69 replies to this topic

[pretyro]

     I’d like a RACI finderscope having a 30 degree or so true FOV (as an adjunct to my 9x50 RACI finder which has a 5 degree FOV).  Would a demagnifying finderscope be more amenable (than a magnifying finderscope) to achieving large true FOV?

     Can anyone suggest a combination of objective, diagonal and eyepiece that would do the trick?  I think that I am looking for a 50 mm diameter objective (although a camera lens might be OK) that has a smaller focal length than the eyepiece, as well as sufficiently large field stops, and diagonal to connect them.
I can’t seem to find any wide FOV finderscopes; demagnifying or otherwise. Perhaps I am just an audience of one (and no one else particularly cares about a wide FOV for a finder).  Of course, there is no inherent requirement to demagnify – I just thought it might be easier to accomplish the goal by doing so.  It could be low magnification like 1.5x (instead of a demagnification) – any combination of smoke and mirrors is fine by me – I just want a 30 degree true FOV (yeah, and suitable eye relief, exit pupil, and acceptable astigmatism etc, and ideally an illuminated reticle). 

     Should’ve asked this question first:  There is a large following for 9x finderscopes and a large following for 1x finderscopes.  Why are there no demagnifying finderscopes (ie 0.75x or 0.5x or lower) with the object of attaining a large true FOV?

 

Background, details, etc

     Demagnification could improve the FOV at the expense of seeing fewer of the faint stars.  I’d be happy with such trade-off.    To test this out, I simply flipped around my 9x50 RACI Celestron finderscope.  By looking through it backwards, the eyepiece (18 mm diameter) became the objective and the 50 mm lens became the eyepiece.  The result: the image circle is really too small (there is constriction somewhere be it a field stop or the reticle apparatus), the image is too dim (although I could see Jupiter, and the nearby tree branches in the “eyepiece”), but the FOV (of this small, constricted image circle is about 30 degrees) was really good.  In fact, both Jupiter and Saturn (20 degrees apart) were in the image circle.  Obviously, a backwards 9x50 RACI is demagnifying more than needed and the 18 mm diameter lens (which functions as the objective but had been the eyepiece) pointed to the stars is perhaps too small. 
     See the image below taken July 11, 2021 at about 1 AM.  Both Jupiter and Saturn are in the image circle.  Due to poor focusing on my part, the images of the two planets are fuzzy and too large.  I used a Sony A5100 camera with a 55/210 mm lens zoomed to 210 mm in order to get the image circle blown up enough to present it.

 

Why
     For me the issue is workflow.  What I like to do is look at the sky with my unaided eyes (well eye, my right eye is too myopic, uncorrected), see something I’d like to observe more closely, then want to point the telescope at it.  The last step is the problem. 
I am having trouble locating stars; as well as recognizing/identifying them.  My Celestron 9x50 RACI finder has a relatively narrow field of view (about 4 or 5 degrees), so I first use a 1x Starpointer Pro.  However, for altitudes above 20 degrees or so, I can Gumby myself in 3 axes and hold it for only long enough to determine that in fact a star is within the reticle of the StarPointer Pro.  That is, I don’t know which star, let alone, which constellation my telescope is pointed at, or if it is even pointed at the region of the sky that I had been looking at.  It would be really handy to have a RACI finderscope having a 30 degree true FOV (which would capture recognizable chunks of constellations). 
     Using the 9x50 RACI finderscope backwards did work.  I pointed the telescope in the general direction of Jupiter, looked into the backwards 9x50 RACI (ie looked down into the 50 mm lens) and there was Jupiter – not centered – but within the image circle, on the first attempt.  So, a 30 degree true FOV finder is large enough such that the target will appear in the FOV after coarsely slewing to it.  However, as stated above, the backwards 9x50 RACI finder image circle was too small and too dim.

 

 

Attached Thumbnails

• 

[Gregrox]

by my quick calculation, 2" eyepieces with the widest field stops would require a focal length of about 90mm for a 30 degree tfov. even the pocket borg, perhaps the smallest telescope sold, has around twice that focal length. and since almost all eyepieces are below 55mm focal length, they would all magnify the sky. For example a 55mm plossl would have a 30 degree tfov, 50 degree afov, and 1.6x magnification, with some 90mm objective.

