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Birding Scope plus Barlow. (1 Viewer)

Tringa45

Well-known member
Europe
A barlow is a negative lens placed between the telescope objective and the eyepiece and effectively increases the focal length of the objective to provide a magnification increase, usually around 2x. It is often asserted that a barlow will increase the eye relief, but this is not true. For the same magnification though, one could use an eyepiece with a longer focal length and this would usually have more eye relief.

In a recent discussion on Cloudy Nights on orthoscopic and Plössl eyepieces one member suggested that the manufacturing and assembly of such tiny elements in the short focal lengths required such precision that it might be better to use a barlow with a longer focal length. This seemed plausible and according to Rutten and van Venrooij in "Telescope Optics" a barlow can also reduce field curvature and astigmatism.

I still have a few astronomical eyepieces, though I don't own an astronomical telescope. I tried out a Televue 60 a few years ago but it didn't really offer any improvement on my Swarovski ATM 65HD and I retained the 3,5 mm Nagler I used in this comparison. It was useful for resolution measurements on the ATM 65 and Kowa 883 but exit pupils of 0,5-0,6 mm are problematic for me now with floaters. The idea was that a barlow in combination with my TV 11 mm Plössl would offer more magnification than the 60x of the Kowa zoom with a still acceptable exit pupil around 1 mm. I ordered a Baader Q-Turret 2,25x barlow, which was reasonably priced.

Well, it didn't work! :( It didn't work either with a 28 mm Edmund RKE, although this would almost have had the same magnification as the 11 mm on its own. I was unable to achieve focus at about 70 m.
However, one can unscrew the barlow lens fom its housing and insert it directly into the eyepiece for 1,3x and, surprisingly this worked with reserves of infinity focus with a 7,5 mm Baader Eudiascopic for a nominal 87x although the Eudiascopic will not reach infinity focus in the Kowa on its own.
It gave me good views of the Tapezium in the Orion nebula although I could discern the A, B, C and D stars at much lower magnifications in the Kowa, and very nearly in the ATM 65 at 30x.

Incidentaly, I don't really understand the preferences of some amateur astonomers for such small increments in magnification and also that many reject large exit pupils in suburban skies and prefer to "darken" the background sky with moderate exit pupils. In suburban skies at half moon I had a superb view of the Pleiades in the Kowa with the 28 mm RKE at 18x and 5 mm exit pupil.

John
 
I have never had problems using Japanese orthoscopic eyepieces of 6, 9, 12, 18 and 25mm focal lengths.
The 4mm does have minimal eye relief.

I also used Kelners with similar focal lengths. Also 50mm and maybe 60mm.

Erfles of 15.5mm, 20mm, 25mm and 32mm.

Huyghenians from 1/4 inch to 40mm.

Plossls from the Clave 3mm upwards.
The 3mm has minimal eye relief.

All of these eyepieces are well made and I don't think that good Japanese, British, French or U.S. makers have problems making short focal lengths.

Barlows from 1.5x to 5x.

Naglers from 7mm upwards. Also 3-6mm.

RKE 8mm upwards.

I didn't have any problems with exit pupils from 0.3mm to 7mm.
Although now 0.5mm is the smallest exit pupil I use.

The magnification used varies with conditions, object, sky brightness etc.
Also this varies depending on the telescope used, the quality of the telescope and whether the mount is driven or not.

One chooses whichever magnification one finds best.

There is no set rule.

Regards,
B.
 
The effect of a Barlow depends where in the imaging chain it is placed as much as its rating (2x, etc.)...
For example I shot the Moon with the following arrangement:
Scope-2x Barlow-Diagonal-Extension Tube-Adaptor-Camera
As I said it's a 2x Barlow, but in this configuration it will be about 3.5x...
 

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  • 5DR27316_ISO1600_Stack-first_AS_P100_lapl4_ap22743_conv_RegistaxWavelets_0-4_0-29_3_ps_flipped...jpg
    5DR27316_ISO1600_Stack-first_AS_P100_lapl4_ap22743_conv_RegistaxWavelets_0-4_0-29_3_ps_flipped...jpg
    769.6 KB · Views: 50
The effect of a Barlow depends where in the imaging chain it is placed as much as its rating (2x, etc.)...
For example I shot the Moon with the following arrangement:
Scope-2x Barlow-Diagonal-Extension Tube-Adaptor-Camera
As I said it's a 2x Barlow, but in this configuration it will be about 3.5x...
The colour in the picture is extraordinary! Much more texture and variety than normal views of the moon.

