Are there any significant disadvantages to using a barlow

[jnb]

Are there any significant disadvantages to using a barlow? I am coming over to the idea of getting a fast newtonian which I could use for DSO work and then adding barlows for planetary or lunar work when required. Are there any disadvantages I should consider?

As far as I can see then only disadvantages are a slight light loss but that would more than offset by the increased aperture I can afford with a newtonian. A possibly more significant issue might be mechanical issues from using a barlow and so extending the focusser, for example a fast newtonian with filters and dslr hanging off of it might be a high load to hang off the focusser if it's then extended by a barlow.

[nmoushon]

The barlow with multiple the f/ratio by its own factor. So a 2x barlow will turn your f/4 into a f/8. For planetary and lunar thats not a problem. With DSOs it is.

There's no light loss that I'm aware of. All it does in extends your focal length.

The barlow will push your equipment out more and put more strain on your focuser. Though a simple 2x barlow doesn't extend it that much so I would worry to much about it. You will have to make sure you can still get focus with it though, which Im sure you can. 

[Pig]

I don't do any AP so I wouldn't know about the effects.

I think the very cheap Barlows can be quite detrimental to views.

Most people seem to be of the ilk that the pro's out way the cons. IE halve the focal length of each eyepiece for a 2X Barlow (2 for the price of 1) and extra eye relief.

[Moonshane]

a good barlow will give approx. the same quality views as an eyepiece effectively of the same length. I personally don't use barlows as I find swapping them over a bit of a faff. they also generally multiply the eye relief by a factor the same as the barlow so e.g. an eyepiece with 20mm eye relief will have 40mm eye relief which can be a problem for some eyepieces but help with others (i.e. those with short eye relief). as above some affect the focus position to some extent. if thinking of imaging, consider a 2" barlow.

cannot comment on imaging really.

[Charic]

jnb.......Hi,  My take on this subject is as follows......The disadvantages are more glass in the optical path, which could pose a problem with the amount of light that passes, unless you buy a quality fully coated Barlow compared to the cheaper plastic variants. They can over power your lowest focal length EP's, and as said above, will increase your f/ ratio, due to the  Barlow increasing the telescopes focal length, which then divided  by the Aperture, gives a higher f/ ratio.

The advantages are, you can virtually change your telescopes focal length by the factor of the Barlow. This allows you to use one eyepiece at two different telescope focal lengths, almost like having two telescopes in one. That also  effectively 'doubles your EP collection',   by the fact the focal length has changed?

This increase divided by the EP focal length determines your power/magnification. My scope is FL-1200 mm and on an 8mm gives me 150x power. If I insert a 2xBarlow, my telescope is now virtually ( because the Barlow cant increase the length of the telescope by physical means) a F-2400 mm telescope, and on an 8mm lens now gives 300x power [ that's your doubling of an  EP? One EP, 2  magnifications ] its still an 8mm EP but with the eye relief of the  8mm (if not slightly greater)   and due to the magnification increase, the field of view will normally decrease as you get into higher magnifications, but your 8mm may have an already wider view in the telescope, than for example trying to use a standard 4mm EP. I've found that my 2.5x is just a little to tight now, and also discovered that Revelation have a 2" Barlow with a 1.25" adaptor. Now I can Barlow  both 2" & 1.25" EPs should I desire, whereas the one I own at the moment wont accept the Panaview 32mm.

[Merlin]

A problem with some Barlows is that dust on the the lens is very evident in the field-of-view. I don't seem to have this problem with the TALs, but definitely with the MEADE.

[Tonys]

Another potential benefit is that because the barlow effectively increases the scopes focal length the acuteness of the light rays entering the eyepiece is reduced, so it's possible the eyepiece won't have to work so hard, so to speak, resulting in a bit better performance. I personally really like Barlowing for higher magnifications, as I struggle with the short eye relief of my short focal length plossls. Another advantage with my Revelation Astro one is that the lens holder part can be unscrewed and a filter inserted between this and the barrel, which saves having to unscrew the filter every time I change eyepieces when using my Newtonian. Quality does matter - my Revelation one shows slight colour fringing on bright objects but my Tal shows none. However the Tal does impart a slight yellow hue.

[ronin]

A barlow is firstly extra glass, so a bit more reflection, a bit more scatter, bit more absorbtion all minimal but present.

They will be generally a negitive achro lens in there, so a bit more CA from that, especially as they will be short focal length. If not at least an achro then really throw it.

Next they are not designed for your scope, they are likely not designed for any specific scope. You don't actually think it has been specifically designed for the radius of curvature of the prime image that your scope delivers? They will not be designed to deliver an optimum image to the eyepiece either.

Something like a plossl, 4 elements, will have edge distortions, we accept that, something like an ES will have much less if any, same for the top end TV. They have multiple elements to take care of these edge distortions. A barlow has 1 or 2 elements - I am willing to bet that edge distortion is not considered at all in most.

Consider this: An achromatic lens is a positive and negative lens combined to deliver you an image that is relatively free from CA. That does not mean that any 2 lens one +ve and one -Ve of different glass types can be put in holder and be called an achromatic lens. So why does picking up any barlow and putting it in the optial path make people think it will work perfectly occur?

If you want a good one then look at the TV barlows, I guess Al takes care in his work and what comes out, but they will cost you about £100, better still for performance get a Powermate, they are £250+.

[itmo]

Biggest disadvantage to barlows is that a bad barlow will turn anything white into a french flag with CA. On the other hand it seems that good barlows (like my televue) arent that expensive compared to cheap ones and they actually improve the image,because they make the light cone easier for the eyepieces to handle. Most of the modern expensive eyepieces have barlow like elements as the first element the light hits. 

[andrew63]

It depends a bit on type of barlow and eyepiece type, but you can find the image does vignette - or give a fuzzy edge. You end up loosing some field of view.  I guess it depends on what you want to compromise on ? 

andrew

[JamesF]

Adding a barlow into an imaging system may extend the exposure time significantly too.  A 2x barlow might mean you need four times longer exposures, for instance.

James

[allen g]

I use my TV 1 2/4 2X barlow with my TV 10 ethos and 8 & 6mm delos, which then gives me 5, 4, & 3mm. I find no difference than EPs at the latter

3 set. I have done the comparision along with others at star parties and no one notices any material difference (of course no difference could be

a function of the quality of TV). thus I have 3 instead of 6 EPs. the only thing is that it is easier to insert a single EP than attach and a barlow to an EP and then insert. Some don't like messing around when changing magnification and others prefer not carrying (and purchasing) that many EPs.