The aim of this page is to help those looking for a telescope to read colour marks on birds, to find the “best” cr-telescope for their budget. There is a difference between a “good” telescope for birding and a “good” telescope for cr-birding: - for cr-birding power is as important as image quality. For similar image quality, choose always a telescope that can give you more magnifications!
If you want to compare some of the "best" cr-telescopes see a test of 100mm telescopes.
After reading the CR-binoviewing
Test of 82º, 100º and 102º AFOV zooms
you might ask what is the solution I prefer? Well
those with >=100º eyepieces
are the most efficient for CR-birding since the
zoom is parfocal i.e. I don't need to refocus when zooming, and the field of
view is larger, so more easy to find marked birds. However, if I'm not
pressed with time to read cr-marks, I prefer the views with the binoviewer.
Yes, the ultimate solution would be a solution with binoviewer,
eyepieces and parfocal zoom but still isn't possible...
- The Pentax PF100 and the Optolyth TBG100 with the TeleVue Ethos 4.7mm 110º;
- The Pentax PF100 and the Optolyth TBG100 with the TeleVue, the Baader, the Siebert or the Denkmeier Bino-viewers and 2 Baader zooms 24-8mm - The idea was good but bino-viewers don't reach focus on birding telescopes, even with the TeleVue 2x OCA corrector... ;
- A good Apo-refractor with image erector, binoviewer and zoom usable from 25 to 125x - I almost accomplished it see it at CR-binoviewing - I would like to test the present material combination on a TeleVue NP101is, on a Takahashi FSQ-106ED, a Skywatcher ESPRIT ED100 and the 95mm lens module of the new Swarosvky X system;
- My TMB92 and the same previous telescopes with the Nikon FSA-L2 and a Nikon HW17, an Ethos21, an ES100_25, and a Docter UWA 12.5.
What I would like that the telescope producers did?
- I would like to see 100 mm versions of Zeiss, Leica, Swarosvky, Kowa and Nikon, as a Compact version of the Optolyth 100 (the new Swarosvky X system has a 95mm lens module but with only 70x of maximum magnification...);
- I would like that Kowa or a eyepiece producer would produce an astro-adapter for the 88mm scope, since is possible and works with astronomical eyepieces (http://www.birdforum.net/showthread.php?t=105037);
- I would like that Swarosvky will produce an adapter that will allow the use of the X lens modules directly with DSLRs and astronomical material;
- I would like a binoviewer that works well with birding scopes, specially without inducing extra magnifications. An alternative would be a birding scope designed to reach focus with binoviewers. Why not? At least, Nikon, Zeiss and Leica produce bino-microscopes. The Zeiss Photoscope could be a base for a Binoscope - it already has a internal 3x zoom...;
- I would like >80º AFOV zooms. I already managed to obtain it with astro-telescopes - see it at Test of 82º, 100º and 102º AFOV zooms!
- I would like 4x or 5x zooms - there are digital cameras with >20x zooms and there are also rifle scopes with 6x zooms. Why not on birding telescopes? Zeiss already released a 3.75x zoom that results in 20-75x in the 85mm model and Meopta released a 3.5x zoom that results on 20-70x!
As the 30x power can make difficult to find marked birds at close distances (it’s a question of get used to it…), I suggest you to use binoculars to find birds. As I do lots of hide resighting, I use a binocular Swarovsky 15x56 to read codes at close distances (with the Leica 10x50 I almost managed to read codes but didn’t most times). It’s more efficient on finding birds and more comfortable.
update - I tested a
(630mm) with the XF zoom (32.5-97.5x) and the Nikon/Optolyth zooms (30-90x),
but only for a short period. For those that don't use eye-glasses the
PF100 with the XF-zoom maybe an CR improvement in relation to the Optolyth
100. For eye-glasses users, the eye-relief is too
small at maximum magnification and the limits of the field are curved and
not focused. In terms of with of field is almost
similar to the Optolyth 100
with the 2 version old zoom eyepiece,
although it increases a little more (the Optolyth 100 with the
compact zoom (1 version old zoom, not the available now, is the one released with the compact telescope),
has a wider field of view and better image quality. Seems to gather more light than the Optolyth and
to be a bit better at high magnifications
(e.g. with the 5.2mm Pentax XL). You can use the Nikon zoom on the PF100
without any adapter and the image quality is better, although the eye-relief
is only a bit better and you loose magnifications.
Both Optolyth zooms don't work well on this telescope because it hasn't enough focus range at great distances.
April/05 update - I tested the Pentax XW 5mm on the Optolyth and it works better than the XL 5.2 mm, being the new reference high power eyepiece for eye-glasses users. It has a wider angle - 70º instead of 65º - and seem to let pass more light than the 5.2 mm although it increases a little more - 126x instead of 121x. The new coatings may explain it, since produce a very clear image and works very good with the more yellowish coatings of the Optolyth. Interesting is the fact that the same testing on the Pentax PF100 is not so positive to the XW since seem to loose contrast due to the fact of the image being so clear/bright.
