2x Barlow question

[smp]

I have been using my TV-85 with a TV 32mm Plossl eyepiece for (kind of) wide field viewing.

Last night, I decided to try my old Celestron 2x Barlow, in order to get on an object (Saturn) and then see the 2x view. I've never really used this Barlow before - it came in a Celestron kit of eyepieces & filters that my wife gave me as a gift many years ago. It worked, but also provided a few issues.

First was the image shift. I was surprised to see that my image shifted quite a bit. Still in the FOV, so not too much of a problem, really. Then, out of focus. I suppose that ought not to be a surprise (maybe?). Last, I felt my view was degraded. A bit more fuzzy than it was, or perhaps a bit more scintillation around the object.

Is this to be expected? Are these things that occur with cheap Barlows? If I were to invest in a "high quality" Barlow, would these issues not occur, or be much less noticeable?

I realize that putting more glass between the object and my eye will degrade the image, but these things were a surprise to me.

Your thoughts and advice are welcome!

smp

[Shabadoo]

I have a 2x and a 5x that I use for lunar and planetary viewing, I have found that the movement out of the field of view increases, and that the point of focus is very, very fine. In other words, on focus, when you achieve it, you are just a twitch away from being out of focus.
It makes it difficult to share the object with another person.

[Don Alvarez]

Last, I felt my view was degraded. A bit more fuzzy than it was, or perhaps a bit more scintillation around the object.

Is this to be expected? Are these things that occur with cheap Barlows? If I were to invest in a "high quality" Barlow, would these issues not occur, or be much less noticeable?

I own two barlows, a Celestron Omni x2 and a GSO 2.5x APO. I can tell you there is a noticeable difference in performance between the two. Not a night and day difference, but noticeable. Typically though, I don't barlow. I'd rather have an EP with the focal length to do the job. I really only like to use barlows with my camera for taking planetary pics.

[notFritzArgelander]

The TV Powermates and Baader 2.25x Barlows are pretty good. I don't remember which cheaper Barlows I have since they were relegated to the "do not use" bin.

[smp]

Thanks very much, guys, for your thoughts and advice.
I'll be looking into (yet another) TV accessory.

smp

[Richard]

I only have a 2 x Meade Barlow , I have not used it for at least a year now but dont remember any focus shift while using it, I used it last on a WO 80mm Refractor , but as I sold it it has no use for me with my current scopes

[Lady Fraktor]

I have been using my TV-85 with a TV 32mm Plossl eyepiece for (kind of) wide field viewing.

Last night, I decided to try my old Celestron 2x Barlow, in order to get on an object (Saturn) and then see the 2x view. I've never really used this Barlow before - it came in a Celestron kit of eyepieces & filters that my wife gave me as a gift many years ago. It worked, but also provided a few issues.

First was the image shift. I was surprised to see that my image shifted quite a bit. Still in the FOV, so not too much of a problem, really. Then, out of focus. I suppose that ought not to be a surprise (maybe?). Last, I felt my view was degraded. A bit more fuzzy than it was, or perhaps a bit more scintillation around the object.

Is this to be expected? Are these things that occur with cheap Barlows? If I were to invest in a "high quality" Barlow, would these issues not occur, or be much less noticeable?

I realize that putting more glass between the object and my eye will degrade the image, but these things were a surprise to me.

Your thoughts and advice are welcome!

smp

Some barlow/ eyepiece barrels will fit a bit loose in the diagonal/ adapter tube so the image shift can be normal. Does yours use compression ring or set screws?
Also remember you were using an eyepiece which provides 27mm FS with almost a 4.5° ExP down to a barlow 18mm -21mm clear aperture 2.3° ExP which may have been (possibly) slightly vignetted by the smaller clear aperture of the barlow.
Try with a Plossl in the 20mm range and see if the result is the same.
Most barlows are not parafocal so some refocusing is to be expected.
A high quality barlow will disappear from view, meaning it does not add any aberrations into the view from its use.
With modern coatings and glass manufacturing using a high quality barlow will result in very minimal light loss. (Older high end ones will not either)
With shorty barlows there is no advantage to a two lens or three lens barlow until you get above 5x magnification. The third lens is then used to correct aberrations of the light cone.
Roland (Astro-Physics) made the comment one time that if there was an actual advantage to using three lenses in a short barlow why have none of the high end manufacturers made them?
PowerMates work well but they are also technically not barlows and there are some eyepiece designs that they do not play well with.

