View Full Version : Telescopes

Mike OMelia
04-25-2014, 8:51 PM
I bought my youngest son an Orion XT8, Dobsonian telescope. It's very nice. But the 25 mm eyepiece (ep) is not very useful. I want to look at planets and eventually deep space objects. Does anybody here know telescopes? Want info regarding EPs


ken masoumi
04-25-2014, 9:29 PM
I could be wrong but I think you need a 2X Barlow lens which essentially doubles the power of your existing 25mm eyepiece:

Michael Gaynes
04-25-2014, 10:14 PM
The XT8 has a 1200mm focal length. With the included 25mm eyepiece you'll get 48x power (1200/25 = 48). You can do the math to calculate other eyepieces and combinations with a Barlow like Ken mentioned. Adding a 2x Barlow with a 9mm eyepiece for instance will give you the combinations: 48, 96, 133, 266

Power isn't everything, however. At a certain point the image may get bigger, but it will be dimmer and detail will suffer. Also, at higher magnifications any shakiness in your set-up and turbulence in the atmosphere will be accentuated.

Mark Bolton
04-26-2014, 8:19 AM
Is agree with the Barlow. I have a C8 on a losmandy g11 Gemini. Longer fl and the mount helps with higher powers but if you added te Barlow and perhaps another smaller ep (maybe a 17 or so?) you'd have a lot of combinations that would likely push your scope about as far as it will go.

The nice thing about the Barlow with the larger ep's is the eye relief. Very small ep with little eye relief can be challenging even with tracking but being able to get your eye back a bit can be very helpful.

For all of it though really try to make sure your collimation is spot on. Will really help with planets.

Kev Williams
04-26-2014, 11:37 AM
I had a telescope a few years ago, nothing major, just a fairly nice one from Shopko-- I remember a couple of times getting Venus in the thing-- for about 10 seconds. As soon as I'd get zeroed in, the earth's rotation took it out of play rather quickly--

So, how do you guys with GOOD telescopes keep what you're looking at in view? (just curious!)

Michael Dedon
04-26-2014, 7:56 PM
Kev, Ask Mark what he spent on the Losmandy and that'll answer the question about tracking what you're looking at.

Seriously a motor drive and "GO TO" mount is what's needed and they're available for any decent tripod/scope setup. FInd a local astronomy club. They're invaluable and may have spare or older equipment for sale.

Tom M King
04-28-2014, 12:14 AM
Most important is a dark sky. Any lights within a few miles make a difference. You should be able to see rings on Jupiter and moons, rings of Saturn with moons, and even a little detail on Mars with what you have, and plenty of other objects. I never liked barlows much. I'd get another eyepiece half the focal length of that one, and that may be all you need for that telescope.

My best friend and I built telescopes when we were teenagers. The largest was a 12-1/2 inch, and we built a little house for it that the roof rolled off of. He's still building telescopes. He's now one of the lead scientists on the JWST, and was on a team that won a Nobel prize in AstroPhysics. Finished mirrors were the same number of dollars in the '60s as they are now, so we had to build them by hand. An equatorial mount with a twist knob works okay for following while viewing (used one for many hours), unless you want to take a photograph.

Mike OMelia
04-28-2014, 12:42 AM
The terms used are confusing, though I do understand optics. I have a copy of Born & Wolf in my reference library. I get the fl ratio for magnification. But I don't understand "eye relief". I also read about 30mm monster eps that are great for Milky Way study, but that seems like less magnification. Then, all 25mm eps must not be created equal since mine clearly cannot see Jupiter sll that well. So, if someone would care to elaborate on the semantics, I would appreciate it. What are the common terms used to describe an EP, and what do they mean?

Michael Gaynes
04-28-2014, 12:17 PM
Eye Relief is the distance from the lens to your eye. This is a factor if you wear glasses and at high magnifications. Your Plössl eyepiece has a relatively short relief.