Is it possible to get similarly vibrant pics with a Nikon P1000, or similar?
 
The colour in the picture is extraordinary! Much more texture and variety than normal views of the moon.

Is it possible to get similarly vibrant pics with a Nikon P1000, or similar?
I probably shouldn't have posted an image along with the comment, sorry if it was confusing. The colour has been brought out rather than visible to the Human Eye (I think this is the only time I've done that). The image is made by stacking. I give a step-by-step guide of what I did here:

Which should still be good (I update it occasionally as the software changes, does need a PC though).

I did stack a bunch of images from a RX10mk4 for someone and they came out nicely so a P1000 should be good if used on a tripod and carefully focused.
 
It is often asserted that a barlow will increase the eye relief, but this is not true.
Possible origin: aren't the huge "short" focal length eyepieces popular today for their eye relief (e.g. 3.5mm Pentax XW) essentially a longer FL combined with a built-in Barlow? (Does this end up improving FOV as well?)
 
A Smyth lens is a bit different to a Barlow and is designed for a particular eyepiece.

I think also called a negative positive eyepiece.

Regards,
B.
 
Possible origin: aren't the huge "short" focal length eyepieces popular today for their eye relief (e.g. 3.5mm Pentax XW) essentially a longer FL combined with a built-in Barlow? (Does this end up improving FOV as well?)
A Barlow would have no effect on the AFoV but I think designs like the Pentax XW (20 mm ER) or Nagler type 6 (12 mm ER) are intended to fulfil a simultaneous requirement for high magnification, large or acceptable ER, and a wide AFoV. Some time ago, Henry posted some cutaway and field curvature diagrams of Pentax XWs showing that each focal length was an individual design and that some had quite a lot of astigmatism.
A Barlow integrated with an orthoscopic or Plössl design would not make much sense, as similar results could probably be obtained with a separate Barlow.
However, there were a couple of "planetary" eyepieces with extremely short focal lengths designed to provide acceptable ER, the Pentax XO and Vixen HR.
The latter had 42° AFoV and 10 mm ER with focal lengths of 3,4 mm down to 1,6 mm! I can't understand how 1,6 mm focal length could provide any visual benefit as even with an f/5 scope the exit pupil would be a mere 0,32 mm with lots of empty magnification.
Apparently Barlows are made with a crown glass negative element and a flint glass positive element, just the opposite of a doublet objective, but that does make sense when you think about it.

Regards,
John

PS:- It seems that the Televue 2-4 mm Nagler zoom eyepiece has also been discontinued.
 
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The colour in the picture is extraordinary! Much more texture and variety than normal views of the moon.

Is it possible to get similarly vibrant pics with a Nikon P1000, or similar?
I had the P1000 and P950, a lot of fun. With someone that knows what they are doing with stacking or a star tracker I expect you could get some very nice pics after editing.
I've attached a shot of the moon from the P1000, I believe it was into digital zoom this close.
 

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    DSCN1841~5.JPG
    1.6 MB · Views: 23
A barlow is a negative lens placed between the telescope objective and the eyepiece and effectively increases the focal length of the objective to provide a magnification increase, usually around 2x. It is often asserted that a barlow will increase the eye relief, but this is not true. For the same magnification though, one could use an eyepiece with a longer focal length and this would usually have more eye relief.

The above paragraph requires some correction.
I had falsely assumed that an eyepiece was an independent entity with its own characteristics, but the objective focal ratio has a considerable influence, the most obvious being the exit pupil (exit pupil = eyepiece focal length/objective focal ratio) and edge sharpness.
A barlow can indeed increase eye relief as stated by Al Nagler on the Televue website. The effects are fairly small, in the region of 20%, so are most noticeable on longer focal length eyepieces.

I did a quick and dirty test with the Baader 2,25x barlow and a 28 mm Edmund Optics RKE on my 65 mm Swarovski scope.
With the barlow the RKE will only focus out to about 45 m, so I checked the eye relief with the focus setting for this distance, with and without barlow, by directing the objective at a diffused light source and projecting the exit pupil onto a sheet of white paper.
Without barlow the eye relief was 30 mm and with barlow it was 35 mm.

