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Demagnifying, wide FOV finderscope

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#26 pretyro

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Posted 19 July 2021 - 07:47 PM

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.

Mamiya RB67 finder.jpg


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#27 Jon Isaacs

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Posted 19 July 2021 - 08:11 PM

Moreover, the Telrad is not a right angle device

 

For about $25 a Telrad can be a right angle device.

 

https://www.bhphotov...iEaAgXlEALw_wcB

 

At 1.6x the largest effective aperture possible is 1.6 x 7 mm (dark adapted pupil) = 11 mm.

 

Jon


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#28 pretyro

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Posted 20 July 2021 - 01:26 AM

For about $25 a Telrad can be a right angle device.

 

https://www.bhphotov...iEaAgXlEALw_wcB

 

At 1.6x the largest effective aperture possible is 1.6 x 7 mm (dark adapted pupil) = 11 mm.

 

Jon

Agreed – on both counts.
     The Telrad has a lot to recommend it; and it does have an option for right angle viewing. 
     And, you have good point; at a 1.6 x magnification (that I discussed earlier), the objective, at 11 mm, would not be particularly large, and wouldn’t collect much light.  I suspect this is why Astrojensen assembled a 50 mm f/1.7 camera lens with a 14 mm ES82 eyepiece (to get 3.57x and a 23° true field of view) which seems to work well (bright and good tFOV) for him. 
     bob4bvm used a 50 mm (focal length) camera lens with a diagonal and 20 mm eyepiece (I’d like to know which camera lens / diagonal combination he used to side step problems with back focus (for example what was the lens' flange focal distance relative to the diagonal's light path length), or was it enough to just chop off the diagonal's barrel to accommodate a lens' modest flange focal distance). 

     As I have been looking into these various options (and in no small part reading the comments on multiple threads by yourself, bob4bvm, astrojensen and a host of others), I plan to try the 50 mm camera lens/RACI diagonal/eyepiece setup.  That is, to your point, a 3x magnification (at the expense of some FOV) is a good trade-off.  And a manual camera lenses can be easily focused, have an adjustable iris and have a pretty flat field – oh, and are not too expensive.
     So, I am getting a Baader Hyperion 17 mm (68 degree aFOV, 20 mm eye relief, 20.3 mm field stop) and a 50 mm f/1.7 Minolta lens and am trying to get a 1.25“ RACI diagonal.  At ~3x, the tFOV will be about 23 degrees (and won’t be otherwise limited by the lens field stop at least).  Naturally, I’ll adjust the f stop to set an entrance pupil that will achieve the appropriate exit pupil. 
     If the Minolta lens does not have sufficient flange focal distance (after shortening the barrel on the diagonal), I’ll look into a Mamiya lens (which has about twice the flange focal distance of the Minolta lens).  And if I made some glaring (or subtle) mistake, hopefully someone reading this thread will straighten me out.


Edited by pretyro, 20 July 2021 - 09:59 PM.


#29 Second Time Around

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Posted 10 August 2021 - 05:08 PM

I have various mini scopes based on Tamron's Wide Field Tele-View.  This a 90 degree Amici prism and 20mm/70 degree eyepiece.  It attaches to any of Tamron's Adaptall camera lenses. 

 

2 of the 3 I've bought have a 49mm internal thread and so with a 42mm step down ring may also take a T thread lens.  When I find a suitable one I'll check if it'll reach infinity focus when set up like this.  It does of course do so with Tamron's Adaptall lenses.  The Tele-View is long discontinued now, but does come up fairly often on eBay, as do the Adaptall lenses.

 

 

IMG_20201213_163228_compress35.jpg

 

 

The picture above shows a 35-80mm f/2.8-3.8 lens attached to the built-in tripod adapter on the Tele-View.  As you can see, I've fitted it with finder rings.

This configuration is an excellent substitute for a Telrad, but it's a right angle correct image (RACI) finder, so much more comfortable to look through.

