Barlows and F5 scopes

[Mark Foster]

Hi guys

I have a f5 Meade Mini Lightbridge 130 (and very nice it is too)

Planning to use it to look at Jupiter over the next few weeks - clouds permitting

I have a 23mm Revelations, a 26mm Meade 4000 and a 18mm Celestron Xcel and a long x2 Meade Barlow 

I was planning to Barlow the 18mm down to 9mm.

 

2 Questions

1. - Will the image be dimmer - narrower? compared to a a 9mm eyepiece?

2 - And if I was to buy one other eyepiece what would people recommend in terms of mm for this f5 scope - a 12mm - a 9mm - 6mm?

 

Looking forward to your suggestions

 

 

 

[Charic]

Based on focal ratio, I would take the 12mm?

A 5mm suitably matches the scope, a 10mm or nearest would be the next in line. anything below 5mm on its own would be too powerful with no real benefits to the image, and very little use,  but anything higher that can Barlow down less the 5mm may be of some help, if/when those special seeing conditions allow.

The 6mm could Barlow to 3mm, Ouch! ok on the moon maybe, just too much to be of general use. The 9mm could give 4.5mm or an 8mm producing 4mm when Barlowed. Unless you experiment and try for yourself, your eyes, you'll never know, and no matter how much experience other users and I have, its all down to your scope, setup, seeing and the quality and experience of your own eyes. I have even 2x Barlowed  a 3mm giving me 800x on my scope! thats not possible, is it? I could still see details on the Moon, but the deterioration of the image was clearly visible. 

I still suggest matching then doubling the focal ratio in order to select the first two eyepieces then Push as far as you want/can, but most seasoned observers will advise that over 250-300 will be your limit for uk skies.

[ronin]

F/5 and 130 gives 650mm FL.

The barlow and 18mm will give a magnification of 72 and should be OK to see Jupiter and 2 bands and possibly the lower one as well. OK it might be the upper one but when I looked through mine it was down towards my feet, no idea where it was on Jupiter. Basically I could just make out 3, but it was not easy. One aspect is as it is a reflector it is a bit less sharp until you claw back from the aperture side, and a barlow will not add sharpness.  With glass it is often easier to keep it simple.

I would suggest the 8mm Starguider for 81x, I was using one and it simply does things well. You will get higher magnifications but the result may not be as good. Wonder what an 8mm TV plossl would do?

In the same scope the 8mm BST performed better then the 6mm Altair - which appears a clone of the WO 6mm planetary= identical specification and body shape, and the 8mm BST was a match in image quality to the 12mm BST and it gave a bit bigger image.

Will prewarn you that when Saturn appears at a humane time then you will want or need around 120x to see that reasonable, and that is 5mm to 5.5mm.

 

[Alfian]

Its sometimes useful to remember that a barlow not so much doubles the power/halves the focal length the eyepiece (with a 2x)  as more effectively doubling the focal length of the OTA.  Sometimes it can  play around with how (or not ) the eyepiece is brought to focus. 

Its also worth thinking about the useful maximum magnification achievable by a 'scope. Its often given (rule of thumb) as twice the aperture  of the scope in mm, which for 130 is 260x. That is for ideal clear dark sky condition, which very few of us ever see and the reality is often much less. Its also worth thinking about the acuity of the eye in terms of exit pupil. Its sometimes cited that an exit pupil of less than 0.7mm  gains little in real terms so 130/0.7 = 185x which is probably much nearer the mark in my experience. There is always the exception and with lunar observation you can push it a bit or sometimes a lot.

[Mark Foster]

Thanks for the comments

If I looked through a 9mm eyepiece and then a 18mm paired with a x2 Barlow would I notice any difference?

[Alfian]

No and yes! Assuming the 2 EPs had the same fov the view should be the same but it would also depend on the quality of the optics. There is the argument that putting more elements in optic path will degrade the image, but when you consider the number of elements there are in some EPs I guess it still comes down to quality. 

An issue I have with Barlows and it's just a personal thing is actually using a barlow in the sense of having a long barlow+ep "protruding" from the focuser or diagonal. Given a choice I prefer to do without.

[Charic]

Both my Barlow's allow me to unscrew the lens cell and attach it straight to the eyepiece, for a reduced factor of approximately 1.5 -1.6 x Barlow, from the standard 2x, not as powerful, but a method I use quite often, so even the Barlow itself has a double use, but not all of them!

As for more glass in the optical train, if you can detect it then so be it, I haven't noticed any issues, and some of the most expensive premium eyepieces have a Barlow in-built, and folk still Barlow those eyepieces, now thats a lot of glass, but its quality, expensive, if you feel you need it.