July/05 update - I tested the Opticron ES100 with the new SDL zoom eyepiece that results on a 27-80 zoom. It has a very good image quality, a wide field of view and a very good eye-relief for eye-glass users. It only exists in angled version (I'm not a fun of it, as already told) and, although waterproof it is not moist proof (as the Nikon ED78 - is sealed but not filled with N). For this last reason you should purchase also the waterproof stay-on-vest but it became more difficult to exchange eyepieces and to turn it for lateral viewing (e.g on a window mount). I tested the Opticron UTA 2.5x booster on the SDL zoom but didn't reach any conclusion about increasing CR potential, since there was too much hot and atmospheric conditions were not satisfactory. For now I can say that the UTA is very easy to put on the zoom and, if it works, can be a nice solution for those that have other telescopes models that do not have high power eye-pieces or can't receive astronomical eye-pieces. The lost of light is evident as also the lost of field of view. For that reason I already whished that Opticron made a UTA that would increase only 1.5 or 2x, that would be much more usable on telescopes (see a Test of 2.5x magnifiers).
September/05 update - on the 1st CR-Birding Meeting (see it at http://www.cr-birding.be) there was a telescope testing with several models. However has it was Sunday and the planned site (a straight road with more than 350m, within the forest) was crowded with people walking and biking witch made the testing impossible - always people in front of the telescopes... The alternative solution was not good enough for the testing since the colour rings and nasal saddles for reading were only at about 120 meters and all telescopes could read it. The only conclusion I made was that the Questar Field Model (see Questar user experience) maintains it very good resolution with the Questar image erector - definitely I would like to test it with conditions and time... I offer bed and food to a Questar user interested in visiting us... :-)
January/07 update - I tested the Televue Radian 4mm in a Optolyth TBG80 Apo (serial number 127467) that results in 105x and it worked fine! I didn't have much time to test it and don't know if if represents an improvement compared to the 5 mm in 80mm telescopes (in this model the 5mm would result in 84x, but the Pentax XW5 didn't reach focus). However it confirmed that 80mm telescopes can also go over 100 magnifications - at http://www.kikkertspesialisten.no/pub_docs/files/tester/testtele.pdf you can find a similar test in a Swarosvky.
After this month I started to organize tests in the following different sections mentioned on the quick page index.
Test of 100mm telescopes (updated on 24-04-08) - the Optolyth, Pentax and Opticron telescopes are the top 100 mm telescopes and some of the best for CR-birding. I put them side by side, with their zooms and higher power fix eyepieces, and tried to reach some conclusions. I used on the Optolyth the 2 generation old zoom (of weight and length similar to the new version), on the Opticron I used the 2005 version with the HR zoom, and on the Pentax I used the XF zoom. On the Optolyth and Pentax I used the Pentax XW5mm and on the Opticron I used the Opticron HDF40861. The following chart summarize most of the information:
**It's possible to put a screw on the Optolyth astro-adapter (see photo below. Adaptation made at http://www.bstar-science.com. Gotthold Barbi gotthold.barbi add web.de found a compatible adapter for using astro eyepieces in Optolyth (31.7mm eyepiece to M37), that already has screws and costs much less than the original http://www.teleskop-austria.at/prod.php?tid=30&lng=de#adap-fgy-hu).
The Opticron is told as a reference in resolution (http://www.kikkertspesialisten.no/pub_docs/files/tester/testtele.pdf), which is a good indicator for the Pentax since they share many optical components (made by the same telescope producer...). The higher power XF zoom turns it more sensible to atmospheric and light conditions, it's not suitable for eyeglasses users (low eye relief at high powers) and seem to not have so high image quality as the other zooms. However for cr-birders that don't use eye-glasses it might increase the cr-birding potential on good light conditions. For more image quality exigent observers you should purchase a 24-8mm zoom (26-78x) - there is a zoom version of Pentax which is waterproof but is heavier, bigger and more expensive than the XF - you can choose the TeleVue 24-8mm zoom instead, that also have a wider angle at lowest power than the XF. Another compromise for CR-birding would be the use of the Nikon zoom, that have better image quality than the XF and would result in 30-90x (you have to open completely the telescope eyepiece attachment to prevent that the eyepiece would be "entangled") - the use of the Nikon zoom is a good solution for those cr-birders using Nikon telescopes. On Opticron you can also use the SDL zoom (27-80x) that has better image quality and a "window view". The "compactness" of the Optolyth could be increased if the compact zoom was used (1 version old zoom - not available anymore), that also has a wider field of view and better image quality than the one analysed, but the new version also should have an improved image quality than the analysed. The Opticron 134x HR eyepiece has good image quality but has a much narrow field of view than the Pentax XW 5mm (126x on Pentax and Optolyth), being more difficult to find a marked bird. The yellowish coatings of the Optolyth produces an apparently less bright image at high power but it doesn't seem a real limitation and, in fact, it can be an advantage since increases contrast and the telescope become less sensible to reflections on the cr-marks (comparing the Pentax XW 5mm on Optolyth and Pentax), and in low light conditions the contrast is also better. Fanatics of image quality and field of view, can also use the 24-8mm zoom of Pentax or Televue on the Optolyth telescope but the Baader zoom is the best solution, if you have a version that will reach focus http://www.birdforum.net/showthread.php?t=73582. The same is applied if you want to maximize the zoom magnifications, using in this case the Pentax XF on the Optolyth. For now I will continue to use the Optolyth because has the better compromise power/eye relief and also the zoom tested can receive the Nikon FSA2 ring for digiscopy with the FSBs (just unscrew the ocular rubber ring, put the FSA2 and screw the rubber ring again - it results more practical than with the Nikon zoom... You can do the same with the fixed eyepieces).