 

With the lens set at 35mm it gives 1.75x magnification with an aperture of 12.5mm and a 40 degree field of view.

 

At 80mm it becomes a 4x21 with a 17.5 deg FOV.

 

I've fitted my Tele-Views with crosshairs (the eyepiece screws out with a bit of force), but found the crosshairs move when the lens is zoomed, so are centered at only one magnification.  I'd be grateful for any suggestions as to why? 

 

Amongst the other lenses I use are a 35-210mm f/3.5-4.2.  This again can be a RACI replacement for a Telrad, giving the same 1.75x magnification with a 40 deg FOV but with an aperture of 10mm.  Set at 210mm it becomes a 10.5x50 with a 6.66 deg FOV.

 

There's a wide range of Tamron lenses that can be attached.  Longer focus ones will of course give higher powers, some of which can be fitted with a 1.4x and/or 2x tele-converter that's like a Powermate.


Edited by Second Time Around, 11 August 2021 - 06:12 AM.

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#30 pretyro

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Posted 11 August 2021 - 08:46 PM

I have various mini scopes based on Tamron's Wide Field Tele-View.  This a 90 degree Amici prism and 20mm/70 degree eyepiece.  It attaches to any of Tamron's Adaptall camera lenses. 

 

2 of the 3 I've bought have a 49mm internal thread and so with a 42mm step down ring may also take a T thread lens.  When I find a suitable one I'll check if it'll reach infinity focus when set up like this.  It does of course do so with Tamron's Adaptall lenses.  The Tele-View is long discontinued now, but does come up fairly often on eBay, as do the Adaptall lenses.

 

 

attachicon.gifIMG_20201213_163228_compress35.jpg

 

 

The picture above shows a 35-80mm f/2.8-3.8 lens attached to the built-in tripod adapter on the Tele-View.  As you can see, I've fitted it with finder rings.

This configuration is an excellent substitute for a Telrad, but it's a right angle correct image (RACI) finder, so much more comfortable to look through.

 

With the lens set at 35mm it gives 1.75x magnification with an aperture of 12.5mm and a 40 degree field of view.

 

At 80mm it becomes a 4x21 with a 17.5 deg FOV.

 

I've fitted my Tele-Views with crosshairs (the eyepiece screws out with a bit of force), but found the crosshairs move when the lens is zoomed, so are centered at only one magnification.  I'd be grateful for any suggestions as to why? 

 

Amongst the other lenses I use are a 35-210mm f/3.5-4.2.  This again can be a RACI replacement for a Telrad, giving the same 1.75x magnification with a 40 deg FOV but with an aperture of 10mm.  Set at 210mm it becomes a 10.5x50 with a 6.66 deg FOV.

 

There's a wide range of Tamron lenses that can be attached.  Longer focus ones will of course give higher powers, some of which can be fitted with a 1.4x and/or 2x tele-converter that's like a Powermate.

 

  Looks like you have a nice setup there.  The zoom feature sounds convenient as well -- zoom from a 40 deg FOV to a 6.66 deg FOV.   At 1.75 x magnification (with that small aperture), what magnitude stars can you see?  btw, what is the diameter of the window (the entrance into the prism) in the adapter?

 

 

 

  I got a correct image RACI (1.25") diagonal recently.  I held a Minolta lens (which has about the same flange focal distance as the Tamron) to one end of the diagonal and an eyepiece to the other; it worked.  I'll get another lens with a little longer flange to focal distance, then print up some adapters and a mount.  I am planning to use a fixed focal length lens; the zoom lens is a bit large and unwieldy.

 

  Then again, a Tele-view  adaptor can be had for $70 on ebay.
 

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  • teleview adapter revA.jpg


#31 Jon Isaacs

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Posted 11 August 2021 - 08:58 PM

I got a correct image RACI (1.25") diagonal recently.  I held a Minolta lens (which has about the same flange focal distance as the Tamron) to one end of the diagonal and an eyepiece to the other; it worked.

