Eyepiece Selection

[Fatrick]

No doubt this has been covered many times, but I cant see the wood for the trees when searching for this topic in here

Had my first go on my Evostar 90 last night, and its clear that I need some more EPs. It came with a 25mm, a 10mm and a x2 Barlow. The Barlow did nothing for me, just made the image more fuzzy and shakey, so I'm willing to buy some EPs to make up a decent range.

The things I noticed is that Jupiter was pretty small with the 10mm, so I'd like something to make it a bit bigger in the EP. And the Pleiades were too magnified in the 25mm so need something to see it in all its glory.

Any suggestions on EPs to cater for my needs will be much appreciated.

Now onto the real question. When looking around at EPs you see stats such as Eye Relief, Apparent FOV and Exit Pupil. Can anyone explain what exactly these mean? Is a bigger number better? Or vice-versa?

Mant thanks

Mark 

[lestergibson]

With a 90mm scope, the planets are always going appear pretty small, even at high magnification.  A better quality high power ep will do a lot to improve the view as you'll see the detail more clearly; that stock 10mm isn't great and a poor quality barlow won't help at all.  You have to spend time observing most objects to tease out more detail too.  Read up on 'averted vision' which is a technique you can use to let your eye pick more detail without looking directly at the object.

I'd imagine that the 25mm should be low enough power to let you see The Pleiades,  though a lower power ep will get more of it into the fov.

Brands to look at as replacement eps would be BST, Williams Optics, Vixen, Celestron Xcel LX.  There are TONS of reviews and discussions regarding these on SGL, as well as loads of info on everything you could think of regarding astronomy so abuse away at the search function:)

Here are a few links to help with your questions:

http://www.celestron.com/c3/support3/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=2377

http://www.celestron.com/c3/support3/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=1646

http://www.celestron.com/c3/support3/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=1652&ratingconfirm=1

Where in Antrim are you?

HTH

[ronin]

First choice is budget - how much do you want to spend on an individual eyepiece?

Your scope is an f/10 scope, 900mm focal length as I recall.

I assume the 25mm is OKish.

For Jupiter you need about 100x, 80x should be OK but lets start at the 100x area.

A 10mm will give 90x, if clear should be OK on Jupiter, a 9mm will give 100x and an 8mm will give 112x, a 7mm will give 128x.

I list all in one go as there is a slight problem, well to me anyway.

The BST Starguiders come in 8mm, the X-Cels come in 9mm and 7mm.

Neither do a 10mm but suggest you consider one anyway.

X-Cel are £65, BST's are £49 (hence the budget question)

For your scope I would look at the X-Cels, the 7mm and 9mm should do very well on Jupiter and Saturn. The BST's come in 8mm then 12mm nice set of two however.

Half suspect that the 5mm in either will be a bit too much.

If you wear glasses the eye relief needs to be up in the 15-16 and above area, it is where the eye has to be to enable you to see "right". Really need a diagram to show it.

FoV is simply with the eyepiece and magnification how wide your view is. It amount to the Eyepiece FoV divided by the Magnification. Simply the bigger the mag the narrower the FoV.

Exit Pupil: Not 100% sure of the explanation, a scope and eyepiece does not actually produce an image. What comes out the eyepiece is (should be) a collimated "beam" of light and this alters in size depending on the magnification. The "beam" diameter is I think the exit pupil. If it is less the 0.5mm I gather the eye has a problem. Read one place that the "optimum" is 1mm which means an eyepiece equal to the f number (conviently).

It is this idea of an exit pupil no less the 0,5mm that give rise to the max mag = 2xDiameter(mm).

Personally I am not sure of the Max Mag rule as scopes are different.

Will say that at f/10 you should be able to use plossl's which cost less, try Vixen's. Eye relief is about 2/3 of the focal length so an 8mm plossl means about 5-6mm eye relief which means you have to get quite close to the eyepiece to view through it.

[houston]

I used to use a 40mm plossl most for scanning the skies and finding the fainter objects with the evostar. The 26mm wasnt too bad but the 10mm not great.

[Fatrick]

Thanks for the replies guys, that helps shed a bit more light on the subject. Now I know what I'm looking at when it comes to EP spec.

I guess budget wise, maybe £40-50 per EP. I think replacing the stock 10mm with one higher and one lower would suit my needs. I'll have to have a look around, and perhaps get a 2nd hand deal somewhere.