 

is 90mm even enough light travel in a 2" diagonal?

 

it's not clear to me how looking backwards through a telescope would compare to looking through a very nice eyepiece of a telescope with a grossly undersized objective.

 

the exit pupil is going to be larger than the aperture of the telescope. which means, as your eye cuts that exit pupil down to size, your eye's effective light gathering against the sky goes down. a demagnifying telescope will always show dimmer stars than your eye would.

[TienDuong]

I think you might be overthinking this. Getting a Telerad might help. It has 4 degree largest ring that would be a bit smaller than FOV of the finder scope. The window on the Telerad is very big. I have never had issues with knowing where I am pointed.

[Bob4BVM]

Well if you can ditch the raci part, what you want is easy with these.:

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

 

 

Built the binos or just half for one eye. But the binos are the way to go, so much nicer using both eyes.  2X mag, 28* TFOV.  Dim stars jump out at you, & the Milky Way starfields become an absolute playground !

CS

Bob

[spazmore]

Put 1/2 of a pair of the 2.1x42 binos on the end of a mirror diagonal and mount it on the scope body like a RACI, using the mirror in the diagonal just like one of those binocular mirror/sky window setups. You'll have to figure out the reticle part, but that should work.

[Jon Isaacs]

From a practical standpoint, both eyes open with a Telrad provides a very large field of view. 

 

For such wide fields, RACI finder's are problematic because you're physically look at 90 degrees to the direction the scope is pointing.. intuition is missing.

 

 

Jon

[Benach]

What I have here, is a finder scope with a TFOV of about 20 degrees and a very low magnification of about 2-4x. I bought it over from a friend of mine. In the longer term I intend to upgrade it with an illuminated crosshair, a bracket and 90 degree mirror. Currently it consists of a small (about 30mm) achromat, focal length about 200mm) with a military (Erfle?) eyepiece. My friend retrofitted that eyepiece with a nice looking housing.

 

It is a great thingy although I rarily use it at the moment because of private reasons. But in the longer term I intend to use it for sure because it can show you most constellations in one view with additional stars because of the increased aperture.

 

Can upload a photo upon request.

[Dick Jacobson]

I have built a small telescope that can zoom all the way down to 1x or less while maintaining a 100 degree apparent field. It uses a zoom camera lens in the front, a macro camera lens in the middle to invert the image, and a 100 degree eyepiece. It's intended as a spotting scope for terrestrial use and is too big and heavy to be a finder on most telescopes and doesn't have a right-angle. I've applied for a patent.

[KBHornblower]

  

I am having trouble locating stars; as well as recognizing/identifying them.  My Celestron 9x50 RACI finder has a relatively narrow field of view (about 4 or 5 degrees), so I first use a 1x Starpointer Pro.  However, for altitudes above 20 degrees or so, I can Gumby myself in 3 axes and hold it for only long enough to determine that in fact a star is within the reticle of the StarPointer Pro.  That is, I don’t know which star, let alone, which constellation my telescope is pointed at, or if it is even pointed at the region of the sky that I had been looking at.  It would be really handy to have a RACI finderscope having a 30 degree true FOV (which would capture recognizable chunks of constellations). 

If you are having trouble identifying stars, I think a reduced view would just make things harder by making the asterisms smaller and dimmer.  If we could make a unit power RACI finder with a 30 degree true field, it would match the naked eye view and spare you the Gumby drill.  From here it is like what an aspiring musician must do to get to Carnegie Hall.  Practice, practice, practice, starting with easy constellations such as Orion.

 

In such a finder, the objective would need the same focal length as the eyepiece, and it might be a challenge to get a roof prism between the objective and its focal plane.  Perhaps someone with better optical knowhow than mine could elaborate on this.