Also I found that I could use Nikon FSA-L2 for reflex digiscopy with the TBG100, after an adaption of the Optolyth Astro-adapter to T male and using an adapter T to Nikon F (a version with reduced length). It works directly with Nikon DSLRs but can work with other brands by using the proper adapter - below a photo with a Sony NEX5.
When doing the 5mm eyepiece test I introduced the use of resolution charts at 32.5 meters and was surprised to note that the Opotyth combo had slightly better resolution than the Pentax combo (both had 950-1100 scale on the EIA1956, but the Optolyth had a score of 5-6 in scale 1 of the USAF1951 bar chart instead of only 5 for the Pentax). Also expected that the Nikon zoom would increase resolution but it didn't, in both telescopes. The Optolyth with the XF went until the 1100-1250 on the EIA1956. Will have to test the Opticron with the charts.
With the test of the 5mm eyepieces, the Opticron lost the advantage of lower cost in the high power eyepieces.
Who never thought of a zoom with more power when trying to read a CR at great distance. For astronomical telescopes there are "Barlows" that can increase the power of a eyepiece 2, 3 or more times. However these don't work on common birding telescopes since eyepieces don't have the 1'1/4 barrel size. Pentax telescopes don't have that limitation but don't reach focus with common barlows. Televue developed special magnifiers that maintain the focus range and the lowest power they have is 2.5x. I remembered to compare it with the Opticron UTA that increases also 2.5x. The test was done with the Pentax PF100 with the Televue zoom 24-8 mm, that result on 26-78x, with the Powermate 2.5x, resulting on a zoom range from 65x to 195x; and the Opticron ES100 with the SDL zoom (27-81x) with the UTA resulting in 67.5-202.5x!!! The concept is similar but the attachment is completely different: the Powermate is attached between the telescope and the eyepiece and the UTA is attached outside the eyepiece (see photo). The result is also different, being that the powermate preserves colours and reduces a little bit the eye-relief, and the UTA has yellowish colours and increases eye-relief of the eyepiece. Has there are no miracles, for equivalent magnifications a fixed eyepiece will have a wider field of view and better image quality (light have to pass through more 4 or 6 lenses, with the magnifiers). If we are trying to read codes at great distances these 2 characteristics are fundamental. We might think that we could use more power to compensate the lower definition but for me the fixed high power eyepieces continue to be better, also because after some values (130-150x) you don't gain nothing on increasing the power since you loose too much light and the atmospheric conditions define the useful limit. The Powermate can be a economical solution to increase the power of your fixed eyepieces and from a 12.5mm you will have a 5mm. The main advantage of the UTA is that you don't have to remove the eyepiece from the telescope being faster to apply and avoids the collection of dust and dirty within the telescope and eyepiece. Both have a common problem - they increase too much and a 2x power would be the ideal for most birding telescopes - better balance of field of view at lower power and not such much problem with light at maximum power, so becoming more useful. I can conclude that the UTA booster is the solution for those telescopes that cannot receive astronomical eyepieces. I'm remembering the Leica and Nikon - for both you don't have the most appropriate ring attachment (Opticron have several diameter rings adapted to their eyepieces) and you will have to do some "bricolage". I did it on the Nikon zoom with the ED78 (25-75x) - I used the Opticron ring that is a bit bigger than the zoom diameter and used glue tape to reduce the inner diameter of the ring to the same of the zoom... The UTA worked fine until about 125x (over that you don't gain nothing), preserving the excellent definition of the telescope with the bonus of good eye-relief for eye-glass users. However the UTA will be more useful in telescopes with 20-60x zoom (would result in a 50-150x) and would like to test it the Leica as also in the Opticron ES80.
I noticed latter that Zeiss has 3x12 monocular that can also work as a magnifier and is told as of good quality (e.g. http://www.birdforum.net/showthread.php?t=91949), being a solution similar to the UTA and might be interesting for those using low power fixed wide angle eyepieces.
On 2009 I noticed that Brightstar http://www.bstar-science.com had a modular barlow from 1.6 to 2.5x. I tested it with a Baader 24-8mm Hyperium Zoom (thanks to Gotthold Barbi gotthold.barbi add web.de by offering a repaired zoom eyepiece), but only reached focus at the infinite with the 1.6x version (the barlow is the black cell at the telescope side).
