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Wide angle finder design

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#1 Ant1

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Posted 05 September 2017 - 06:58 AM

Hi all,

 

I am trying to figure out how to build a wide angle finder. I would like a very wide FOV such as 20°.

So far I'm thinking of a very simple design such as getting a telescope eyepiece, an achromat objective lens, and some tubing.

Now here's the difficult part.

 

1. I have a 30mm 80° eyepiece.

2. 4x magnification should get me 20° FoV.

3. I therefore need an objective of 4x30=120mm focal length.

4. Focal plane diameter is 2x30xtg(80/2)=50.3mm.

5. Objective diameter needs to be bigger than this, otherwise I won't get to see the entire field.

6. 120/50.3=2.4 and I am unlikely to get correct performance from a f/2.4 objective

7. Opting for an eyepiece of different focal length doesn't change the f/2.4 problem

 

Am I computing something wrong or is it just a fact of life that 20° FoV can't be achieved with simple objective technology?

Any hints how to achieve my goal?

 

Thanks

Ant1



#2 DLuders

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Posted 05 September 2017 - 07:15 AM

Recommend playing around with the online, interactive "Field of View Calculator:  https://astronomy.to.../field_of_view/ .  I selected the Moon, entered the specs you provided, and saw the output.  I personally think that an f/2.4 finder telescope would distort the edge stars so much that it would be unusable. 



#3 Jon Isaacs

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Posted 05 September 2017 - 07:21 AM

Am I computing something wrong or is it just a fact of life that 20° FoV can't be achieved with simple objective technology?

 

There are a number of issues you are facing.  The field curvature of such an objective is tremendous, I calculate about 6mm..  The eyepiece will have serious issues.. 

 

If you want a wide angle magnifying finder, I'd look over at the binocular forum where there's a thread or two about using Nikon Coolpix TeleConverters to build wide angle binoculars.

 

 https://www.cloudyni...idefield-binos/

 

These are based on Galilean optics and are available both 2x and 3x versions though only the 2x version is practical for binoculars.  Eye relief is minimal, the 2x version provides ~27 degree TFoV. I measured the TRoV of the 3X version, if I am not mistaken, it's about 17 degrees.

 

Jon



#4 DLuders

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Posted 05 September 2017 - 07:38 AM

Here is an extreme example of very low-power (2.1x), Ultra Wide Field -- the Vixen SG 2.1x42 Super Wide Binocular:  http://shop.opticspl...BCABEgJy8PD_BwE .

 

Some additional specs for those binos appear in https://www.cloudyni...eld-binoculars/



#5 Mr. Joey

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Posted 05 September 2017 - 07:39 AM

Hi Ant1,

I think there may be some confusion regarding true field of view versus apparent field of view (FOV / AFOV).

I say this because if I'm understanding you correctly, you want to achieve a true field of view of around 20 degrees...

I've never seen anything like that.

 

True field of view is normally calculated as a function of an eyepiece's apparent field of view divided by magnification:

True Field = Apparent Field ÷ Magnification.

 

Extrapolate magnification as the focal length of the instrument divided by the focal length of the eyepiece and we have:

True Field = Apparent Field ÷ (focal length telescope ÷ focal length eyepiece)

 

Here is a simple example:

http://www.telescope...22&type=product

 

Here is a more in depth discussion:

http://brayebrookobs...WIDER_AFOV.html

 

I like to remember the basics, and put things in practical terms. The full moon is about .5 degree (half a degree). A good pair of wide field binoculars can often yield a true field of view of around 7 degrees... Or a circle with a diameter of 14 full-moons. That's a pretty big true-field circle. Most finder scopes do less than that. In practice, and from my own very limited experience, I've never seen anything that could present a true 20 degree field of view, or a circle with a diameter of 40 full moons!

 

Others with more technical knowledge may chime in here and prove me wrong. Still, I have a pair of binoculars with a true 7.5 degree field of view and I've never seen a finder scope that could present the same, or greater true field of view.

 

Regards,

Mr. Joey



#6 Jon Isaacs

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Posted 05 September 2017 - 07:57 AM

Ant1 calculated the diameter of field based on the magnification it should be done based on the field stop of the eyepiece.  

