Eyepieces recommendation needed

[Somnium (Aidan)]

Currently, the only eyepieces I have are the 10mm and 25mm 1.25" ones that came free with a very entry level scope , needless to say these do not provide stellar views. While 99.9% of the time I will have my camera connected to the scope, I was looking for a decent couple of eyepieces to use, 1 for planetary work and another for Galaxy/nebula viewing. I am not looking to spend a whole lot, I was looking nice for something in the gap range but will work well with the following gear

14.5" cdk
F6.7
I also have a 2" 2x Barlow and a 5x 1.25" powermate.

Any suggestions? Is there a standard recommendation, I.e What is the 8" dobsonian of eyepieces?

[N1 (Mirko)]

Originally Posted by Somnium

Is there a standard recommendation, I.e What is the 8" dobsonian of eyepieces?

What I like about "standard recommendations" is that you can do the exact opposite and not be worse off. Want one anyway? Aim for a 2mm exit pupil.

[Atmos (Colin)]

As Mirko said, you want ~2mm exit pupil for doing galaxy viewing, this equates to about 13mm eye piece. What comes to mind for that is a TeleVue Nagler VI 13mm
http://www.bintel.com.au/Eyepieces-a...oductview.aspx

From there you can either get a 2.5x barlow:

http://www.bintel.com.au/Eyepieces-a...oductview.aspx

Or go down the path of getting a good quality 5mm plossl or ortho, you don't need to worry about FOV doing planetary, just need a good quality tunnel vision bit of glass

[Somnium (Aidan)]

Thanks guys, though $480 is probably a bit more than i was hoping for.

i would have thought that you want a larger lens, like a 20 for galaxies and nebulas and a 10 - 13 for planetary ... is that incorrect ?

[Atmos (Colin)]

There is a 13mm Baader Hyperion for $210, I have heard good things about these eye pieces, never used one though.

http://www.ozscopes.com.au/accessori...-eyepiece.html

You can probably keep the 13mm for galaxies and planetary, the 2mm exit pupil is a pretty good all rounder

[Somnium (Aidan)]

Cool, thanks

[N1 (Mirko)]

A combination of your two existing focal extenders and one decent 1.25" EP between 24mm and 32mm (plus an adaptor for the 2" barlow) may actually get you close enough to everything mentioned in this thread so far. If that was my approach, I'd go for low glass (Abbe or Plossl), but that's just me and other designs will probably work just as well.

A concern may be what powers your seeing usually supports.

[Somnium (Aidan)]

Originally Posted by N1

A combination of your two existing focal extenders and one decent 1.25" EP between 24mm and 32mm (plus an adaptor for the 2" barlow) may actually get you close enough to everything mentioned in this thread so far.

I already have a 1.25" adaptor. Any specific Ep design recommendations?

[MattT]

What is the 2x Barlow...GSO or other?

From my extensive mucking around with Barlow's. I found adding a 35mm extension tube to a GSO 2" Barlow, makes it a 2.5x Barlow. Guessing at this with visual comparing with higher mag eyepieces, and 4 eyepieces will then get you a whole range of magnifications.

My suggestion...GSO Plossls 32 25 20 and 15 with the 'modified' Barlow.

A more expensive version is TV Plossls and a 2.5x Powermate.

32 25 20 15 add the Barlow @2.5x and put the 32 in again and work down from there to get...12.8 10 8 6...no duplication of mags.

[Somnium (Aidan)]

Originally Posted by MattT

What is the 2x Barlow...GSO or other?

From my extensive mucking around with Barlow's. I found adding a 35mm extension tube to a GSO 2" Barlow, makes it a 2.5x Barlow. Guessing at this with visual comparing with higher mag eyepieces, and 4 eyepieces will then get you a whole range of magnifications.

My suggestion...GSO Plossls 32 25 20 and 15 with the 'modified' Barlow.

A more expensive version is TV Plossls and a 2.5x Powermate.

32 25 20 15 add the Barlow @2.5x and put the 32 in again and work down from there to get...12.8 10 8 6...no duplication of mags.

Both the powermate and the Barlow are televue

I will have a look into it, I think I need to go back to a star party and look through some eyepieces , I haven't done that for a while

[janoskiss (Steve H)]

8" Dob of EPs: good plossls or related improved designs, e.g., Orion Ultrascopic, Parks Gold Series. A 15mm would be a good all-rounder for your scope. Televue Plossls are highly regarded though I prefer Parks GS and the like. In Dobs at least, the TV Plossls vignette somewhat, especially when barlowed. Also the colour is bit too warm for my taste.

If you want cheap as chips but good 20mm GSO plossl is decent and will also give you a usable high magnification when 2x barlowed. (15mm is not so good in that series: narrower below-spec FOV, but last I checked one out was ~7 years ago.)

The 2mm exit pupil rule is good, but it breaks down for very large scopes. Your scope is borderline getting too big in aperture for 2mm ex.pup. to be your default DSO EP (depends on what targets you're most interested in though). You're looking at a moderately high ~165x magnification, where many of the larger DSOs will not fit in the FOV (unless you get an ultra- or hyper-wide, but that won't be no 8" Dob of EPs).

[N1 (Mirko)]

The advantage of Plossls and Abbes is that even a good one will not cost the Earth, and so you could get several focal lengths if you wanted to experiment. They don't provide spacewalk vistas though.

[Somnium (Aidan)]

cool, looks like at least a 20mm Plossl is the way to go.

