Astronomy for the Absolute Beginner - eyepieces and filters

Astronomy for the Absolute Beginner
Eyepieces and Filters

Astronomy for the Absolute Beginner

Getting Started

I want a Telescope

Time for a new Telescope

Eyepieces and Filters

Observing Session Basics

Book Reviews

Free Astronomy Software

Our First Night Out

Orion SpaceProbe 130ST First Light

Handicapped Astronomy
Page 1

Handicapped Astronomy
Page 2

Astro Links

My Home Page-

God, Genesis and the big bang

Eyepieces and Filters

My Deep Space Hunter came with two eyepieces (25mm & 9mm) and a moon filter. The Moon is too bright to view comfortably without the filter. It messes up your night vision for when you go to look at something else. Without the moon filter, when you step away from the eyepiece you will be pretty much blind for several minutes.

The eyepieces are plossl type. This is the most common type used by backyard astronomers. There are apparently better types (Nagler for instance) but they cost more than my whole scope. I am not likely to ever own one of them. There are also far worse type eyepieces you can waste money buying. If you stick with a plossl ep until you know what you are doing you will have a good general purpose workhorse. I bought some nice plossls at a great price from Owl Services . My experience with them was very positive.

The eyepiece determines your magnification. The bigger the number on the eyepiece the lower the magnification. The actual magnification is determined by dividing the focal length of your telescope (1200mm on my DSH) by the eyepiece size.

To figure magnification divide telescope focal length mm by eyepiece size mm
example: 1200mm / 9mm = 133

There is a limit to how much magnification a telescope can actually handle.

The maximum magnification of a telescope = aperture in inches x 50 or aperture in mm x 2
example: 8" scope x 50 = 400 same as 200mm x 2 = 400

On most nights the actual usable magnification of a telescope is closer to 200 - 250 because of atmospheric interference. You can reach 250 power with a 5" scope, so why get a bigger one? While a bigger scope won't help with more magnification it will gather more light therefore allowing you to see fainter objects.

Another important consideration when looking at eyepieces is known as eye relief. This is the distance your eyeball needs to be away from the lens to use the eyepiece. The lower the number on the ep the closer your eye must be to the lens. This distance varies depending on what type eyepieces you are using.

For me, plossls between 32mm down to 12mm are very comfortable to use. A 9mm is borderline for my eyes and a 4mm is near impossible. It's weird but eye relief does not change when a barlow is used to double the magnification. This offers us a way around the eye relief dilemma. A 12mm ep with a barlow is the same magnification as a 6mm ep but the eye relief remains very comfortable. Likewise a 9mm and barlow is the same as a 4.5mm ep but the eye relief is much more usable.

I have read that the beginner ought to have three eyepieces. One for high, mid, and low magnification. The 25mm that came with my DSH works for the low. The 9mm hits between high and mid. By using a Barlow lens I can double the power of the two lenses I have, in essence giving me the equivalent of 25mm, 12.5mm, 9mm, and 4.5mm eyepieces. The 9mm with the Barlow is the same as a 4.5mm eyepiece, which gives 266 power magnification on my scope. This will probably be too high to be usable.

I thought if I had around a 12mm and a Barlow I would have a good range up to 6mm (200 power on my scope). When I checked prices I realized these two pieces together could cost easily a hundred dollars or more. Then I found Celestron's Accessory Kit on Amazon.com. It comes with 5 eyepieces, a Barlow lens, seven filters, and an aluminum case which at the time sold for around $100. Cool! I had to wait forever to get it (back ordered) because it is a really good deal.

There is some duplication with what I already have (the 9mm and the Moon Filter) but so what. Maybe I'll work out a trade later. These are not great eyepieces but they are terrific for learning what works and what doesn't. Later as you understand what you really need you can buy better quality eyepieces one at a time. The first item you might want to replace is the barlow. Buy a good one as this is an important item.

As far as the filters go, I haven't a clue what to do with them yet. Being really honest here. Supposedly they are good for planetary viewing. Making features stand out better. I have tried using them some and haven't personally found a need for them.

One thing I think I want to add someday is a Nebula filter or O-III filter. So the theory goes these filters make the nebula easier to view by blocking out interferring light rays. I am holding off getting one until I can look though one at a star party. They are expensive so I want to make certain it is money wisely spent. Another item I am interested in is an adapter that lets you hookup a video camera to the telescope. Saw one being used at a star party and thought it was a neat idea.

I hope you found something of interest here. If you did drop me a note. If I have written something that is not correct please point it out to me and if you convince me I will get it changed.

Clear Skies!