Barlow and expectations

[Dujon]

I am considering purchasing a barlow for bright planetary observing. Having never used one of these devices I'm wondering what benefits or demerits might attach to their use.

I might add that I've had a bit of a look through the site before asking this question but there doesn't seem to be an answer (or I've missed it).

Firstly: What I'm expecting.

1) A slightly less bright image (which is a positive in some respects, particuarly as I found the moon filter helpul last evening when viewing Jupiter).
2) A slightly larger image.
3) Possible distortion due the extra lens(es) introduced into the light path.

Secondly: What I don't know.

1) When a barlow is described as 2x or 3x (or whatever) is this linear or 'volume'?- I suspect the latter but am unsure.
2) Is chromatic aberration likely to be a problem on brighter objects?
3) Given that I have a 2" Crayford focusser, is it better to invest in a 2" barlow (with adapter) rather than the 1.25"? I ask this because at this point in time I only have a 26mm 2" eyepiece (don't you just love the mixed measurements!) the other two being 1.25" plossls (15 & 9mm).

I am considering purchasing an el cheapo in order to work out whether or not this is the way to go (rather than investing in a dedicated eyepiece), so if anyone has an unused unit that they are happy to part with - cheaply - I'll think about it.

Initially though, your various advice would be much appreciated.

[janoskiss]

John, A good barlow will not noticeably dim the image for a given magnification, e.g., 7.5mm Plossl, and a 2x barlowed 15mm plossl should be as bright as each other. But the higher power view will be naturally dimmer as you have the same amount of light spread over a larger area.

No, a good barlow won't produce any "distortions" of the image.

The magnification factor refers to the size of things, e.g., Jupiter will have 2x the diameter when you place a 2x barlow between EP and focuser, compared with the unbarlowed view.

Chromatic aberration is not an issue with good barlows IME. Having owned the shorty-plus and the meade #140 3-element apo barlows, and three 2-element barlows that easily outperform both of these, I believe that the "apo" barlow concept is mostly a gimmick.

2" vs 1.25" is up to you, but the 2" gets a bit awkward with 1.25" EPs when you have to fumble around in the dark with the big barlow and one or two 2-to-1.25" adapters. But if you have 2" or 1.25"/2" hybrid EPs (like Stratus, Hyperion or LVW) that you definitely want to barlow, 2" barlow might be the way to go.

You can get an el cheapo GSO 1.25" for around $20 from Andrews, but chances are you will be disappointed. If you're getting it because you want higher power now but cannot afford to spend any more, then grab one, but if it's to see if you would like barlows then don't. For that I'd recommend no less than a televue barlow.

My recently acquired 1.8x 1.25" televue barlow is a very good one - I assume the newer 2x would be at least as good. Antares 1.6x is a good 2" barlow and fairly compact - I got mine from Scopestuff. The 1.25" Televue is just a little better at controlling stray light from bright objects, and has visibly better barrel blackening and lens coatings.

One tip: You can increase the power of barlows using a barrel extension. E.g., the Antares 1.6x becomes a 2x with a 2" extension, or a 2.4x with a 4" extension without any noticeable degradation in performance. So a lower power barlow is desirable IMO because you can always up the power with an extension tube, but you cannot go the other way.

[Dujon]

Thanks, Steve, that's given me more than one thing to think about.

You are right: the budget won't stretch to a Televue at the moment (around $180) but I am in fact trying to get an idea as to whether or not the Barlow is the right way to go.

With the two 1.25" plossls I presently own the barlow 2x will give me the equivalent of 7.5mm and 4.5mm eyepieces. A 3x would give me 5mm and 3mm - which might be a better bet. The latter could be a right pain when it comes to keeping the subject in the eyepiece but it could be handy on those rare occasions of perfect (?) seeing.

It's a right quandary I must admit and I guess most of you have pondered over similar situations in your observing lives, so in that respect I'm sorry (but most appreciative of your reply) for taking your time.

I'm tempted to buy a cheap unit, such as the one you mentioned, just to get an idea as to what I might gain from a decent Barlow. It might be a couple of months before I could afford to buy a quality unit as I'm in the process of researching a replacement for my current vehicular transport but think that I could outlay a few dollars prior to that investment in order to test the water.

Again, thanks, Steve.

[Starkler]

That sentence contains a contradiction.

There is no cheap barlow that I feel offers decent performance.

IMO you are far better off buying a good 1.25" barlow instead of a mediocre 2" unit.
Cheap barlows such as those made by GSO have inferior lens coatings, resulting in light scatter and reduced contrast. This is noticeable when viewing the moon and bright planets as a bright background and gives a view analogous to driving at night with a dirty windscreen.

[ving]

and dont forget that you can only go so far with planets before the image degrades... using a 6mm eyepiece and a 2x barlow in your 1500 fl scope will wash the image out. you wont see the detail you are seeking. there is a maximum recommended magnification for your scope so keep that im mind.

[xelasnave]

Would not an Eyepiece of a shorter F/L be a safer bet?
I have a 2x and a 3x cheap ones I guess and they work in certain situations but after the novelty wore off I rarely use them for viewing and they dont get past siting in the guide scopes these days. Probably the fact that they were cheap creates my lack of passion for them.
alex

[janoskiss]

I agree with Geoff, cheap barlow will not give you any indication of how a good one will perform.

You will get a lot more use out of a 2x barlow than a 3x, which will give too much power most of the time. (and if you want more power than 2x use a barrel extension tube)

Shorter FL EPs that are comfortable to use and work well in an f/5 Dob will cost at least as much each as a good barlow.

[ving]

of course it goes without saying that if you are using the barlow for astrophotogaphy then its a must to get image scale... hope we arent confusing you...

[Dujon]

Pedant alert . . . Pedant alert . . . ;)

Gerrof, Starkler, you know full well what I meant. Anywhatsit, both yours and others opinions are telling me that I have to bite the bullet and either buy a decent Barlow or a dedicated eyepiece and not even entertain the thought of a cheap item.

It's probably going to be money well saved. At least I now have the impression that there's nothing wrong with (decent) Barlows and can put that money I might have spent into a 2x of quality.

Thanks, everyone.

[Starkler]

That will teach me for posting replies in my lunch break when Im hurriedly scanning the forums and skimming posts.:shrug:

[Dujon]

Thank heavens you have a sense of humour, Starkler. I was thinking about my post last night and began to worry that you might take offence - me being a newcomer and all. :doh:

Incidentally; the only reason I asked my question was because 20 to 30 years ago Barlows tended to be frowned upon - presumably because of the quality of the offerings and the size of affordable 'scopes at the time.

[Harpspitfire]

i use the Antares 2X (better model) which is the same as the celestron ultima and parks gold- i doubt youll see a differecene on low power barlowing, but on high power image degrade is alot better using a better barlow- i think this more an ER issue if you use barlow, i use the ultima and elite EP,s for high power but needed something in the 7mm range- i got the apgoee 7.8mm abbe ortho form apogeee inc- $39 or $49 USD- it looked cheap being lightweight and all plastic- but visually its a super high power EP

[dugnsuz]

Don't go for a GSO ED barlow for planetary work - lots of reflections from brighter objects.
Cheers
Doug

[janoskiss]

Most barlows will produce reflections to some degree. The only ones I found that do not are the UO 2" 2x barlow and a Televue barlow.