Expanding EP Collection

[Robny]

Hi All

Within the next week or so I'd like to expand my collection of eyepieces, my kit is as follows (also seen in my sig)

• SW Quattro S8
  
  • FL 800mm
    
  • F/4
    
  • 8"
    

  [*]Vixen NPL 25mm

  [*]Vixen NPL 15mm

I'd like to continue with Vixen NPL's but add:

1. EITHER two more eyepieces
   
2. OR one eyepiece and a Barlow
   

There lays my indecision and any advice would be welcomed.

I was thinking of including a Vixen NPL 6mm and Vixen NPL 10mm for option number 1

I was thinking of a revelation x2 ED Barlow and a......

Ultimately I enjoy DSO's which i use my 25mm for - but have a craving to see Jupiter's Bands and Saturn's Ring's which is why I'm going for the 6mm and 10mm to fill a gap. Or maybe i could get the 40mm and 6mm with no Barlow and that would give me - 40mm, 25mm, 15mm & 6mm.

Please help

Yours confused

Rob

[The Flinty Fox]

Hi Rob.

It were me I would not get the 40mm, too much exit pupil (about 10mm). I would go for the 10mm plus barlow as then you would be able to barlow the 15mm also. This would give 32x 53x 80x 107x and 160x which would do a nice job on the planets and moon.

Hope that helps but am sure someone more knowledgable will step in with better advice.

Paul.

[The Flinty Fox]

But then again you could do the 6mm plus barlow giving 32x 53x 107x then 133x and 266x (with the 6mm and 6mm + barlow) for that extra fire power on really good nights. Hope this is helping.

[John]

Your scope is an F/4 which has a couple of implications for eyepieces:

1. Any eyepiece with a focal length longer than 25mm (at most) will be producing an exit pupil (the disk of light you actually look at when you stare into an eyepiece) that is too big to fit through your iris. So you will in effect be reducing the effective aperture of your scope.

2. The fast focal ratio (F/4 is fast !) will mean that wide angle eyepieces, unless very expensive, will show distorted stars as you look away from the central area of the field of view.

The above are factors with all fast scopes, not just yours !.

Unless you have a large eyepiece budget, I'd stick with the Vixen NPL's (which are nice plossls) and add a nice 1.25" barlow such as the Baader Q-turret 2.25x @ £45. Used with your 25mm and 15mm this would give you 11.1mm and 6.6mm. You would also find a 4mm eyepiece for 200x useful but a 4mm Vixen NPL will have very tight eye relief so will not be pleasant to use so perhaps the 4mm Skywatcher Planetary eyepiece @ £39.00 which manages to give better eye relief and decent views.

I've outlined a relatively low cost approach above. There are other much more expensive options of course

[Qualia]

I've been thinking a lot about EPs since I got my new 10" f/5 and I'm not sure if it is at all sound to conclude the following, but here are a few thoughts I've arrived at.

Taking the following as a working guide and not a rule written in stone, my journal has a number of observations on exit pupil which conclude: 3mm to 4mm - nice contrast, good on star-fields; 2.5 to 1.5mm - nice for most DSOs; 1.5 to 1mm - nice contrast and resolution on planets and globs; 0.7 to 0.5mm - nice resolution and fine detail on planets tweaked.

From this I worked out that if I took one of my telescope's aperture, say, the 250mm, and divided it by 2 , I ended up with 125. If I looked at that in terms of magnification I would require a 10mm EP. I then divided that 10mm by my scope's focal ratio and ended up with the magic exit pupil number of 2mm. This I figured would be the general work-horse for the 10".

For your 200mm, I'd work along similar lines. I honestly feel you'd also be able to use something between 90x to 125x as your general workhorse EP.

From this reasoning, I figure little steps could be made on a 1.3 or 1.4 ratio. So, for example, from my hypothetical 125x mag, or 10mm workhorse EP, I'd divide the mag or multiply EP's focal length by 1.3 and 1.4. So, I end up with 96x to 89x, or an EP of around 13mm to 14mm, or an exit pupil of between 2.6mm to 2.8mm. With a 2x Barlow, that gives a useful range of magnifications: 90x, 125x, 180x, and 250x (weather and atmospheric conditions permitting). Cross-referencing these figures with my journal entires I see that with just 2 hypothetical EPs and a barlow, I've covered a lot of adequate ground. Sure, there is room for improvement, a lot of room, you might fancy 150x for Jupiter, for example, it's a rare night one can use 250x on Saturn, but you get an idea of what is possible with just 2 EPs.

So, I'd buy a powermate or barlow and I'd stick with your 25mm and 15mm. Thus, you achieve, 32x and 53x and barlowed 64x and 106x. From there, I'd think a little about selling the 25mm and I'd want to get around my hypothetical 125x but bearing in mind the barlow. So, I'd go for something between a 8mm to 6mm, depending, of course, on what exactly you really fancied viewing and general seeing conditions where you live.

I'm not sure if this helps, but I feel the numbers offered do help getting a general gist of what is possible and some kind of map to work around. Being a fast scope, I'd stick with good quality plossls or orthos. Either that, or start buying into premium EPs which - and I'm fast coming to this conclusion - may not be such a bad move in the long term and I feel may save money in the end.

[Stargazer_00]

Good posts above offering sound sensible advice.

Yours is a fast scope and that punishes lower costing eyepieces. In truth an F4 is closer to an imaging newtonian than a visual one simply because of the effect the speed will have on eyepieces.

