting post. I couldn't see how to tighten this in the field.
2. The right diopter adjustment needed to be rotated almost all the way clockwise to be adjusted for me. When I'm wearing my glasses, normally this is set to pretty much "0" on all other binoculars. Something must be misadjusted here. It is fine for me, but for someone else it might not be able to be rotated enough.
When I got home I found and read Ed's Cloudy Night's article on binocular collimation. It indicated that horizontal divergence was not acceptable (contrary to the BigBinoculars site). I set them up again last night and did the collimation on a bright star as Ed suggests. I was able to bring them to perfect collimation and still have a perfectly round exit pupil. I didn't look at much last night since I was still really spoiled from TSP. I did put them on Jupiter and it looked much better than before, so I'm hoping things will be much improved when I give them a good workout.
I was able to get the cap off of the center mounting post and using a spanner, tighten up the movement of the binoculars on the post. It is now quite acceptable. I will email Kevin at Oberwerk about the diopter adjustment. I seem to recall others mentioning this as an issue with there OB's.
I will probably be mounting a Stellarvue red dot to the side of the 20x90's to help with locating objects.
Posted 25 May 2004 - 11:31 AM
Posted 25 May 2004 - 11:58 AM
A few comments.
There is a highly regarded binocular website and even a portion of a well known book that states the tollerances for Convergence (crossed over) should be stricter than the tollerances for divergence (spread apart). This is incorrect. It should be the other way around. And as far as vertical error, there is almost no allowable error.
Before I published my article on collimation, I wrote to the author of the book in question and discussed with him the values for tollerances. I was puzzled because the author explained at length in the text of his book how the eyes have no muscles to accomodate divergence. He confirmed what I am saying above. I then wrote to a well known binocular repair specialist. He confirmed the same.
As magnification increases the tollerances become much smaller. You mentioned above that error before you adjusted may have been about 1/20th the fov. Just for example sake, if your binocular has a 2.5° fov with a magnification of 20x, the allowable error in collimation convergence (according to standards) would be 4-5 arcminutes or 1/30th to 1/40th the fov. Personnaly I cannot even tollerate half that much error. In my 20x binoculars I cannot tollerate ANY vertical error and divergence/convergence needs to be within 1 to 2 arcminutes for me to be able to get comfortable views. Anything beyond that and I have difficult bringing objects to focus with two eyes.
Posted 25 May 2004 - 12:04 PM
That's too bad you had to do some much work to get your new obies to perform somewhat better. With your next clear night can you see how well EdZ collimation guides worked for you. It seems that every binocular is unique in a way, despite coming from the same supplier with the same specs. Perhaps your obies got knocked around during shipping to explain the mis-collimation.
I did use Ed's procedure and I think I have then basically perfect. At least when relaxing my eyes and pulling them 6" away, I see no discrepency between the left and right.
I'm sure they did get knocked in transit. The molded styrofoam the binocs were cradled in was crushed and broken somewhat as if the box had been jared and the subsequent binocular movement smashed the styrofoam.
Posted 25 May 2004 - 12:31 PM
In addition to seconding the points made by Ed with regard to CORRECT collimation tolerances , I would just like to thank you for providing such a detailed , honest and prompt report.
It's been said before and I'll say it again.
HIGH powered binoculars need PERFECT collimation.
Perfect collimation needs very sturdy construction to be maintained.
I'm as delighted as the next person about the fact that prices for large binoculars seem to be getting much more affordable , but I wonder if it co -incidence that we hardly ever hear about collimation problems with large binoculars from Fujinon or Nikon ?
Surely a box is a box to a carrier ?
Regards , Kenny.
Posted 28 May 2004 - 08:48 AM
"This one is an easy fix. The 20x90's objective cells have screw off covers
(just unscrew by hand). Then you will see that each objective cell is
locked in place with three setscrews. You can loosen the setscrews, then
spin the objective in or out to obtain perfect focus at a diopter setting of
"0". Once it's perfect, tighten the setscrews and spin the cover back on.
Note that you will probably have to touch up collimation a bit after doing
Posted 30 May 2004 - 05:30 PM
It appears it will be a few days before I get to use them again. I'm looking forward to the Milky Way area around Sagittarius.