[pretyro]

by my quick calculation, 2" eyepieces with the widest field stops would require a focal length of about 90mm for a 30 degree tfov. even the pocket borg, perhaps the smallest telescope sold, has around twice that focal length. and since almost all eyepieces are below 55mm focal length, they would all magnify the sky. For example a 55mm plossl would have a 30 degree tfov, 50 degree afov, and 1.6x magnification, with some 90mm objective.

 

is 90mm even enough light travel in a 2" diagonal?

 

it's not clear to me how looking backwards through a telescope would compare to looking through a very nice eyepiece of a telescope with a grossly undersized objective.

 

the exit pupil is going to be larger than the aperture of the telescope. which means, as your eye cuts that exit pupil down to size, your eye's effective light gathering against the sky goes down. a demagnifying telescope will always show dimmer stars than your eye would.

 

 

Agreed, I have no compelling reason for demagnification.  If a 1.6x system would provide 30 degree true FOV, it would be great. 

 

I don’t know if a 90 mm lens has enough light travel in a 2” diagonal either.  (Astrojensen, reply 8, https://www.cloudyni...e-finder-design, indicated “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”).   Do you think such an eyepiece would do the trick?

 

Actually, I was going to make my post on the “wide-angle-finder-design” thread, but my topic seemed to be tailored to particular wide angle finder of demagnifying and RACI.  It looks like I don’t need demagnification, especially if a 1.6 x finder would work – just need the right hardware though.

 

 

[pretyro]

I think you might be overthinking this. Getting a Telerad might help. It has 4 degree largest ring that would be a bit smaller than FOV of the finder scope. The window on the Telerad is very big. I have never had issues with knowing where I am pointed.

 

 

From a practical standpoint, both eyes open with a Telrad provides a very large field of view. 

 

For such wide fields, RACI finder's are problematic because you're physically look at 90 degrees to the direction the scope is pointing.. intuition is missing.

 

 

Jon

The Telrad, of course, is a really nice instrument.  If I could sight along the telescope parallel to tube into the Telrad, I can only use one eye (my other eye is too myopic).  So, I can’t get leverage the “both eyes open” FOV.  Moreover, the Telrad is not a right angle device.  I just can’t Gumby myself into a position of fovea / reticle alignment for long enough for it to be useful.  Using a 1x finder, I always have issues knowing where I am pointed; unless the telescope is within a few degrees of the horizon.

Although I recognize that the right angle devices are not as intuitive as straight-through, I have gotten more accustomed to them (both my 9x50 finder scope and telescope have eyepieces located at right angles). 

Additionally, I’d like to use the wide angle RACI (or at least right angle) finder as a device to observe / peruse the stars, not just for a moment to find them.

[pretyro]

Well if you can ditch the raci part, what you want is easy with these.:

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

 

 

Built the binos or just half for one eye. But the binos are the way to go, so much nicer using both eyes.  2X mag, 28* TFOV.  Dim stars jump out at you, & the Milky Way starfields become an absolute playground !

CS

Bob

 

I really do need the right angle element (the correct image part of RACI would be nice as well).  Both my telescope and 9x50 finder scope are right angle.  I can’t really get my eye behind a straight-through scope (for any length of time) unless the it is pointed at the horizon.
Actually, there is a post from you (I think, in 2017) showing a right angle finder using a camera lens (bob4bvm, reply 24 & 25, https://www.cloudyni...inder-design). 
Could you expand a bit on the right angle scope shown in pic 4 (Pic4: 50mm with 1.25 diagonal & EP. A really sweet compact WF finder. I use it at 2-5X, dep on EP selection; cf reply 25).  Which 50 mm lens, which diagonal and which EP does it use – and what is the true FOV.  Would use of a 2” diagonal improve the FOV (or is something else limiting)?
Did you have any trouble with the back focus (reply 8 in the same thread indicated that when using camera lens in a telescope:  “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”).
 

[pretyro]

Put 1/2 of a pair of the 2.1x42 binos on the end of a mirror diagonal and mount it on the scope body like a RACI, using the mirror in the diagonal just like one of those binocular mirror/sky window setups. You'll have to figure out the reticle part, but that should work.

