Homemade Bayonet Mount for Astro EPs in Kowa 883 (1 Viewer)

I have recently made a cheap but very nice astronomical eyepiece adapter for my Kowa 883. I was looking for some way to use higher powers than the 60X of the zoom eyepiece. The adapter I have made is designed around the TMB/BO Planetary eyepiece, as sold by Burgess Optical, or an equivalent Planetary sold by Teton Telescopes (available for about $60). The specific eyepiece I purchased is a 5mm focal length with 16mm eye relief and 58 degree AFOV. This eyepiece gives me about 100X with the Kowa 883. I communicated this information to Henry Link, who suggested I put the design on BirdForum.

I am including a few photos of the fabrication process for anyone who is interested in making one of these. Please examine the photos as you read. I have about 20 photos, but can only submit five. If the photos you see here are not enough for you to get the idea, let me know and I will email the others to you.

The adapter is basically two pieces of PVC with a total cost of about $1.00. The fabrication can be done in a few hours if you have the right tools. You will need a good pair of calipers for measuring diameters precisely.

The two pieces of PVC will be described below as the Inner Ring and the Outer Ring.

The Inner Ring Is a small joint of 1.25 "/32mm(inside diameter) PVC. It needed to be reamed out to 1.30"/33mm to accept the eyepiece, and cut to a length of 1.65"/42mm. The outside diameter of this Inner Ring was 1.65"/42mm. It needed to be ground down to 1.50"/38mm to fit into the scope body. The grinding down of the outside diameter is the major time consuming job, and is probably best done with a grinding wheel of some type.

Once the grinding of inside and outside is done on the Inner Ring, it will fit snugly into the scope body, and the eyepiece will fit snugly into it. Cutting the bayonet mount comes next. Mark the sections to be cut by using the little cap which comes with the scope. Depth of cut is 0.25"/6mm, and the "hook" opening of the bayonet mount is 0.14"/3.6mm. I made these cuts using a Dremel tool. These cuts allow you to push in the eyepiece holder and twist it to the right so that it locks in tight, just like a Kowa eyepiece.

The Outer Ring is a small joint of PVC which has an inside diameter of 1.65"/42mm. The Outer Ring has an octagonal end which will fit perfectly against the Kowa scope body if the points of the octagon are ground down just a little.
Only a short section of this Outer Ring is needed. Cut a piece 0.70"/18mm. This piece will include the octagonal end.

The Inner Ring and Outer Ring will need to be glued together, flush at the top. See the photos. The gap between the two rings is about 0.10"/2.5mm. A fast-setting epoxy can be used to connect these two rings, but be very careful to keep the two rings centered, and to wipe off the excess epoxy with a damp cloth before it hardens.

You will also need to drill a small hole into the side of the eyepiece adaptor to insert a set screw for holding the eyepiece securely. The set screw should be no more than 0.5"/7mm in length. It will easily make its own threads as it is screwed into the drill hole. If you want your adapter to click into locking position, you will need to cut a small notch into the under side of Outer Ring at the point where the little steel pin sticks up.

I could go into more detail, but I am afraid I would get too bogged down. Just look at the photos in sequence and you will see what is involved. Again, if you need to see more detail, send me an email.

I will note that the length of the Inner Ring is chosen specifically to allow this eyepiece to be aproximately parfocal with the 60X in the Kowa Zoom. In other words, when the zoom is removed and the 5mm with adapter inserted, very little focus wheel adjustment is needed.

John Wright.

Good job John,

Now, would it be possible to get a report on the performance of the Kowa 883 with the TMB?

Kimmo

Kimmo,

I will post a performance report on the TMB 5mm lens as soon as I get the time and good weather. Perhaps a couple of days.

John Wright

Kimmo,

I have spent a little time comparing the Kowa 20-60X zoom eyepiece ($500) to the TMB-type 5mm Planetary eyepiece ($60) which I have put in my home made bayonet mount. As I stated previously, the magnification with the 5mm is about 100X.

Actual field of view, as would be expected, is considerably less in the TMB lens, about 60% of the field in the Kowa eyepiece.

