BSTs

[Dom1961]

What kind of effects?

[John]

A barlow lens makes the eye relief of the eyepiece longer. That can be useful when it's used with eyepieces that have short eyerelief but can lead to an awkward eye position when the eyepiece already has a fair amount of eye relief.

[djpaul]

The 3.2mm is fantastic for planetary viewing of Jupiter when all is stable

My SW 120ed's max magnification is 245, but this eyepiece pushes it to 280 with no problems on good nights

Paul

[djpaul]

I use the 5mm a lot as it gives me magnification of x180

Paul

[SpaceWalker]

What kind of effects?

There are two kinds of Barlows, *regular* or telenegative (usually of 2-3 optical elements) and telecentric (4 elements arranged into 2 doublets). The former type extend an eye relief ~10% and it's magnification factor depends on the position in optical train, so you never know the exact magnification factor with any certain eyepiece unless estimate empirically. Most of 2x Barlows work generally as ~1.8x-2.2x. The consequences of longer eye relief John has explained pretty clear.  If one use a telenegative Barlow with an eyepiece with eye relief over ~24mm, e.g. 32mm Plossl, blackouts can be expected. Some wide angle eyepieces, especially of longer focal length, also can interact with it in a similar manner. At the same time, it's a routine tactics of many planetary observers to use telenegative Barlows with medium and short-medium focal length Plossls and Orthos (e.g. 10-18mm) to have comfortable eye relief  and avoid short FL eyepieces (e.g. 5-6mm) with practically non-existent eye relief. Also, so-called *shorty* telenegative Barlows usually vignetting longer focal length wide angle eyepieces like 14mm+ 82*AFOV.

The latter type of a Barlow, called telecentric, is actually not a Barlow but focal extender. It consists  of a negative doublet (actually a 2-element telenegative Barlow) on the bottom and a positive doublet on the top which corrects light beams coming from the negative doublet. The resulting image is flat, sharp and bright. Telecentric Barlow doesn't affect eye relief,  doesn't vignette anything, provide the claimed magnification which is not affected by its position in optical train.  Telecentrics are, for example, the TV Powermate or cheaper but comparable optical quality the ES Focal Extender, Bresser SA Barlow etc. Even though I have several Barlows I've been using exclusively my Meade TeleXtender since I got it, it's so good. So, I'd easily recommend you to look into the telecentric, it worth every penny.

[alro]

Looking at getting 25mm and 5mm to bookend my herd,

Those with 25mm, will the area of sky view be similar to that of my 32mm plossl but bigger?,

i cant seem to get on to well with 32mm plossl for some reason, all others i can.. 6.4 to 26 occasional 40

al

[John]

The 25mm BST shows a little less actual sky than a 32mm plossl - about 15% less. The apparent field of the 25mm BST is larger though so the field will look larger despite not actually showing quite as much sky.

It's difficult to convey this

[alro]

Thanks john,

me be worthwhile, cant seem to get on with 32mm plossl which does seem strange.

i had thought of hyperion or maxvision with 68 fov, but i know i find starguiders comfortable to use for me.

al

[Roysten]

3.2...wow, whats that like? I had not considered going that far on my 150.

I've got the 5mm BST for my 150P and am finding it giving some nice sharp images of Jupiter, I'd say noticeably better than the supplied 10mm + 2× Barlow.

I've tried to Barlow the 5mm which for me would be 300× magnification, the views are ok but no where near as sharp as at 150×. The 3.2mm would 234× which I imagine could give me quite views, though obviously never to be Barlowed with that scope!

I've personally be pondering the 8mm BST, as that with the Barlow at 188× would probably be a reasonable limit for good, sharp magnification.

[ronin]

The "problem" many find with a 32mm plossl is that the eye has to be well back from the eyepiece, often your head will be sort of floating in space with no contact to the eyepiece. This makes getting your eye in the right place not so easy, then keeping it there is plain difficult.

Getting a 3.2 BST is again odd.

You will find yourself getting eyepieces sort of shorter and shorter until one doesn't work.

The point being at that stage you have bought one that does not work.

The 3.2 seems a little short to expect it to operate a reasonable percentage of times, so if you go for one realistically do not expect nightly usage of it. A 3.8mm may have been a better item to produce. It is pretty however.

Just seen where Hartland is, it looks like it will not suffer from light pollution a great deal.

[John]

The "problem" many find with a 32mm plossl is that the eye has to be well back from the eyepiece, often your head will be sort of floating in space with no contact to the eyepiece. This makes getting your eye in the right place not so easy, then keeping it there is plain difficult....

It depends on which 32mm plossl you go for. Some have a more recessed eye lens or even better an ajustable height eyecup in which case the "floating in space" eye position is not needed. The Vixen 30mm NPL is a good and affordable example of this. The Baader 32mm plossl comes with an extension piece which moves the position of the eye cup upwards by around 15mm for those who find that position works best for them.

My point is that all 32mm plossls are not the same.   

[nightfisher]

It depends on which 32mm plossl you go for. Some have a more recessed eye lens or even better an ajustable height eyecup in which case the "floating in space" eye position is not needed. The Vixen 30mm NPL is a good and affordable example of this. The Baader 32mm plossl comes with an extension piece which moves the position of the eye cup upwards by around 15mm for those who find that position works best for them.

My point is that all 32mm plossls are not the same. 

I half guess the Revelation 32mm photo visual plossl might score here as you can slide the eyecup up or down 

[alro]

I'll be buying them one every two months due to pocket money, so I won't be buying all at once

dom,

there are a couple of bst starguiders at auction, 8mm and 15mm on STL, may be worth your while

al

[Dom1961]

Don't have any money yet

[spaceboy]

Ask for a sale pending to be put on them

[John]

Ask for a sale pending to be put on them

It's OK to ask but please be open and honest about payment timing and don't be offended if the seller politely declines. They are often selling to fund other acquisitions so "cash flow" will be required pretty promptly.

[Dom1961]

Yeah, I've asked him to let me know when he has customer returns, he offered me those the other day and I told him I have no money yet. So he kindly offered that I can let him know when I have enough and he will let me know what he has and he will give them to me discounted

[Dom1961]

Okay, I've got some money, which 2 shall I buy first, 8mm and 12mm or 8mm and 25mm?

[alro]

I havent yet got 25mm so cant comment on that.

i would maybe go for the 8 and 25 if you havent already got a 25mm stock ep.

if you have , i can highly recommend 18mm, its probably my most used.

but if you are using barlow x2 you might not want 8 and 18!

al

[Dom1961]

I've got a 25mm already and no Barlow, is the 25mm much better than the stock 25mm?

[alro]

To be honest i dont know, however there is plenty of reviews on here to suggest it will be a significant improvement.

have you considered contacting directl to strike a deal for the 8 and 15 at auction!

Al

[Dom1961]

I was about to call him to see if he had any other returns, I do not yet want a 15mm, I thought I'd see peoples opinions on here, hmmmmm

[Dom1961]

Because I definitely want an 8, but I don't know for the second one...

[alro]

Ah ok, you will do whats best for your own needs,

for what its worth, the 15mm won a bbc sky at night best in group review a while ago.

the review is on here somewhere.

al

[Dom1961]

Just ordered a 8mm and a 15mm after Alan recommended it, I asked him which he recommends and he recommended the 15:D