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Matching eyepieces with barlow

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#1 mv1612

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Posted 24 November 2012 - 03:23 PM

Hello,

My first post here, I guess it's a simple question. I have a small Orion 6" f/8 dobsonian, and I want to buy some affordable eyepieces. It will probably be either the 10 and 15mm SWA or the 9 and 15mm EWA eyepieces from Agena. I will also buy Barlows, I was thinking about the 2x and 2.5x GSO Barlows.

I read sometimes about eyepieces not reaching focus with a certain Barlow. So my question is: will the mentioned eyepieces reach focus with those Barlows? In what situation en eyepiece would not reach focus with a Barlow?

Thanks very much,
Virgil.

#2 JamesL

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Posted 24 November 2012 - 04:46 PM

I just got a televue 2x barlow for 115.00 it is not a shorty and would probably work but im not sure. One thing I am sure is that it is the best barlow that I have ever looked through and I will never be sorry for buying it...jim

#3 csrlice12

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Posted 24 November 2012 - 05:10 PM

If you only plan on viewing, the shorty barlow will perform fine. The GSO shorty and most any of its clones are a good barlow and a good buy for it's price.

#4 faackanders2

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Posted 24 November 2012 - 05:18 PM

Since you only have a 6" scope I would go with the 2x barlow. 6" scopes are limited on higher powers than larger scopes.

#5 faackanders2

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Posted 24 November 2012 - 05:21 PM

Go with widest AFOV if you can afford them to minimize the desire to upgrade later.

#6 mv1612

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Posted 24 November 2012 - 05:38 PM

Thanks guys. So I guess I'll not have a mismatch problem.

>> good buy for it's price
Yeah, that's what I'm looking for at the moment, they seem to be good eyepieces and Barlows for the price.

>>Go with widest AFOV if you can afford them to minimize the desire to upgrade later.
Well, there's no way my 1200mm scope will give me the field of view I want. Incidently I love wide TFOVs, I bought this scope when I had no clue about this stuff... So I'm already thinking about a small refractor, maybe an Explore Scientific ...
So the stuff I want to buy for now is OK...

Thanks,
Virgil.

#7 faackanders2

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Posted 24 November 2012 - 06:03 PM

Eyepieve Apparrent Field of View (AFOV) are properties of the eyepiece.

Normally options are 50-55 deg for Possils, 68-70deg SWA, 82-84 deg UWA, and now 100-120deg.

Explore scientific has pretty good prices for wide FOVs. If your scope only takes 1.24", I'd recommend the 14mm 82 AFOV for $99. 24mm 68 AFOV provides widest TFOV for 1.25" eyepiece though.

#8 mv1612

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Posted 24 November 2012 - 07:42 PM

>> Eyepiece Apparrent Field of View (AFOV) are
>> properties of the eyepiece.

Of course. I was talking about the max TFOV my telescope can take, which is 1.29 deg by my calculations, being small compared to 2-3 deg TFOVs that I would like to have. For these TFOVs I plan buying that small refractor.

Virgil.

#9 faackanders2

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Posted 24 November 2012 - 09:17 PM

>> Eyepiece Apparrent Field of View (AFOV) are
>> properties of the eyepiece.

Of course. I was talking about the max TFOV my telescope can take, which is 1.29 deg by my calculations, being small compared to 2-3 deg TFOVs that I would like to have. For these TFOVs I plan buying that small refractor.

Virgil.


I often have binos in addition to my big 17.5" dob. And they are easier to take on trips (including my camera tripod mounted 25x100).

#10 Keith

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Posted 25 November 2012 - 12:19 AM

also take into account the issue of exit pupil, there are reasons to have more than one focal length in the max tfov range of 1.25", such as a 32mm or 40mm plossl, as well as a 24mm SWA, or 14-20mm UWA.

At f8 coma is not an issue, and astigmatism in less expensive widefields is not as bad, and eyepieces like the ES 68 and 82 deg are pretty much close to perfect to the edge. Barlowing would effectively make it an f16 scope, and edge correction improves greater still, but there ARE issues to watch out for with any barlow, shorty's in particular.