Eye Relief of the two lenses is about the same. The amount of eye relief with the TMB is fairly good for a 5mm lens. The eye relief of the Kowa is adequate for a zoom eyepiece. I can use my glasses with either lens if need be, but prefer to remove my glasses for higher powers. One reason for needing good eye relief is that with a spotting scope, pushing your eye up against the eyepiece creates shakes which typical tripods just can't handle.

Depth of focus, as would be expected, is less in the TMB lens.

Brightness, as would be expected, is considerably less in the TMB lens. This would be a problem in low light situations or with heavy overcast, but would be no problem on a sunny day.

A sheet of graph paper viewed through each showed essentially straight lines edge to edge with both eyepieces. Neither shows any noticeable distortion.

Edge sharpness is perhaps slightly better in the TMB lens.

Contrast is difficult to judge, but may be a bit better with the Kowa lens. I base this on several low contrast objects which I viewed.

I attempted to compare resolution with the two lenses using the well-known 1951 USAF chart. The TMB lens gave fractionally better resolution, but considering its higher magnification, this would be expected.

Basically, the 100X TMB Planetary eyepiece used with the Kowa 883 breaks no new ground. The slight increase in resolution is probably not enough to make up for the loss of brightness and very shallow depth of field.

Achieving critical focus with the Kowa 883 at 60X is already touchy, given that the fine focus wheel is not really all that "fine". With the higher power lens in place, achieving critical focus is even more of a problem.

On the positive side, using the 5mm TMB lens makes it clear that the Kowa 883 has almost zero chromatic abberation, as the higher power lens still shows focus to be very crisp, with none of the image breakdown we are accustomed to seeing with lower quality spotting scopes at higher powers. In other words, this scope can easily handle the higher power. The only questions is, does it add significant detail. My impression is that is may, but only in bright light and with excellent seeing conditions.

Obviously, there are better, brighter, higher contrast eyepieces out there, but probably not many for the $60 price I paid for the TMB Planetary.

I am left with the impression that if Kowa decided to make a high quality high power lens, it would be better than the astronomical eyepiece which I am using. It would also certainly cost a lot more.

Am I glad I went to the trouble to make the home made bayonet mount? Yes. Do I wish I had more perfect lenses to go with it? Yes.

If anyone would like to lend me a really nice 5mm eyepiece to try out with the Kowa 883, please feel free to send it.

John Wright

John,

Thanks for the review. When I tested the Kowa 883, I used the zoom mostly as it is, but also with a 3x booster to check for resolution at very high magnifications. My finding was also that the Kowa takes magnification exceptionally well for a spotting scope, and like you I found exceedingly little chromatic aberration in the image, and very little image breakdown even at 180x.

Your 5mm TMB will probably be very nice for planetary viewing, but for daytime high-magnification identifying you would probably do best with a 6mm eyepiece, giving you around 83x. At this magnification, you would make nearly all the available detail visible, without yet sacrificing as much brightness, contrast and color rendition as with the 100x. Henry could perhaps recommend the ideal ca. 6mm eyepiece for you, now that you have the adapter ready.

Kimmo

Nice report John. It echoes some of the experiences I had in finding a high mag EP to use with my 80mm Swarovski. Fortunately they already make an adapter for the scope to accept 1.25" astro eyepieces, but my point is really about the eyepieces rather than the adapter (nice job by the way-though it's a shame with a top scope like this that Kowa haven't already saved you the effort and made their own. I'm sure lots of owners would make use of this facility.)

I don't know much about the quality of TMB eyepieces, but from your comments I assume that they are not as good as say, Pentax or Televue?

My first purchase was a 5mm Baader (giving 92x), which seemed fine on the sunny day on which I bought it, but I quickly realized it wasn't up to much when I got it home and tested it properly, so it was quickly returned. Huge great thing too!.

Next I saw a 5mm Televue Nagler on ebay, and snapped it up. It is a big improvement in terms of image quality, but I have to ask myself if there is enough difference in resolution compared to my own (excellent!) Swaro zoom. Obviously there is a noticeable difference in image size, and I'm slowly coming to realize that this is really the advantage for me with this EP. There isn't very much extra detail available to the eye, but it is somehow easier to see because it is bigger. I hope I am being clear about this point, as it might just be something peculiar to me that I like the image to be bigger, even if there isn't that much extra detail available. Many posters on here claim that 40x or 50x is all they ever need!

