help! beginner alert

[demonicrage]

hi, i was looking at jupiter a few weeks back, but all i was seeing was a large white dot and i could make out the moons, i used my moon filter to turn it green and block out the white light, but i couldnt even see the planet close enough to make out it was jupiter. i used a 25 or 24mm eyepice then tried a 2x barlow and erecting eyepiece 1.5x. my telescope is a reflector seben with a 6 inch mirror, the length from the base mirror to the reflector into the eyepiece is 14 inces, im not sure if this helps but if anyone can give me an idea on how to get maximum magnification this would be a great help.

cheers

Kris

[pvaz]

Your scope seams to have a very short focal length (14inches = 355mm).

So with a 25mm ep you get 14x (355mm/25mm). so that's 28x with a 2x barlow. You need about 100x to start getting some details out of the planets.

It seams too short so it's likely you have a "seben style" telescope with a built in corrector lens barlow which doubles the FL to 700mm. If that's the case you get 56x (still a bit low) and the combination of the built in barlow, + 2nd barlow and EP will give you a poor quality, hard to focus, image. That's mainly because this kind of scopes (seben, tasco, etc) use plastic on their lenses and it's hard to get a decent view out of them.

[demonicrage]

Thanks alot, it makes a little sense, I done a maximum magnification calculator online, I put my mirror diameter in and focal length which I am guessing is the 14 inches from base mirror to eyepiece and the 25mm eyepiece, unfortunately I could make sense of it. Could u give me a beginner lesson on the measurements I need and where to take them so I can figure out what the max magnification would be using your equation? Thanks again

Kris

[pvaz]

Sure.

Magnification = scope focal length / EP focal length

Max Useful Magnification for a scope = 2 x Primary Lens/Mirror Diameter (aka Aperture)

Recommended Max Magnification for a Reflector = 1.2 x Scope Primary Mirror Diameter (aka Aperture)

so for a 6" scope, like yours:

Max Useful Magnification = 300x

Recommended Max Magnification = 180x

You need to go to the manual and get the right focal length of the scope (if it does have a builtin corrector lens it will be longer the the actual size). Then divide it by the EP focal length and you get the magnification. When you add the barlow it doubles that specific EPs mag.

[ronin]

May as well ask:

What is written on the scope?

As in what numbers etc.

If you could list whatever is on the scope in a message it would help to determine what the specification is.

Please be aware that Seben scopes are not the best.

As it is a reflector have you collimated it? This is essential for decent viewing.

[demonicrage]

Thanks fellas

There is no marking or numbers on the scope just seben.com.

I'll get a tape measure on the job and measure the base mirror which I'm sure it is 150mm and the distance between the base mirror and the reflecting eyepiece thingy.

May I ask what collimated means?

Cheers

Kris

[pvaz]

Collimation = aligning the 2 mirrors on a reflector scope

It can be a bit complicated at 1st. Here's a very good guide: Astro Babys Guide to Collimation

[demonicrage]

thanks for the guide, i found a sticker telling me the diameter (150mm)and focal length (1400mm), so with a 6.5mm plossi i can get 215x magnification, what could i see with this say if i looked at jupiter? how would i know if the scope needs collimating?

Thanks

[pvaz]

Looking at jupiter you would see a disk about the size of a large pea at arms length. It's moons will be points of light.

In the disk you can see the weather bands, maybe the great red spot (it's large but a lot fainter then in the photos). At the right time you may see a moon desapearing behind the disk or crossing it.

The amount of detail depends largely on the atmosphere, the scope quality and the amount of time you spend looking at it. At high mags objects shimmer but every 5 min (or so) you get a few stable seconds when you can glimpse more detail.

[callisto]

Hi there,

I found this useful for collimation also: Collimating a Newtonian

Mark

[pvaz]

how would i know if the scope needs collimating?

