Eyepieces recommendation needed

[Somnium]

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]

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]

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-and-Barlows/TeleVue-/Nagler/TeleVue-Nagler-VI-13mm/187/productview.aspx

From there you can either get a 2.5x barlow:

http://www.bintel.com.au/Eyepieces-and-Barlows/TeleVue-/Barlows-and-Powermates/TeleVue-Barlow-br--2x--1-25--/876/productview.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]

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]

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/accessories/eyepieces/baader-eyepieces/baader-planetarium-hyperion-eyepiece.html

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

[Somnium]

Cool, thanks

[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. 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]

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]

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]

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]

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]

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

Thanks

[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

[Somnium]

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]

Hi Aidan

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

Cheers
John B

[janoskiss]

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

[Somnium]

A 14.5 iDK on a paramount MX

[ausastronomer]

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]

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

[Renato1]

I go along with the notion that a 2mm exit pupil is best viewing galaxies at a dark site, which translates to a 13.4mm eyepiece. Which in practical terms is either a 13mm or 14mm eyepiece.

On my dob, I wanted a very wide field of view because of the pushing I was doing, whereas on my C8 a normal wide angle sufficed.

There's no need to rush out and buy an expensive new eyepiece. 13mm Naglers show up here or on Ebay every so often. So too does the old Meade 14mm Ultra Wide angle (great eyepiece, rarely expensive, but darn heavy), and so do various other eyepiece brands that have good reviews. Some patience is all that is required.

For high power, the typical recommendation is an eyepiece that delivers a 0.5mm to 1mm exit pupil (3.3mm to 6.7mm eyepieces). So in the first instance, after you get your eyepiece for galaxies, just use it in your 2X Barlow. For 10% higher power, don't put the eyepiece all the way into the Barlow.
Regards,
Renato

[janoskiss]

John B (ausastronomer) delivers rare informed insight yet again. Listen to him. The large secondary will indeed be a problem for high powers. I've only ever looked through one astrograph, but my limited experience correlates with optical theory that it's basically a difficult to control blackout effect that is no issue for a camera sensor because you position it precisely at the focal plane, but for visual you can't help your head moving back-and-fro and side-to-side a little, and a large SO will be an intolerable annoyance at mid-high powers.

LVW 22mm discontinued? Oh no! http://astroshop.com.au still have it listed...? It's a great EP, one of the best.

[SkyWatch]

Thanks for the comments John. I am interested in the idea that eyepieces can perform poorly in an f10 SCT because of the fast primary, but well in a refractor or newt. Do you have references to any studies about this? It would be an interesting read. I am guessing that it is much more an issue for wide-field eyepieces than the more traditional 40-50 degree fields of plossl's, orthos etc?

Empirically I haven't really noticed a major problem with relatively poorer performance in SCT's. For example I (and a couple of experienced friends) tried a 19mm Panoptic alongside a 20mm GSO SV in a range of scopes, including a 8" SCT, an f5 dob (with and without coma corrector), and an f8 refractor. The edge sharpness and useable (sharp) field of each eyepiece in each telescope was surprisingly similar, especially given the huge cost difference. I ended up returning the Panoptic and bought a 18mm Radian which was a huge improvement (giving sharp stars right across the field in the range of scopes), and my friend with the SCT kept using his GSO SV20 as he felt there was no justifiable reason to "upgrade" to the Panoptic in terms of sharpness of the view...

I suggested the GSO's because the IP said that he wanted thoughts about an "8" dob" of eyepieces. I interpreted this to mean low-price but still producing reasonable quality views. The GSO SV's I have used have performed surprisingly well given their price point, so I thought they might fit his requirement.

I hadn't noted the size of the secondary on his scope though: it does make a big difference. I agree that the Vixen LVW would be worth checking out if he wants to spend more. The 24mm Panoptic is also a nice eyepiece (I remember trying one of them at the same time as I checked out the 19mm version, and it performed much better).

All the best.

Dean

[Somnium]

thanks for the details John. i know these scopes are not ideal for visual and i didn't really intend on putting an eyepiece on it, just wanted to know my options. i will toss up the options of getting a dedicated visual scope (a dob) or a ~20mm eyepiece. Thanks again

[N1]

Aidan, it would have been interesting to hear what the view through your instrument was like. I thought it was a great idea, and I hope you will one day put an eyepiece in it and let us know your observations.