[smp]

Thanks very much for this info, Gabby!
Very much appreciated!

Right now, I'm kind of torn between simply getting a TV DeLite 18.2mm to do pretty much the same as a 2x Barlow, or start experimenting with Barlows.

I have a hidden constraint. The TV eyepiece has the proper connection to my new Night Vision monocular. The 2x Barlow was a very inexpensive way to go to 2x with my 32mm eyepiece. The TV 18.2mm DeLite is on the order of $250, and the TV 2x Barlow is on the order of $130. Switching the 32mm for the 18.2mm would essentially not put anything more in between the object and my monocular. Adding in a Barlow would add more glass into the train.

Lots to think over before making my next move.

Thanks again for the info!

smp

[Lady Fraktor]

One thing I will mention from my compression ring/ set screw question.
Once you start getting into shorter light cones (i know f/7 is mid range) alignment of the optical train becomes more important.
A set screw can tilt or push an eyepiece off center, a compression ring may not really be centered.
I have an eyepiece that I had to put a layer of electrical tape around the barrel to remove the excessive slack between the barrel and focuser tube.

[Bigzmey]

Stock/budget barlow definitely degrade image quality. Loss of contrast, fine details and sharpness are fairly common with those.

A good quality barlow like Televue 2x can actually improve the views and I feel that you should have at least one in your EP box.

As for which to get first TV barlow or 18.2mm Delite is a tough call. I like both. 18.2mm Delite is such a nice match for F10 SCTs for DSO hunting, since it delivers exit pupil close to 2 mm.

Maybe you should wait for good deals around Christmas. I got my TV 2x on Christmas sale a few years ago.

[smp]

One thing I will mention from my compression ring/ set screw question.
Once you start getting into shorter light cones (i know f/7 is mid range) alignment of the optical train becomes more important.
A set screw can tilt or push an eyepiece off center, a compression ring may not really be centered.
I have an eyepiece that I had to put a layer of electrical tape around the barrel to remove the excessive slack between the barrel and focuser tube.

Yeah, the Celestron Barlow has a single set screw, so that could indeed be the culprit of the image shift. (All my TV stuff has compression rings, but even there, there is only one set screw.)

Thanks very much for all your thoughts and advice.

smp

[smp]

Stock/budget barlow definitely degrade image quality. Loss of contrast, fine details and sharpness are fairly common with those.

A good quality barlow like Televue 2x can actually improve the views and I feel that you should have at least one in your EP box.

As for which to get first TV barlow or 18.2mm Delite is a tough call. I like both. 18.2mm Delite is such a nice match for F10 SCTs for DSO hunting, since it delivers exit pupil close to 2 mm.

Maybe you should wait for good deals around Christmas. I got my TV 2x on Christmas sale a few years ago.

Thanks for your thoughts, Andrey. Yes, all this gear is quite useable with my C6, also. I'd sorely love to get my hands on that DeLite, but I'm a bit down on budget after all the spending I've been on with the NV setup. Good advice to hold on a bit and see what deals my come about between now and Christmas.

smp

[Lady Fraktor]

Depending on the height of the eyepiece barrel undercut a set screw may seem tight but is just pressing against the edge of the groove.
Moving the telescope around can cause the eyepiece to shift against the screw.
Issues when things like useless undercuts are not a standard height.

[smp]

Depending on the height of the eyepiece barrel undercut a set screw may seem tight but is just pressing against the edge of the groove.
Moving the telescope around can cause the eyepiece to shift against the screw.
Issues when things like useless undercuts are not a standard height.

Ha! I never thought about that with the undercut. Of course, my TV 32mm Plossl has an undercut with beveled edges, so I'm going to have to see where it seats, and if the set screw edds up hitting against a beveled edge. That could lead to the image shift issue, for sure.

Thanks again for all your thoughts and advice. Greatly appreciated.

smp