When you look through the center of the focuser, without an eyepiece in it, you should see everything centered like this:

http://www.astro-baby.com/collimation/actual%20view%20of%20secondary.jpg

if it's not centered then you need to collimate. A collimation cap, which is a plastic cap you put on the focuser, with a tiny hole in the center where you peek through, will allow you to see it more accurately. If the scope didn't bring one, you can use a photographic film plastic canister and make a 2mm hole in the center. The film canister's are usually 1.25" wide and will fit the focuser perfectly.

[mrwhippy14]

Interesting to read guys. Whats a collamation cap ? does it just fit on to the eyepeice ?

So your saying place a 2mm hole in the center and use it to give u more of an accurate (steady ?) view ?

Thanks!

[ronin]

You could put a 6.3mm in it, get a value of 215x but I would sure that you will see nothing.

A big image is just one aspect and a much overrated one.

The scope is f/9.3. Stick to a 9mm or a 10mm eyepiece. These will give more then enough to see Jupiter, and when it is in a good position enough to see Saturn. I would say try a 15mm on Jupiter if you can borrow one to try.

If you try for much more then a 9mm I am afraid that the scope probably won't handle it. As said Sebens are not the best so accept the limitations.

I don't know if Sebens are parabolic or spherical, f/9.3 could be a long parabloic or a spherical that is long simply so that the images aren't too bad.

Overall eyepieces like 9, 12, 20 and 30 would be useful if you decide to start collecting them.

As Pvaz said collimation is the alignment of the primary mirror and the secondary. If not aligned then the image is poor.

[pvaz]

Interesting to read guys. Whats a collamation cap ? does it just fit on to the eyepeice ?

So your saying place a 2mm hole in the center and use it to give u more of an accurate (steady ?) view ?

Thanks!

It's a piece of plastic that fits on the focuser, without an eyepiece, and it haves a small hole in the center to check if the mirrors are aligned or not. It is used for newtonian (aka reflector) scopes' maintenance.

[demonicrage]

Thank you all for the advice

So it would be worth investing in a 9mm eyepiece?

And getting the scope collimated?

Just one last one, my finder scope isn't lined up with my main scope, is there a easy way of lining the two up so it is easier to look at whatever I'm finding in my finder?

[brantuk]

Collimation is pretty esential - it's also a very personalised thing. You can make a collimation cap by drilling a hole dead center of a drawtube cap. There are various types and grades of other collimators right up to a self centering laser (my preferred tool).

Using a cap with hole, first line up the secondary mirror. Make sure it's square on to the focuser draw tube by looking in the hole. You should see your eye reflected back at you dead center, and you should also see the three primary mirror clips evenly spaced around the view. To turn it, slightly loosen the center screw above the spider and turn till it's square on (don't loosen too much or it may drop on the primary).

Once square (using only tiny turns) the three adjustment screws can be adjusted to ensure it's tilted at exactly 45 degrees to the draw tube. The primary mirror center marker should be dead central in the view. All circles you see around it should be perfectly concentric.

Then do the primary mirror. Locate the 3 pairs of screws behind the primary mirror. Loosen the locking screws slightly (not totally as they are spring loaded and a pain to get back in) and turn the adjustment screws until the mirror is level and square and producing concentric circles at the draw tube/cap hole end. When satisfied retighten the locking screws just "pinch tight" to hold in place.

Do this all in the light of day and with the tube horizontal so you don't drop tools down the tube onto the primarry mirror.

Once out at night do a star test. Find a bright star and focus on it. Then take it slightly out of focus one way then the other, again ensuring concentric circles in both directions. Adjust if needed.

That's roughly it (with a basic tool). Should take no more then 5-10 mins once you are well practised with the procedure. You'll find diagrams online showing the expected views at each step and other more refined techniques with different tools. Try AstoBaby's guide in the "Primers" section - it's excellent

[brantuk]

Lining up the finder is easier. During the daytime, point the tube at a distant object 1-2 miles away (e.g. top of a chruch steeple or a pylon/tall building/arial mast). Get it central in the view of a medium powered ep. Once there - adjust the finder until it too is central in the finder view (usually on cross hairs or a reticle of a red dot finder). And you're there. At night time - refine on a star using a higher powered ep.

[demonicrage]

thanks for all the advice, cheers