However it worked very well (both on the Optolyth TBG 100 and Pentax PF100), and as the Baader Zoom has a wide angle at maximum power (about 70º), the zoom with the 1.6x barlow can be a good and economical alternative to the Pentax XW5 (same magnification and field at maximum power of the zoom), with the bonus of wider field at lower power zoom positions - I was waiting for a Ethos 5mm 100º from TeleVue, or clones from other brands, but the zoom with the barlow maintain wider fields at lower magnifications... I tested resolution of the combo and it's similar to the one of Pentax XW5 (see it at Magnifications vs. resolution) but the Pentax maintains better light and colours (when compared to the zoom+barlow at 8mm position which results in 5mm but is visible a bit of colour aberration at this position on the Optolyth - didn't tested on field conditions with the Pentax). Nowadays I preferably use the zoom+barlow instead of the XW5, since it allows better fields for scanning for marked birds - I have to test this barlow in a new eyepiece to confirm colours and image quality since the tested is a home repaired one. The use of this barlow is also a way of the Baader zoom reaching focus on Pentax scopes and it doesn't affect the aesthetics of this brand scopes.
Test of image erectors
(updated on 21-08-12 with the Nikon FSA-L2
- without any image erector - image erected in the computer to make easier to compare fields;
- with the Celestron 45º prism;
- with the Vixen image erector - it should be lens and not prism;
- with the Siebert prism;
- with the Optolyth photo image erector;
- the TBG100, to show the field at 20x - nest of Ardea cinerea included as a bonus... :-)
I already had noticed that without prism the image was much more bright but only now I noticed that it also result from a additional increase in the focal length of the telescope when using an image erector - it's not only more glass with the image erector...
The Celestron prism is ok for low powers but not suitable for high powers since make focus of the details impossible, as I already knew...
The Vixen allows correct focus at high powers, maintaining a light level similar to the Celestron prism, although probably increasing magnifications a little bit more. Only not so good detail is that is long - 14.3 cm - the C5 looks at least a bit strange... The refractors, that are already long, will be even longer... This might be a limitation for bino-viewing since will increase even more the complete set, although it's robust enough. An important quality is that it can be used in all telescopes since don't need to move focus range - it even focus in the Pentax PF100...
The Siebert prism maintain also the fine focus at high powers but vignettes a bit (much more in a Celestron Ultima 30mm), and, more important, it decreases the light too much. The main advantage is the compactness;
The Optolyth photo erector (also lens, as the Vixen), allows also fine focus at high magnifications and preserves light availability. It increases magnifications more, which can be limiting. This solution can also be done with the Nikon photo erector but needs more adaptors (at least an extra reducer ring). It might be a economical solution for those who already have these photo adaptors, otherwise the Vixen is a better and more economical solution.
As conclusion the Vixen is the solution for those who want to have terrestrial viewing from an astro telescope, preserving the ability of using high magnifications without focus or image quality problems. I wish that it would be not so long... The ideal would be a solution of the length as the one I "built" for the Siebert. Also for refractors a 2" erecting solution would be better, and I can imagine the fanatics of wide fields using 2" wide angle eyepieces for terrestrial viewing!!! This would also be useful for the 2" bino-viewers and since only Siebert produces these, why not them to develop a good straight 2" image erector - a barlow like I did please...
In the case of the C5, besides the use of a 6.6 focal reducer, the reducing of the length of the 1.25" eyepiece holder will also reduce magnifications - this will allow wider fields. Also for bino-viewing this will help since in this telescope magnifications will increase 1.25x.
When testing the 5mm eyepieces in the resolution charts I tested the C5 with the Vixen and the mentioned Optolyth zoom and it obtained the best score with 1600 on the on the EIA1956 (maximum) and a score of 2-3 in scale 2 of the USAF1951 bar chart. Will test the solution in field on a study area where ducks are at bigger distances, to see if it results more efficient than the Optolyth, but for now astro telescopes should return to be a solution for cr-birding at high distances.
During 2009 I found that the increase of power induced by the Vixen erector is higher than I expected since is in fact around 1.5x with eyepieces!!! Vixen publicity never mention it... After finding that it increased 3x when using a binoviewer I started to search for an alternative. When I saw the Nikon FSA-L2 for reflex digiscopy with the new EDG telescopes I thought of using it as an image erector (as the Optolyth equivalent worked and also the FSA-L1...), and has it doesn't induce increase of magnification at lower power (it works as a sort of 3.5x zoom), I expected that it also wouldn't induce increase of magnifications when working as image erector with eyepieces. After finding the pieces needed to use it on astro-telescopes (the easier way I found was to use T connections, so in one side a female Nikon F-mount to T and on the other a male Nikon F-mount also to T. For using astro-eyepieces is needed an adapter T to 1,25" or T to 2"). So the good news are that it works and has excellent image quality and it works both with 1,25" and 2" eyepieces - on this last one it vignettes on the C5 but it doesn't on the ED80/600 or on the TMB 92/506.