Issue #1 vignetting, robbing you of the AFOV and TFOV you paid for. Many wide eyepieces, especially in the 24mm 68deg range, do not like being put in shorty's, even high quality ones like the ultima. A "long" barlow such as the televue, or a meade #140 (people unload them for cheap used, try to get a japan made version as the chinese ones are not so hot), will not have that issue.

#2, funky exit pupil placement. With long eye relief eyepieces like the 32mm and 40mm plossl's, which are the best value if you want a bright view, when used with a barlow can extend the eye relief out to a point where you can't get close enough in to see the full field of view without getting blackouts. This was especially true with the big 35mm panoptic, which led uncle al to come out with the powermate. It is probably not as bad with the 32mm plossls, but it could still happen depending on how far in the eyelens is recessed from the eyecup, which varies by brand. Consider that it also makes more sense to NOT barlow a 32mm or 40mm plossl, since you will probably have a 15-20mm eyepiece with a 65-72deg field which will show the same mag with a wide field than the barlowed plossl.

Barlowing a nice 24mm 68deg does make sense, and this is where the shorty's can vignette. The Televue barlow is the way to go if you can afford it, and not have to worry about upgrading later. It also has compression ring to not scratch your eps.

1200mm is in the semi bummer zone when it comes to being limited to 1.25", but I had a 6"f8 for a while, and it still gives a wide enough field for most objects.

40mm 30x = 5mm exit pupil, essential for getting the most from dark skies, or if you want to use an OIII filter on faint nebulae, from either dark or lp skies. The catch here is that you are limited to 43-44deg AFOV.

32mm 38x = 4mm exit pupil, good for deep space, bright OIII filtered objects, and most UHC filtered objects good at this power too, limited to 52deg AFOV, not quite as clausterphobic as the 40mm.

24mm 50x = 3mm exit pupil, darker sky background increses contrast in light pollution, but may not be as good of a view of faint extended nebs when a narrowband OIII/UHC filter is used. Limited to 68-70 deg.

Those are the common 3 max FOV ranges in 1.25, in 82deg most companies do not push the limit past 16mm (televue), meade had an 18mm series 5k, and I think the new one is 20mm crammed into 1.25, but I have not seen it yet. The 18mm design actually worked, but looked very awkward physically. I have not seen the 20mm. I personally have a widescan III 20mm 84deg, which actually worked quite well in the f8 dob if you can get ahold of one.

Considering you are at f8, a 16mm is the ideal, and there are older nagler type 2's about $150 used if you can find one where the seller is not asking type 5 prices. This is the 2mm exit pupil sweet spot for brighter DSOs, galaxies and globular clusters especially. 14mm was/is offered by meade and ES in 82-84deg UWA, and celestron has a good 15mm in the axiom LX/Luminos line.

If it was me, and on a budget, I would figure out if I just wanted it wider, or if I also wanted it brighter too, when deciding where to spend the $, since you are likely sitting on a more than adequate 25mm sirius plossl, getting a 32mm to get max field and a little more brightness might make sense to you, or you might find you like the 25's power, and just want to widen it out to a 23-24.5mm range superwide.

My advice would be grab a cheap 32mm plossl, and put the rest of the money into a 14-16mm ultrawide, the 14mm meade 5000 UWA should be going for about $70-80 used, as with the ES 14mm, but considering the backorder, I see people selling them for close to new prices. There is also the older mushroom top ES14 which should also claim a lower price used. And if you search the net, you might find new 15mm Axiom LX for a good price, since they were replaced by the luminos line, and some of the more obscure dealers, or big box closeout places are sitting on old stock and blowing it out.

Warning, once you go 82 it is hard to go back to narrower fields. I am curious to see how the meade 1.25" 20mm 82deg performs, but I would not be surprised if it is not as good as the 18mm was.