 

Was the object at optical infinity?

 

Jon



#32 pretyro

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Posted 11 August 2021 - 10:26 PM

Was the object at optical infinity?

 

Jon

nail on the head.

 

  The object was only ~30 feet away and I had trouble getting the eyepiece close enough to the exit of the diagonal to get focus.  So, I had to flip the EP over and use it backwards.  In doing so, I could get focus and had a cm or two of room to spare (ie I held the EP about 2 cm from the exit of the diagonal -- any closer and it went out of focus in the other direction).  Hence, I am planning to get a lens with a longer flange focal distance to provide a bit more operational flexibility..

 

Lens:  Minolta 50 mm  F/1.7 (I held it against the entrance of the diagonal as best I could; I think I set the camera lens at infinity)

diagonal:  Orion Correct Image 1.25"  (I unscrewed and removed both the entrance barrel and the EP barrel to shorten the light path length)

EP:  Celestron 25 mm Plossl (the apparent FOV is not great but OK for now)

 

  Specifically, I don't know what constitutes optical infinity for this setup.  The lens focus ring varies from 1.5 feet to 30 feet where the next step is infinity.  I am assuming that 30' is close to optical infinity and as long as I have another cm or two of travel between the EP and diagonal, I'll be able to make it work.  I'll try it at a longer distance soon.



#33 Jon Isaacs

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Posted 11 August 2021 - 10:53 PM

Orion Correct Image 1.25"  (I unscrewed and removed both the entrance barrel and the EP barrel to shorten the light path length)

 

 

Hopefully that buys you enough back focus to make it all work.  I calculate that 30 feet is pretty close to optical infinity, infinity would focus 0.3 mm closer in.

 

Jon


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#34 Second Time Around

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Posted 12 August 2021 - 10:46 AM

@pretyro

 

I've never measured the limiting magnitude but when my health allows I'll do so and report back.

 

The diameter you asked for is approx 23mm.

 

I don't have a 3D printer but your idea certainly sounds interesting.  For those who are unaware of the flange distance of lens mounts see https://en.wikipedia..._focal_distance

Edit note: I've just noticed that there's a list of flange distances earlier in the topic.

 

I'm therefore limited to Tamron Adaptall lenses (and possibly screw fit lenses as I mentioned earlier).   Here's some useful sites giving data and test reports on Adaptall lenses.

 

https://www.apotelyt...daptall-catalog

 

https://www.pentaxfo...lenses-i98.html

 

http://allphotolense...stems/c_59.html

 

I've tried lenses mainly with the SP designation as these were Tamron's top line.

 

The zoom lenses are larger and heavier, but have more space to mount finder rings.  Fixed short focal length lenses may be more difficult though.  This may not be a problem if you have access to a 3D printer of course. 

 

If you don't, you'll need a dovetail where the distance between the rings can be adjusted.  After an awful lot of searching I found these in the Star Pal store on Ali Express: https://www.aliexpre...0127933644.html  These mini dovetails fit standard Synta bases.  I bought them as a package with 82mm finder rings at https://www.aliexpre...0495081864.html

 

As I mentioned earlier, when I zoom in/out the object viewed doesn't stay centered.  I'd love to have ideas as to why and any fix.


Edited by Second Time Around, 12 August 2021 - 05:04 PM.

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#35 pretyro

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Posted 12 August 2021 - 06:05 PM

@pretyro

 

I've never measured the limiting magnitude but when my health allows I'll do so and report back.

 

The diameter you asked for is approx 23mm.

 

I don't have a 3D printer but your idea certainly sounds interesting.  For those who are unaware of the flange distance of lens mounts see https://en.wikipedia..._focal_distance

Edit note: I've just noticed that there's a list of flange distances earlier in the topic.