Thanks

[SkyWatch (Dean)]

You could try the GSO SV 68-degree series. 1 1/4's are 15mm, 20mm, and 2" are 30mm, 42mm and 50mm.
They should be OK in f6.7 optics. Andrews have them for 10% off at present...
The Long Perng planetary series (same as Orion "edge-on" but 1/2 the price) are excellent, with great eye-relief. 3mm-18mm available also at Andrews.

The "1-2-4 rule" is a good one to go by:
1 mm exit pupil for high magnifications (globular clusters, binary stars, bright small planetary nebulae)
2 mm exit pupil for galaxies, open clusters, and details in bright emission nebulae
4 mm exit pupil for low surface brightness objects, and for very large objects.

All the best,

Dean

[Somnium (Aidan)]

Originally Posted by SkyWatch

You could try the GSO SV 68-degree series. 1 1/4's are 15mm, 20mm, and 2" are 30mm, 42mm and 50mm.
They should be OK in f6.7 optics. Andrews have them for 10% off at present...
The Long Perng planetary series (same as Orion "edge-on" but 1/2 the price) are excellent, with great eye-relief. 3mm-18mm available also at Andrews.

The "1-2-4 rule" is a good one to go by:
1 mm exit pupil for high magnifications (globular clusters, binary stars, bright small planetary nebulae)
2 mm exit pupil for galaxies, open clusters, and details in bright emission nebulae
4 mm exit pupil for low surface brightness objects, and for very large objects.

All the best,

Dean

Thanks for that Dean. i guess you can only really understand how these things impact the view when you see the difference. for me, eye relieve, exit pupil, FL (for EP) are numbers. i get what they mean conceptually but there is no substitute for direct comparison.

[ausastronomer (John Bambury)]

Hi Aidan

What telescopes are you planning to use the eyepiece in and what mounts are those scopes on?

Cheers
John B

[janoskiss (Steve H)]

SV will still show a fair amount of aberrations in the outer 1/3 of the FOV at f6.7.

[Somnium (Aidan)]

Originally Posted by ausastronomer

Hi Aidan

What telescopes are you planning to use the eyepiece in and what mounts are those scopes on?

Cheers
John B

A 14.5 iDK on a paramount MX

[ausastronomer (John Bambury)]

Originally Posted by SkyWatch

You could try the GSO SV 68-degree series. 1 1/4's are 15mm, 20mm, and 2" are 30mm, 42mm and 50mm.
They should be OK in f6.7 optics. Andrews have them for 10% off at present...

Dean

Hi Dean,

With compound cassegrain design telescopes like SCT's, Maksutovs, Classical Cassegrains, Ritchey Chretiens, Dall Kirkhams and in this case a "corrected Dall Kirkham" you cannot relate eyepiece performance to the overall F-Ratio of the optical system, as many of the aberrations manifest based on the F-ratio of the primary (usually very fast) and not on the F-ratio of the system.

For instance, lets use a standard F10 SCT as an example. Most of these use an F2 primary with 5X secondary amplification, the Celestron C9.25 being an exception using an F2.5 primary. These scopes will exhibit coma and field curvature based on the F2 primary and not on the F10 overall system. Consequently many eyepieces which perform very well in an F10 refractor or newtonian, perform poorly in an F10 SCT.

That all having been said while the telescope in question (14.5" corrected Dall Kirkham) is not a great visual telescope due to its imaging optimisation and very large secondary obstruction (>50%) I would still be inclined to aim a bit higher than a $60 eyepiece in over $30,000 worth of telescope and mount.

Cheers,
John B

[ausastronomer (John Bambury)]

Originally Posted by Somnium

A 14.5 iDK on a paramount MX

Hi Aidan,

I suspected that from your original post, but I just wanted to be sure there were no other different telescopes involved. As you would be aware this telescope is designed and optimised as an imaging platform and not as a visual telescope. It has a secondary obstruction of 7.5" (51.7%) which will cause major issues with some aspects of visual optical performance. The large secondary obstruction does not impact its performance as an imaging platform.

For visual use the large secondary obstruction causes significant diffraction and will have a major effect on the MTF curves, which will result in poor performance at higher powers, which you might normally use on a 14.5" telescope for lunar, planetary and double star work. Consequently, I wouldn't be spending a lot of money on high quality eyepieces for high power work. As a telescope for low to medium power work on DSO's and other extended objects the telescope will do a fair job. The 7.5" secondary obstruction will reduce its light gathering power to be the equivalent of an unobstructed 12.4" telescope. Whilst the large secondary obstruction does not affect the resolving power of the telescope itself, it will limit your ability to run higher powers and take advantage of the resolution capabilities of the telescope.

All of the above having been said the telescope warrants "decent" eyepieces IMO for low to medium power use. The telescope has a focal length of 2.47 metres (fairly long). I would be aiming to get an eyepiece which gives around 100X to 130X for general Deep Sky use in this scope, which means about a 20mm to 25mm eyepiece. Two good ones which immediately come to mind are the 22mm Vixen LVW and the 24mm Televue Panoptic. The Panoptic can still be sourced new, but the 22mm Vixen LVW would have to be sourced 2nd hand, but they are reasonably available on the used market. With patience you should be able to find either eyepiece for under $300 second hand. These eyepieces generate about a 3.5mm exit pupil which is ideal for use with visual deep sky filters like UHC and OIII also.

Cheers,
John B