 

In any event, the field curvature and eyepiece aberrations will make this finder basically unusable.. 

 

The exit pupil is will be 30mm/2.4 = 12.5mm, so things are pretty odd there too.  

 

I have a pair of binoculars that provides a nominal 11 degree TFoV, tain't pretty.  I have a hand held telescope that provides a 9 degree TFoV.. It's a 50mm finder objective mounted to accept a 2 inch eyepiece.

 

50mm with 2 inch diagonal.jpg
 
20 degrees is very wide but possible with the Teleconverters. 
 
Jon


#7 BGRE

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Posted 05 September 2017 - 08:03 AM

Achieving a real field of view of 20 degrees is relatively easy.

If the eyepice has an apparent field of say 60 degrees then a magnification of 3x suffices.

However with a 5mm exit pupil an objective diameter of 15mm is all that's usable.

If you eye pupil is say 7mm then a 21mm diameter objective is about right.

Use a long focal length eyepiece to keep the aberrations down with a simple objective.

Not convinced that a 15mm or 21mm aperture finder is that useful for anything other than bright objects though.

Simpler eyepieces will have lower apparent fields of view making the magnification and consequently the effective objective diameter even smaller. 

A unit power telrad style finder is probably more effective if you really need such a large field of view.


Edited by BGRE, 05 September 2017 - 08:06 AM.


#8 Astrojensen

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Posted 05 September 2017 - 08:49 AM

A small finderscope with 20° true field of view is very easy and cheap to make. You simply use an old camera objective combined with a suitable eyepiece. 

 

Let's use a 50mm f/1.7 together with a 14mm ES82 eyepiece, as an example. This combination gives 3.57x and a 23° true field of view. Because the camera objective is designed to yield a flat field more than 20mm across, the performance with the eyepiece is terrific. There is no field curvature and no coma. The objective may suffer from pretty dramatic spherical aberration at f/1.7, but it's still usable as a finder. 

 

The exit pupil is 8.23mm, which is not much larger than neccesary and the objective can be stopped down to f/2 for exactly 7mm exit pupil or f/2.8 for 5mm. Stopping the objective down to f/2.8 will give extremely good performance and extremely tight stars from edge to edge. f/2 is already much better than f/1.7, at least in the case of my objective.  

 

The best thing is that 50mm objectives with T2 threads can be found for next to nothing in thrift stores and on ebay. You can then attach an eyepiece adapter to them via Baader Planetarium's T2 adapter system. And you've got fine focus built in from the start. The downside is that the back focus is tight, so using a diagonal between the objective and the eyepiece is not possible, except if you can find an eyepiece with the focal plane quite far ahead of the barrel. It is not a problem, if you can build it yourself.  

 

And yes, it really does work. I've tried it. 

 

 

Clear skies!

Thomas, Denmark



#9 Astrojensen

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Posted 05 September 2017 - 09:05 AM

 

A unit power telrad style finder is probably more effective if you really need such a large field of view.

Wrong, in my opinion. The 50mm f/1.7 - f/2.8 objective, combined with a 14mm ES82 eyepiece in my above example, show FAR more stars than the naked eye and with considerably better resolution. This is a HUGE advantage in more light polluted skies. The true field of view is large enough, that entire constellations, except the very largest, fit inside. 

 

 

Not convinced that a 15mm or 21mm aperture finder is that useful for anything other than bright objects though.

If this was true, then the Telrad is even less useful, because it only has 7mm aperture... These mini-finders with low magnifications and huge apparent and true field of view can be considered as a sort of "Telrad on steroids", as they allow views of entire constellations, but with many, many more stars visible than the naked eye. The point, just like with the Telrad, is that you may not see the object you're interested in them directly, because it's too faint, but you can point the scope to the correct location among the stars and see the object in the main scope.

 

I haven't tried it, but I can imagine, that such a finderscope would be really well matched to an atlas such as the S&T "Pocket Star Atlas", because of the close match between limiting magnitude and field of view.

 

 

Clear skies!