Another factor to consider is Apparent Field of View (AFoV) (http://en.wikipedia.org/wiki/Eyepiece#Field_of_view). This is the "measure of the angular size of the image viewed through the eyepiece". The Plössl has an AFoV of 50°. Some eyepieces have AFoV's of 68° to even 100°. A wide field of view is a benefit in two ways: The first is simply being able to view large swaths of sky, either to see large objects like nebula or star clusters, or to simply get your bearings before you zoom in at higher magnifications. The second advantage is that the object you're looking at stays in the frame longer as the Earth rotates.

With your 25mm EP, Jupiter should look like a small disk in that relatively large 50° field of view. When you find the planet and get it in focus, try to place to one side of your view and observe it as it floats through the field. Then, you can nudge your scope a little and let it float through again. With careful observation (and dark, clear, steady skies) you should be able to make out cloud bands. Up to four moons are easily distinguished even in binoculars.

Magnification isn't always the most important factor and many times gets people in trouble - They can't find what they're looking for. If they do find it, it moves out of the field of view too quickly. The image jiggles around too much, looks fuzzy and they have trouble focusing. Plus, they have to jam their eyeball into the lens because of the shorter eye relief.

As people always recommend with woodworking tools, use the heck out of what you have, learn their (and your) limits and then buy what you need once you have some experience.

Mark Bolton
04-28-2014, 1:38 PM
Michael is spot on. Remember too that "zooming in" is effected by many things, optic quality, light pollution, steady skies, and so on. Often times you'll get a much better image at lower powers. Also remember staring right at an object is not always best. Using averted vision will show much more detail. The periphery of your eye is much more sensitive than the center.

You can of course break the bank on eyepieces. Naglers and such are phenomenal and will give you higher magnifications while retaining wide field of view but at an astronomical cost.

I like the televue barlows but again they are costly. The only criticisms of barlows is that your putting more glass (hence more defect) inbetween you and the photons hitting the primary mirror. But if you have a collection of 4 nice ep's for 200 bucks or less you will instantly have 8. Of course the argument could be to invest in two phenomenal eyepieces but I don't care for that option. I have a half a dozen good eps and with a 2x Barlow and a 2.5x powermate I have virtually the best option for any skies I'm under because of the ability to jump up or down in power based on seeing.

And yes, mounts are costly. The g11 Gemini goto as I have it (lot if extras), about 3.5k. That said, I had a permanent observatory, ccd imaging, and multiple scopes/configurations but now I'm down to that scope and a 90mm short tube refractor for a quick grab.

My advice is to get another ep or two (plossl are fine by me) and make sure your collimation is spot on, and just keep observing. You'll have nights of very clear seeing where you'll get to see what your scope is capable of (usually best after a rain). It'll make sense over time.

Mike OMelia
05-04-2014, 10:49 AM
Is it possible to see Saturn with any of these EPs?

Myk Rian
05-04-2014, 12:02 PM
Get a 12.5mm eyepiece, and a 3x Barlow. The Barlow can be used for the 25 and 12.5.
You could also try a 9mm eyepiece. Getting down to 4mm is a little iffy, and can be hard to focus.
We've looked at Saturn with a 12.5 and 9mm on my refractor scope.

Mark Bolton
05-04-2014, 12:46 PM
Id be concerned the 3x Barlow may be pushing it for that scope. That'd effectivly be a 4mm ep. May fly but it would seem optics and seeing would factor in heavily.

You can see Saturn in binocs if they are on a mount (stable) so with your scope and any option mentioned (including your stock ep) you would be able to see Saturn and Jupiter easily.

Tom Stenzel
05-04-2014, 1:09 PM
Is it possible to see Saturn with any of these EPs?

When I was 14 I received a Sears Discoverer telescope for Christmas. A 2" refractor with an eyepiece that gave 75X power. I didn't use it much, I couldn't see anything. Years later I found that 75X was a ridiculous amount of power for that size lens and got an eyepiece that gave 30 -35 power. With that I could see 4 or 5 moons of Jupiter and the rings of Saturn.

With the telescope you have you should see a lot more than I ever did. The kid in me is way jealous.


Greg Portland
05-05-2014, 4:46 PM
I want to look at planets and eventually deep space objects
What are your expectations when you say "look at"? A small colored dot or your entire FOV filled with a planet?