 

I'm therefore limited to Tamron Adaptall lenses (and possibly screw fit lenses as I mentioned earlier).   Here's some useful sites giving data and test reports on Adaptall lenses.

 

https://www.apotelyt...daptall-catalog

 

https://www.pentaxfo...lenses-i98.html

 

http://allphotolense...stems/c_59.html

 

I've tried lenses mainly with the SP designation as these were Tamron's top line.

 

The zoom lenses are larger and heavier, but have more space to mount finder rings.  Fixed short focal length lenses may be more difficult though.  This may not be a problem if you have access to a 3D printer of course. 

 

If you don't, you'll need a dovetail where the distance between the rings can be adjusted.  After an awful lot of searching I found these in the Star Pal store on Ali Express: https://www.aliexpre...0127933644.html  These mini dovetails fit standard Synta bases.  I bought them as a package with 82mm finder rings at https://www.aliexpre...0495081864.html

 

As I mentioned earlier, when I zoom in/out the object viewed doesn't stay centered.  I'd love to have ideas as to why and any fix.

 

     I think your setup is really quite nice.  It sure seems like a nice idea to have a RACI finder that you can zoom from a convenient wide FOV (to get your bearings) down to something useful for centering a star and the like.  It looks like you and I are in the minority on this one around here though.
     I don’t know how or why your image via the Tele-view with Tamron zoom lenses does not stay centered when zooming in and out (just don’t have the background).  And I don’t know to what extent that it is normal either.  For cameras, a small change in center when zooming would not be as important an issue as for astronomy I suppose (in photography, you often move around a bit to frame the shot anyway, and your subject normally is less than a million miles away and subtends more than a fraction of an arc second); perhaps the design criteria for this is not very demanding in a camera system such as the Tamron.  Yep, it would be nice to have the image stay centered for astronomy uses.  Hopefully someone on CN can ferret this out.
     Btw, I am a little unclear how your mounting rings are affixed to your lens in relation to the zoom mechanism.  That is, when you zoom in, is the front portion of the lens held in place while the back portion (and Tele-view adaptor) slides backwards?  If either the lens or Tele-view adaptor are unsupported, perhaps the free end is bowing down thus introducing small deviation to the co-axial alignment that could perhaps contribute to misalignment during zoom.

     As a finder, the device really doesn’t need to be heavy.   Even some plastic lenes (they can be 3D printed now, and the niceties of apochromatic and such as not really important for an astronomy finder), a plastic prism, and PEEK housing should suffice.  But it would need to stay centered when zoomed.  With 8 more years of college, I bet I could make one myself (well, maybe 12 more years ;)

     Thanks for the measurement.  Your measured diameter of 23 mm is bit larger than the diameter of the opening of my Orion RACI diagonal at any rate. 
     I hope you get back feeling well enough soon.  And hope we get the zoom misalignment figured out.

 



#36 Second Time Around

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Posted 14 August 2021 - 09:15 AM

          Btw, I am a little unclear how your mounting rings are affixed to your lens in relation to the zoom mechanism.  That is, when you zoom in, is the front portion of the lens held in place while the back portion (and Tele-view adaptor) slides backwards?  If either the lens or Tele-view adaptor are unsupported, perhaps the free end is bowing down thus introducing small deviation to the co-axial alignment that could perhaps contribute to misalignment during zoom.

     

Sorry, I should have mentioned that I use a lens hood.  One ring fits on this, the other on the body of the lens.  The position of the latter varies from lens to lens.



#37 pretyro

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Posted 14 August 2021 - 02:02 PM

Sorry, I should have mentioned that I use a lens hood.  One ring fits on this, the other on the body of the lens.  The position of the latter varies from lens to lens.

Thanks for the update.

 

     btw, when you say that when zoomed in/out the object viewed doesn't stay centered, how much does it go out of center by?  For example, when starting from zoomed-out with moon centered in the eyepiece, upon zooming in, does the Copernicus crater become the new center or is the new center all the way out to Langrenus crater --  or is the moon no longer even in the field of view? 