Thomas, Denmark 



#10 PETER DREW

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Posted 05 September 2017 - 10:56 AM

My interest invites the question "Why a 20 degree field?". If it is for guiding a telescope to an object it should be possible to do this just as accurately with no optical aid. I have the Vixen SG's and the view is good but I wouldn't immediately think of one half as a finder.



#11 MitchAlsup

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Posted 05 September 2017 - 11:24 AM

Now here's the difficult part.

 

1. I have a 30mm 80° eyepiece.

2. 4x magnification should get me 20° FoV.

3. I therefore need an objective of 4x30=120mm focal length.

4. Focal plane diameter is 2x30xtg(80/2)=50.3mm.

The focal plane stop in the 30mm 80º FoV EP is 46mm. This is all you have to illuminate.

 

5. Objective diameter needs to be bigger than this, otherwise I won't get to see the entire field.

Err, no; a pinhole objective illuminates at least 45º !

 

But the suggestion of using a 100-120mm FL camera lens is spot on.

These are often corrected for field curvature. But they may cost more

than the scope you are putting it on.



#12 HarryRik9

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Posted 05 September 2017 - 11:43 AM

I think Thomas is correct in his suggestion. My 135mm Soligor camera lens came with an adapter and 1 1/4 inch right angle diagonal. The limiting factor is the size of the camera field lens at the output. In my case it is 30mm making a good match to a 1 1/4 eyepiece. That 30mm 80 degree eyepiece requires a bigger field lens than my camera lens provides. So I am limited to about 12 degrees with my 32 mm eyepiece. 



#13 Astrojensen

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Posted 05 September 2017 - 01:26 PM

 

 

But the suggestion of using a 100-120mm FL camera lens is spot on.
These are often corrected for field curvature. But they may cost more
than the scope you are putting it on.

 A quick search on ebay shows many 50mm f/1.4 - f/1.8 objectives starting at $10. There's a lot of decent ones, like old Canons and Nikons, and even Zeiss, starting at $20. 

 

Get one with either T2 or M42 threads, the neccesary adapters are available from Baader Planetarium and you're in business. 

 

The error the OP is making is thinking that you need a large, long focal length eyepiece to get a large field of view. You don't, you just need a short focal length objective. And if you use an old camera lens, you get flat-field correction built in, which means fantastic performance.

 

 

Clear skies!
Thomas, Denmark


Edited by Astrojensen, 05 September 2017 - 01:29 PM.


#14 Astrojensen

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Posted 05 September 2017 - 01:45 PM

My interest invites the question "Why a 20 degree field?". If it is for guiding a telescope to an object it should be possible to do this just as accurately with no optical aid. I have the Vixen SG's and the view is good but I wouldn't immediately think of one half as a finder.

The advantages of using a small, ultra-wide finderscope instead of something like a Telrad are:

 

- it shows many more stars, which is an advantage, especially in light polluted skies

 

- it is actually SIGNIFICANTLY smaller than a Telrad... 

 

- because of the magnification, it can show a lot of large, dim deep-sky objects directly, such as M33, etc., even in less than perfect skies.  

 

- it is significantly more precise than the Telrad, since the resolution is much, much higher, even just at 3x, because you can see many more stars and the perceived distances between guidestars are much larger. 

 

- a second hand 50mm camera lens is so cheap, you can build a ~4x25 finderscope with AWESOME field correction CHEAPER than what a new Telrad or a new 6x30 finderscope costs, provided you already have a decent wide-field eyepiece. You don't have to dedicate the eyepiece to the finderscope, though, as you can just preset the focus and just insert the eyepiece into the finder, when you need it and then use the eyepiece on the main telescope, when neccesary. 

 

- it doesn't require electricity...

 

 

People have critizised small finderscopes for decades, and not without reason, as most of them have been much too dim, but the problem with small finderscopes is often not that they are small, but that their magnification is much too high for their aperture, leading to a dim view with a much too restricted field of view. A small finderscope with low magnification and correspondingly large exit pupil and bright views is a whole different animal entirely. Couple this with a modern 80° eyepiece and a flat-field objective and you're in for a VERY different experience. 