I hadn’t thought of that.  The bino/mirror devices seem to comprise a mirror positioned between the bino objective and stars.  You’re proposing, placing the mirror (in this case the diagonal) between bino objective and the stars.  I am concerned that the diagonal might limit the FOV a bit.  I’d have to try it though.

[pretyro]

What I have here, is a finder scope with a TFOV of about 20 degrees and a very low magnification of about 2-4x. I bought it over from a friend of mine. In the longer term I intend to upgrade it with an illuminated crosshair, a bracket and 90 degree mirror. Currently it consists of a small (about 30mm) achromat, focal length about 200mm) with a military (Erfle?) eyepiece. My friend retrofitted that eyepiece with a nice looking housing.

 

It is a great thingy although I rarily use it at the moment because of private reasons. But in the longer term I intend to use it for sure because it can show you most constellations in one view with additional stars because of the increased aperture.

 

Can upload a photo upon request.

Sounds nice!  What are the specs of the eyepiece?  Yes, I would like to see a photo of it.  Could you please upload it?

[pretyro]

I have built a small telescope that can zoom all the way down to 1x or less while maintaining a 100 degree apparent field. It uses a zoom camera lens in the front, a macro camera lens in the middle to invert the image, and a 100 degree eyepiece. It's intended as a spotting scope for terrestrial use and is too big and heavy to be a finder on most telescopes and doesn't have a right-angle. I've applied for a patent.

 

That is in interesting device.  On your patent application, have you had a first action on merits yet?  Will you file an international application as well?  Good luck!
For me, at least, a 2x to 8x (or 1.5x to 9x) zoom RACI finder might be nice; if the 2x had a nice wide FOV; perhaps that would be small enough to mount on a telescope too.
 

[spazmore]

I hadn’t thought of that.  The bino/mirror devices seem to comprise a mirror positioned between the bino objective and stars.  You’re proposing, placing the mirror (in this case the diagonal) between bino objective and the stars.  I am concerned that the diagonal might limit the FOV a bit.  I’d have to try it though.

The clear aperture on the GSO 2" 99% diagonal is 46.6mm; a 2.1x42 is, well, 42mm. So you shouldn't lose much FOV. But you'll be upside down due to 1 reflection. I think an Amici diagonal would fix that. But you're looking at $200 for a 2" amici diagonal.

[pretyro]

If you are having trouble identifying stars, I think a reduced view would just make things harder by making the asterisms smaller and dimmer.  If we could make a unit power RACI finder with a 30 degree true field, it would match the naked eye view and spare you the Gumby drill.  From here it is like what an aspiring musician must do to get to Carnegie Hall.  Practice, practice, practice, starting with easy constellations such as Orion.

 

In such a finder, the objective would need the same focal length as the eyepiece, and it might be a challenge to get a roof prism between the objective and its focal plane.  Perhaps someone with better optical knowhow than mine could elaborate on this.

 

  I recognize that low magnification will make the objects appear smaller and dimmer.  I have been magically thinking that, by using a larger objective, the objects could be at least brighter.  Optics has its way of making the design tradeoffs apparent -- very quickly.  That is, using a larger objective introduces other problems.

  There may be a RACI finderscope (or at least an erecting right angle one, sans CI) based on camera lens that will do the trick.  1x, 1.5x 2x, 0.75x --  I mean, a little magnification is fine (good in fact).   I just would like to have 25 - 30 degrees true FOV.
 

Edited by pretyro, 15 July 2021 - 09:06 PM.

[Spectral Joe]

Any unit magnification system will have an exit pupil the same size as the entrance pupil, conversely, if you limit the size of the exit pupil (like with your eye's pupil) you effectively stop down the entrance pupil (aperture). You can imagine a system that gets around this, but it will remain imaginary. A demagnifying system is worse in this respect. I would suggest this: buy a low light C mount security camera, a C mount lens with a 30 degree field of view and a little LCD monitor. All of these are readily available (Supercircuits, for example) and together will give you a brighter image than you'll get through a unit mag imaging system.

