Replacement Barlow.

[John]

Cheers John, many thanks.

Because the field lenses in my BST's are all different distances from the end of the tube that they sit in, would it be the same that its not exactly 2x Barlow when used as a Barlow?

Thats a good question and I don't know the answer to be honest with you. To add to the complexity, not all barlows are quite what they claim to be in terms of amplification. The Celestron Ultima 2x for example has been measured at 2.2x. It's a quality barlow all the same.

My guess would be that the amplification that the barlow element will give is based on it's distance from the optical plane of the eyepiece, thats where the field stop that defines the edge of the field of view is placed. If the BST's are more or less par-focal then the focal plane position within them is consistent despite the varying position of the field lens set. 

Eyepiece optical design is somewhat more complicated than scopes I reckon ! 

[Charic]

....too true.

There's so much to learn and to try and understand!  Wiki is telling me that a standard Barlow lens is housed in a tube that is one Barlow focal-length long, and if a focusing lens is inserted into that Barlow tube, separation between the lenses  will  one Barlow focal-length, and as such produces a 2x Barlow magnification. If I double the Barlow tube, the lenses are now separated by 2 Barlow focal-lengths giving 3x Barlow magnification. So I assume by separating the lens cell from its tube and attaching directly to an eyepiece, there will be a loss of  Barlow magnification, as you have already described, but by how much, that's my next search. I'm not certain that my  EP's are fully par-focal, as I need to adjust them slightly.  Maybe I should lock the focuser, in-case its me being a little to ham-fisted.

I also came across this link,  as you mentioned about field stops and calculations. http://astunit.com/astunit_tutorial.php?topic=barlow   It agrees with your comment, in as much as the stated magnifications of Barlow's are generally nominal?

[AlexB67]

Just to add to the pot, I have the skywatcher barlow, the 1.5x element mode does not barlow well.  Essentially having it much closer to the eyepiece asks more of he barlow optics to maintain good correction, or sharpness of field.  The same is true of shorter versus longer barlows. That is not to say a good short barlow cannot work well when optimised for that specifically, but generally it is true that the adapter barlows will involve compromises and work best at the higher magnification multiplier.

My barlow in 1.5x mode it really does begin to fall apart in terms of sharpness and clarity of image when I screw it onto my pentaxes and TVs. I notice it in terms of off axis performance, planetary details degrading and so on. The one exception to that is my ortho where my 1.5x works out a bit better, but I think that is in part because the field stop is quite high up, and even in 1.5 mode works more like an 1.7x, as well as with an ortho you are not asking for that much off axis field with only 42 degrees FOV.

In 2x mode considering it is a basic cheap one, it works remarkably well, It is not one of those premium barlows and I can tell it is there, but very useable.

Even the more illustrious more expensive barlows like the baader modular, the baader classic 2.25 also have this problem form reports I read. Noting that the image begins to degrade when used in 1.6x mode or less, but work better in 2x mode. Your mileage may vary as they say and it may not sufficiently bother you, it will depend a bit on scope and eyepiece combo and how fussy you want to be. 

If you want a good barlow that is dedicated for a nominal magnification of around 1.5. I would think about the Antares 2 inch 1.6x, very decent quality for the money I gather, without having tried one but it gets great reports everywhere.  The 1.5x adapter would do as a sort of temporary solution I feel, but for best performance I would not use it in the long run. At least,  it is what I am finding anyway having put it to the test for some time now, and reports elsewhere and the basis of how the optics work in barlows seem to fit with what I am finding/observing with my current one. 

[Charic]

Cheers AlexB67........this perfectly shows a situation, that for  person "A" works very well, yet  for person "B" not so well? I have found that the nominal '1.5x' lens is my preference when using the 5mm and the 8mm. I`ll be doing some more tests, if / when the skies clear, but have found myself just using the lens cell in preference to the Barlow tube, and my reason for wanting another 1.5x cell?

I too have seen references about the Antares 2" 1.6x, as the nearest thing to a 1.5x.  My problem there is, all my EP's except the panaview are 1.25" EP's. I dont  need to Barlow the 2" panaview (yet?)

My own eyes are part of the equation too, The kit here might just be perfect, its just the way I see things,  but it just goes to show how the same equipment works at different levels / standards for different  folks!

[SunBear]

Just sold my Revelation 2.5x Barlow, too powerful for Jupiter on my system on anything below 8mm?