Some EPs to look for on the used market to consider, 35mm Ultima/Ultrascope/Parks GS5, has the max field at 49deg, not as clausterphobic as a 40mm plossl, but still a tad brighter than the 32s. These 5 element japanese eyepieces are uber sharp and crispy with excellent contrast.

Celestron 23mm Axiom (original japanese), not as well corrected as panoptic, or ES68/5kSWA, but really nice japanese glass similar to ultima but stretched to 70deg. This will be sharper on axis than todays taiwan/china 68s (panoptic/ES68), but stars at the edges will not be perfect.
24mm Televue widefield, and 24.5mm Meade 4000 SWA are also similar in this regard, japanese glass, but an older design not as perfect at the edges.

20mm KK Widescan Type III, hard to find but a neat way to get the max field at the lowest power in 84 deg, good japanese glass, but do not expect perfect edges.

On the low cost end, there are a ton of 15mm-20mm 68-72 deg eyepieces out there, the classic is the GSO superview, but the burgess SWA (now the agena SWA among others like OWL), meade QX series,Orion Q70 and the ES 70deg series, are all out there in the $30-60 price range, and share similar designs. Think of them as china/taiwan versions of the classic Televue Widefield/Meade SWA designs, not as well corrected, but less $$$.

#11 CosmoSat

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Posted 25 November 2012 - 05:08 AM

The Agena 1.25" Enhanced Wide Angle Eyepiece - 9mm and the GSO 1.25" 2x Achromatic Barlow Lens should work well for planetary viewing with your telescope. For DSO's the 25mm plossl the came with the scope would be sufficient for now.

Clear Skies!

#12 mv1612

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Posted 25 November 2012 - 02:25 PM

Thanks Keith. I’m quite good in making calculations and I know things from reading forums and such, but my actual observing experience is quite limited, and because of this your post is really valuable to me.

>>#1 vignetting, robbing you of the AFOV and TFOV you paid for.
>>Many wide eyepieces, especially in the 24mm 68deg range, do not like
>>being put in shorty's, even high quality ones like the ultima. A "long"
>>barlow such as the televue, or a meade #140 (people unload them for
>>cheap used, try to get a japan made version as the chinese ones are not
>>so hot), will not have that issue.

If I stay with my choice of Barlows and use them in the 10-15mm, 60-70deg range, would I be OK as of vignetting?

>>#2, funky exit pupil placement. With long eye relief eyepieces like the
>>32mm and 40mm plossl's, which are the best value if you want a bright
>>view, when used with a barlow can extend the eye relief out to a point
>>where you can't get close enough in to see the full field of view without
>>getting blackouts.

I suppose I will not have this problem, since I don’t want to use barlows on these.

>>Barlowing a nice 24mm 68deg does make sense, and this is where the
>>shorty's can vignette. The Televue barlow is the way to go if you can afford
>>it, and not have to worry about upgrading later.

This is something to keep in mind. For now, I only want to use barlows for planetary observations, so in the 10-15mm range.

>>1200mm is in the semi bummer zone when it comes to being limited to
>>1.25", but I had a 6"f8 for a while, and it still gives a wide enough field
>>for most objects.

My favourite object, the Pleiades, should barely be observable in one shot in my scope. Looking forward to having the eyepiece for that. I guess the 24mm 68deg should do it.

>>40mm 30x = 5mm exit pupil, essential for getting the most from dark
>>skies, or if you want to use an OIII filter on faint nebulae, from either
>>dark or lp skies. The catch here is that you are limited to 43-44deg AFOV.

>>32mm 38x = 4mm exit pupil, good for deep space, bright OIII filtered
>>objects, and most UHC filtered objects good at this power too, limited
>>to 52deg AFOV, not quite as clausterphobic as the 40mm.

>>24mm 50x = 3mm exit pupil, darker sky background increses contrast
>>in light pollution, but may not be as good of a view of faint extended
>>nebs when a narrowband OIII/UHC filter is used. Limited to 68-70 deg.

