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Demagnifying, wide FOV finderscope

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#51 pretyro

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Posted 29 August 2021 - 06:31 PM

Thanks for the info.  Dang, I applied a strap wrench and about 20 foot-pounds of torque to no avail.  I'll try again later.  

 

  btw, how close is the bottom lens (of the EP) to the prism?  The Tamron EP is pretty short as it is.  Let me know if a standard 1.25" EP can get focus at infinity.



#52 Second Time Around

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Posted 01 September 2021 - 11:29 AM

  btw, how close is the bottom lens (of the EP) to the prism?  The Tamron EP is pretty short as it is.  Let me know if a standard 1.25" EP can get focus at infinity.

Whether or not a 1.25" eyepiece will focus at infinity depends on the design of the eyepiece and where the focal plane is situated.  Where the focal plane is inside the eyepiece, like many modern long eye relief designs, you won't be able to focus at infinity.  If all the eyepieces in the series have the same eye relief regardless of their focal length then almost certainly they'll have internal focal planes.  With an external focal plane, for instance with Plossls and Erfles, you may be able to focus at infinity, although you may have to shorten the barrel. 

 

I took the Agena 20mm/70deg partly apart (it's probably an Erfle) and it wouldn't focus at infinity.  Interestingly though, when I turned it upside down it would focus at infinity.

 

I'm now going to see if I can adjust one of the Tamron lenses to focus past infinity.  I've been speaking to a very helpful camera repair guy called Ian at Newton, Ellis in Liverpool.  It may take a while before I can try this, but if I'm successful I'll report back.



#53 pretyro

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Posted 01 September 2021 - 02:20 PM

  Sounds good.   I am looking forward to hearing from you. 

  It would be nice, too, if the tele-view wide angle could be adapted to accept lenses other than Adaptall lenses (ie like a Nikon lens). 



#54 pretyro

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Posted 01 September 2021 - 03:03 PM

Whether or not a 1.25" eyepiece will focus at infinity depends on the design of the eyepiece and where the focal plane is situated.  Where the focal plane is inside the eyepiece, like many modern long eye relief designs, you won't be able to focus at infinity.  If all the eyepieces in the series have the same eye relief regardless of their focal length then almost certainly they'll have internal focal planes.  With an external focal plane, for instance with Plossls and Erfles, you may be able to focus at infinity, although you may have to shorten the barrel. 

 

I took the Agena 20mm/70deg partly apart (it's probably an Erfle) and it wouldn't focus at infinity.  Interestingly though, when I turned it upside down it would focus at infinity.

 

I'm now going to see if I can adjust one of the Tamron lenses to focus past infinity.  I've been speaking to a very helpful camera repair guy called Ian at Newton, Ellis in Liverpool.  It may take a while before I can try this, but if I'm successful I'll report back.

btw, on this forum (https://www.cloudyni...l#entry11159819) at response 17, they discussed adjusting a lens past infinity.


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#55 Second Time Around

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Posted 01 September 2021 - 03:42 PM

btw, on this forum (https://www.cloudyni...l#entry11159819) at response 17, they discussed adjusting a lens past infinity.

Many thanks!



#56 pretyro

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Posted 01 September 2021 - 10:48 PM

  I found a link on-line that indicates that use  of Pentax 2x tele-converter will extend the infinity focus enough to permit use of some third party lens (ie a Nikon) with the Tamron Wide Field Tele-View via the M42 ring.  Please see (https://www.dpreview...s/post/50899303) for details.  Don't know if this will help any.  Good luck.


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#57 Second Time Around

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Posted 02 September 2021 - 04:35 AM

  I found a link on-line that indicates that use  of Pentax 2x tele-converter will extend the infinity focus enough to permit use of some third party lens (ie a Nikon) with the Tamron Wide Field Tele-View via the M42 ring.  Please see (https://www.dpreview...s/post/50899303) for details.  Don't know if this will help any.  Good luck.

Will take a look. Thanks!



#58 pretyro

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Posted 07 September 2021 - 10:46 PM

  It looks like the Adaptall lens' have a fairly long flange focal distance (50 to 55 mm).  If an old T-mount style lens has sufficient flange focal distance, I'd be tempted to make an adaptor for my Tamron raci (the wide field Tele-view adapter which has a plain-old unthreaded Adaptall mount).