 

 

Clear skies!
Thomas, Denmark



#15 BGRE

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Posted 05 September 2017 - 03:26 PM

 

 

A unit power telrad style finder is probably more effective if you really need such a large field of view.

Wrong, in my opinion. The 50mm f/1.7 - f/2.8 objective, combined with a 14mm ES82 eyepiece in my above example, show FAR more stars than the naked eye and with considerably better resolution. This is a HUGE advantage in more light polluted skies. The true field of view is large enough, that entire constellations, except the very largest, fit inside. 

 

 

Not convinced that a 15mm or 21mm aperture finder is that useful for anything other than bright objects though.

If this was true, then the Telrad is even less useful, because it only has 7mm aperture... These mini-finders with low magnifications and huge apparent and true field of view can be considered as a sort of "Telrad on steroids", as they allow views of entire constellations, but with many, many more stars visible than the naked eye. The point, just like with the Telrad, is that you may not see the object you're interested in them directly, because it's too faint, but you can point the scope to the correct location among the stars and see the object in the main scope.

 

I haven't tried it, but I can imagine, that such a finderscope would be really well matched to an atlas such as the S&T "Pocket Star Atlas", because of the close match between limiting magnitude and field of view.

 

 

Clear skies!

Thomas, Denmark 

 

The wide angle eyepiece allows a higher magnification but the pupil diameter still limits the effective aperture to around 20mm or so for a 5mm pupil.

This gains about 3 magnitudes in the faint star limit

Unit power finders do have a 5mm to 7mm pupil you can't rewrite the laws of optics to suit your own preconceptions.

Using one is like looking through a window this doesn't magically increase the effective pupil area.

Unit power finders arent useless **** they make it easier to achieve initial coarse aiming at the object of interest.

However with a large telescope capable of high motor driven slew rates using a Telrad or similar can be dangerous.

A biocular unit power finder with a sweet spot larger than ones head allows it to be used effectively while staying well clear of a fast slewing telescope.



#16 Astrojensen

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Posted 05 September 2017 - 04:17 PM

 

 

However with a large telescope capable of high motor driven slew rates using a Telrad or similar can be dangerous.
A biocular unit power finder with a sweet spot larger than ones head allows it to be used effectively while staying well clear of a fast slewing telescope.

Don't you mean that with a telescope with high slew rate, a FINDERSCOPE is dangerous (because the scope can poke it in your eye while you're looking through it)?

 

And that a TELRAD or something similar with very long eye relief allows you to be clear of the fast moving telescope?

 

 

Unit power finders arent useless **** they make it easier to achieve initial coarse aiming at the object of interest.

I never said that Telrads were useless. If you think I did, you have misinterpreted what I wrote. I am personally not a huge fan of them, for various reasons, but I never said they were useless.

 

 

Clear skies!

Thomas, Denmark



#17 BGRE

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Posted 05 September 2017 - 09:40 PM

 

 

 

However with a large telescope capable of high motor driven slew rates using a Telrad or similar can be dangerous.
A biocular unit power finder with a sweet spot larger than ones head allows it to be used effectively while staying well clear of a fast slewing telescope.

Don't you mean that with a telescope with high slew rate, a FINDERSCOPE is dangerous (because the scope can poke it in your eye while you're looking through it)?

 

And that a TELRAD or something similar with very long eye relief allows you to be clear of the fast moving telescope?

 

 

Unit power finders arent useless **** they make it easier to achieve initial coarse aiming at the object of interest.

I never said that Telrads were useless. If you think I did, you have misinterpreted what I wrote. I am personally not a huge fan of them, for various reasons, but I never said they were useless.

 

 

Clear skies!

Thomas, Denmark

 

Telrads are far too small to be useful in that application.

A large area sweet spot means that ones head doesn't have to slew at the same rate as the telescope when homing in on the target.

Yes using high slew rate when looking through a finder is not particularly safe.

That's where a unit power finder with in effect enormous eye relief comes in handy.

Laser beam pointers have parallax issues with a bright sky and one isn't standing almost in line with them.

