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To 2.5x or not to 2.5x that is the question

ChrisG3484

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ChrisG3484

ChrisG3484

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Hi All,

I am looking and getting another Barlow lens for my telescope and I am stuck between a 2.5 x barlow and a 3x barlow. I know if I get a 3x barlow I won't be able to use it with my 10mm eyepiece as the magnification would be beyond the best usable magnification for my scope. So if I was to go for 2.5x barlow, what are good budget brands ?

I am keeping an eye on the Classified section but can't post anything on Wanted yet :(

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Schorhr

Schorhr

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What eyepieces do you have? If it's just the stock 25 and 10mm, get a 32€ UWA or 45€ HR Planetary instead of barlows, almost the same price but way better performance ;-) Though on faster telescopes the HR P are better. Both have better eye relief then the 10mm SW and nice wide angle views.

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RikM

RikM

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I use a 2.5x Barlow with my 10mm eyepiece only on the very best nights. The air is seldom steady enough to get a clear view at that magnification with either my f/5 or f/4.7 scopes. I would have thought that a nice 2x Barlow or a 5mm EP would probably be more use. 

Is eye relief important for you? Plossl and Orthoscopic eyepieces tend to have quite short eye relief; you need to get your eye quite close the lens to be able to see all the field. The first 'upgrade' I made was a £40 6.4mm Plossl and it felt like you had to lick it and stick it to your eyeball to see anything through it. Horrible experience.

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Schorhr

Schorhr

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HR/tmb Planetary 58 degree apparent field of view

http://m.ebay.co.uk/itm/160858485287

Uwa 66 degree apparent field of view

http://m.ebay.co.uk/itm/140717462903

Perform similar to Plössl on faster telescopes, at f/6 acceptable, at f/5 a matter of taste, the 6mm on f/5 isn't as bad as I had expected, I like to use it.

I also have a 3 and a 2x achromatic barlow, the 3x is a cheap one from Meade, 19€ shipped, not as well black coated as the 2x (Seben Orbinar 16€) and the color fringe gets more obvious, but both work fine, but never as good as a single eyepiece (even if it has a barlow element itself like the hr p, but these are usualy designed to fit the lens design well).

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Schorhr

Schorhr

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No problem, sorry for beeing so crypic ;-)

The 66 Degree eyepieces are a modified Plössl as far as I know, and are sometimes sold as UWA (ultra wide angle) due to their amazing 66 degree apparent field of view. Apollo feeling!

I used HR-P to avoid writing hr planetary once more, that's the one from the other link with 58 degree afov but sharper outer field on telescopes with f/5 aperture ratio.

There are similar/identical eyepieces like this, some newer, some just re-branded. I think the TMB one's where the original.

A HR planetary has a barlow element on the bottom. It can be unscrewed and by changing the distance you can actually change it's magnification. This can be done via diy extension tubes or you can buy a AstroZoom that will alow you to turn eyepieces with barlow element into a zoom eyepiece that won't change the aparent field of view (as in contrary most cheap zooms do).

As many modern eyepiece designs contain a barlow element to reach higher magnification while keeping the eye relief constantly comfortable, barlowing them further can cause a decrease of contrast.

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RikM

RikM

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Neither of the linked eyepieces are actually 'HR' or 'UWA' I am afrad !

TS HR Planetaries are similar to the 58° planetary eyepieces above. They are High-Resolution Planetary Eyepieces supplied by Telescope-Service in Germany http://www.telescope-service.com/eyepieces/start/eyepiecesstart.html#hr, yes they probably have a built in barlow to give the more comfortable eye relief.

UWA normally means Ultra Wide Angle. This usually refers to an eyepiece that gives 82° or even more apparent field of view. The 66° eyepieces would be SWA or super wide angle. It is difficult to get super/ultra/mega wide field eyepices that are both cheap and good !

Most modern eyepiece designs with wide apparent fields or long eye relief include some kind of barlow element built in.

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Okeydokey

Okeydokey

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Hi all sorry to be a pain but what is a hr-p and uwa ?

So does a planetary eyepiece have a barlow elements built into it?

Sorry for all the beginner questions.

I'm with you on this Chris,  I spend most of my time unpicking the answers.... but I guess I learn alot when I do.  Must admit the hr & uwa bit sounds like it does a great job, just wish I knew how to plumb it into the numbers I am trying to figure out at the moment.

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RikM

RikM

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As Marcus says, there are a few of the 58° planetaries about. The TMB were the original but there are a couple using the TMB name and I don't think anyone is exactly sure which are which. They all work about the same and are a good step up from the kit eyepieces that come with your scope.

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Schorhr

Schorhr

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Agreed, these days UWA means 80 degree or more... But some time ago those where labled as different things uwa, swa... It's sadly not standarized :-)

In Germany those are often referred to as gold-edge due to the first imports having the golden line on the top.