     At any rate, it sounds like this could be due to the lens elements not being perfectly centered ( “If you center something at the exact center of the image, then zoom in, you’ll find that the center goes sliding off to a different location”).  And I found this snippet on-line, so it must be true ; ). 

     Perhaps you can get the lens elements re-centered to correct this issue.  For example, this guy, https://www.youtube.com/watch?v=rLSFJTHrivo, got his lens recentered.  Since your lens is no longer available, it might be more difficult doing so though.



#38 Rutilus

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Posted 21 August 2021 - 01:23 AM

A few nights ago I used a  50mm f/1.7 prime focus 35mm camera lens with standard 1.25" prism diagonal.
This diagonal has a wider aperture compared to that of my Acimi prism diagonal. Stars down to magnitude

4.5 were visible with the naked eye.
With the camera lens finder (plus modified 25mm eyepeice so that it sits lower in the eyepiece holder,

the only way I could get it to come to focus, and diagonal nose piece barrel removed) I had a 22-23 degree

field of view and was able to observe stars down to mag.5.3 - 5.5.

 

Using my home built 90 degree binoculars (using 2" diagonals) with 46mm lenses.
I had a FOV of 16 degrees, with stars down to mag.7.5 -7.7 visible (mag 8.0 when used on better nights).

 

Though my 2" diagonal  set-up has a  smaller FOV, I found it much better as a finder scope. Even with a

flipped image (right to left) I had no problems finding my way around the largest constellations.

Both systems at the magifications  being used produce  large exit pupils.

Even with the large exit pupil the finder is still  useful  for someone like me that has a fused spine and

can not use straight through finder scopes.

 

Since making those observations, I have now made a finder which is attached to my equatorial mount. 

Here is a photo of my binocular set-up and the finder scope.

Attached Thumbnails

  • wf-bino-cn.jpg

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#39 pretyro

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Posted 21 August 2021 - 01:54 PM

A few nights ago I used a  50mm f/1.7 prime focus 35mm camera lens with standard 1.25" prism diagonal.
This diagonal has a wider aperture compared to that of my Acimi prism diagonal. Stars down to magnitude

4.5 were visible with the naked eye.
With the camera lens finder (plus modified 25mm eyepeice so that it sits lower in the eyepiece holder,

the only way I could get it to come to focus, and diagonal nose piece barrel removed) I had a 22-23 degree

field of view and was able to observe stars down to mag.5.3 - 5.5.

 

Using my home built 90 degree binoculars (using 2" diagonals) with 46mm lenses.
I had a FOV of 16 degrees, with stars down to mag.7.5 -7.7 visible (mag 8.0 when used on better nights).

 

Though my 2" diagonal  set-up has a  smaller FOV, I found it much better as a finder scope. Even with a

flipped image (right to left) I had no problems finding my way around the largest constellations.

Both systems at the magifications  being used produce  large exit pupils.

Even with the large exit pupil the finder is still  useful  for someone like me that has a fused spine and

can not use straight through finder scopes.

 

Since making those observations, I have now made a finder which is attached to my equatorial mount. 

Here is a photo of my binocular set-up and the finder scope.

Wow.    That is really nice. 

 

    I recognized, as I started to try this (ie to do what you have done), that it would be a long row to hoe.  So, I went and bought the Tamron RACI diagonal ("Tele-view adaptor wide field") as suggested by SecondTimeAround -- I'll get a lens or two for it soon.  The Tamron RACI though does not permit use of different eyepieces.  However, it is trivial to use different lenses with it (because it uses the Adaptall connector).  Since it does not have a reticle,  I'll get at least one lens that has a bit of a zoom to it as well.

 

 

    btw, which camera lens (which make/model) did you use?  And, do you have any convenient way to align the device with the optical tube?

 


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#40 Second Time Around

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Posted 22 August 2021 - 06:03 AM

Thanks for the update.