Apart from a manually driven scope a wide angle finder with small eye relief isn't all that useful/safe as one is tempted to use a high slew rate to centre the target quickly.

Another danger is falling when attempting to use a finder whilst slewing rapidly.



#18 PETER DREW

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Posted 06 September 2017 - 05:08 AM

 

My interest invites the question "Why a 20 degree field?". If it is for guiding a telescope to an object it should be possible to do this just as accurately with no optical aid. I have the Vixen SG's and the view is good but I wouldn't immediately think of one half as a finder.

The advantages of using a small, ultra-wide finderscope instead of something like a Telrad are:

 

- it shows many more stars, which is an advantage, especially in light polluted skies

 

- it is actually SIGNIFICANTLY smaller than a Telrad... 

 

- because of the magnification, it can show a lot of large, dim deep-sky objects directly, such as M33, etc., even in less than perfect skies.  

 

- it is significantly more precise than the Telrad, since the resolution is much, much higher, even just at 3x, because you can see many more stars and the perceived distances between guidestars are much larger. 

 

- a second hand 50mm camera lens is so cheap, you can build a ~4x25 finderscope with AWESOME field correction CHEAPER than what a new Telrad or a new 6x30 finderscope costs, provided you already have a decent wide-field eyepiece. You don't have to dedicate the eyepiece to the finderscope, though, as you can just preset the focus and just insert the eyepiece into the finder, when you need it and then use the eyepiece on the main telescope, when neccesary. 

 

- it doesn't require electricity...

 

 

People have critizised small finderscopes for decades, and not without reason, as most of them have been much too dim, but the problem with small finderscopes is often not that they are small, but that their magnification is much too high for their aperture, leading to a dim view with a much too restricted field of view. A small finderscope with low magnification and correspondingly large exit pupil and bright views is a whole different animal entirely. Couple this with a modern 80° eyepiece and a flat-field objective and you're in for a VERY different experience. 

 

 

Clear skies!
Thomas, Denmark

 

Thanks for your reply Thomas. I have no argument against the points you mention, it's just that as I find no difficulty in pointing a regularly supplied finder accurately enough for the object sought for to be somewhere in the field of view before centering it on the crosshair in the eyepiece. I don't immediately appreciate the benefit of a much wider field and lower magnitude threshold.  smile.gif



#19 Jon Isaacs

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Posted 06 September 2017 - 06:08 AM

Telrads are far too small to be useful in that application.
A large area sweet spot means that ones head doesn't have to slew at the same rate as the telescope when homing in on the target.
Yes using high slew rate when looking through a finder is not particularly safe.
That's where a unit power finder with in effect enormous eye relief comes in handy.
Laser beam pointers have parallax issues with a bright sky and one isn't standing almost in line with them.
Apart from a manually driven scope a wide angle finder with small eye relief isn't all that useful/safe as one is tempted to use a high slew rate to centre the target quickly.
Another danger is falling when attempting to use a finder whilst slewing rapidly.

 

 

If one wants to use a high slew rate, I guess you better stand back.  

 

But precise pointing with a Telrad or a magnifying finder is best done slowly where you can see exactly what you are looking at and then position the scope.  

 

Jon



#20 Ant1

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Posted 06 September 2017 - 06:44 AM

Hi

 

Thanks for all your replies.

I understand my question raised a lot of concern about the usefulness of a wide angle finder but I won't argue whether such a device is best suited for everyone's use or not.

I have included below some very basic raytracing to show what I have in mind, using a 30mm aperture objective, the 10° beam escapes the 2" eyepiece.

A smaller diameter objective makes the problem worse.

 

Captureray.jpg

 

I will look into the camera lens idea, but I'm not sure how to select one with sufficient viewing angle, this is not often mentioned on the lens itself and makes selection difficult.

I have two camera lenses already but both were restricting the FoV when testing with a 23mm 62° eyepiece and I suppose using a 80° 30mm will only make things worse!

 

Regards,

Ant1



#21 dan_h

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Posted 06 September 2017 - 11:21 AM

 I have two camera lenses already but both were restricting the FoV when testing with a 23mm 62° eyepiece and I suppose using a 80° 30mm will only make things worse!

