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Hi All,

First of all id like to say what a warm welcome we have had on this forum, so thankyou :)

We are complete novices and i have a couple of questions.

I am a little confused about what magnification eyepieces are and how to calculate it, my telescope is a celestron 80gtl (computerized version) it has a focal of 400mm. It came supplied with a 25mm and a 10mm eyepiece.

Capricorn suggested i purchased a 5mm eyepiece, what type should i buy and i presume it has to be a 1.25" fitting ??

Is the sun filter safe to use that was supplied, it fits over the "big end" lol !!! ??

what is the best eyepiece to use to view saturn and how clear will the image be ??

Sorry for all these questions but we really are so very new at this and i dont want my daughter to get bored with it as we did spend 125.00 on this telescope and it has captured my interest as well, be nice to have a common hobby :o

Thanks for any help

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Hi RobbieW!

Let's take your questions one at a time.

Telescopes gather and focus light, eyepieces magnify the image and control how wide a field of view we see when we look through the scope. Most eyepieces supplied with scopes these days are 3-4 element designs that give a reasonable 40-50-degree apparent field of view.

Magnification is calculated by dividing the focal length of the telescope by the focal length of the eyepiece. This also means that the same eyepiece will give different magnifications on different scopes!

For you: 400mm scope / 25mm eyepiece = 16x magnification, your 10mm lens will give 40x. Instead of a 5mm, you might be happier with a 2x barlow lens - this will make the 25mm into a 12.5mm and the 10mm into a 5mm (two more magnifications for the price of one!) A moon filter might also be a wise purchase.

To view Saturn, begin with the LOWEST power eyepiece and find the planet. This is vastly easier at low power than at high power because of the wider field of view! The view of the planet itself will be very unsatisfactory at 16x - you will be able to see that it is a disk - not a star - but that is about all. However, you may be able to see a faint point of light near Saturn - its moon Titan!

With Saturn in focus and centeredin the view, switch up to your 10mm lens and re-focus. The view is much more satisfactory now, and you should easily see the ring! Look for Titan again at higher power, it may be anywhere within several ring-diameters, and it orbits in the ring plane - so look in the direction that the rings extend to find Saturn's moons! If you had a barlow lens or a higher power lens, the process for going for more magnification is the same - center the object, then change the eyepiece and re-focus!

You will also find that tracking Saturn becomes more of a challenge at high power. The planet is larger, but seems to move across the field of view rather quickly. If you have an equitorial mount or a GOTO telescope, the motors and gears take care of this for you - if you have a push-n-go telescope that you aim yourself, tracking takes a bit of practice. If Saturn slips away and you lose it, go back to lower power eyepieces to find it again, then back to high power observing!

I'm also attaching a few astronomy activities you can download and try with your daughter. (I teach astronomy for a living! :o ) It will give you a few new things to do with that new toy that will keep the interest up and help you have more Dad & Daughter time at the eyepiece! :)

Let me know how it goes!


Spring 2011 Labs.zip

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Welcome to the lounge.

First off to calculate magnification you divide the focal length of the scope by the focal length of the eyepiece.

I don't know what sun filter you have so can't comment on that.

If you are that new to astronomy and have only just got the scope i wouldn't rush out to buy eyepieces.

Give it a while, get used to the scope then when your ready you'll be able to make your own more informed decision.

Clear skies

Regards Steve

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As to the sun filter, I think I'd leave it alone for now. The filter on the aperture (large end) is the correct way to go, by the way!

To use this:

1. check the filter visually for holes, flaws, cracks etc. DO NOT USE IF IT IS NOT IN PERFECT WORKING ORDER. Toss it in the bin if it is not in proper shape! A badly working solar filter is like having a stick of dynamite in the kitchen drawer... it will hurt someone eventually - probably badly!

2. Attach the filter to the scope - make sure that it fits properly.

3. Make sure the finder scope is capped or removed - focusing the sun through the finder can burn out your crosshairs!!! :(

4. Now point the scope - watch the shadow of the scope on the ground - when the shadow is a circular shape - the scope is properly pointed at the sun! :o

5. HOLD YOUR HAND IN FRONT OF THE EYEPIECE WHERE YOUR EYE NORMALLY GOES FOR 10 SECONDS! This one is so obvious - but so often ignored. :) No burning sensation??? Your ready to observe!

6. Observe and adjust focus.

7. Pick jaw up off the ground - the sunspots are AWESOME! Most of the "little ones" you see are larger than the Earth!

Have fun - safely!


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If the solar filter fits the front of the scope then it may be made of a film for the purpose, Mylar. More expensive is a solar filter that is in effect very close to a mirror and transmits just 0.001% of the light. Without seeing it it is hard to say. People will be a little nervous of giving a definite "Yes it's OK" in case it is wrong, myself included.

The scope being 80mm dia and 400mm focal length is an f/5 scope. At the shorter f numbers the eyepiece usually has to be a bit better for the resulting image to be good.

A 5mm plossl has a small bit of glass to look through. You will need a 5mm eyepiece to get 80x magnification. (Magnification = Scope Focal Length/Eyepiece Focal Length).

To see Saturn I think you will need smaller then a 5mm, 80x should show it but only as a small object. A 4mm eyepice would give more magnification (400/4 = 100x) and this may be better, however we are now in the realm of a good eyepiece. A standard plossl will be pushing your luck, depending on the make, quality and therefore budget..

Try the site Sky's the Limit, Sky's the Limit, they have some planetary eyepieces that are good and people seem to have purchased from them without problems.

A warning is that you can/will end up buying quite a few eyepieces. Saturn will be small always, what you need is the contrast and detail to come through.

Any chance of a picture of the items for people to get a better idea?

Edited by ronin
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Thankyou Dan, that was very kind of you to offer such a decent explanation, i will source a barlow lens, sounds like a good thing to have, when is the best time to view saturn and does it affect what you see if there are a lot of street lamps about, we have one right outside our front garden gate but we do have access to a really dark field just accross the estate from our house, would this be better ???, it is so cloudy tonite, cant see anything !!!!, i hope this weather clears soon

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All planets, and anything else for all that, is best viewed when it is highest in the sky. Closer to the horizon usually means more turbulent air and more 'swimming' of the image - especially at higher magnifications. More problems with lights at the horizon, too!

Darker area is ALWAYS better! If you can't get to the dark, you can try putting a towel over your head (really! :o) to screen out unwanted light - you would be amazed at how much it helps. There is a firm that even sells a vest with a "Monk's hood" on it to shield you at the eyepiece. :)

Have fun with it!


PS: Did you download the astro activities???

Edited by Ad Astra
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