[Dick Jacobson]

That is in interesting device.  On your patent application, have you had a first action on merits yet?  Will you file an international application as well?  Good luck!
For me, at least, a 2x to 8x (or 1.5x to 9x) zoom RACI finder might be nice; if the 2x had a nice wide FOV; perhaps that would be small enough to mount on a telescope too.
 

I haven't received any response yet from the patent office. My application was filed last April. I don't expect to file an international application. One patent attorney sounded skeptical about my chances, saying that the PTO has a negative attitude toward new optical configurations.

[pretyro]

I haven't received any response yet from the patent office. My application was filed last April. I don't expect to file an international application. One patent attorney sounded skeptical about my chances, saying that the PTO has a negative attitude toward new optical configurations.

btw, if you are over 65, you can file a petition to make special with the patent office.  In doing so, the patent office will start the examination process more quickly (so you don't need to wait as long to get an office action).

Edited by pretyro, 16 July 2021 - 09:10 AM.

[KBHornblower]

If you are having trouble identifying stars, I think a reduced view would just make things harder by making the asterisms smaller and dimmer.  If we could make a unit power RACI finder with a 30 degree true field, it would match the naked eye view and spare you the Gumby drill.  From here it is like what an aspiring musician must do to get to Carnegie Hall.  Practice, practice, practice, starting with easy constellations such as Orion.

 

In such a finder, the objective would need the same focal length as the eyepiece, and it might be a challenge to get a roof prism between the objective and its focal plane.  Perhaps someone with better optical knowhow than mine could elaborate on this.

My bold.  What was I using for a brain?  A 35mm single lens reflex camera fits a roof prism between the focal plane and the eyepiece, and with a 50mm lens it gives very nearly a unit power view with about a 35 degree field of view.  In a RACI unit power finder the prism would go between the objective and the focal plane, as in the 9x finder.

[Gregrox]

 

I don’t know if a 90 mm lens has enough light travel in a 2” diagonal either.  (Astrojensen, reply 8, https://www.cloudyni...e-finder-design, indicated “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”).   Do you think such an eyepiece would do the trick?

 

it occurs to me if the objective is very small, you could put the diagonal in front of the objective. I think I've seen binocular mounts which do this. Maybe a 2" Diagonal -> 2x ultrawide monocular as described https://www.cloudyni...idefield-binos/ here.

 

[pretyro]

My bold.  What was I using for a brain?  A 35mm single lens reflex camera fits a roof prism between the focal plane and the eyepiece, and with a 50mm lens it gives very nearly a unit power view with about a 35 degree field of view.  In a RACI unit power finder the prism would go between the objective and the focal plane, as in the 9x finder.

 

I've ordered a Minolta 50 mm lens (an old manually operated lens) to try this out (using https://www.cloudyni...inder-design thread as reference). I'll scare up a diagonal and EP as well.  Some SLR lenses have a greater flange focal distance than others (https://briansmith.com/flange-focal-distance-guide/); see below.  Looks like the Minolta I am getting has about a 45 mm flange focal distance.  If I trim down the diagonal a bit, it should work using a 14 to 20 mm EP and judicious use of the F stop settings to get a manageable exit pupil.  

 

 

Lens Mount "Flange Focal Distance"