No matter the lable, if they have 66 degree and are available in 6-9-15-20, that's the ones. As said they perform similar to Plössl but eye relief and wide angle view are great.

The 58deg eyepieces should be similar to http://www.astroversand.com/Skywatcher1-p94h8s37-Sky-Watcher-TMB-BURG.htm but I may be wrong. I do have some of those and the ones from TS, none from the link above, I just chose the link from the picture and specs.

Probably running of the same factory anyway :-P

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ChrisG3484

ChrisG3484

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Hi guys, 

Ok so looks like planetaries might be the best ones to go for. I shall do a trawl of the classifieds on here. So if I Barlow a planetary I have good chance of reducing the image quality, is that correct. Does the normal formula for calculating magnification still hold true for planetaries or do I have to take into account the barlow part of the eyepiece ?

So are all planetary eyepieces HR ones?

http://www.firstlightoptics.com/skywatcher-eyepieces/skywatcher-planetary-eyepieces.html I assume these have barlow elements in it. 

Again sorry for all the questions

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RikM

RikM

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The normal formula applies.

The built in barlows (more properly called Smyth lenses or elements) are a standard part of most modern eyepieces and are included in the eyepiece specification. So a 5mm with a built in Smyth element, gives the same magnification as a 5mm without a built in Smyth element. i.e. the '5mm' applies to the eyepiece as a whole regardless of the internal design.

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Schorhr

Schorhr

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Oh look, they are labled UWA too ;-) Really hard to name eyepieces if everyone lables them differently, though their case are rather easy recognized.

Don't worry, magnification is allways focalLength devided by EyepieceFocalLength (in millimeters), the barlow element is already calculated into the labled focal length. It just get's more complicated once you try to change it's distance...

A barlow will allways reduce contrast a bit and increase color fringe, the cheap ones the most, the achromatic type less, the ED even less...

Some eyepieces will work better with barlows then others, what they do have in common though is that you will see every dust spec on their lens...

A barlow makes sense if you want a 400x magnification for a 8" dobsonian, as a dedicated eyepiece would be a waste of money (cLear nights with such good seeing are rare), or if you have a real good eyepiece... Most of the time it's a good strategy to get a few good eyepieces first, and don't waste too much money on barlows.

For example, a 2x achromatic barlow is a great idea if you have two stock eyepieces and need to experiment, but before you get another barlow a good wide angle eyepiece will give you a better experience.

Of course there are many strategies to expand your eyepiece case, and everyone has their own views on the subject :-)

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RikM

RikM

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Oh look, they are labled UWA too ;-) Really hard to name eyepieces if everyone lables them differently, though their case are rather easy recognized.

Don't worry, magnification is allways focalLength devided by EyepieceFocalLength (in millimeters), the barlow element is already calculated into the labled focal length. It just get's more complicated once you try to change it's distance...

A barlow will allways reduce contrast a bit and increase color fringe, the cheap ones the most, the achromatic type less, the ED even less...

Some eyepieces will work better with barlows then others, what they do have in common though is that you will see every dust spec on their lens...

A barlow makes sense if you want a 400x magnification for a 8" dobsonian, as a dedicated eyepiece would be a waste of money (cLear nights with such good seeing are rare), or if you have a real good eyepiece... Most of the time it's a good strategy to get a few good eyepieces first, and don't waste too much money on barlows.

For example, a 2x achromatic barlow is a great idea if you have two stock eyepieces and need to experiment, but before you get another barlow a good wide angle eyepiece will give you a better experience.

Of course there are many strategies to expand your eyepiece case, and everyone has their own views on the subject :-)

This is the same way I did it. I have a few nice wide angle eyepieces and then a nice Barlow (half the price of my EP's) for those few exceptional nights. I have a 2.5x because of the EP range I have, a 2x is not so useful. I have 28, 20, 14, 10, 7mm eyepieces and with the 2.5x Barlow I have 8, 5.6 and 4mm equivalents.

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Schorhr

Schorhr

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I managed to Get a astrozoom cheap, so for me that's enough :-) Somehow getting better eyepieces allways gets delayed by some other gizmo I need to buy... Or simply the lack of money ;-)

I'm pretty happy wit Erfle, Plössl, Planetary and the like, those alone cost more then my h130p for example. It's amazing how much money can be spent on eyepieces.

I actually have a 2.5mm hr planetary as it was on sale for 30€ new, and as I have the 130/650 it's actually quite nice to have when the nights are clear. It's just a tad better then the 6mm plus barlow, but Saturn and even m51 where amazing... Most of the summer I can't use it though.

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Schorhr

Schorhr

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That will be a great "every day" magnification as it is usable most of the time :-)

Yes, with a 2x barlow it will be too much (but you can use it on the moon for example anyway, as it's bright enough).

If you like to tinker, try to extend the distance between the barlow element bottom from the HR Planetary eyepiece and you can crank it up to 200x or more ;-)

Good luck!

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