Thanks, I never really understood why Al Nagler pushes observing deep sky with large AFOVs, the exit pupil decreases and so the brigtness should decrease… I read his essay about the ‘majesty factor’, increasing the contrast by increasing the magnification (blacker sky) and I had the impression that he’s really implying that deep sky is better observed at large AFOV. Maybe I missinterpreted him. Anyway, you confirm what I thought, that using a bigger exit pupil is the best for observing faint objects, if we accept the trade off of having a small field of view.

>>Considering you are at f8, a 16mm is the ideal,… This is the 2mm exit
>>pupil sweet spot for brighter DSOs, galaxies and globular clusters
>>especially. 14mm was/is offered by meade and ES in 82-84deg UWA,
>>and celestron has a good 15mm in the axiom LX/Luminos line.

So I suppose this is what Al Nagler was reffering to…good to know...

>>If it was me, and on a budget, I would figure out if I just wanted it
>>wider, or if I also wanted it brighter too, when deciding where to
>>spend the $, since you are likely sitting on a more than adequate
>>25mm sirius plossl, getting a 32mm to get max field and a little more
>>brightness might make sense to you, or you might find you like the
>>25's power, and just want to widen it out to a 23-24.5mm range superwide

I would like to have an eyepiece for max brightness allowed by my scope. Looking on the Agena site, I found the following eyepieces giving me roughly 1,3 deg on my scope:
ES 24mm 68deg 3mm exit pupil
Vixen NPL 30mm, 50deg 3,8mm exit pupil
GSO 32mm, 52deg 4mm exit pupil
Vixen NPL 40mm, 40deg 5mm exit pupil
Celestron Omni 40mm, 40deg 5mm exit pupil

So I should choose one of those 40mm eyepieces for maximum brightness, right? Is looking through a 40deg eyepiece that bad?

>>My advice would be grab a cheap 32mm plossl, and put the rest of the
>>money into a 14-16mm ultrawide

It’s just that for now I’m preparing for the planets, Jupiter and Saturn through 2013, so the ultrawide will have to wait.


>> Warning, once you go 82 it is hard to go back to narrower fields.

:) :)

>>Some EPs to look for on the used market to consider, 35mm
>>Ultima/Ultrascope/Parks GS5, has the max field at 49deg, not as
>>clausterphobic as a 40mm plossl, but still a tad brighter than the
>>32s. These 5 element japanese eyepieces are uber sharp and crispy
>>with excellent contrast.

Thanks, I’ll be looking for those…

>>On the low cost end, there are a ton of 15mm-20mm 68-72 deg
>>eyepieces out there, the classic is the GSO superview, but the burgess
>>SWA (now the agena SWA among others like OWL), meade QX series,
>>Orion Q70 and the ES 70deg series, are all out there in the $30-60 price
>>range, and share similar designs. Think of them as china/taiwan
>>versions of the classic Televue Widefield/Meade SWA designs, not as
>>well corrected, but less $$$.

Yes, I’m really into those ones for now.

Thanks Keith,
Virgil.

#13 mv1612

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Posted 25 November 2012 - 02:27 PM

The Agena 1.25" Enhanced Wide Angle Eyepiece - 9mm and the GSO 1.25" 2x Achromatic Barlow Lens should work well for planetary viewing with your telescope. For DSO's the 25mm plossl the came with the scope would be sufficient for now.


Thanks CosmoSat.

#14 mv1612

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Posted 26 November 2012 - 10:22 AM

One more question. I saw the TMB planetary eyepieces, I'm thinking of buying the 9mm TMB instead of the 10mm Agena SWA. It's more expensive but not by much. Would it be significantly better on planets than the Agena?
Thanks.

#15 CosmoSat

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Posted 26 November 2012 - 11:41 AM

If u are still looking for more options in the inexpensive class hve a look at this one too...The 8mm 1.25" Paradigm Dual ED.. it wil give a 1mm exit pupil in ur f/8 scope and 150x magnification.. with the 1.25" barlow lens u would get 225x and 300x.