[Somnium]

I will put an eyepiece in regardless but it is just a matter of whether I buy one specifically for the scope. I will let you know how it goes

[ausastronomer]

Hi Aidan,

I wasn't trying to discourage you from using the scope as a visual instrument, only to help you set your expectations at the right level. At low to medium power it will do a fair job and perform like a 12.4" scope because of the light loss due to the large central obstruction. At low and medium powers the image quality should still be reasonable. At high powers the images will definitely fall apart. I am not sure at what point this will happen. You should at least be able to run to about 150X with decent images as this is still only ~10X per inch of aperture. Hence my recommendation of using a 20mm to 25mm eyepiece. It may well go somewhat higher than this before it falls over. There's a lot going on here to affect the high power images and knowing where it breaks down is just going to be trial and error. As I mentioned in my previous post the large secondary obstruction will have a major affect on the MTF curves and it will also cause significant diffraction. In addition to this however there will be introduced spherical aberration because of the change in focus position. All cassegrain telescopes achieve focus by changing the intercept distance between the primary and secondary mirrors. In most cases the primary mirror moves relative to the secondary mirror and the telescope tube itself. This is a good thing for imaging because it creates a large back focus distance range for small amounts of mirror movement. Unfortunately it also introduces a problem which doesn't manifest when imaging, because the telescope is designed and optimised for this purpose. Cassegrain telescopes have specific curves on their secondary mirrors calculated for a given overall telescope focal length, a given intercept distance between the 2 mirrors and a given secondary amplification factor. In other words the secondary is ground to be correct for a specific overall focal length only, incorporating a known back focus distance and this is usually optimised based on the telescopes intended use, which in this case is imaging. As soon as you vary the distance between the primary and secondary mirrors the curves on the mirrors are no longer correct and spherical aberration is introduced into the system. The amount of spherical aberration introduced is dependent on many things including the focal length and ratio of the primary mirror, the secondary mirror amplification and the change in intercept distance between the two mirrors. As a rough guide for an average type F8 Cassegrain Telescope using an F2 to F3 primary mirror, if you change the back focus distance by 35mm you introduce a 1/4 wave of spherical aberration. If you change the back focus distance by 50mm you introduce a 1/3 wave of spherical aberration. When you use an eyepiece in this scope it will require somewhat more focuser out travel than is normally required when imaging, this causes introduced spherical aberration and the optical system will become over corrected. The same scenario applies to all cassegrain type telescopes notwithstanding that different cassegrain type designs use different curves on their primaries and secondaries, to achieve correction for spherical aberration. In the case of this telescope which is a Corrected Dall Kirkham it uses an ellipsoidal primary mirror with a spherical secondary mirror. Assuming the focus position between camera and eyepiece changed by 35mm the telescope system would be 1/4 wave over corrected and that's presuming you started with perfect optics. If you used a barlow which would generally require even more focuser out travel than just a native eyepiece the system would become further over corrected and the images would deteriorate even further due to the additional over correction. If the eyepiece in fact required additional focuser in travel compared to the focus position with the camera the system would become under corrected by an equivalent amount based on the change in back focus distance. With an imaging platform like this, the back focus distance is calculated and optimised to achieve focus on a CCD chip not on the focal plane of an eyepiece. Consequently you have a large secondary obstruction and a significantly over corrected optical system wreaking havoc on the high power image quality. These issues are a lot less of a concern at low and medium power.

Cheers,
John B

[ausastronomer]

Hi Dean,

I tried to put this into really simple terms and unfortunately it's not that simple. I was trying to get the point across that you can't just make a general blanket statement that because a telescope has a slow F-ratio an eyepiece should perform fine in it. There is a lot more involved than just the F-ratio of the telescope system. Some people presume that if a telescope has a slow F-ratio that most eyepieces, including cheap ones which will not perform in fast scopes, will perform well because of the slow F-ratio and this isn't always the case, particularly, when it comes to the various Cassegrain designs. The overall eyepiece performance in any given telescope system is based on 3 factors:-

1) The aberrations inherent in the telescope
2) The aberrations inherent in the eyepiece
3) The aberrations introduced or removed by the interaction of the particular telescope and eyepiece.

One of the most predominant aberrations is field curvature of both the eyepiece and the telescope. This can be reduced when each component has a different direction of curvature and compounded when they have the same direction of curvature.