As I tested it with a 30mm 2" UWA (>80º), that has almost the largest field possible with 2" eyepieces, this means that fanatics of wide angled eyepieces can use this solution, even with a TV Ethos 21mm/100º and having a 3.5x zoom!!! For 1,25" eyepieces it can even reduce power about 0.95x. For 2" it seems to induce a bit of increase in magnification that could be solved by reducing the length of the 2" barrel piece, so getting closer the lens of the eyepiece to the ones of the FSA-L2. Besides the high cost, the other not so good details are that is long and for binoviewing it still induces a 2x increase... It isn't also parfocal when zooming.
Since it worked well I decided to improve it by removing the Nikon male F mount piece (of the camera side - it doesn't damage the chip for Nikon cameras), and asked Brightstar http://www.bstar-science.com do produce a piece with male T. Like this the 30mm 2" UWA is parfocal along the 3.5x zoom and the minimum position doesn't induce extra magnifications. On the other hand the 1.25" see magnifications reduced to 0.77x - using an extender you can find a position where there is no change in magnifications and the zoom become parfocal. With the Baader MarkV binoviewer the actual increase is about 1.7x.
As the 30mm 2" UWA doesn't work well with the fast F5.5 ratio of the TMB92 (it shows too much field curvature), I tested the combo with a 17mm TV Nagler type 4 but this eyepiece shows blackouts during day use and the eye-relief isn't good enough for a eye-glass user like me. So I had to choose a 22mm TV Nagler that has better eye-relief and no blackouts during the day. The result is a 23-81x parfocal zoom with 82º AFOV! The resulting image quality is very good but I would prefer the 30-104x zoom range of the 17mm ep for cr-birding, if it worked well. Tested the BS 1.6x barlow with the 22mm, but it vignetted at lower power. So found a similar barlow but 2" (a GSO produced, sold with several brand names), that resulted on 1.44x increase factor so results on a 33-116x zoom. The image quality still is good but the field became a bit too much curved to allow a well focused area to read the codes in >50% of the field.
Meanwhile I had a opportunity of testing an Ethos 17mm with this combo and, surprise or not, the image quality still is better than with the Nagler 22 and, although I don't manage to see the all 100º AFOV at the same time, I can see about 90º with my eye-glasses! See in the photo bellow how it looks like!
Test of 82º, 100º and 102º AFOV zooms (31-03-2013)
As the Nikon FSA-L2 works well with 82º and 100º eyepieces, I organized in the following text the information from tests I did to compare several solutions.
The Ethos17 deals very well with the field curvature of the TMB92 showing a completely flat field for the resulting zoom of 30-96x - I noticed by measuring the TFOV that, with complex eyepieces like this, the zoom factor of the Nikon FSA-L2 is inferior to 3.5x, having in this case a zoom factor of 3.2x. This flat field allows the use of the 2" barlow cell that results in a increase factor of 1.36 to 1.29x (over 1250mm the first lens of the FSA-L2 start to be closer to the barlow cell reducing the increasing factor - the global zoom factor result on only 3x), maintaining a very good image quality that allows the reading of codes at >85% of the field. The resulting magnifications and resolutions are summarized at the following table that also show the Nagler 22 values that result on a zoom factor of 3.3x with the FSA-L2, but the FSA-L2 also increases 8.3% the magnifications when used with the Nagler 22 i.e. is equivalent to a 20.3mm eyepiece (the Nagler is another complex eyepiece...). At the same day I obtained the following results with the Optolyth 100: 30x - 2 (scale 0) on USAF1951, 400 on EIA1956; 90x - 1 (scale 2), 1000-1100.
I prefer the version with the barlow cell (in the following photo), since it increases the eye-relief a bit, allowing me to see 85 to 97% of the field of view (with and without rubber eyeguard, respectively), as opposite to 82 to 90% of the Ethos17 without barlow - don't forget that I use eye-glasses - but also found that the Nikon FSA-L2 increases the eyerelief at the minimum magnifications since without it I only can see 55 to 62%, respectively. So now I have a 41-124x zoom with 100º AFOV, being that the TFOV is bigger than with the Optolyth 100 - at minimum magnification is +70% and at maximum magnification is +16%, even with the +37% magnification increase!!! By the way with only the Ethos 17, the increase in fields are +130% and +50%!!! - this version is more interesting when the higher magnifications aren't needed and we can benefit from it's larger field of view. I should also inform that, although the factory field stop of the Nagler 22 is wider than the Ethos 17, with this combo I verified that the Ethos 17 has a bit wider TFOV than the Nagler 22 (as I told before the Nagler results in an equivalent 20.3 mm eyepiece with the FSA-L2). Higher magnifications are possible inserting T spacers between the FSA-L2 and the barlow - using a T spacers with 17mm the resulting magnifications with the Ethos17 are 48-144x! The image quality is good but I still didn't resolution tests.
These results made me to think if would be better to have a 12.5 Nikon HW eyepiece since would not need the barlow (so with better image quality), once it is supposed to have better eye-relief than the Ethos, and the resulting magnifications could be higher, after the minimum zoom position, and might result in higher resolutions besides having 102º AFOV... It took me some months to obtain one and the following photo shows how the combo result.
