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Running out of focus with 2 x barlow


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Hi

I have a 4.5 Tasco and am having problems using a 2x barlow, I run out of focus travel at the scope end (even with a 25mm EP). I have a 6mm plossl that I can focus with no problems so I dont think its over magnification

The barlow is a very cheap one from an old refractor that I had and it works fine with that scope.

Any thoughts as to why this might be and what I can do to rectify the problem?

Many thanks

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Hi Symbiosis, I beleive this is a catadioptric telescope which means it has a barlow already fixed in the focuser, if this is the case it will be hard to get focus with by adding a barlow doubling the magnification once again, it will be too much for the telescope's ability and too much in UK seeing conditions, the maximum mag would be around 228x though the optimum mag would probably be around 100x.

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Thanks for the reply Robin but I'm sure my scope is a newtonian reflector (its the Tasco reflector 302012). It didnt seem to have anything pre-fitted to the focuser.

When I recieved it, it had 0.965 EP's and barlow but once I took the adaptor from the focuser it accepted 1.25 EP's. I had heard that this can be a common problem with Newtonian's but am not sure how to rectify it?

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Hi Symbiosis, my apologies, when I looked at the model online it did look like what is known as a short-tube reflector, i.e. The focal length did not look like the 900mm I thought the model had, and so I made a big assumption as in some scopes like these a Barlow or negative lens is included in the optical train somewhere to increase (usually double) the focal length. If this is not the case and you have a ‘pure’ Newtonian then all the better as this is much easier to work with. I was assuming it is the Tasco Galaxie version of the Newtonian that I believe has a 114mm mirror and a focal length of 900mm, giving it a focal ratio of f/7.89. If this is not the case please let me know the details of your particular telescope, it should be printed in a label somewhere on the scope.

There is no absolute solution to the problem I don’t think as you cannot obtain any extra inward focus (only outward with the addition of an extension to the focuser) but I hope the following will offer some 'food for thought':

a) The problem may indeed be poor quality optics in the optical train as you suggest but whilst this hinders I do not believe the quality will be bad enough for you not to come to focus, especially as the Barlow has been proven to work with the short focal length eyepiece, the 6mm plossl. The Barlow also works fine in your refractor, so not the problem in my view.

B)

From what I understand the problem is with the ‘inward’ travel, very little can be done about this, but have you tried racking the focuser all the way out and trying to see if there is another focus point somewhere along the line? I suggest you try this to see next time you are out. I presume you are trying this on astronomical targets as daytime targets respond differently! Best to focus it on one bright star to see if you can achieve focus with this first.
c) When you add a Barlow to the optical train it usually doubles the eye-relief of the eyepiece, it therefore follows that the exit pupil will also be doubled and this can be a problem as the pupil can only dilate to 5-7mm or so, less so in older eyes (like mine!). This means that the eye will not be able to take in all the light that is offered by the view and this will translate as not being able to focus. I had experience of this when I Barlowed my 32mm PanaView, it would just not come to focus, so I have come to the conclusion that it is not always possible to Barlow a low power (especially a wide-field) eyepiece, as the exit pupil is just too large for the eye to absorb.

I think c) is probably the most likely reason for the problem and the only thing I can suggest is to not Barlow but to get a 12 or 12.5mm eyepiece to get the resolution and magnification your require. A 12.5mm plossl would work well, as would the 12mm BST Exporer/StarGuider from Sky’s the Limit: http://www.skysthelimit.org.uk/telescope%20eyepieces.html

I do hope this is helpful. Let us know how you get on.

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Thanks for trying to help me with this Robin, much appreciated.

Yes, I'm trying to get this to work with Jupiter (which I'm getting fantastic views of with my 6mm plossl, so my optics cant be too bad overall). I have tried racking out the focuser to full extent in both directions but it seems to "nearly" achieve focus only at the tube end. What I'm seeing, as I'm attempting to come into focus, appears to be the reflection of the secondary in the primary.

Its very frustrating as I have recently bought a 9mm EP and had hoped to get (probably the max for my scope) a 4mm use out of it with the barlow

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The shadow of the secondary is sometimes a sign that the mag you are endeavouring to use is too low for the optics of the scope, as well as focus issues, however, the 25mm ep should be perfectly useable in your scope. Rather frustrating I think. I have used a 114mm telescope some time ago but don't recall having such problems, mind you, I didn't try it with a Barlow either. Perhaps if you can borrow a 20mm that might be intersting to try, do you belong to a club or anything?

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I think it's important to document the solution to these cases (for those that my have similar issues in the future), so with that in mind....

I borrowed a friends 2 x barlow that came with his 6" Reflector tonight and found I could focus perfectly with it. I even used my 6mm plossl in it and got some good very close up views of the moon (even through a rainy sky).

It would appear that all barlows are not alike - I knew this was the case in terms of quality but didn't know if this was the case in terms of design

Comparing them side by side, this is what I found:

Two differing barlows with lenses in different positions 1

As you can see, the lenses of the barlows are in different locations - one being at the very tip of the barlow and the other nearly half way up. You can also see this here:

Two differing barlows with lenses in different positions 2

This is the one that worked with my reflector:

The 2 x barlow that worked in my reflector

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How odd. I've not seen a barlow with a lens in that position before (I don't get out much :)

All mine have the first lens group within a few millimetres of the end of the barrel (I have a Revelation 2x, Ultima 2-ish x, Revelation 2.5x, Tele Vue 3x, ImageMate 4x and Revelation 5x).

James

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How odd. I've not seen a barlow with a lens in that position before (I don't get out much :)

All mine have the first lens group within a few millimetres of the end of the barrel (I have a Revelation 2x, Ultima 2-ish x, Revelation 2.5x, Tele Vue 3x, ImageMate 4x and Revelation 5x).

James

Nope, I dot think I do - no wonder i was having problems! The joys of being a begginer I guess - any recommendations on a decent 2 x barlow James?

PS - nice to hear from a "local"

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Nope, I dot think I do - no wonder i was having problems! The joys of being a begginer I guess - any recommendations on a decent 2 x barlow James?

PS - nice to hear from a "local"

I'm happy with my 1.25" Revelation 2x barlow, but I can't recall seeing it for sale outside their eyepiece set recently (though there is a 2" version). The TAL 2x barlow is also spoken of highly but seems to be made of unobtainium this days.

The Celestron Ultima 2x is also very well regarded, but is more like 2.3x and not being sold new any more, as is the Orion Shorty 2x (same hardware, different brand).

If a 2.5x would suit then the Revelation 2.5x barlow is very good for the money. I don't tend to use barlows for visual, but I use this one heavily for planetary imaging and drop back to the Ultima if I can't quite get the quality of image I'm after.

You'll probably find there are quite a few "East Devon" SGLers hiding in the woodwork :)

James

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