 

     btw, when you say that when zoomed in/out the object viewed doesn't stay centered, how much does it go out of center by?  For example, when starting from zoomed-out with moon centered in the eyepiece, upon zooming in, does the Copernicus crater become the new center or is the new center all the way out to Langrenus crater --  or is the moon no longer even in the field of view? 

     At any rate, it sounds like this could be due to the lens elements not being perfectly centered ( “If you center something at the exact center of the image, then zoom in, you’ll find that the center goes sliding off to a different location”).  And I found this snippet on-line, so it must be true ; ). 

     Perhaps you can get the lens elements re-centered to correct this issue.  For example, this guy, https://www.youtube....h?v=rLSFJTHrivo, got his lens recentered.  Since your lens is no longer available, it might be more difficult doing so though.

By memory it was several moon diameters, so a very large amount.

 

I've tried it with 3 Tele-view adaptors and 3 different Tamron lenses, with a similar result each time.



#41 Second Time Around

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Posted 22 August 2021 - 06:20 AM

Wow.    That is really nice. 

 

    I recognized, as I started to try this (ie to do what you have done), that it would be a long row to hoe.  So, I went and bought the Tamron RACI diagonal ("Tele-view adaptor wide field") as suggested by SecondTimeAround -- I'll get a lens or two for it soon.  The Tamron RACI though does not permit use of different eyepieces.  However, it is trivial to use different lenses with it (because it uses the Adaptall connector).  Since it does not have a reticle,  I'll get at least one lens that has a bit of a zoom to it as well.

 

 

    btw, which camera lens (which make/model) did you use?  And, do you have any convenient way to align the device with the optical tube?

I glued crosshairs at the bottom of eyepiece once I'd removed it from the Amici prism.  It does take a bit of force. 

 

Thinking about it further, I seem to recall it wasn't just that the crosshairs that moved, I'm fairly sure it was the whole field of view.  So it may well be an alignment issue.  I'll have to check.



#42 pretyro

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Posted 22 August 2021 - 11:36 AM

I glued crosshairs at the bottom of eyepiece once I'd removed it from the Amici prism.  It does take a bit of force. 

 

Thinking about it further, I seem to recall it wasn't just that the crosshairs that moved, I'm fairly sure it was the whole field of view.  So it may well be an alignment issue.  I'll have to check.

 

 

By memory it was several moon diameters, so a very large amount.

 

I've tried it with 3 Tele-view adaptors and 3 different Tamron lenses, with a similar result each time.

   I put my Sony A5100 (it is an APS-C mirrorless camera; using a 55 - 210 mm zoom lens) on the tripod.  When I zoomed in and out, the moon stayed more or less in the center (it would be easier to assay if I had some reticle rings on the display.  This does not have any direct bearing on your Tamron lenses.  But I bet the Tamron lenses could also be expected keep a target centered when zoomed in and out.  It is odd that all three of your lenses don't maintain center. 



#43 RichA

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Posted 22 August 2021 - 05:49 PM

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

Low power finders tend to be a bit pointless unless you really don't know the sky or where the object might be.  Even then, low powers mean almost no object is visible that isn't visible to the naked eye.  You might as well just use a tube with no optics in it.  A 50mm 8x finder brings hundreds of objects into view, allowing them to be "found."  One of the most shocking things I've noticed is that the reason some people avoid real finders is they just don't know how to align them with the scope, so they are reduced to "gun-sighting" along the edges of tube rings or some such method as that.



#44 pretyro

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Posted 22 August 2021 - 08:39 PM

Low power finders tend to be a bit pointless unless you really don't know the sky or where the object might be.  Even then, low powers mean almost no object is visible that isn't visible to the naked eye.  You might as well just use a tube with no optics in it.  A 50mm 8x finder brings hundreds of objects into view, allowing them to be "found."  One of the most shocking things I've noticed is that the reason some people avoid real finders is they just don't know how to align them with the scope, so they are reduced to "gun-sighting" along the edges of tube rings or some such method as that.