 

  In the meantime, I bought a Tamron model 01A Adaptall camera lens.  It has a modest zoom (35 - 80 mm) which corresponds to a magnification of 1.75x to 4x.  With an f stop range from 2.8 at 35 mm to 3.8 at 80 mm, it  is not all that fast; but fast enough.  It turns out the the lens' aperture could be easily adjusted while the lens is connected to a Nikon or Olympus adapter.  However, with the lens connected to the directly to the Tamron raci (ie via the Adaptall connection), the aperture was simply locked in the fully dilated position (which is not a problem). 

 

  The upshot -- it worked great.  Using the Tamron model 01A lens with the Tamron raci I could see more stars than I could by naked eye.  The FOV was quite large, enabling me to quickly finds objects of interest, and, upon centering them in the Tamron raci eyepiece, the objects would be easily near the center of the FOV of the 9x50 finderscope for further refinement.  Note, with the Tamron camera lens set at 50 mm and Saturn centered in the Tamron raci eyepiece, and Jupiter was visible on the edge of the FOV, the FOV was ~35 degrees.   It was nice. 

 

  Here is a picture of the Tamron raci (with the model 01A lens) perched on my telescope.

 

 

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  • tamron on telescope revA.jpg

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#59 pretyro

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Posted 18 September 2021 - 10:21 PM

Whether or not a 1.25" eyepiece will focus at infinity depends on the design of the eyepiece and where the focal plane is situated.  Where the focal plane is inside the eyepiece, like many modern long eye relief designs, you won't be able to focus at infinity.  If all the eyepieces in the series have the same eye relief regardless of their focal length then almost certainly they'll have internal focal planes.  With an external focal plane, for instance with Plossls and Erfles, you may be able to focus at infinity, although you may have to shorten the barrel. 

 

I took the Agena 20mm/70deg partly apart (it's probably an Erfle) and it wouldn't focus at infinity.  Interestingly though, when I turned it upside down it would focus at infinity.

 

I'm now going to see if I can adjust one of the Tamron lenses to focus past infinity.  I've been speaking to a very helpful camera repair guy called Ian at Newton, Ellis in Liverpool.  It may take a while before I can try this, but if I'm successful I'll report back.

 

  Perhaps the Kasai FPX filter can help with your backfocus as well  (www.kasai-trading.jp/fpxfiltere.html).  Good luck and let me know how it goes.



#60 Second Time Around

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Posted 25 September 2021 - 06:52 AM

  Perhaps the Kasai FPX filter can help with your backfocus as well  (www.kasai-trading.jp/fpxfiltere.html).  Good luck and let me know how it goes.

It looks an interesting product. However, I need a lot more than 1mm extra.

 

I'm going to try using a T2 Lens when I can find a suitable one in conjunction with a 49mm to 42mm step down ring.  That should give me up to another 4.3mm compared with an Adaptall lens depending on the design of the ring.



#61 Second Time Around

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Posted 19 November 2021 - 04:29 PM

I've now bought a 35mm f/2.8 T2 lens. Unfortunately though my step down adapter doesn't allow the lens to focus at infinity.

 

I've therefore ordered a custom-made adapter from SRB Photographic.  Due to Covid there's a 3 month+ lead time, but when they're ready to make it I'll send the Tamron Teleview and the T2 lens to SRB to make sure it works.

 

I'll report back in due course.



#62 pretyro

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Posted 21 November 2021 - 02:52 PM

I've now bought a 35mm f/2.8 T2 lens. Unfortunately though my step down adapter doesn't allow the lens to focus at infinity.

 

I've therefore ordered a custom-made adapter from SRB Photographic.  Due to Covid there's a 3 month+ lead time, but when they're ready to make it I'll send the Tamron Teleview and the T2 lens to SRB to make sure it works.

 

I'll report back in due course.

     Nice – sorry about that 3 month wait looming ahead.

 

      If you will be making a custom adapter, is there any need to make it for a T2 lens?  Newer lenses (which often have better optics than older lenses) are generally not T2 mounts.  If your adapter fit a more common mount (like Nikon etc), especially one that has a wide array of manual lenses, you would have a wider range of lenses to choose from. 

 

     Regarding the back focus problem, could lenses be added to the custom adapter to address that?  The Kenko Lens2Scope already solved this problem somehow (though for a 45 degree not a 90 degree viewer).
 


Edited by pretyro, 21 November 2021 - 02:54 PM.


#63 Second Time Around

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Posted 22 November 2021 - 03:10 PM

     Nice – sorry about that 3 month wait looming ahead.

 

      If you will be making a custom adapter, is there any need to make it for a T2 lens?  Newer lenses (which often have better optics than older lenses) are generally not T2 mounts.  If your adapter fit a more common mount (like Nikon etc), especially one that has a wide array of manual lenses, you would have a wider range of lenses to choose from. 

