Understanding eyepieces

[jet486]

I'm hoping to purchase a 12" Dob but I need some help as I am confused to say the least on eyepieces and I was hoping if someone can direct me to a forum/web site which can explain what type of eyepieces are available. What does eye relief mean? Do I use 2" or 1.25" eyepieces? What is a Paraccorr and a Barlow and where would I use them? What does 35mm or 4.5mm mean? Then on top of that you have filters to contend with. I just find it difficult to comprehend the amount of information on eyepieces where all I want to do is just go out in my backyard and observe the night sky in the western suburbs of Sydney.Cheers

[astro744]

1. A search for eyepiece fundamentals gave me the following address: http://www.rocketroberts.com/astro/eyepiece_basics.htm Also a search for eyepiece designs gave me the following address: http://www.quadibloc.com/science/opt04.htm Two important eyepiece parameters are focal length expressed in millimetres & aparent field of view (AFOV) expressed in degrees.

The barrell diameter (1.25" or 2") determines the maximum true field of view (TFOV) an eyepiece can have with a given telescope. Typically the 1.25" eyepiece can have focal lengths from 2mm to 40mm. 2" eyepieces are typically used for lower powers with wide apparent fields but focal lengths can vary from 6mm Ethos to 55mm Plossl in the Tele Vue range.

2. Eye relief is the point at which your eye is located away from the eyepiece eye lens to take in the image fully. Long eye relief is typically 20mm or more and is suitable for people who observe with glasses on. 20mm eye relief means your eyeball is 20mm from the eye lens of the eyepiece. If you wear glasses for astigmatism then you may want to observe with glasses on especially if the astigmatism is severe. If no astigmatism is present in your eye but are long or short sighted then you can adjust the focus for your eye by moving the telescope focuser in or out. Note another observer will have to re-focus.

Therefore in answer to your question, you will use both 1.25" and 2" eyepieces depending on what magnification and field of view you want. Magnification is simply telescope focal length divided by eyepiece focal length. True field in its simplest form is eyepiece AFOV divided by magnification although I prefer another formula that involves the field stop diameter of the eyepiece which I wont go into now.

3. Your 12" will probably be f5 (focal ratio = focal length of telescope divided by diameter of primary mirror). All parabolic mirros have coma which causes stars to appear as tear drops at the edge of field. A Paracorr sharpens the edge stars by reducing the coma significantly. Some people are annoyed by coma and some are not. My advice is get a quality eyepice first so that you only see coma and nothing else and then decide if it bothers you or not. Poor eyepieces have field curvature and astigmatism than makes the edge stars out of focus and/or look like seagulls. Some lesser quality eyepiece perform poorly at f5 but are very good at f10.

4. A Barlow lens magnifies the image by the Barlow factor. eg. A 2x Barlow makes your effective telescope focal length twice as long OR your eyepiece half as long whichever you prefer (same result). A Powermate is a complex Barlow that retains certain eyepiece parameters and is available in higher multiplying factors, (can also be used for imaging).

5. 35mm could refer to eyepiece focal length, film format, anything else that is 35mm long. Lets say 4.5 mm is an exit pupil number (could be focal length too). Exit pupil is eyepiece focal length divided by telescope focal ratio. It is the diameter of the light circle leaving the eyepiece, ie. what your eye sees. Your pupil can expand to 5mm to 8mm depending on age and if the exit pupil is larger than your pupil diameter you are not taking in all the light available.

6. Filters are available in various colours and are used mainly for observing planets. There are also wide and narrow bandpass filters which let through specific wavelengths that most nebulae emit and thereby enhancing the contrast significantly.

Hope this all helps.

[dugnsuz]

What does eye relief mean?
Distance from your eye to the glass in the eyepice. Long eye relief = more comfortable view - some eyepieces (eg Orthoscopics - simple designs) require the eye to be very close to the lens and stationary ie short eye relief.

Do I use 2" or 1.25" eyepieces?
Your 12" dob will allow the use of both - it has an adapter as standard.
2" eyepieces are larger/ heavier often more complicated and expensive designs. Once again your Dob will probably ship with a 2" and a couple of 1.25" eyepieces.