As John has said the wide field eyepieces will cost alot more but in my opinion they give amazing views as well. Due to the speed of your scope you'd need to get TeleVue eyepieces to see no effect of the scopes speed at the eyepiece but if you can tolerate slightly worse views (but still good) there are plenty of moderately priced offerings around £100-£150 an eyepiece. Explore Scientific 82 degree, Meade 5k UWA, Skywatche rNirvana, Celestron Luminos. All good quality glass with nice widefield views.

Eyepieces will likely be the more stationary part of your astronomy kit, as telescopes come and go, which inevitiably they do, your eyepieces will likely remain far more constant. It's worth investing in good ones.

I'd suggest you get along to a astro group and have a view through a widefield eyepiece so you know what is possible, you may be amazed - i was and I've never looked back.

Qualia makes some very sound points above and lays out a good, solid strategy to purchasing your eyepieces. It's very much worth doing all the sums on a piece of paper or spreadsheet before you buy your eyepieces so you know what you will get from each purchase. I always keep a spreadsheet which cross references all my scopes, eyepieces and barlows so I know exactly what I have available. Once my eyepiece case settles down a little I'll print and laminate it and keep it as an insert in my eyepiece case too!

[Pete Presland]

Yours is a fast scope and that punishes lower costing eyepieces. In truth an F4 is closer to an imaging newtonian than a visual one simply because of the effect the speed will have on eyepieces.

they are designed for imaging , as is my 200PDS.i found mine to be very harsh on eyepieces, with the Baader Hyperions almost unusable.the Televue plossl's work very well, also you may want too consider a comma corrector it really does make the view through much better.

[John]

Some good alternative suggestions in this thread but they do highlight that having a budget guide is useful in shaping suggestions - I think we have solutions here which range from £85 to £hundreds to implement !

IMHO, if you really want to go wide field at F/4 you need to budget for a coma corrector as well as quite pricey eyepieces.

[Robny]

Thank you John, I agree - great advice and gives me something to think about, I don't have a large budget at the moment for eyepieces which is why I was looking at the Vixens. I understand that faster scopes demand better eyepieces, but having said that I have been very happy with the quality of view from my 25mm eyepiece. That is probably largely due to the fact I have nothing to compare it against, which is no bad thing.

I'm more interested in imaging so the chances of me having an expensive eyepiece collection is unlikely as most of the time I will have a camera strapped to the end of it, I'm fully aware i need a Comma Corrector and that will come with time (gotta do this is stages - otherwise my wife will NOT be impressed ) i just want a basic eyepiece collection that will satisfy my urges.

So....I have the 25mm and 15mm, I'm thinking to go for the 6mm to add to my collection , now i'm thinking to sell my 25mm as suggested above and put the cash towards a premium quality wide angle and not get the Barlow and the money I was going to use on that put towards the premium eyepiece??

Sound sensible - Or way of the mark

BTW - IK lie the idea of the spreadsheet, thanks Spaceball

[Stargazer_00]

Here's a tip for you:

http://telescopes.net/store/meade-99-reflectivity-diagonal-and-24mm-ultra-wide-angle-eyepiece-kit.html

Buy that, it's very cheap. $159, after postage (use the cheap option) and import tax mine landed for £168. In this country that eyepiece is £220 and the diagonal is £145. Sell your 25mm and sell the diagonal in this kit, for - I dunno, £80-£90? More?

You get yourself a premium 24mm 82 degree 2" eyepiece for peanuts! Mine is a really lovely eyepiece. I use the diagonal too in my ST120 so this was the deal of the century.

[Robny]

Here's a tip for you:

http://telescopes.ne...epiece-kit.html

Buy that, it's very cheap. $159, after postage (use the cheap option) and import tax mine landed for £168. In this country that eyepiece is £220 and the diagonal is £145. Sell your 25mm and sell the diagonal in this kit, for - I dunno, £80-£90? More?

You get yourself a premium 24mm 82 degree 2" eyepiece for peanuts! Mine is a really lovely eyepiece. I use the diagonal too in my ST120 so this was the deal of the century.

Thanks so much that is a GREAT deal and something I will keep in mind when I'm aloud to spend more money.

Rob

[Stargazer_00]

Bored so I just knocked up a spreadsheet quick for you and/or anyone else to use.

Just fill in the white boxes, the spreadsheet will calculate everything else you need to know.

Telescope potential TFOV

Mags, TFOV (by eyepiece), exit pupil

also put a barlow multiplier in there which will apply across the entire eyepiece section. Use 1 for no barlow as indicated.

Hope it helps.

EDIT: If you havent gotten some sort of Excel installed then download a google docs plugin for Chrome or Mozilla Firefox. You can open xslx file in browser I believe. Most windows 7 computers have offie starter 2010 installed anyway so most have access to excel.

Stargazing00's astro-Calculator spreadsheet.xlsx

[Robny]

Bored so I just knocked up a spreadsheet quick for you and/or anyone else to use.

Just fill in the white boxes, the spreadsheet will calculate everything else you need to know.

Telescope potential TFOV

Mags, TFOV (by eyepiece), exit pupil

also put a barlow multiplier in there which will apply across the entire eyepiece section. Use 1 for no barlow as indicated.

Hope it helps.

EDIT: If you havent gotten some sort of Excel installed then download a google docs plugin for Chrome or Mozilla Firefox. You can open xslx file in browser I believe. Most windows 7 computers have offie starter 2010 installed anyway so most have access to excel.

Thank you so much this is great - saves me searching every time i want to figure out the mag with any specific eyepiece because i never remember which formula to use.

Thank again

[Stargazer_00]

No worries Rob.