Kowa TE-11WZ zoom on astro telescope? (1 Viewer)

Apropos the vignetting issue, I removed the TE-11WZ from the scope and at the 40x marking the field stop started to blur. This increased up to the 60x setting, where there was a darkened periphery making up perhaps 20% of the radius. Viewing terrestrially with the eyepiece back on the scope this was no longer apparent except fo a somewhat fuzzy field stop.

In view of the otherwise excellend performance of the TE-11WZ with its wide AFOV (see: https://www.birdforum.net/showpost.php?p=3753347&postcount=58), flat field and negligible astigmatism, I don't regard this as a serious fault.

Btw, the 883/884 is assumed to have a focal length of 500 mm, so I found it a little confusing that the Kowa specs for the 500 mm telephoto lens show magnifications of 28-66x with the TE-11WZ. This would suggest that the FL of the 883/884 is actually shorter, around 455 mm. However, when I compared the same astro eyepiece on my Swaro ATM 65HD (460 mm FL) and the 883, the latter showed a significantly narrower FOV, so I think the 500 mm figure is about correct. Explanations anyone?

John

Tringa45 said:

Btw, the 883/884 is assumed to have a focal length of 500 mm, so I found it a little confusing that the Kowa specs for the 500 mm telephoto lens show magnifications of 28-66x with the TE-11WZ. This would suggest that the FL of the 883/884 is actually shorter, around 455 mm. However, when I compared the same astro eyepiece on my Swaro ATM 65HD (460 mm FL) and the 883, the latter showed a significantly narrower FOV, so I think the 500 mm figure is about correct. Explanations anyone?

John

Click to expand...

John,

I think the explanation is that the prism unit used to adapt Kowa eyepieces to the telephoto probably contains a Barlow lens in front of the prism. You can see the extra lens in the cutaway below:

https://store.shopping.yahoo.co.jp/loupe/kw-tp556.html

Unfortunately it's too small to tell what kind of lens it is, but a 1.1x Barlow would nicely explain the higher magnification. It's probably needed to move the focal plane back a bit so the eyepiece can reach focus.

There was a thread about the telephoto lens/scope when it first appeared. I recall Rick (RJM) stating in that thread that the 883/4 scopes have 510mm focal lengths.

Henry

Edit: Here's the thread. See post #23.

https://www.birdforum.net/showthread.php?t=165593&highlight=Kowa+lens+scope

The adapter to use Minolta MD lenses on the newer (1985 onwards) A mount autofocus Minolta contains a single element magnifying about 1.2x. This is to enable the lenses to focus throughout the range to infinity.

Several other makes of similar adapters contain single lens elements.
Generally simple adapters without lenses cost about £15, whereas the ones with a lens cost about £35.

With the newer Sony mirrorless system the flange distance is much smaller and nearly any lens can be used including fast f/0.95 movie lenses. With the very high ISOs possible with some of these Sony cameras one can photograph things more or less invisible to the unaided eyes.

I used to use a good Nikon 500mm f/8 mirror lens with a two times Barlow plus an adapter with a 1.2x lens. With an APS size sensor this worked out at about 1800mm equivalent focal length. Good photos of the Moon were possible hand held.

Barlows are only roughly the nominal magnification as the actual magnification is sensitive to position. The same happens with the nominal 1.2x single lens adapters.
The only way to know the true focal length is to actually measure it. With photographs one can measure the image size of the Moon at any given time.

Thank you for those insights, Henry and David.
Incidentally, the Kowa prism unit (presumably Porro II with a single element Barlow?) costs €650, so the conversion of the 500 mm telephoto lens to a scope entails an investment of ca. €1300.

John

Here I have a SRB-Griturn Canon FD adapter marked Made in Japan 7000/9000 +CF with red and green dots very nicely placed around the circumference. It is black and well made.
The soft back cap is marked 495, the hard front cap CA Japan.
(The 7000/9000 means Minolta 7000/9000, autofocus A mount cameras, the same as Sony A mount. The 495 probably 49.5mm).

I think that some of these adapters are single element.
In this case there are two coated uncemented elements with negative outer surfaces at least. The lens is thin.

The clear aperture is 22mm.

The magnification is about 1.15x.

I got this because a shop mistakenly thought that a Sigma 300mm f/2.8 lens in Canon FD mount was almost useless.
It isn't.
The lens is in excellent condition with case.
Not only does this adapter make it very usable, it is also a good quite modern lens. And it has an operational aperture ring.
The price was £100, a fraction of the normal price for other mounts of the same lens.

I also have a tiny Canon own adapter that makes Canon FD lenses functional on more modern cameras. Unfortunately this adapter is so tiny I can't remember where I put it.

The Sigma lens is probably about 292mm focal length, so with the SRB Griturn adapter it is about 340mm f/3.2.
In the rush for newness one can forget that older lenses can be very good.
This Sigma lens can also be used with visual adapters, but f/3.2 is fast, so a 1.4x, 1.7x or 2x teleconverter may be needed.

I wonder if there may be other cheaper ways to make the Kowa 500mm lens usable with eyepieces.
There are numerous adapters out there.

The Novoflex adapters are very good but expensive.
SRB Griturn own make adapters are usually tested by them before sale and seem pretty good.
Getting an adapter from China may be risky, as the quality may be pooor.
Also the Korean visual lens adapters are very poorly assembled with dust in all the four mounts that I have and narrow fields, although long eye relief and tripod foot.
There are angled and straight versions.