 

     Regarding the back focus problem, could lenses be added to the custom adapter to address that?  The Kenko Lens2Scope already solved this problem somehow (though for a 45 degree not a 90 degree viewer).
 

Although an adapter could be made for other 35mm lenses the problem is that they all have a shorter flange distance than Adaptall lenses (50.7mm).  This means that they wouldn't be able to focus at infinity.  The flange distance of T2 lenses is 55mm, so they should work with a suitable adapter.

 

Large format lenses have an even longer flange distance, but are too large in diameter.

 

Supplementary lenses may work, but that would add even more to the already not inconsiderable cost.

 

Keep the ideas coming though!

 

I'm also going to send SRB Photographic the 20mm/70 degree eyepiece that accepts a Dioptrx, and see if they can get this to work with the Tamron Amici prism.


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#64 pretyro

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Posted 07 December 2021 - 01:53 PM

Designing it to accommodate a Dioptrx is nice idea.  Which T2 mount lens are you considering?  btw, would a Dioptrx also correct for astigmatism created by the optical system (rather than just compensating for the astigmatism of the eye)?  For that matter, I don't know if or how much astigmatism might be caused by a lens/prism/eyepiece system.



#65 Second Time Around

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Posted 07 December 2021 - 02:50 PM

The T2 lens I've bought is mainly to test out the idea.  It's a 35mm f/2.8, so could be useful for a very wide field of view.

 

No, the Dioptrx just corrects for astigmatism in the eye.



#66 Tom Austin

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Posted 10 December 2021 - 12:15 AM

Now, this would be a nice RACI finderscope -- the Mamiya  RB67 camera.  It is right angle, correct image, and multiple people can observe it at once.  Be nice if there was a device like this sans camera.

attachicon.gifMamiya RB67 finder.jpg

Reminds me of the setup I use.  I gave up on Telrad and finderscopes because it's too inconvenient for me to have to look through them.  Now I just look AT my 8" tablet that's mounted beyond the focuser.  It's a cheap Samsung Galaxy tablet connected to an ordinary webcam like what I get for $5.00 at garage sales.

 

https://www.cloudyni...am-as-a-finder/


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#67 pretyro

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Posted 10 December 2021 - 09:30 AM

Reminds me of the setup I use.  I gave up on Telrad and finderscopes because it's too inconvenient for me to have to look through them.  Now I just look AT my 8" tablet that's mounted beyond the focuser.  It's a cheap Samsung Galaxy tablet connected to an ordinary webcam like what I get for $5.00 at garage sales.

 

https://www.cloudyni...am-as-a-finder/

   Yeah, that camera/computer approach of yours looks pretty good.  The Mamiya RB67 view finder is just too dim.  Those darn laws of physics and optics can be soooo inconvenient at times smile.gif .


Edited by pretyro, 10 December 2021 - 09:33 AM.


#68 GlennLeDrew

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Posted 24 May 2022 - 09:25 PM

@ Second Time Around,
You described the reticle center as rotating while you rotated the zoom lens ring in order to alter the focal length.

This is a bit strange.

I would expect a linear drift, resulting from an improperly centered reticle with respect to the the lens' optical axis.

If there is a cyclical rotational component to the image position with respect to the reticle center, that would imply one or more lens elements being decentered with respect to the rest, causing the image translate circularly. But that could happen only if this lens or lenses rotated with the zoom ring.

Most Zoom lenses have the movable elements slide longitudinally, with no rotational motion at all.

So I'm still a tad confuzzled. More detail on the construction/operation of the system would be useful.

#69 Second Time Around

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Posted 25 May 2022 - 08:55 AM

Many thanks for the reply, Glenn. It's good to have you posting again!

I obviously haven't explained myself well. The drift is indeed linear. If I centre an object at low power the object appears to move in a straight line as I zoom in.

It happens with several different zoom lenses and 3 different eyepiece/prism/reticle combinations.

Is this something I could fix simply by ensuring that I've got the cross hairs precisely centered?

If so, what would be the easiest way to do this?

Thanks again.

#70 GlennLeDrew

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Posted 25 May 2022 - 10:59 AM

Yep, just get the crosshair centered. How best to do that will depend on the construction of the rig. Either the crosshair support is made a bit undersized so that it can be translated across the image, or the entire diagonal attachment has provision for translational movement.

 

The latter approach is what I took for the objective barrels on one of my binocular projects. On the barrel back end was an undersized, tapered flange which, when inserted into the 2mm larger diameter receiver on the body, permitted easy translational adjustment of the entire barrel via 3 thumdscrews.


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