The other point about the Nagler is that it has a huge FOV (82 degrees I think), though it is difficult to see all of it at once as the eye relief is so short with the eyepiece. You have to get your eye so close to the glass to see through it properly, that you risk either knocking the scope and causing vibrations, or you get greasy marks all over the glass.

I used the eyepiece yesterday to grill some distant geese on a cold sunny afternoon, and found it very useful. There was a distinct advantage over 60x for this purpose, and I am now 90% sure it is worth keeping, even if its use is only occasional. My only reservation now is that Televue's own Radian eyepieces are almost the same in quality- if not identical, but with FOV of around 60 degrees, but more importantly, much better eye relief. Needless to say one is currently on my radar for the next year or so.

Steve

Kimmo and Steve,

Thanks for the comments about eyepieces. Kimmo, perhaps I will look for a suitable 6mm lens. Steve, I am familiar with the attributes of several of the high end astronomical eyepieces (although I have never purchased one.... yet). I know that I don't want to deal with short eye relief lenses, no matter the quality and FOV. I also know that I don't want to end up with a collection of eyepieces I don't use. I was led to believe that the TMB Planetary was a pretty good eyepiece for not much money. I still think that is the case. But I will keep looking.

John Wright

John,

I don't think you can do much better than the BO/TMB Planetary for this purpose no matter how much money you spend. With any decent eyepiece it's really the telescope optics (even diffraction limited) that limit visible detail at such small exit pupils. The TMB is a very clever design that somehow manages to extract excellent edge to edge sharpness with very little distortion from a relatively simple 5 element, 3 group eyepiece. The expensive astro eyepieces with complex designs (like Naglers, Radians and Pentax XWs) are not visibly brighter, sharper or higher contrast than the TMB. IMO the Radians are dimmer and lower contrast. I don't like Naglers for daylight use because of kidney beaning type blackouts, severe pincushion distortion and short eyerelief. I do like the 70 degree field and long eye relief of the Pentax XWs, but the 5.2mm is a very large and heavy eyepiece. Only well executed simple designs like orthos and TMB Super Monocentrics have a slight edge in brightness and contrast over the TMB Planetary, but that comes at the expense of eye relief and field width.

Great work with the eyepece adapter! Maybe you could start a cottage industry of PVC adapters for the DIY challenged.

Henry

Henry,

Thanks for your comments. I like the TMB Planetary, but I guess I just can't expect to squeeze much more detail out of the Kowa 883 than I have already done.

John Wright

John,
Well done!
Thanks,
Greg

John,
Nice post and hope that Kowa or an eyepiece producer (Baader???...) will do an adapter as you done. You showed that it's possible and works - the Kowa 88 started to be more interesting to me...
Regarding the eyepieces, I bought the one you have (the European version - Telescope Service), also bought the Baader 5mm and have the XW5mm - I'm waiting for TV Nagler 5mm that is another of our study areas to do a test of all in Optolyth 100 and Pentax PF100. From the contact of the 3 first I still prefer the XW but I have to spend more time with all - XW has more light, wider field, better eye-relief and easier to adjust.
Regarding if you can obtain more detail/power from the scope, it will depend on your field conditions. I have lucky with mine, so use the Optolyth with the 30-90x zoom regularly and go to the XW 5mm, when need and it usually work well - 126x. I have a TV Radian 4mm that don't use since there is no gain on it, with this scope. However I think it maybe useful in some 80mm scopes - I tested in a Optolyth TBG APO 80 and liked it... However that scope is only a 420mm (result on 105x) and the Kowa is around 500mm (would result on 125x).
An interesting eyepiece, on paper, is the Antares 5-8mm Speers-Waler Eyepiece that has not bad reviews (e.g. http://216.92.113.163/item.php?item_id=331). Although it's very big - wouldn't be a problem for me... The main problems can be a reduced eye-relief (although some publicity gives 17mm eye-relief) and a need of too much in-focus and don't know if it would work on birding scopes. If someone tested it, let us know about it...
David

John,
You mention you have twenty photos and displayed five. If you could attach a few more photos of your handy work would be helpful.
Thanks,
Greg