Poor Combo: BST's with a Barlow?

[Special K]

Not to diss the BST's one little bit (as they have given me such pleasure and I will recommend them all day long) but I've noticed several on the forum who also have difficulty getting a decent focus when one is paired with a Barlow. I have a Celestron X-Cel LX Barlow which has one tightening screw which engages a collar. This collar contacts the cylinder of the EP of the BST and if tightened it will cause an elbow effect as shown below. This is due to the Barlow cylinder not being the same diameter up the neck and this groove causes the bend with a Barlow with only one tightening screw. Would this bend cause a distortion making it harder to focus on objects?

[Special K]

[nightfisher]

the 5mm bst is not really up to barlowing, as it already has a kind of barlow element built in, not sure what you can do about the eyepiece "leaning" when in a barlow

[faulksy]

i had a similar problem with my meade telextender i screwed a old filter into the end of it with out the glass this lifted the ep so it was griped slightly higher up

[russ.will]

The mechanical combination, as opposed to the optical, are differing issues.

I've had a few odds and sods that were a poor mechanical match, although that is normally because of a chance mismatch between a thumbscrew/compression ring and an undercut position or shape. I don't reacll my BSTs being drastically undersized, so I'm wondering if this case is a rather over-large 1.25" barlow?

EPs sporting leading negative doublets (basically a barlow element in the the barrel) tend to be quite long eye relief. Barlowing them doubles this, making them more finicky to use with respect to eye position - I'm thinking kidney beaning and blackouts, etc.

As for focusing - A 5mm anything barlowed in a 650mm FL scope is not ideal - 260x magnification is pushing it under UK skies, not to mention it's right at the theoretical magnification limit for the 130p anyway - 50x each inch of aperture is a rough rule of thumb, with seeing and/or transparency pushing this lower, depending on the subject. If you ignore the atmospheric limits for the moment, there comes a point when you simply reach the maximum resolution allowed by the scope's aperture. At this point, increasing magnification will yield a larger (and duller) image, but no more detail. It will look more blurred than the smaller, brighter, lower magnification image.

With planets, you may find that the best nights to really push the magnification limits aren't the crystal clear ones of good transparency. Often clearish nights with high level haze deliver greater stability (better seeing) and whilst this can wash out DSO fuzzies, bright subjects like planets can really tollerate some serious magnification. You'll still not gain anything beyond the scope's native limits, you'll just get closer to them.

Russell

[Stargazer_00]

I used to barlow all my BSTs and never thought anything of it to be frank. Looked perfectly acceptable to me. I also used to use a bog standard Skywatcher barlow and I didn't really see any issues introduced by it either. It's been a while since I've used a barlow though as I'd sooner do without the extra glass.

[russ.will]

That was very much my point of view too, until I tried a 4 element telecentric multiplier as they work very differently. But, in defence of the barlow, it's not a given that their presence should degrade image quality. There are EPs that barlow better than others and used intelligently, they can extract some remarkable performance out of modest EPs.

I'd rather barlow a 12mm Plossl/Ortho than look through a 6mm Plossl/Ortho simply because you'll get the same magnification, but whereas the 6mm will have very tight eye relief, the barlowed 12mm will actually have more than it would by itself. In the case of a BST Explorer, it goes too far the other way as it's not exactly lacking in the first place. A side benefit in the use of a barlow, is that it effectively doubles the scopes focal ratio. Thus your modest EP, rather than be tortured at the edges by an F5 scope, will now act like it's sitting in an F10. Using the barlow nosepiece by itself can give a half-way house of a lower magnification increase (typically 1.3-1.6x) and slightly better edge correction, so it's worth remembering that a barlow offers more than one magnification boost too.

Finally, with respect to barlows; Every Nagler (and their clones like the ES82s, Meade UWAs, etc) are effectively long focal length, tight eye relief EPs, with a barlow in the nose piece to increase magnification, eye relief and edge of field performance and you don't hear many people complaining about them! That is because you are talking about a leading negative doublet designed to precisely match the rest of the eyepiece, but it's a clue as to what you should look for in an eyepiece that will barlow well.

As regards telecentrics, well there are several reasons why they are frequently prefered. These four element designs are better corrected for false colour, plus as you don't find any bargain basement telecentrics, they tend to have better glass and coatings anyway. They tend to be more transparent optically, but not necassarily any more so than a decent barlow.

In a barlow, the magnification is achieved by taking the (virtually) parallel light rays that enter and causing them to diverge. Depending on the distance that the EP is set from the barlow elements, is how the magnification is set. In a cheap range, the reason the 3x barlow is half as long again as a 2x barlow is simply to achieve the extra magnification with extra seperation from the EP. Zoom barlows of yore, took this distance range to a variable level.

In a telecentric (these being TV Powermates, ES Focal Extenders, Meade Tele Extenders, etc) the magnification is still achieved by the leading negative doublet, but this is then followed by a positive doublet that then turns the magnified light rays back to parallel. As you don't have diverging rays, your eye relief remains unaffected. As these more expensive telecentrics 'tend' to be used with more expensive EPs that don't 'generally' benefit from, or need, increased eye relief, they tend to disappear into the optical chain, both optically and ergonomically in a way that a barlow doesn't.

On the flip side, a telecentric is a one magnification device. It delivers it's rated magnification regardless of the setting distance, so even if you can unscrew the nosepiece, there's no point. If you are lacking in-focus on your scope for imaging purposes, then a telecentric won't help. If yu love your 9mm BGO, but find a 5mm BGO too tight, then a telecentric won't help, etc, etc.

It's horses for courses. Take a Televue Powermate and a Televue Barlow. There is no difference in the quality of optics, although you'd struggle to find anybody bigging up the Barlow over the Powermate. But what if you're not a widefield EP obsessive and you have a nice range of TV Plossls in your case? Then there's a very good reason for buying the Barlow over the Powermate, if you fancy a spot of planetary/double star dabbling, without glueing an EP to your eyeball.

I must say, I love my ES FE for visual and I tried a few of the budget barlows along the way, including the 'world famous' TAL 2x - Didn't like any of them, but then my EPs were the BSTs and latterly the ES82s - Enough eye relief for me. However, recent dabbling in Solar imaging caused me to look at barlows again due to a lack of in-focus on the refractor - The ES was no use. I bought a GSO 2.5x 3-element Apo barlow and obviously couldn't resist trying it for visual. I still don't need the extra eye relief, but I was extremely pleasantly surprised by the quality of views it gives, with a bit of ergonomic effort. I'm tempted to say it's a Celestron X-Cel LX Barlow on the cheap and it may be no coincidence that the Celestron is a 3 element design too - Still a barlow, but better corrected for chromatic aberation. Either way, I personally prefer it to (Shock! Horror!) to the TAL 2x and it's only £4 more than the TAL used to cost.....

Russell

[Special K]

Thanks all for the very useful info and many characteristics I was not aware of. Russell this is a good point that I have hit the ceiling of mag which is likely going too far under typical UK seeing conditions. In the future I may consider a 6 or 7mm of different design as an added variety to my collection; and something that will be more compatible with a Barlow. This gives me a lot of research to get busy with! The BST on its own is still a great piece of equipment. The 5mm on its own gives a fine view of planets.

Regarding the elbow issue, I think this can be fixed with a layer of paper taped around the cylinder of the BST to fill that unwanted groove. The added benefit for me is it also decreases slop in my focus tube.

Thanks again!