  Agreed.   That is why I am trying to set up a Tamron raci with an attached camera lens.  It appears that  a  2 to 3 x magnification would provide a low enough magnification for a nice wide angle, yet a high enough magnification to make more stars visible than a 1x paper towel tube.  Since the Tamron raci eyepiece has about 20 mm focal length, a 50 mm lens will yield a 2.5 magnification. I am considering use of a modest zoom lens.   And, yes, I really don't know the sky.  However, when I do get a star centered in my Celestron 9x50 raci finder -- I can also see it in my Mak; so I think I have figured out alignment.   

 

  btw, last night I saw the moon and Jupiter (and some of its moons) with the Tamron raci and a 500 mm fixed focal length lens -- it was pretty nice.  Due to poor conditions, on any given night around here, only a dozen or so stars/planets are visible, so there are not many landmarks.  And a tripod camera mount is a lot easier to use than a motorized mount to look around for stars.
 



#45 pretyro

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Posted 26 August 2021 - 11:35 AM

I glued crosshairs at the bottom of eyepiece once I'd removed it from the Amici prism.  It does take a bit of force. 

 

Thinking about it further, I seem to recall it wasn't just that the crosshairs that moved, I'm fairly sure it was the whole field of view.  So it may well be an alignment issue.  I'll have to check.

  Since you have managed to remove the stock Tamron eyepiece from the Tele-view RACI, have you tried to insert other eyepieces?  There are a number of eyepieces that have an illuminated reticle (for example, https://www.astronom...-eyepieces.html). You wouldn't need to glue a reticle in then.   I don't know the specifics of the Tamron eyepiece (other than as you said, it appears to be 20 mm focal length, 70 degree that can be focused a bit by twisting it).  



#46 Second Time Around

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Posted 28 August 2021 - 11:07 AM

  Since you have managed to remove the stock Tamron eyepiece from the Tele-view RACI, have you tried to insert other eyepieces?  There are a number of eyepieces that have an illuminated reticle (for example, https://www.astronom...-eyepieces.html). You wouldn't need to glue a reticle in then.   I don't know the specifics of the Tamron eyepiece (other than as you said, it appears to be 20 mm focal length, 70 degree that can be focused a bit by twisting it).  

Yes, but the Tamron eyepiece is threaded and has a diameter of approximately 1 1/2 inches.

 

No doubt something could be made using a 3D printer, but I don't have access to one.



#47 pretyro

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Posted 28 August 2021 - 05:51 PM

Yes, but the Tamron eyepiece is threaded and has a diameter of approximately 1 1/2 inches.

 

No doubt something could be made using a 3D printer, but I don't have access to one.

    I don’t have a 3D printer either (although, if you intend to make a lot of prints, it is cost effective to buy one).  But there are a number of shops than can made 3D prints based on your drawings (here as well as in the UK).  I just sketch it out using OpenSCAD (ver 2021.01), generate an .STL file and send them the STL file.  It’ll take a couple weeks for them to make the part. 
    Based on the dimensions you mentioned, this routine (below, the attached .SCAD file has the “metric thread” routine) will get pretty close to making of a sleeve you need (so you can physically place a 1.25” eyepiece into the Tamron RACI).  You may need to adjust the dimensions a bit, and you will need to enter the correct thread size, pitch, depth and such.  I’ve attached the .SCAD file (the text file used by openSCAD) and the .STL file (a file generated by openSCAD that provides instructions to a 3D printer). 

 

   I have not been able to find a reticle eyepiece that has a 70 degree FOV (like that of the Tamron RACI). 

 

This text file -- just change the extension to .SCAD and open it in OpenSCAD.  Once you have opened it, hit F6 to render it (it will take about 15 to 20 minutes), then generate an .STL file (and send it to your local 3D print shop if you like).  This should get you started.  You may may need to edit the file a bit to adjust the sleeve radius and, of course, the thread pitch and depth.