In practise the HW showed to have lower useful eye-relief for eye-glass users than the Ethos+barlow cell combo since the rubber eye-guard induces higher loss of eye-relief than the one of the Ethos. Also, even without the rubber eye-guard there is no better eye-relief. Regarding magnifications, the resulting FSA-L2 zoom factor is about 3.4x, being that this combo has wider TFOVs at minimum magnification and the same at higher magnification (although the 2º AFOV difference seem higher...), but the useful fields with glasses are more reduced than with Ethos17 + barlow solution. More problematic is that the HW deals less well with the field curvature of the TMB92 so isn't possible to focus all field at once...
When testing the 2" barlow cell with the HW, to verify if there was an increase on eye-relief, I noticed that the field curvature of the TMB92 was no longer a problem (the barlow corrects it...), so did a practical test to verify If I would prefer the the views with the HW12.5 + barlow cell or with the Ethos17 + barlow cell + spacer, for having similar minimum magnifications. I seem to prefer the Ethos combo views but have to do more tests including resolution estimations... It will be comparing a 53-178x 102ºAFOV zoom to a 48-144x 100ºAFOV zoom!!! At minimum magnifications de TFOV are similar...
Before having the HW12.5, I thought of using a HW17 with the barlow but unfortunately this eyepiece most probably wouldn't be parfocal when zooming the FSA-L2...
For wider TFOV but lower magnifications / resolutions there are the Ethos 21, the ES100 20 (at Cloudy-Nights forum the ES100 25mm isn’t recommended for daylight use. If you use eyeglasses the ES100 20mm isn’t recommended since has less eye-relief than the Ethos…), and on 82º there are the Nagler 31 and the Celestron Axion 31 – although still not tested the Nagler 31, it’s assumed as one of the best eps of this focal length. I tested the ES82 30mm, that didn’t work well with my material (too much field curvature and distortion...), and tested the Celestron Axion 31 that worked great – see how it looks on the following photo!
If was today, I would purchased it for testing it on digiscopy since the image was completely flat and well corrected, the useful eye-relief and brightness were high and the resulting zoom range would be 16 to <= 57x...
On 2008 I tested 2 low cost high power eyepieces of good quality against my reference - Pentax XW 5mm. At that period I also introduced the test of resolution charts to test resolution. On this update I included a TeleVue Nagler 5mm (82º) and tried to reach a similar power with a 100º AFOV eyepiece - I used a 9mm TMB100º with a BS 1.6x barlow (used one filter cell as extender reaching an equivalent of 5.2mm. On the PF100 was possible to use a second filter cell resulting on 5 mm - on the Optolyth this solution didn't reach focus...)
The eyepieces were tested in the Optolyth TBG100 and the Pentax PF 100 (not the last 2 eps). Test was made after 5pm, with both telescopes and charts in the shadow.
The result table show that both TS and Baader eyepieces have good image quality, being good low cost alternatives to the Pentax XW. One detail not possible to evaluate is the durability and resistance of the coatings of these eyepieces. Also not objectively quantifiable is the fact that the XW goes very well with the Optolyth coatings, increasing light without compromising contrast. TS and Baader increase the contrast in the PF100 but reduce light a bit in the Optolyth. Although having a good eye relief, Baader eyepiece is not so comfortable for eyeglasses users since there is no finer eyeguard adjustment. There is a APM UWA eyepiece 5 mm 70° that seems to have a similar system of Pentax.
TS eyepiece eyerelief can be limiting for some eyeglass users and the reduced angle of view makes much more difficult to find/follow cr-birds (8 or 10º seem a small difference but the result is completely different).
The TV Nagler 5mm is the less user-friendly eyepiece, due to the reduced eye-relief, being difficult to find a comfortable eye position. In the case of eye-glasses users, like me, the reduced eye-relief is even more problematic since only allow a useful AFOV of about 60º... It also shows pincushion distortion that is more problematic in birding than in astro use.
The TMB100 is the cheaper and more compact of the existing 100º AFOV eye-pieces models. It's reduced eye-relief results from a bad conception of the rubber eyeguard that induces a loss of 2-3mm of eye-relief... For eye-glass users the TMB100 only allow about 70º useful AFOV, being that we must move the head to see the remaining field of view. For non eye-glass users it's a good alternative of the 82º eyepieces since are more comfortable although this model only allows the viewing of about 85º AFOV at the same time because if the eye becomes closer to the lens (to see all 100º), kidney bean / blackout effect appears in day-light use - to see the remaining field we must move our eye... It seems that the TV Ethos don't have these 2 problems so will try to test one in the similar way (a 8mm with the BS barlow result in 5mm) or the new Ethos SX 4.7mm, this one with 110º AFOV!
On May 2012 I managed to do a short test with an Ethos 3.7mm on the Optolyth100 but the result was disappointing since the eye-relief is too short - with glasses I can only see <50º AFOV, besides the problems the excess of power problems you can read in the following paragraph. The 4.7mm version seems to have the same useful eye-relief so should be avoided by those that need to use eye-glasses with telescopes.