Is there any benefit in buying two separate Barlow's  or just  a 2"  Barlow with the 1.25" adaptor.

I still feel the need  for a Barlow for extreme work on the Moon, but mainly  to Barlow my laser when tweaking the primary out in the field. I thought I'd get a way with the plastic Celestron 3XBarlow..........lol

The new Barlow will be 2x 

Hi Charic,

What is the advantage to Barlowing your laser for field tweaking?

I would think it would tend to distort the beam.

[John]

Hi Charic,

What is the advantage to Barlowing your laser for field tweaking?

I would think it would tend to distort the beam.

It does distort the laser beam and thats why it's very useful for collimation. Have a look at this:

http://www.smartavtweaks.com/RVBL.html

[Charic]

SunBear......Hi, John got here first, but Barlowing a Laser is a very effective and quick method of setting your primary mirror. It also does not matter if the Laser is accurate/collimated, which is why I sigh when folk throw away their lasers? If you Barlow a Laser it diffuses/diverges the Laser beam which strikes the primary mirror and creates a reflection of the Mirrors centre Spot, which is most clearly visible back at the laser face. You then just adjust the mirror until the grey reflected circle is centred ( instead of chasing the laser dot into a hole) on paper, It would appear to be an easier method, and having used this method for some time now, I  totally agree with it. Also because I cant adjust the primary mirror and look through a Cheshire at the same time, I prefer to use a Lasered Barlow when out in the field, when I have no assistance.

The Laser is not effective for everyone though. If you have a short tube Newtonian that has a corrector lens (Barlow) permanently fixed inside the base of the focuser tube, and you try to collimate the whole telescope using a Laser, then you will have problems from the start ( Its at this stage that you could be trying to adjust the secondary mirrors tilt )  due to the divergence of the laser beam, you cant put a laser spot onto the primary mirror unless you remove that  corrector lens ( which is not easy or practical) however, if you align your secondary mirror with a Collimation cap or a Cheshire sight tube, then you can still use a Laser for the final  primary adjustment only.

Most,  if not all, short tubes with a corrector lens (Barlow) installed, you will find the primary mirror does not have a centre spot (like my old Celestron 127EQ) so for the laser method to work, you would need to remove the mirror cell, and spot that mirror. I did just that to the Celestron. My Skyliner was already marked.

[Charic]

.........Just to reiterate, for those contemplating throwing their lasers away. If you are setting the primary mirror on your Newtonian, It does not matter if the Laser is uncollimated or loose fitting in the focuser tube. Its not the Laser spot that is important here (If there were any inconsistencies you can see them by rotating the laser and watching the dot rotate around or across the spot on the primary mirror) What is important here, is that the divergent laser illuminates the mirrors surface,subsequently, the mirror spot is illuminated and reflected. If you were to move the laser now, you will see that the reflection of the spot/donut does not move! It only moves by adjusting the screws. so there is your accuracy with the mirror placement. So simple.

[SunBear]

Charic & John,

Thanks so much for the info. I never would have thought!

Went and tried it and it worked like a charm. I can clearly see why you like it.

I think all beginners should learn this as part of their basic repertoire. (At least as soon as they recover from collimation panic.)    -- Dave

[rwilkey]

Cheers rory......seen that one, its  probably twice what I want to pay for at present, and I know, at the end of the day, you only get what you pay for, with regards to quality. The 2.5 I had was good. I think it was your recommend to me. But I only really used it on the Moon, as it was too powerfull for anything else planet wise, especially Jupiter, but now its gone, I cant Barlow my laser and dont have the 300x + power for the Moon when needed?

Edit. Having the Panaview brings an added dimension, so maybe get a 2"  2XBarlow with 1,25" adaptor. Still thinking about the Revelation version.  Just wandering if there is any reason not too? 

Hi Charic, the PanaViews do not Barlow very well, so don't base buying a 2" Barlow for use with the PanaViews.  I don't think widefields Barlow very well anyway, at least that is my experience.

[Charic]

.........cheers Robin, I think you've told me that before in another thread and I took on-board your advice. SunBear has quoted a reference to me from February!. I`ve no intention or need to Barlow the Panaview.

I was probably thinking, that If I purchased a 2" with a 1.25" adaptor, I would have both options covered if I ever needed another 2" of a higher magnification. Unless someone forces me to look through a TeleVue and I win the lottery, I think I'm done with buying eyepieces. especially for the price I paid, and the quality they produce.