If your unsure, use the Barlow supplied or buy a cheapie, then if you like it, you could go to the expense of a dearer one, see if there's any difference looking with your eyes?
 

[Alfian]

Yes that's a good point. I have a 1.6x  screw on "magnifier" that has been handy sometimes and adds little to the mass of the eyepiece. Again it's useful to be reminded that many well liked good EPs have additional "barlow"elements in their construction, and good advice.

[Rocket_the_Raccoon]

Hi! Mark,

One thing haven't been mentioned yet is eye-relief. All your eyepieces have relatively long eye-relief's because of their focal lengths. Using a barlow with them would basically retain their comfortable eye-relief's. Of course, there are short focal length eyepieces that have long eye-relief's, but "Short FL" + "Long Eye-Relief" + "Sharpness" + "Contrast" + "Wide AFOV" = "VERY EXPENSIVE" :-)

Personally, I uses a Meade 3x Short-barlow + basic Plossl 10mm on my portable 70mm f5.6 for planets and doubles.

P.S. The Meade 3x is actually a ~2.7x

[Dave In Vermont]

Regards 'dimming the image' - this is where an excellent Barlow will shine. My TeleVue® Barlows, 2x & 3X, will effectively 'vanish' in the optical-pathway. In other words, they hardly have any impact on the quality and contrast of the image without notable dimming. They cleanly magnify the view only.

Others have noted the same thing with other top-end Barlows, but I'll stick to the TeleVue's as these are the one's I know best.

Hope it helps -

Dave

 

 

[Mark Foster]

Thanks for the feedback - really useful

Got great view of Jupiter last night - using my 26mm Meade 4000 with a 2x Meade Barlow and a 3x Celestron Barlow.

Is there any benefit in getting a dedicated good quality 6 or 9mm eyepiece for looking at Jupiter ? Or would the views be the same as using an 18 or 26mm and barlows?

 

[John]

45 minutes ago, Mark Foster said:

Thanks for the feedback - really useful

Got great view of Jupiter last night - using my 26mm Meade 4000 with a 2x Meade Barlow and a 3x Celestron Barlow.

Is there any benefit in getting a dedicated good quality 6 or 9mm eyepiece for looking at Jupiter ? Or would the views be the same as using an 18 or 26mm and barlows?

 

I've owned lots of barlows, Powermates, Telextenders etc. They worked pretty well at the time but now I just use dedicated eyepieces for the short focal lengths. Are the views any better ? - slightly I think. A bit less light scatter anyway, which I don't like when viewing the planets. 

Barlowing long focal length eyepieces can lead to awkwardness in eye placement because a barlow pushes the eye relief outwards. For an eyepiece which already has longish eyerelief the results can mean, when the barlow is fitted, having to position your eye above the rubber eyecup, sort of "hovering" well above the eye lens, which is not ideal IMHO.

[Mark Foster]

John  - thanks for the reply

Sounds like a nice to have rather than a definite. Something to put on a Christmas list

Here's another related question - I really like my 26mm 4000 Meade - but would a BST Explorer (apart from the wider field) be any better? - Sharper - More Contrast??

[Alfian]

2 hours ago, Mark Foster said:

using my 26mm Meade 4000 with a 2x Meade Barlow and a 3x Celestron Barlow.

Thats quite an assembly!

[Mark Foster]

28 minutes ago, Alfian said:

Thats quite an assembly!

Yep - it would be !

Probably tip  over the scope and poke my eye out!

[N3ptune]

With my F5 scope, I use a barlow Xcel LX 2x often with a 25mm Xcel-LX, 18mm Xcel-LX and 7mm orthoscopic. I don't see any dimming, or narrowing of the image. Although, I notice the eyerelief is different with my 18mm, it's less comfortable a bit with the barlow, I can live with it.

Has below, I could get a 12mm and a 9mm to replace the barlow, but it's never getting high on my priority list.. The barlow is not bad at all and it can double the power of an infinite quantity of 1.25" eyepiece too. (a plus)

25mm = 40x / 80x with 2X barlow = a 12mm EP
18mm = 55x / 111x with 2x barlow = a 9mm EP

But it's absolutely just me, you can establish your own level of tolerance to imperfection in relation with the prices you wish to pay. With more available budget, I would probably buy separate EPs to gain a bit of comfort.

[Peter Drew]

One attribute of an eyepiece is its ability to cope with the cone angle presented by the objective. This is why a more sophisticated eyepiece is needed to cope with the wide cones from fast optics. The performance of a "borderline" eyepiece for a given objective focal ratio can be improved by using a decent Barlow as it will narrow the native cone angle of the objective.