In 2008, I also tested the Televue Radian 4mm - it confirmed my field experience, with these 630mm focal length scopes of 100mm aperture, that there is no gain on resolution by using a 4mm eyepiece (compared to a 5mm), and there is a great lost in field of view and light availability. On scopes of 80mm aperture with lower focal length this might be not true.
Resolution charts (24-04-08)
From reading several posts at http://www.birdforum.net/forumdisplay.php?f=286 I decided to start using resolution charts to try to standardise tests and better allow comparing different materials. Charts were obtained also through some posts of the same link, being that the EIA1956 chart was printed in the original dimensions but the USAF1951 bar chart had to be printed in landscape with adapting size to allow the total printing (in portrait I didn't found a printer that printed all chart detail...). Test was done with the chart at 32.5m from telescopes and the distance will stay the same for future tests. On the USAF1951 bar chart is not visible the 2 scale group but has lower dimensions then the 1 scale group visible in the centre of the chart. On the EIA1956 I considered the middle group of lines as going from 800 to 1400 (one side goes from 200 to 400, the other from 400 to 800). The curved field of the photo is only the result of the lower zoom position of the photo camera...
Magnifications vs. resolution (04-06-10)
I'm always searching for higher useful magnifications and I know that the increase on resolution isn't directly proportional to the increase in magnifications. I remembered to measure the resolution of several possible magnifications - see resolution charts. For that I used the same telescope (Optolyth TBG100) and just changed eyepieces.
If you compare this values with the ones of Test of 5mm eyepieces you will notice that some values changed - this might be a result of different light and atmospheric conditions, some dust in the Optolyth ocular hole (this time the Pentax PF100 had slightly better results than the Optolyth) and/or different eyesight - it varies along the day and within days and I also changed lens of eyeglasses... Interesting also to note that I obtained the same resolution values with a Nikon ED78 with the zoom, both at 25x and 75x.
CR-binoviewing (updated on 14-08-2012)
Viewing with both eyes is much more comfortable than with only one eye. Also resolution increases since 2 eyes can resolve more detail than one. The problem is that there aren't binotelescopes, with maximum magnifications over 50x, that are practical to use for cr-binding. Since there are binoviewers for astronomical telescopes I thought of using them with normal birding telescopes but that is not possible. After testing various solutions (see CR-binoviewing tests), I reached a practical solution with good Image quality and high resolution (see table below - at the same day I obtained the following results with the Optolyth 100: 30x - 2 to 3 (scale 0) USAF1951, 400 EIA1956; 90x - 6 (scale 1) to 1 (scale 2), 1000).
The resulting combination of material is not short nor light but the image quality and the confort of viewing compensate it. I usually used the HR25 since have wider AFOV and better resolution - the true field of view is similar within all tested eyepieces, for each focal distance i.e. at 103x the HR25 have the same field than the NLV40 at 64x (see the last pargraph of this section). Also at minimum magnifications I have a wider field of view with this combo than with the Optolyth100 with the zoom at 30x.
Compared to a normal telescope I just have to additionally mount the erector+binoviewer+eyepieces combo on the telescope so it takes just a few seconds more to mount it on the tripod. In the case I also take the pair of TS Superview 32mm for situations where light conditions aren't good. If the transport case would be few centimetres shorter it would be easier to transport in my cars (note that also did a bit of bricolage with the box foam...).
I had a chance to compare the combo with a Swarosvky 80 HD with the new 25-50x zoom and I prefer the views through the present bino-combo - 2 eyes and the higher magnifications are another dimension of cr-birding!!! For birding the same is true since the "bino effect" is increased with closer distances.
The summary is that views are great (seeing birds at close distances we enter in a new dimension of birding!!!) and comfortable, the minus are the dimensions and weight, and that I must refocus when zooming - on practise this can result in an advantage - when you are following a bird that is moving and approaching you, by reducing zoom correctly, you will maintain the bird in the field and focused (the opposite is also applied i.e. when following a bird that is moving away you must increase zoom).
In the last material combination I replaced the HR25 by a pair of Meade 5000 26mm/60º since found that these last ones result in a flatter field of view - see the end of the following CR-binoviewing tests. Also found that the zoom factor of Nikon FSA-L2 is reduced when using this last combo (3.25x, probably also the above ones), being the resulting magnifications of 33x, 65x, 95x and 107x (at similar FSA-L2 positions of the anterior table), and the resulting resolutions similar to the ones of the Ethos17 with the barlow, on monoviewing, although with the bino-combo the light availability is a more limiting factor.
CR-binoviewing tests (updated on 05-02-2012)
As binoviewers for astronomical telescopes don't reach focus on birding telescopes, I turned to my old C5, that with the Vixen erector lens has high quality image. As 24-8mm zoom eyepieces result in too high magnification on this telescope (52-156x), I though of using the Siebert PMW for of 0.6-2.5x (result similar to a 4.2x zoom). However I only received a 1-3x PMW and then I had to replace the 1.6x element by a 0.5x element, resulting similar to a 6x zoom! As I read on the web that one of the problems of binoviewers were that can induce increase in magnifications I had to choose one that allowed to use the maximum field of traditional 1.25" astroeyepieces. So I opted for the Baader Mark V, considered one of the best, and that is European made (easier to send to repair - binoviewers are considered not much robust...). Regarding the eyepieces I wanted the maximum field of view and not too much magnifications with the C5, so I chose a pair of Vixen NLV 40mm, that also have twist eyecups, important for those like me that must use eye-glasses. After testing several solutions with focal reducers, the best solution I tested was using a Brightstar 0.5x focal reducer before the image erector and the PMW from 1 to 3x. The result combo is a bit long and strange - see it compared to an Optolyth 100...