Pentax Q-mount      9.20 mm
D-mount      12.29 mm
CS-mount       12.50 mm
Nikon Z-mount       16.00 mm
DJI DL-mount       16.84 mm
Nikon 1-mount       17.00 mm
C-mount       17.526 mm
Fujifilm X-mount       17.70 mm
Canon EF-M-mount       18.00 mm
Sony E-mount       18.00 mm
Hasselblad XCD mount       18.14 mm
Sony FZ-mount       19.00 mm
Micro Four Thirds       19.25 mm
Canon RF mount       20.00 mm
Leica L-mount        20.00 mm
JVC 1/3″ bayonet mount       25.00 mm
Samsung NX-mount       25.50 mm
Fujifilm G mount      26.7 mm
Pentax Auto 110       27.00 mm
RED ONE       27.30 mm
Leica M-mount       27.80 mm
Nikonos       28 mm
Leica M39 mount        28.80 mm
Olympus PEN F         28.95 mm
Contax G-mount         29.00 mm
Contax RF-mount         34.85 mm
Nikon S-mount         34.85 mm
1/2″ TV bayonet mount         35.74 mm
Minolta V-mount         36.00 mm
Sony 1/2″ TV bayonet mount         38.00 mm
Olympus Four Thirds System         38.67 mm
Konica AR-mount         40.50 mm
Konica F-mount         40.50 mm
Canon FD-mount         42.00 mm
Canon FL-mount         42.00 mm
Fujica X-mount         43.50 mm
Minolta MC/MD mount         43.50 mm
Arri LPL         44.00 mm
Canon EF-mount         44.00 mm
Pentaflex         44.00 mm
Praktica B-mount         44.00 mm
Sigma SA-mount         44.00 mm
Minolta / Sony A-mount         44.50 mm
Rollei / Voigtlander QBM         44.50 mm
Exakta         44.70 mm
M39x1         45.46 mm
M42         45.46 mm
Pentax K-mount         45.46 mm
Contax C/Y-mount         45.50 mm
Kodak Retina DKL-mount         45.70 mm
Voigtlander Bessamatic DKL         45.70 mm
Voigtlander Vitessa T DKL         45.70 mm
Yashica MA-mount         45.80 mm
Olympus OM-mount         46.00 mm
Nikon F-mount         46.50 mm
Leica R-mount         47.00 mm
KMZ Zenit DKL-mount         47.58 mm
B4 2/3″ TV bayonet mount         48.00 mm
Contax N-mount         48.00 mm
Arri B         52.00 mm
Arri PL         52.00 mm
Arri STD         52.00 mm
Leica S-Mount         53.00 mm
T-mount         55.00 mm
YS mount         55.00 mm
Panavision PV-mount         57.15 mm
Mamiya 645         63.30 mm
Contax 645         64.00 mm
Pentax 645         70.87 mm
Rollei SLX         74.00 mm
Pentacon Six         74.10 mm
Hasselblad 500 / 2000         74.90 mm
Hasselblad 1000F / 1600F         82.10 mm
Pentax 6X7         85.00 mm
Rollei SL66         102.80 mm
Mamiya RZ67         105.00 mm
Mamiya RB67         111.00 mm

 

 

 

 

 

 

 

[pretyro]

it occurs to me if the objective is very small, you could put the diagonal in front of the objective. I think I've seen binocular mounts which do this. Maybe a 2" Diagonal -> 2x ultrawide monocular as described https://www.cloudyni...idefield-binos/ here.

 

 

  I have to admit some of those 2x binoculars look pretty good.  The binocular mirror setup use relatively large mirrors (about the size of a notebook computer).  Perhaps the RACI mirror setup you proposed may not have a large enough "mirror".   If I get a wide angle binoculars (or a 2x teleconverter sometime, I will give it a shot.  In the meantime I will try connecting a camera lens to a diagonal (as was proposed on some earlier threads). 
 

[pretyro]

Any unit magnification system will have an exit pupil the same size as the entrance pupil, conversely, if you limit the size of the exit pupil (like with your eye's pupil) you effectively stop down the entrance pupil (aperture). You can imagine a system that gets around this, but it will remain imaginary. A demagnifying system is worse in this respect. I would suggest this: buy a low light C mount security camera, a C mount lens with a 30 degree field of view and a little LCD monitor. All of these are readily available (Supercircuits, for example) and together will give you a brighter image than you'll get through a unit mag imaging system.

Your suggestion would certainly solve the problem.  In fact, I could connect the camera to an iPad to not only achieve a wide FOV, but a display larger than any exit pupil.  I've been looking for more simplicity (I suppose it is debatable though which invokes more simplicity, a camera/monitor system or an lens system), or maybe it just something about capturing those photons in my retina; photons released from a star a thousand years ago.

  Agreed, I will need to contend with the realities of entrance and exit pupil.  I will try making a camera lens/diagonal/eyepiece device as has been proposed in some earlier threads.