Any point to a 2" EP for a f/4.7 scope?

[Admiral Crispy]

Hi all, I have just been looking at several 2" EPs and had been debating whether to go and get one or not.

My scope is a Skyliner 250px Dob, f/4.7, so ideal for DSO and I was thinking of getting a 2" EP for say a 70 degree view, so that I can get a wider angle of several Messier objects (eg. M42/3, M31/2 etc.). However, I have read that the fast focal ratio of the scope makes it hard to get decent results out of a large EP as astigmatism sets in towards the edges of the field, and sometimes markedly so. Problem is, I don't really want to spend a huge amount of money on a TV Panoptic or the like for over £250.

I had seen that GSO do their own brand of 2" EPs for £50, but was told that they would suffer with fast scopes.

All of this has led me to wonder, is there any point to a 2" EP for a fast f/4.7 scope? Is it even possible of getting a decent one for a less than extortionate cost that the TV brand has (even if the cost is deserved)?

And if there are decent alternatives to this, what are they?

All of this is putting me off the 2" EPs, and am unsure if this is a pursuit I should abandon, so any advice would be very welcome!

[Admiral Crispy]

Oh, and just to add before, would I be better off sticking to my 1.25" Plossl eyepieces instead?

I had been thinking if getting an OIII filter to improve views of Veil, Ring and other nebulae, so should I go in this direction instead?

Please help!

[Damo636]

Hi Chris. At f4.7 your 250px is pretty fast and will demand £££ being spent for decent edge correction in a widefield eyepiece. I have used the GSO 2" Superview at f6 and agree it would be a poor performer at f4.7. A 32mm Televue plossl would be a fine compromise giving the maximum fov in the 1.25" format. They are tested down to f4 so would give excellent views in your scope. An Oii will help with the Veil and the Owl nebula but i personally haven't found it any additional help with the Ring. A UHC is considered more of an all rounder.

[Chris H]

I use a Skywatcher Nirvana 28mm ep in my 10" F4.7 Dob and it works great, not as expensive as a Nagler 31T5 (£475) but still not cheap at £245. Combined with a UHC filter it gives me excellent views of the Veil but my skies are pretty dark.

[JamesC]

Don't forget about exit pupil and coma.

Exit pupil is entrance pupil divided by magnification, or eyepiece focal length over focal ratio. So in your case a 28mm eyepiece would give you 28/4.7=5.9mm exit pupil. If your pupil is smaller than that then you will be 'wasting' light from the edge of your mirror. Mind you I think people worry about this excessively. Sometimes you just want that wide angle view and who cares if you are using a little less of your mirror?

I find that at f5 I'm not overly bothered by coma in 1.25" eyepieces though others might disagree. However when I go the wider fields offered by a 2" it does bug me. Coma correctors aren't cheap but they do improve the view. I'm not sure I'd like to offer advice about coma correctors. I use the Baader MPCC photovisual kit. Trouble is the draw tube isn't 2" all the way down, so I can't get the eyepiece all the way in with the corrector on the back. I'm also short sighted which means I need yet more inward travel so I end up using the 14mm adapter rather than the 28mm one, but am still satisfied with the view. Maybe ask around what would be suitable for your scope.

In a fast scope you need good eyepieces. I like the Meade 5000, although I haven't tried a lot of alternatives. I tried cheaper eyepieces, didn't like them so increased my spending until I found something I liked. In your case I'd be tempted by the 28mm SWA. GSO do some nice kit but without having tried the 2" eyepieces you are talking about, I doubt you will be satisfied at f4.7.

OIII filters are great. To be honest I've only really needed it on the Veil, but it was worth it. So much spending. Maybe I've put you off. I'd hate to add up what I've spent over the last few years...

[JamesC]

Oh yeah, about the filter. I only observe from dark sites which is probably why I haven't found as much benefit from filters as others. Dramatic effect on the veil though, even from a dark site.

[John]

The Owl Nebula and a few others show a big difference when an O-III is used on them but otherwise mine tends to stay in it's case quite a lot too. I prefer the unfiltered views of M42, M27, M57 etc.

[cs1cjc]

Two midrange wide angle eyepiece types which ought to perform well at f/4.7 are the Baader Hyperion Aspheric (31mm and 36mm) and the Skywatcher Aero (30mm, 35mm and 40mm). I certainly hope so because I intend to buy one. Currently I am tending towards the 31mm Aspheric, apparently giving a slightly wider true field of view at a reasonable exit pupil than the Aero.

[Damo636]

I don't think the Aero will perform well at f4.7. I had the 35mm recently and it was only sharp till about 80-85% of its 68 degree fov in my f6 dob.

[TheThing]

There is a lot of misinformation about fast scopes and EPs. Truth be told, you don't need to spend mega bucks to get good results.

Best advice? Try them and see!

To be perfectly honest, who pays much attention to the edge of the FOV? You look at the centre, look around a bit a your OTA if needs be!

[John]

I don't think the Aero will perform well at f4.7. I had the 35mm recently and it was only sharp till about 80-85% of its 68 degree fov in my f6 dob.

To be honest that's pretty good performance I reckon. To get better you would have to double or triple the investment

I know that astigmatic stars in the outer parts of the field of view bothers some people (including me ) more than others though. But the price to pay for "sharp right across" at F/4.7 is pretty steep.