Regarding the image quality, it's not much good at 1x since the field is too much curved and the central obstruction of the C5 becomes visible. Other magnifications are sharp but lacks a bit of contrast, compared with the image resulting from the use of the same setup without any focal reducer. The main problem is that this solution still have much high magnifications - see the images below resulting from (1x, 2x, 2.5x and 3x, on the left), compared with the Optolyth 100 with the 80mm model 20-60x zoom that result in 30-90x (on the right at 30 and 90x). I should mention that the same combo without the focal reducer produces high quality images but with too much high magnifications, so not usable due to reduced field.
As I found that the use of the binoviewer with the Vixen erector lens result in an extra increase of 2x (3x!, in total, compared to the use of an eyepiece without both bino and erector), I tested the combo in a 600mm ED80 Celestron/Skywatcher (thanks Pedro http://celestron80ed.blogspot.com/). The 0.5x focal reducer can not be used due to lower focus range (never focus). In the opposite of what I read before, with the Vixen erector and a binoviewer, the problem is not lack of in focus but lack of out focus. This limitation was solved by using 2 Brightstar T extender rings. The resulting combo is even longer (the image below is also longer due to the use of the objective protection).
If the combo compactness is not good, the image quality is incredible good, for the resulting magnifications, and I never had so nice high power views - looking with the 2 eyes is really a big plus. However, the high magnifications was again a problem because reduces the usable field. So I have to find a <=500 mm apo-refractor, eventually with a 0.8x reducer.
Since the Nikon FSA-L2 resulted in the best image erector I tested and works as sort of 3.5x zoom, I thought I could use a Nikon telephoto lens as an scope. I tested it in a 300mm/2.8 (thanks Loureiro http://joseloureirophotography.blogspot.com/), that resulted in the more compact solution I tested - see photo below.
However the result isn't the best because the image quality suffers from lack of contrast and isn't possible to use all zoom range since the focus range of the lens isn't enough - back to an astro-refractor solution.
I opted for the TMB APO triplet 92/506 with two eyepieces Meade 5000 26mm/60º. With this combo (still big and more heavy...), I can do cr-binoviewing with larger fields than my Optolyth 100 with the 30-90x zoom, being that I'm working with magnifications from 40.5 to 142x!!!
However I detected a lack of resolution at high power (the Optolyth at 126x has better apparent definition) that I thought that might be a problem of not perfect collimation of the telescope (It spent too many time travelling...) and/or the binoviewer (It felt down accidently from 1.6m...). However I did a star test and all things are collimated - the Baader bino proved to be robust!!! After replacing the lens cell things improved much. I also found that the Meade 5000 26mm/60º eyepieces induces aberrations with this combo so returned to the Vixens that resulted on 26-92x. With the improvements made on the Nikon FSA-L2 the resulting magnifications were reduced to 21-75x with the Vixens 40mm. This mean that I have to find another pair of eyepieces - something between 32 and 24 mm - the advantage is that I will have better AFOV.
After searching the market for eyepieces that could be used and that have regulation of eyeguard (I use glasses), I ordered a pair of 32 mm Teleskop Service (TS) SuperView (52º) and a pair of 25 mm TS Planetary HR (60º). These models have Infinitely adjustable eyeguards that are better then the Vixen system that only have 2 positions. Also the SuperViews eyeguards have bigger adjustable distance than the Vixens - the NLV40 have high eyerelief and users without glasses can find the longer position of the eyeguards too short to hold a comfortable viewing.
To improve contrast and image quality I added a light reducer black ring (made from plastic from a CD box... see image below), to only allow the entrance of light needed in the FSA-L2 - it seems to work when light is abundant.
After this improvement the Meade 5000 26mm/60º also showed better image quality but is more difficult to find the best eye positions since my nose contacts frequently with the eyepieces - the TS HR are much smaller (and lighter...), so is much easier to find the correct eye positions and these are my preferred, even having a field slightly curved. In terms of true fields of view, the NLV40 and the Meades have the same value (in the Mark V), slightly bigger than the fields of the HR25 and SuperView32 (similar within both), but the differences are insignificant.
After noticing that the field of view with the HR25 is a bit curved I tested a pair of TV Panoptic 24/68º but the field still resulted curved and the reduced eye-relief didn't allowed to see all field. After re-testing the Meade 5000 26mm/60º I found that these resulted in the flatter field and the eye/nose position problems could be eliminated by not extending the eyeguards. The present combo now includes a pair of these instead of the HR25.
Questar User experience
Nikon ED82 user experience