14 replies to this topic

[Scott in NC]

Now for something a little different, a scope which I'd never seen before, and one which was generously loaned to me by a fellow CN'er.  This is a Hino Optics Mizar GT-65, which is essentially a finderscope, but with nice enough optics that it can be used as a stand-alone refractor in its own right.  It's a doublet achromat with a clear aperture of 65mm, and a focal ratio which I've estimated (but have not precisely measured) at somewhere between f/6.5 and f/7.  I'm guessing that this was produced somewhere in the 1970s, but really don't know for sure.  

 

Here are some pics of the scope.

 

[Scott in NC]

Now for the DPAC results:

 

Inside focus:

 

 

At focus:

 

 

Outside focus:

 

[Scott in NC]

This has pretty nice spherical correction, especially in green and red, and pretty decent control of chromatic aberration for a fast doublet achromat. I also see a central zone and a small turned down edge, but nothing too objectionable considering that this was likely meant to be used as a finderscope.

 

Overall, I'm very pleased with the way this scope turned out, and I thank my friend who generously loaned me this scope for testing purposes.

 

If any of you know more about this scope, please fill me in.  There's very little about it on the internet.

[Scott in NC]

Maybe I should consider moving this thread to the Classics forum, as no one over here (myself included) seems to know much about this scope.

[Scott in NC]

Okay, I’ve moved this topic to Classics.  Do any of you that frequent this forum know anything more about this scope?  It’s not labeled, and to be honest I’m not even 100% sure that I have the designation correct. When Googling “Mizar GT-65,” this is the only reference that I can find on the internet:

 

https://m.facebook.c...70976876274967/

[sdedalus83]

That looks like a 65mm Borg objective. The focuser is definitely Mizar but it doesn’t look original to the tube.

https://img.aucfree....313046843.1.jpg

[Scott in NC]

Very interesting.  The objective on this scope looks exactly like the Borg objective that you linked too.

[Scott in NC]

And regarding the focuser, I have no idea if it’s original to the scope or not, but the visual back certainly isn’t, as it looks like this one sold by Astro-Hutech:

 

https://astrohutech....-1-25-holder-s/

[Kasmos]

Never heard of a GT65. I'll bet it's not all Mizar.

If it was the all of the components like the objective cell and the collar where the focuser mounts would have the same color paint.

 

IIRC Mizar made a GT68/600 guide scope and also a GT68/800mm. 

Both could be bought as a stand alone scope.

Typical Mizar 68mm cell

 

 

[Scott in NC]

So maybe a “Frankenscope,” with Borg lens cell, Mizar focuser with Astro-Hutech visual back, and aluminum tube by either Borg, Mizar, or perhaps none of the above?

 

I found this site, describing a homemade scope using a Borg 65mm f/6.9 achromatic objective, which is what I’ll bet this scope has: http://yamaca.in.coo...ouko/e65rfr.htm

 

And I also found this description of a Borg “roughly a 65mm f/6.5” travel scope: https://skyandtelesc.../travel-scopes/

[sdedalus83]

I’ve seen a few 65mm Borgs come up on Yahoo auctions. They were in the all plastic series 80 tube that also came with the cheapest 100mm achromat package. You can still get the lens cap for the 65. It also fits the 50ED.

 

I was wondering how a short focus achromat could be as well corrected for CA as your results show. I can’t find the link now, but I once read a blog post about Japanese enhanced achromats using an abnormal dispersion flint like the Zeiss AS objectives. The post indicated that the Vixen A80SS, Tomy Famisco 60, and all of the Borg achromats use similar glass.

[Scott in NC]

And regarding the focuser, I have no idea if it’s original to the scope or not, but the visual back certainly isn’t, as it looks like this one sold by Astro-Hutech:

 

https://astrohutech....-1-25-holder-s/

I just found out that the visual back was made by Borg and added on to the Mizar focuser at a later date. 

[deSitter]

This has pretty nice spherical correction, especially in green and red, and pretty decent control of chromatic aberration for a fast doublet achromat. I also see a central zone and a small turned down edge, but nothing too objectionable considering that this was likely meant to be used as a finderscope.

 

Overall, I'm very pleased with the way this scope turned out, and I thank my friend who generously loaned me this scope for testing purposes.

 

If any of you know more about this scope, please fill me in.  There's very little about it on the internet.

I would put in a ring to stop it down to 60 or 55mm and the performance will be outstanding.

 

-drl

[DAVIDG]

This has pretty nice spherical correction, especially in green and red, and pretty decent control of chromatic aberration for a fast doublet achromat. I also see a central zone and a small turned down edge, but nothing too objectionable considering that this was likely meant to be used as a finderscope.

 

Overall, I'm very pleased with the way this scope turned out, and I thank my friend who generously loaned me this scope for testing purposes.

 

If any of you know more about this scope, please fill me in.  There's very little about it on the internet.

Scott,

 As a finder used at low magnification it is fine  but in reality it  is not  that good.  It is zoney  with large hill and an even higher one in the center along with a turned edge.  This causes the color correction to be off as well since the white light image is showing  blue not focusing anywhere close to the other colors.  The bottom line is that this a 1/2 wave or worse over wave front.

   The concept that  I keep seeing getting missed is that a lens can have a perfect Strehl  or very good Strehls at  a number of given wavelengths but this is  only looking at the spherical aberration at those wavelengths. It does not  tell you anything about the over chromatic aberration which can be terrible. I can made a very long focal length singlet and it will have perfect  a Strehl from the blue all the way to the red. So if you just use these test results you would say the lens is excellent but the chromatic aberration is huge !  Here is an example and the longitude chromatic plot shows it.  Over all this lens as couple of waves of chromatic aberration. 

 

 

 

   Here is another  example of 3" f/15 achromat. It has near perfect Strehl in the green of 0.999. So you would think all is well, but if you calculate the total Strehl over the visible range it drops to 0.57   which is the second  calculation and that is if the lens is made perfectly. Start throwing zones and turned edges in and both spherical and chromatic get even worse. So again the bottom lines is that you can not say a lens is good by only calculating Strehls from single wavelength DPAC images  and then say the lens is "good"  You need to look at the chromatic correction as well. 

 

 

 

 

  - Dave 

Edited by DAVIDG, 09 July 2023 - 11:31 AM.

[Scott in NC]

Thanks for the input, David.  I still think that chromatic control isn’t really that bad for a fast achromat. Green and red appear to focus pretty close to one another, and blue isn’t too terribly far out.  Sure, this doesn’t have apo level color correction, but for an f/6.9 doublet achromat?  As a finderscope, it’s meant for low power sweeping anyway.