 

A camera lens is designed to evenly illuminate the physical size of the film/sensor it is to be used with. The angular size of the view is is related to the lens focal length. For a 50mm lens, 20 degrees image is about 17.6mm diameter. This should be a good match to your 23mm eyepiece.

 

In your drawing, the objective focal length is much longer than 50mm and that is why the field is restricted. FOV is a function of focal length, not aperture.  

 

If the two camera lenses you have are longer focal lengths, then you can expect to cover a smaller field with any given eyepiece.

 

dan 



#22 MitchAlsup

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Posted 06 September 2017 - 11:22 AM

I will look into the camera lens idea, but I'm not sure how to select one with sufficient viewing angle, this is not often mentioned on the lens itself and makes selection difficult.

I have two camera lenses already but both were restricting the FoV when testing with a 23mm 62° eyepiece and I suppose using a 80° 30mm will only make things worse!

Camera lenses are designed to illuminate <well> a camera image frame. If we are talking about dSLRs--mainly 24mm*36mm, cropped lenses illuminate 15mm*22.5mm, and there are camera lenses that are smaller (4/3rds), and camera lenses that are larger (4*4 format and 4*6 portrait).

 

Which two camera lenses do you have?



#23 BGRE

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Posted 06 September 2017 - 03:22 PM

 

Telrads are far too small to be useful in that application.
A large area sweet spot means that ones head doesn't have to slew at the same rate as the telescope when homing in on the target.
Yes using high slew rate when looking through a finder is not particularly safe.
That's where a unit power finder with in effect enormous eye relief comes in handy.
Laser beam pointers have parallax issues with a bright sky and one isn't standing almost in line with them.
Apart from a manually driven scope a wide angle finder with small eye relief isn't all that useful/safe as one is tempted to use a high slew rate to centre the target quickly.
Another danger is falling when attempting to use a finder whilst slewing rapidly.

 

 

If one wants to use a high slew rate, I guess you better stand back.  

 

But precise pointing with a Telrad or a magnifying finder is best done slowly where you can see exactly what you are looking at and then position the scope.  

 

Jon

 

I find that a supersized biocular finder works very well at high slew rates, its very easy to point the telescope well enough that the target object is centered in the finder field, its often in the FOV of the main scope. A Telrad is just too small yo be useful in this application. 



#24 Bob4BVM

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Posted 06 September 2017 - 03:47 PM

Thomas is right on concerning using camera lenses.
I have been doing this since the 80's, and many of my film camera lenses have done a lot of double duty as small scopes and WF finders.  I totally agree on the value of WF and star-count gain over Telrad devices.  I do use telrads on my RFT type scopes, but much prefer some magnitude gains for use on longer FL scopes
Rigging a lens up as a scope is utterly trivial if you do this:

1-take a secure-fitting (screw or bayonet attachment) REAR lens cap for the camera lens in question
2-bore a ~ 1.1" hole thru it
3-attach a piece of 1.25" ID tube to cap (hot glue or epoxy work fine)  A 1" alum. pipe coupling works for the tube if you open up the ID slightly.
4-insert eyepiece of choice, experiment with a few.  Some lenses have enough focus range to allow use of a 1.25 diagonal. For precise targeting use a guiding (crosshair) EP.

 

Below and in next post are a few pix to illustrate how simple this can be:

 

Pic1:

At top is 400mm x 60mm telephoto with 2" extension added for 2" EPs- a nice little scope in its own right !

Bottom is 50mm lens with 1.25" EP adapter as described above

IMG_5668s.jpg

 

Pic2:

Bottom is a 80-210mm zoom w/ 1.25 EP adapter

IMG_5669s.jpg

 

 

 

 



#25 Bob4BVM

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Posted 06 September 2017 - 03:52 PM

Couple more pics

 

Pic3:

One of the adapters as described above

IMG_5671s.jpg

 

Pic4:

50mm with 1.25 diagonal & EP. A really sweet compact WF finder. I use it at 2-5X, dep on EP selection

IMG_5672s.jpg

 

CS

Bob




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