Lester: I'm currently in Ballyclare but hope to be buying a house closer to ballymena in the very near future.

[completenovice]

I use vixen NPL's which are around the £35-£50 mark and I have noticed a step up from the basic one's supplied. I usually start with the 30mm and work my way up to my 8mm for more detail.

Seem well made and I'll be sticking with them for a while until I can justify the price of TV's

BST's also get  good reviews and are rated highly on here and are similar in price so maybe worth checking them out also.

[ronl]

My first scope was an Evo 90mm eq2, I still have it and still use it, my forward thinking was that I will soon 

be upgrading to a bigger scope, so setting my sights on a 200p I started collecting the BST Starguiders,

which I used on my Evo, and they did a good job, good views, but even better in the 200p, I now have a

full set, which I am so pleased my forward thinking worked, I still use them in my Evo, I use it as my grab

and go, when the weather is is a bit iffy, it takes no time to set up, and no time to cool down, so only my

opinion, the Starguiders suit me perfectly, I think they are very good, and really good value for money.

Hope this helps. BTW, aperture fever will get you one day. 

[lestergibson]

I'm in Ballymena, mark so if you fancy a hook-up when you get this length, give me a shout. The skies have to clear up sometime this year!

[faulksy]

all good advice above, but when i had the bsts i didnt like the yellow tint, i found the views through the ts h-rs a lot better nice natural tones and very close to the bgo,s

[Blazar]

You also may find this superb thread by Qualia help set some other expectations on what you are likely to see, though it is based on using a 4" (100mm) TAL (close to your scope in size), and some very good eyepieces.

What can I expect to see.

Going to a society and looking through other scopes and eyepiece set-ups might be beneficial.

Good luck.

[Fatrick]

I did see that threat before Blazar, it was more of less what I got, just mine wasn't as sharp. I was figuring that the stock 10mm was a bit poor, so the reason for looking for a replacement. 

[lestergibson]

Here's a good read on eps from SGL member, Robin Wilkey:

http://www.swindonst...s/eyepieces.htm

[Fatrick]

Thanks for that link Lester. Helps me to understand what the actual differences are between the various types of EP are. Looks like the BSTs are the way forward, good middle of the road EP.

[bingevader]

It looks like some of the Maxvisions are still available, worth a look and the 16mm at least is in budget.

Cheers

[Fatrick]

OK, think I'm going to try a BST 8mm and see how that goes. 

Im currently trying to make up a FOV template to use with a Sky Atlas to help with star hopping. My question is, what effect does a 2x Barlow have on the FOV? I know it will double the mag, but will it half the FOV? Will it leave the FOV unaffected? What will it do?

Thanks

[lestergibson]

It looks like some of the Maxvisions are still available, worth a look and the 16mm at least is in budget.

Cheers

I love the those unbranded Meade 5000 MV's; an awful lot of ep for not a lot of dosh.  Just a pity they don't do any short ones.  Impressed enough with them to be thinking of springing for the Meade 5000 5.5mm to use as my highest power ep.

[LDW1]

No doubt this has been covered many times, but I cant see the wood for the trees when searching for this topic in here

Had my first go on my Evostar 90 last night, and its clear that I need some more EPs. It came with a 25mm, a 10mm and a x2 Barlow. The Barlow did nothing for me, just made the image more fuzzy and shakey, so I'm willing to buy some EPs to make up a decent range.

The things I noticed is that Jupiter was pretty small with the 10mm, so I'd like something to make it a bit bigger in the EP. And the Pleiades were too magnified in the 25mm so need something to see it in all its glory.

Any suggestions on EPs to cater for my needs will be much appreciated.

Now onto the real question. When looking around at EPs you see stats such as Eye Relief, Apparent FOV and Exit Pupil. Can anyone explain what exactly these mean? Is a bigger number better? Or vice-versa?

Mant thanks

Mark

Google the N.A.A. Telescope Calculator and it will explain everything in detail that you can understand and as well you can enter scope dimension and EP sizes and it will calculate all the performance specs for your scope / EP's. Very simple and very comprehensive !

[Fatrick]

Just figured out my Barlow query. As the fov and zoom are proportional, a Barlow EP will have half the fov. I knew my a-level maths would come in handy one day