But for a small aperture f/8 scope I personally would still consider the 9mm EWA 66° eyepiece mentioned earlier for its wider apparent field of view and sharp views.

Clear Skies!

#16 mv1612

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Posted 27 November 2012 - 12:51 PM

I guess I will order the EWAs (6, 9 and 15mm) and the 2x GSO Barlow. A very nice combination of magnifications with my scope, good value and great shipping price from Agena to Canada. I still need to decide on a big exit pupil eyepiece and then I'll make the order.
Thanks everyone.

#17 csrlice12

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Posted 27 November 2012 - 01:41 PM

with a 6" scope, you also don't want to go too big a f/l eyepiece as you will start seeing the shadow of your secondary. I probably wouldn't go much over 32mm with a 6" scope.

#18 mv1612

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Posted 27 November 2012 - 01:47 PM

with a 6" scope, you also don't want to go too big a f/l eyepiece as you will start seeing the shadow of your secondary. I probably wouldn't go much over 32mm with a 6" scope.

csrlice12 , would you elaborate a little bit on this?

#19 csrlice12

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Posted 27 November 2012 - 02:07 PM

To a degree, it appears I was wrong. The lowest useable mag for the 6" is 21X (according to the factory), that's technically about a 50mm eyepiece. but you aren't going to find a 50mm 1.25" eyepiece and the 6" has a 1.25" focuser. A 32mm plossel will probably be the biggest you could find in a 1.25" eyepiece. I believe the ES68* 24mm (or Pan Equivalent)will be the largest 1.25" widefield you will find. This is due to the field stop of a 1.25" eyepiece (this is about 27mm).

#20 CosmoSat

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Posted 27 November 2012 - 02:42 PM

The 2x GSO barlow can also be used as a 1.5x...so coupled with the 9mm eyepiece u effectively get 6.7 and 4.5mm focal lengths too...giving u exit pupils range of 1.12mm, 0.84mm and 0.56mm. with 133x, 177x and 266x magnifications respectively. The 6mm eyepiece with a 0.66mm exit pupil and 200x mag can just be an option really.

And while the 6mm and the 9mm EWA's hve been getting very good reviews..the 15mm and the 20mm arnt so highly rated by most folks here..And if at all u are considering investing in a reportedly mediocre performer like the 15mm ewa.. I would rather u consider this 17mm Agena 1.25" Wide Angle (WA) Eyepiece. which nobody here seems to hve yet. If u get it ur review might be useful to atleast a small part of this community. Both the eyepieces btw seem to hve the same design (4 lens elements in 3 groups) and it wouldnt be surprising if they come frm the same factory too...

On the lower end..about 30x magnification and a 5mm exit pupil should be about as low as u can go..which in ur 1200mm fl f/8 scope can be achieved with a 40mm eyepiece. tho I really will not suggest one with the 45° field of view. The 32mm Plossl is another option u hve..



Clear Skies!

#21 mv1612

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Posted 28 November 2012 - 12:14 PM

Clear Skies!


Yeah sure... in Montreal... :mad: :mad:

#22 WAVT

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Posted 28 November 2012 - 09:43 PM

The 6mm & 9mm Agena SWA are good eyepieces, as is the GSO 2x barlow. The GSO 32mm Plossl Link is pretty decent, and with the barlow you have 16mm and 21mm covered. Agena is a good vendor.

#23 mv1612

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Posted 29 November 2012 - 09:14 AM

OK. I ordered the 6, 9 mm Agena EWA, 17mm Agena WA, 32mm GSO Plossl and 2x GSO Barlow. I pay $12 for shipping everything, which is pretty amazing considering what other vendors are charging for shipping to Canada.

Thanks for your help,
Virgil.

#24 CosmoSat

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Posted 06 December 2012 - 10:26 PM

Would like to know how u find the 17mm wa performance.

Clear Skies!

#25 mv1612

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Posted 07 December 2012 - 09:51 PM

Strangely, I haven't received them yet. When I click on the tracking number, I get some information until the 1st of December, than nothing. I'm not sure what to do...






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