Another common aberration is coma of the telescope and to a lesser extent internal coma from the eyepiece.

and another common aberration is off axis astigmatism from the eyepiece. This is always a predominant aberration with cheaper eyepieces and it sometimes swamps and masks all the other aberrations going on at the same time.

With a slow F-ratio telescope the off axis astigmatism is always reduced irrespective of telescope design. This occurs because the angle of incidence of the light rays entering the field lens of the eyepiece is reduced and the light rays enter the eyepiece in a much more parallel fashion, rather than at a steep angle. Internal eyepiece Coma is also reduced in slower telescope systems regardless of design.

An F10 Cassegrain Telescope (irrespective of design) still has the light rays entering the field lens of the eyepiece at the same angle of incidence as any other type of F10 telescope, so it will in fact offer reduced off axis astigmatism, compared to faster newtonians and refractors and cheaper eyepieces like GSO superviews will definately perform better in an F10 SCT than they will in say an uncorrected F5 newtonian, in terms of off axis astigmatism and internal eyepiece coma.

Where it gets complicated is that depending on the type of cassegrain design some of the telescopes aberrations correlate to the F-ratio or the focal length of the primary and not to the overall F-ratio of the system. For instance a traditional Schmidt Cassegrain exhibits Coma and Field curvature correlating to the focal length of the F2 primary and not the overall F10 system. Depending on the eyepiece interaction with the telescope, these can manifest as pretty strong aberrations. With a traditional Dall Kirkham (non corrected) the predominant aberration is off axis coma which again correlates to the focal length of the primary mirror. In slow F-ratio dall Kirkhams (F15 to F20) using a fairly slow primary mirror of say F4 to F5 then eyepiece performance will be very good as coma will not be severe. In say an F8 DK with an F2 primary coma will be severe and eyepiece performance will suffer.

I don't know where you would find any emperical testing data on this. I have done a bit of testing on this over the years and my findings have been consistent with historical optical theory. If you are interested in understanding a bit more about this and other aspects of telescopes, eyepieces and optical performance there are a couple of good books well worth reading. Historically the best resource to learn about this type of stuff was a book called Telescope Optics - A Comprehensive Manual for Amateur Astronomers by Rutten and Van Venrooij. (http://www.willbell.com/tm/tm6.htm) This was originally released in the late 1980's. My edition is about a 2002 reprint. Whilst this doesn't have information about the latest in eyepiece and telescope designs included it still does an excellent job of explaining a lot of the basic optical principles which still apply with todays modern eyepieces and telescopes. A much more recent release book which is really excellent on how it explains things and also the fact it incorporates all the latest optical designs is Telescopes, Eyepieces and Astrographs - Design, Analysis and Performance of modern astronomical optics by by Gregory Hallock Smith, Roger Ceragioli and Richard Berry (http://www.willbell.com/tm/TelescopesEyepiecesAstrographs.html ) This is a 2012 publication and contains the latest of everything. I highly recommend it.

Cheers,
John B

[SkyWatch]

Thanks John.

Regards,

Dean

[Somnium]

Hi John

thanks for the response, the real reason why i was keen to get an eyepiece on this is just to take advantage of the aperture and get some decent planetary views as well as a couple of DSO's. but from what you say it wont really be possible. it was very much an after thought, i got a telescope to put a camera on the back, not really for visual but it is nice to let people look through it when they come down and see my observatory. there is nothing that will connect you to space more than seeing Saturn through an eyepiece. but if it is better to do it through a 10 - 12" dob then i will do that.

just one thing you mentioned that was new to me was around the focusing of cassegrains. you say that all cassegrains achieve focus by manipulating the distance between the primary and the secondary, i understand that is the case for SCTs but with a CDK the primary and secondary are fixed. if extra travel is needed there is no manipulation of the primary to secondary distance, you just move the eyepiece further back. so how would this impact the optical performance or am i fundamentally missing something here?

[Atmos]

From what John was saying it sounds like the focus thing shouldn't effect your CDK as it isn't what would be considered a classical SCT. From memory the DK design works with two hyperbolic mirrors as opposed to having a parabolic primary on the SCT. The CDK also has a corrector (possibly in front of the secondary??).

As for how this translates to visual performance outside of the large secondary... I have no idea :)

[Somnium]

Hi Colin,

the CDK has a parabolic primary and a spherical secondary. the corrector lives just in front of the focuser so basically smack in the middle of the primary mirror itself.
those were my thoughts too, given that the primary and secondary are not being shifted to achieve focus, i am not sure how the optical performance will be impacted. for sure the secondary is going to be a problem, particularly for high contrast ... but is it going to have the same issue as a SCT. John probably knows a heck of a lot more about this than i do so keen to get his thoughts, i might have missed the mark in terms of understanding the issue.