 

Attached File  Tamron raci EP adaptor sleeve revE.txt   19.72KB   1 downloads

 

 

 

//---------------------Execution section - begin-------------------//

// all dimensions are in millimeters
l_length=24;  // length of sleeve, based on the length of an eyepiece barrel
r_outer=19.1; // major radius based on the 1.5" diameter dimension provided
r_inner=15.9;  //minor radius based on the 1.25" diameter of a standard eyepiece barrel

 

// this is an example of an unthreaded sleeve
difference(){  //outer-innner
   cylinder($fa=0.5, $fs=0.5,h = l_length, r1 = r_outer, r2 = r_outer, center = true); // outer cylnder
   cylinder($fa=0.5, $fs=0.5,h = 1.1*l_length, r1 = r_inner, r2 = r_inner, center = true); // inner cylnder.  note, the subtracted pieces cannot share common surfaces, so length of this cylinder is a bit longer than this object that it is subtracted from
} // close difference

 

// this is an example of a threaded sleeve
translate([0,2*r_outer+4,0])
difference(){  //outer-innner
  translate([0,0,l_length*(-0.5)]) //for shifing threaded cylinder for alignment
  metric_thread (diameter=2*r_outer, pitch=1, length=l_length); // outer threaded cylnder.  This has been abitratily threaded in this example.  The desired thread should be used here as needed to match the Tamron EP holder.  Threads can be customized using some of the routines below
   cylinder($fa=0.5, $fs=0.5,h = 1.1*l_length, r1 = r_inner, r2 = r_inner, center = true); // inner cylnder. 
} // close difference

//---------------------Execution section - close-------------------//

Attached Thumbnails

  • sleeves.jpg

Edited by pretyro, 28 August 2021 - 05:57 PM.


#48 Second Time Around

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Posted 29 August 2021 - 10:25 AM

Many thanks, pretyro.

 

I've found out my brother-in-law has a 3D printer so I'll see if he can help. I certainly would like to use different eyepieces.

 

There are a number of 20mm 70 deg eyepieces available, probably all variations of the same design.  I have one from Agena Astro that happens to accept a Dioptrx as a bonus (other variations may do so as well). However, I can't then see the whole field of view, neither can I do so wearing glasses.  On the other hand the same goes with the eyepiece on the Tamron, that in fact is even worse.



#49 pretyro

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Posted 29 August 2021 - 12:23 PM

 No problem at all -- I hope it helps gets you started on it.  I don't know how well home 3D printers can handle overhanging, unsupported elements (like a deep screw thread, a ball joint or raceway with ball bearings).  I've made screw threads (and other such features) using commercial SLS (selective laser sintering) printers where all the work is supported by a bed of nylon particles.  I found that nylon is a nice material in that it can handle fine detail and is really quite temperature resistant (if placed in boiling water; a proxy for sitting in a hot car, it does not deform). 

 

  Dioptrx -- great idea, thanks much.  I did not know it existed.   I will look into it.  Use of other eyepieces would be handy.  It looks like it will be easier for me to adapt (or just use as it) the Tamron Tele-view raci than make one using a telescope diagonal as was done by Rutilus and others.  Moreover, there are a number of Tamron lenses (having sufficient flange focal distance) that can be simply connected to the Tamron Tele-view raci

 

  btw, how did you remove the Tele-view wide angle eyepiece?  When I rotate it (in either direction ) it hits a stop.  I am concerned that the application of more torque will damage the stop or EP.  Could you post a photo of the eyepiece (after it has been removed from the raci)?



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Posted 29 August 2021 - 03:49 PM

The bit that unscrews (anticlockwise) is at the base of the eyepiece where it joins the prism housing (see the picture). It takes just a moderate amount of force.

 

IMG_20210829_214126_compress63.jpg


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