Which eyepieces in 6mm for dob?

[glend (Glen)]

Which eyepieces in the 6mm range provde the best eye relief used in a fast 12" Dob? My old eyes struggle with the 6mm GSO plossl.

[Wavytone]

Eyepieces with ~20mm eye relief should be comfortable.

Examples include Vixen LV6mm (old but still excellent, secondhand around $100), LVW 5mm, Takahashi LE, Televue Radian... Some were also rebadged eg the Orion/Celestron copies of the vixen ones, Orion Stratus and Baader Hyperions....

Plossls and orthoscopic have relatively little eye relief. Worst are the monocentrics, almost no eyerelief, meaning you have to be brave enough to put your eye in contact with the lens. I once had a 4mm ortho like that and froze my eye to it in a cold winter night, very painful.

[ColHut (Colin)]

Perhaps also the TMB series - they too have good eye-relief.

regards

[mental4astro (Alexander)]

I agree with Wavy about the eye relief. Plossls were the bees knees of EPs thirty years ago. Today there are much better designs that leave plossls dead in the water, and are not too expensive either. They employ newer optical designs and newer glass types.

But, be aware that oils ain't oils when it comes to all EPs. Some perform better than others depending on the optical design of the scope they are used in. AND, within an EP model range, there can be a difference in performance.

Like Colin, the TMB Planetary Type II range perform very well in Newtonians. But the 6mm isn't the strongest performer. They can be had cheaply too new for around $70, and on eBay of all places. The 9mm is one of my favourite EPs in my kit.

The Baader Hyperions are an optical mismatch with Newtonians, except for the 5mm. I've had many Hyperions on the strength of reading many reviews, but what I hadn't picked up on was all the positive reviews had these EPs being used in SCTs and Maks. In these scopes these EPs are excellent. But like I said, only the 5mm is good in Newtonians. The only Hyperion in my kit now is the 5mm. The nice thing of the Hyperions is there excellent eye relief, big eye lens, and a generous 68 degree AFOV.

The Orion Stratus range is the exact same EP as the Hyperions. The performance of the 5mm will be just as good. There is a 5mm Stratus up for grabs in the classifieds too for $90.

The Vixen LVWs are all fabulous in Newtonians, but they are all much more expensive than the TMBs and the Hyperions. Other EP designs are excellent in Newtonians, but they too are significantly more expensive than plossls.

Mental.

[barx1963 (Malcolm)]

I had a 6mm Delos for a while. The eyerelief was certainly excellent, the views were crisp and 72deg field was nice, but I found it kidney beaned a lot and was a little tricky to use. That was in my 12" f5 GSO dob.
I am sure with a bit of practice it may have been OK and easily beat any other EP in that range that I tried, found I had sharper stars than with the 7mm Nag T6 that I had.
I am unable to comment on the other EPs as I have not really had the pleasure!!

Malcolm

[Wavytone]

Glen there is a 6mm Radian FS here on IIS, this is about as good as you'll get in 6mm... http://www.iceinspace.com.au/forum/s...d.php?t=108245

[glend (Glen)]

Thanks Gents, I have taken the advice onboard. I have found a new TMB Planetary Type II (9mm) for sale on ebay so I have gone with that for now. I already have good eye relief from my GSO Superviews in 30 and 15mm, and this TMB should be a good addition to the collection. As to 6mm I will probably park that for now as I can't afford the cost of the good ones.

Can I ask one other question, what are the views on the use of 2x Barlow tubes with these EPs, and are they all the same? I have a Celestron 2x Barlow that came with my old Celestron 130 but when used on the 12" Dob it doesn't seem to be very impressive compared to the differences that the change to a good EP made.

[mental4astro (Alexander)]

Barlow lenses are curious creatures. Like EPs, some work better in one scope design than they do in others. I've got two, one a humble 1.25" achromatic, the other a 2" ED hot shot, both 2X. They each perform very differently with different scopes I've got and then also with the different EPs I've got. For me, I don't believe they maintain the same viewing properties of an EP as when the EP is used neat. They can't. They alter the shape of the light cone entering the EP, so it will mean that the EP will react differently. For example, my 30mm Superview with the 2" 2X barlow is not a good combination in my fast dobs, giving a worse quality image than the neat Superview, which is a reasonable image. But, the same Barlow with the 5mm Hyperion in the same scopes is a cracker.