[Damo636]

It's good advise to try them and see, i did and found i couldn't stand seagulls, even if was in the outer fov.

[Moonshane]

The problem is that in a fast scope like yours you will see any deficiencies in the eyepiece (like astigmatism etc) and this will mask the coma in the primary mirror. Once you buy a very expensive eyepiece with no (or few) deficiencies then you see the coma in the primary mirror!

One option is a good coma corrector. I have an f4 newt and the difference with a Paracorr (I got one used for about £165) is incredible with all or most of my eyepieces - it makes the scope perform like an f8.

I suspect that you might get better performance from other eyepieces too although I have not tried a budget eyepiece with it in truth.

As others have said though, these things bother some more than others and give a used wide field budget eyepiece or around 25-30mm a go and see what happens. If you buy used then you can sell on again at about the same price if you don't like it.

[DOBBY]

a 32mm panaview from skywatcher + a decent OIII filter in an F4.5 dob suprised even hardened A/P ers

[cs1cjc]

It's good advise to try them and see, i did and found i couldn't stand seagulls, even if was in the outer fov.

To me "seagulls" sound like coma, which originates in the mirror, not in the eyepiece. I find a coma corrector cleans up the edge a lot even in a cheap 38mm Adler Skarp.

[John]

My understanding is that coma makes star images look like mini comets, with their tails pointing away from the centre of the field of view. With astigmatism the star image is extended in two directions, hence the "seagul" reference although astigmatism can take other forms as well.

I find this web page helpful on this:

Got Aberrations?

[cs1cjc]

With astigmatism, a star shows as a cross at best focus. On one side of the focus it will show as a radial line and on the other side a tangential line. With coma, the star is smeared outwards radially and to a lesser extent tangentially, but the diagrams here show that, off the spot, most light lies on the lines splaying outwards.

Coma aberration

[Damo636]

To me "seagulls" sound like coma, which originates in the mirror, not in the eyepiece. I find a coma corrector cleans up the edge a lot even in a cheap 38mm Adler Skarp.

I don't have the experience to differentiate between coma & astigmatism Chris but i would hazard a guess it was more likely the latter. The 35mm Panoptic that replaced it was sharp right across the fov. The 28mm UWAN that i eventually ended up with is also sharp for almost its entire 82 degree fov. I agree with John in that some folk are more tolerant to edge of field abnormalities than others. I personally hate mushy stars anywhere in the fov.

[rwilkey]

As mentioned above, I would say the SW PanaView is a safe bet, a great ep though not used it in anything faster than f/5.

[Admiral Crispy]

Hi guys, thanks for all the advice and discussion, it has been very useful! I have to say, I would probably be very happy with 70% ish of field sharp in an eyepiece, as I don't get too fazed by some astigmatic stars! After all, I can always move the EP more to get a sharper view of certain areas.

I have been advised by some friends to get a GSO Revelation 2" , and some say a Panaview, and wondered what people thought considering my fairly OK field of view acceptance for astigmatic stars?

I will look into a UHC filter as well, as I got to say, any improvement on nebulae, which are my fave DSOs, will be worth it!

[rwilkey]

I really do think the SW PanaView will be more rewarding and a better quality view than the GSO.

[great_bear]

It's worth mentioning that large exit-pupils do lead to a somewhat washed-out sky if you've not got dark skies. For me - with city-levels of light pollution - an exit pupil approaching 5mm or more spoils the view with its washed-out appearance.

[Admiral Crispy]

Which would you go for out of a 26, 30 or 32mm EP for a f/4.7? I take your point great bear and I agree that the sky looks darker the higher the magnification. But I also like the view out of my 32mm plossl as well and how wide it is. So what do people think?

PS. Thanks to all for the advice, and convincing me that 2" EPs are worthwile, as I was getting a bit sceptical!

[Moonshane]

personally I went for a 26mm 82 degree eyepiece which is as big an exit pupil as I wanted at 6.5mm (in fact effectively 5.6mm as the coma corrector I use creates a slightl barlow effect of 15%).

30mm are a little more common though I think and this might be a cheaper option.

[russ.will]

FWIW, my Skywatcher Panaview 2" 32mm (70deg FOV) is my most used EP. Okay, I'm not badly affected by street light polution (just the occasional neighbours drive lights) but it does seem to give the highest contrast views of the sky and it's like putting your head inside the view. I state that against my limited experience of a particularly wide range of EPs, beyond a few snatched views here and there. I'm also not particularly bothered about the outer few % of the FOV, so your mileage may vary.

It's in another league compared to my 2" 26mm Revelation, which is a lot fussier about eye positioning, no better at the edges and even when you've achieved all of this, seems to lack the contrast 'pop' of the Panaview. It's a lesson in buying second hand. I'll probably flog it at no loss to someone who it suits better and wonders what the dickens I'm on about!

What I would add is that I recently made dew shield out of cheap foam sheets - less than a fiver in fact. That has made large difference to contrast with all of my EPs. I didn't make it because of any particular dew issue, but just to see if shielding the secondary from any stray light, however limited, helped. It does, even when viewing something bright like Jupiter.

Russell