[ausastronomer]

Hi Aidan,

You didn't miss anything. I didn't word my post very well at all. I should have said "a lot" as opposed to "all"; cassegrains achieve focus by changing the distance between the mirrors". I don't even know why I said that because I have used several cassegrains that had a moving focuser train as opposed to moving the mirrors.

On the basis that the intercept distance between the mirrors remains fixed there will not be any introduced spherical aberration. The only issue which will affect the high power images is the large secondary obstruction which will affect the MTF curves and cause diffraction.

I have prepared some simulated images of Jupiter using Aberrator software, which is a free download from this linked website, to attempt to show the likely impact on the high power images of the large central obstruction. (http://aberrator.astronomy.net/)

The left side simulated image below shows Jupiter as you would expect it to appear in good seeing using an unobstructed 368mm (14.5") telescope, assuming perfect optics.

The right side simulated image below shows Jupiter as you would expect it to appear in good seeing using a 368mm (14.5") telescope, assuming perfect optics, but in this case introducing a 51.7% Central Obstruction.

If you're not a discriminating visual observer you may well be very happy with the images you get, notwithstanding they won't equal what you would get from a high quality 10" to 14" newtonian. The only way to find out is to throw an eyepiece in it and see what happens. As I mentioned previously low to medium power should be fine, you just lose some light gathering from the large Central Obstruction, high power you just have to suck it and see if you're happy with it.

I have used other Corrected Dall Kirkhams (CDK's) for visual astronomy including a Takahashi 250 Mewlon and a 14" Planewave. The Tak Mewlon has a relatively small central obstruction (~28%) compared to other CDK's and performs very well as a visual instrument and IMO is just a little behind a high quality equal aperture Newtonian. The Mewlon also has a slower F Ratio (F10) compared to the specialist imaging CDK's. The 14" Planewave, which has a ~49% Central Obstruction, came up well short of the best 14" scopes I have used. That doesn't mean however that you wouldn't have been happy with it. Over a long period of time I have used some of the best visual telescopes on the planet, so I can be a bit more discriminating at times than some observers.

Cheers,
John B

[ausastronomer]

Hi Colin and Aidan,

Neither of these are correct. By definition a Dall Kirkham uses a concave elipsoidal primary mirror and a convex spherical secondary mirror. If a telescope has some mirror configuration other than this it isn't a Dall Kirkham. A Corrected Dall Kirkham introduces a sub aperture refractive corrector, to correct off axis aberrations inherent in the standard Dall Kirkham design. The corrector I believe is likely to be a 3 element modified Wynne Corrector.

For those interested the following Cassegrain Telescope Designs have the following mirror configurations:-

Schmidt Cassegrain = Spherical Primary and Spherical Secondary with a full aperture Schmidt Corrector Plate

Maksutov Cassegrain = Spherical Primary and Spherical Secondary with a full aperture Maksutov Corrector Plate.

Houghton Cassegrain = Spherical primary and Spherical Secondary with a full aperture Houghton corrector which is a multi element corrector. Unlike the Schmidt and Maksutov correctors which are single element.

Classical Cassegrain = Parabolic Primary with a Hyperbolic Secondary. A Classical Cassegrain will often have a Newtonian focus point in addition to the Cassegrain focus point where the hyperbolic secondary is swapped out for a diagonal flat.

Dall Kirkham = Ellipsoidal Primary and Spherical Secondary

Press Carmichael = Spherical primary and Ellipsoidal Secondary

Ritchey Chretien = Hyperbolic Primary and Hyperbolic Secondary

Cheers,
John B

[Somnium]

Thanks for that John , in my mind I was inventing physics to explain why a fixed primary secondary distance varied :) needless to say it didn't quite fit, though I am sure there are some string theory equations in there ... Anyway, that type of aberration is fine. I would be more than happy to get that type of view. I have a fair few people come to my observatory and they all want to look through the telescope and showing them an image on the screen is a little less satisfactory than putting an eyepiece on and showing them Saturn . I think I will win a few astronomy fans with that. The scope will always be an imaging not scope but it is good to know I can put an eyepiece on it and still have a decent view.