John
 
The above paragraph requires some correction.
I had falsely assumed that an eyepiece was an independent entity with its own characteristics, but the objective focal ratio has a considerable influence, the most obvious being the exit pupil (exit pupil = eyepiece focal length/objective focal ratio) and edge sharpness.
A barlow can indeed increase eye relief as stated by Al Nagler on the Televue website. The effects are fairly small, in the region of 20%, so are most noticeable on longer focal length eyepieces.

I did a quick and dirty test with the Baader 2,25x barlow and a 28 mm Edmund Optics RKE on my 65 mm Swarovski scope.
With the barlow the RKE will only focus out to about 45 m, so I checked the eye relief with the focus setting for this distance, with and without barlow, by directing the objective at a diffused light source and projecting the exit pupil onto a sheet of white paper.
Without barlow the eye relief was 30 mm and with barlow it was 35 mm.

John
BTW if it can't focus at infinity if possible reduce the length the light has to travel. For example some Barlows can be screwed into a 45 or 90 degree image erector after removing the external tube on that side, or screwed directly into some eyepieces.

I thought a Barlow usually helped with infinity focus and an extension tube or diagonal did the opposite?

Also how much a Barlow magnifies depends on where in the optical path it is. For example before or after a diagonal. I get my 667mm scope up to about 2200mm with my 2x immediately after the scope and before the diagonal (e.g. in my Moon example linked above - oh and P.S. you REALLY can't fit the whole Moon in a photograph at 2500mm equivalent - e.g. a 1250mm scope with a 2x crop camera).
 
BTW if it can't focus at infinity if possible reduce the length the light has to travel. For example some Barlows can be screwed into a 45 or 90 degree image erector after removing the external tube on that side, or screwed directly into some eyepieces.
Unforunately not an option on a birding scope.
However, with the barlow lens screwed directly ino the 7,5 mm Eudiascopic that gets me under 6 mm for a theoretical 87x on the Kowa.
In reality that's probably somewhere between 80x and 100x and a 1 mm exit pupil, which is the bottom limit for me.
If we get some clear skies sometime I might even resolve the E star in the Trapezium.
In any event it was a relatively inexpensive experiment.

John
 
Hi John,

I remember having seen Theta Orionis E (easy) & F (with difficulties) in my ED120 at around 150x in a fairly good seeing at what we call a dark site near Frankfurt (north of the Taunus ridge so the terrible light pollution from the cities and the airport is mostly hidden).
No dice for me with the ED80 at comparable magnification... an eagle eyed friend claimed to see E with averted vision.

I would like to hear if you manage E with 80mm aperture at 90x... some on CN claim to have done this... but usually with a $$$$ scope (Tak, Televue and AP were mentioned) and probably in the desert in a once in lifetime night...

Joachim
 
I have had a few barlows over the years , the televue 2x 1.25" barlow used to be my favorite . I ended up with a telecentric lens because of it's ability to maintain the eyepiece's eye relief . Televue powermates are telecentric . My telecentric lens is a Meade 2x TeleXtender , I like it better than the televue barlow I had .
Perhaps someone with more knowledge than I will offer up a bit of info on telecentric lenses .
 
Say the zoom eyepiece is 70x.
2.25x, which may not be accurate, gives 160x.

Few zoom eyepieces go to 100x, although the Mirador 70mm Maksutov has a 30x-120x eyepiece
The Yukon 100mm spotting scope has a 25x-100x zoom eyepiece.
My Japanese 80mm binocular has 25x-135x eyepieces but maxes out at 80x in terms of best image.

Regards,
B.
 
Distancia focal 600: 8 mm de distancia focal de 8-24 mm del Zoom Baader Hyperion IV = 75X

( 75X ) x 2.25X Lente barlow = 168,75X

si, tu operación es aprox.OK!!

Intento lo mismo pero con el extensor Lens ES 3X....
I Will tell you!!
 
Hi Granpoli,

Amazingly I understand your post.

A really good 80mm astro scope can be used at 225x in very good atmospheric conditions.

But I doubt a spotting scope could use this, although it is possible, especially if the focal length is 600mm or f/7.5

F/7.5 is unusual for a spotting scope, but may be common for an 80mm ED astro scope.

Regards,
B.
 

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