What is a Paraccorr
An expensive corrective lens made by Teleview to flatten out the inherent curvuture of the field due to the shape of the mirror. as the mirror is slightly curved, stars in the centre of the field are sharp but start to elongate towards the edge of the field of view - a Paracorr flattens this abberation out.

and a Barlow
A barlow is a lens which multiplies the power of the eyepiece by a factor dependant on the power of the barlow (ep 2x or 3x are common) So, if you're using an eyepiece that yields a 100x magnification...using a 2x barlow will result in a magnification of 200x (easy?)

...and where would I use them?
Probably to push the magnification a bit on Planets

What does 35mm or 4.5mm mean?
These are the focal lengths of the eyepieces and put simply these numbers represent the 'magnification power' of your eyepiece...a bit of calculation is necessary though. To work out the magnification for each eyepiece you must know the focal length of your scope - usually designated as F=1000mm for example. To obtain the magnification for an EP (eyepiece) one simply divides the Telescope focal length by the EP focal length...1000/35 = 28.6x magnification 1000/4.5= 222.2x magn'.
You can see then that the smaller the focal length of the EP - the greater its magnification.

Hope this has helped a little - others will add responses too to answer your other questions I hope
All the best
Have Fun
Doug:thumbsup:

[dugnsuz]

D'Oh - you beat me to it astro744!!!
:lol:

[mental4astro]

Doug & astro744 have got the it goods on the gear.

You can also get yourself to a star party or observation session ( see under 'star parties' forum ), with or without a scope, and see what all this gear looks like & how it's used before you spend the big bucks.

[wavelandscott]

I want to further reinforce this recommendation...if at all possible get to a viewing night or two before spending your money.

An evening (or two) invested in this way will no doubt give you a chance to get your questions answered and also give you a chance to try out some gear before you use your money.

[dugnsuz]

Agree 100% - BUT, (the ominous big but!!) if you can't access viewing evenings/ star parties...acquaint yourself thoroughly with review sites such as IIS, Cloudy Nights etc etc. There will be items which are outstanding and consistently reviewed as such at a variety of budgets. A few spring to mind...Televue 13mm T6 Nagler, Televue 24mm Panoptic and (not EPs but...) Orion ED80 refractor, Telrad etc.
It must be remembered though that many EPs are reviewed by users of varying experience and objectivity so one must take any recommendation or criticisms with that in mind.

By checking out these various sources you'll pick up lots of information on the journey too!

I have found however that the equipment which is consistently reviewed favourably (ie 100s of reviews) tends to be because it is deserved.

This is my experience with optical gear in general - astronomy accessories and camera gear.

BUT, (the big but again) - if you can get "hands on" - do it!!!
All the best
Doug:thumbsup:

[wavelandscott]

I don't disagree ...as with all things "posted" you need to be careful about the source and situation of the information (myself included).

With the OP living in Sydney there are many options to catch-up with people or even go to a shop or two and try things out.

[Paddy]

I would heartily agree with this approach, but would also suggest that for a while you stick with the eyepieces that will come with the telescope (as they usually do with production scopes) so that you get an idea of the focal lengths that you most use etc. Often the eps that come with the scope are reasonable (except for some of the long fl eps like the 32mm FMC that came with my Bintel 12") and will keep you happy until you're clear enough about the significant investment in better ones.

[hulloleeds]

I feel almost as I'm a salesman for andrews but I just bought their 12 and the forte of buying their 12 is that it comes with a range of (low cost) EPs. Plus, they are in your city so no freight ($110 to bris). As far as new prices go, they are the cheapest, too.

http://www.andrewscom.com.au/site-content-section-10-guansheng.htm

But, 12's can go for wayyy cheaper second hand. If you're lucky, that is. Kind of amusing that I dropped 1k on a 12 and two months ago the same scope I bought was sold here for 250 (local, too).

Depends how patient you are, clearly I am not. :)

[jet486]

What a great site! Thanks everyone.