I think your Barlow though will do well with the 9mm TMB. Mind you, you will be pushing the scope to approx. 333X, which for really clear, sharp image you will be limited to only a few times a year. When those occasions do happen, you will be happy you've got it up your sleeve. When you first try this combination, and the image is shimmering like a looking through hot water, don't blame the Barlow or EP, its the thermal currents in the atmosphere your scope is picking up. You'll need to wait for that occasion when the atmosphere is more cooperative for high magnification to make the final call on the EP and Barlow combination.

Ultimately, only trial and error will tell you what works with what.

[Wavytone]

Glen, a 2X Barlow combined with the 15mm will work like a 7.5mm eyepiece. Having had a fast 12.5" Newtonian before , this will be more than enough magnification most nights - a 6mm will be overkill when the seeing is less than perfect, you might have 1-2 nights a year when it's worthwhile.

Optically, a generic Barlow with an average 15mm eyepiece will be outperformed by a real 7.5mm eyepiece, usually by having fewer elements (less scattered light) and sharper image.

The trade off is that you could use the Barlow with your other eyepieces, vs the cost of a singe eyepiece around 6-7.5 mm.

I've used Barlows before, and I don't own one, preferring to use dedicated eyepieces.

[Steffen]

On the topic of Barlows, what do people do to really figure out the effective EP focal length, or Barlow factor, in case it matters to them? Other than timing the passing of a star through the field of view.

I find that there is a *huge* difference in the position of the focal points of eyepieces in relation to their barrels, for example between any TV eyepiece and any ortho I've owned. The variation here is 2-3cm, which must make a big difference in the effective Barlow multiplier.

Cheers
Steffen.

[Wavytone]

Timing a star is the only reliable way. Do it first without the Barlow, then insert Barlow and repeat.

If all else is the same (star,eyepiece and scope), the ratio of the times will give you the actual power of the Barlow directly without any further calculations or assumptions regarding focal length. Very simple and accurate.

And yes the power does vary depending on how far back the eyepiece focal plane is located from the Barlow lens. Whether this really matters ... I don't think so.

But what it does mean is that a Barlow chosen from brand X is highly unlikely to be optimal for eyepiece brand Y, in the sense that the designer of one has no control over what you're going to mate it with.

The best results should be from a Barlow specifically designed to match your eyepiece (there are some, for example Edmund had one for the RKE's, TV have the Powermates presumably for their eyepieces, and Vixen made one for the LV series, and Clave did too).

But... Also note that many modern eyepieces actually have a Barlow built-in as part of the eyepiece design, specifically designed to compensate the aberrations from the rest of the eyepiece - this is the trick in the Vixen eyepieces, Hyperions, TV Naglers, ES ultrawides, and many more. Sticking another (unmatched) Barlow in front isn't going to be as good as using the right eyepiece with the right focal length. It's called a Smythe lens (after the inventor). The thing is, with the exception of the Hyperions, the eyepiece won't work well with the Smythe lens removed, so the eyepieces aren't designed to let you remove it in the first place.

Barlows were a pretty common item back in the 19th century. It was common knowledge that putting a Barlow in front of a low power Plossl, Orthoscopic, or Erfle made a pretty good high-power eyepiece with good eye relief. Smythe took the obvious step to permanently mate the Barlow in the eyepiece, then re-optimise the overall design to reduce aberrations. The only snag in Smythes era was the lack of antireflection coatings at that time, so while it was a great idea, it never became reality as there were too many air-glass surfaces - light losses and reflections made a practical eyepiece impossible.

When modern good multilayer dielectric AR coatings became standard in consumer-grade eyepieces in the 1980s... voila, Masuyama, Vixen, Al Nagler all jumped on board and we have modern ultrawide eyepieces with long eye relief, virtually all stemming from Smythe's idea.

[Steffen]

Thanks Wavy, that's what I thought. Incidentally, I sold my last Barlow a couple of years ago, so my question was purely academic. Thanks for your trademark comprehensive reply

Cheers
Steffen.

[PeterHA (Peter)]

I can recommend the TeleVue Delos, very good eye relieve, very good optics and if the adjustable and lockable eyecup is set correctly they are great to use. I have several and all work well with my seasoned Celestron Ultima 2x Barlow. When I purchased my Barlow in 1993 I compared TeleVue, Celestron and Meade on my scopes and the Celestron Ultima was the winner. Not sure if they are still made.

[Richard Gamble (Richard Gamble)]

You are likely to get the TMB clones on ebay.
The genuine TMB is a cut above.