help from a newbie

[kiltedscotsman]

hi everyone my name is andy, and im very new to astronomy , just got a meade etx 90. i live high up overlooking the firth of forth , and have been getting great views , weather has been rubbish for star gazing though did see jupiter.

now i have a few questions i was hopeing to get some answers to some questions

i got 2 eye pieces a plossal 25 mm which is all glass on top and get a great view over the forth the other eye piece is a super plossl 7.5mm which is different on top a small hole to look through i get a bigger image with this but is very murky .

could this be the the eyepiece is old or dirty , and is the smaller the number is that the higher the magnification ?

is there any other way to increase the magnification can this be done with barlow lens

also ive noticed the the hole on top for lenses is a different size than the one on the end of tube how do i no to get correct size

does any one recommend a good eyepiece set or good sites to purchase lenses

can the telescope itself be cleaned

thx for all your help andy:)

[ronin]

The ETX 90 has a focal length of 1250. So just under f/14

The 7.5mm plossl will be, in most circumstances, too much magnification for the scope.

Friend has the same scope and I lent them a 12mm amd a 25mm eyepiece. These were good in the scope. They were the BST Explorers.

The 12mm is as much as you will generally get in use, occasionally a 10mm eyepiece may be usable. Which 10mm I hesitate to suggest.

The ones you have will (should) be plossls.

As the exit pupil is small with these there is no point in making the lens at the top big, it does nothing and costs money. If there is a "rod" of light 2mm across coming out of it then it is wasting money to put a 15mm lens there. 13mm is not used.

The "murky" is that the image is too big and ill defined and also too big so dim. You collect light over the 90mm front aperture and as you make the image bigger then the light is spread out thinner and thinner. So it is dimmer. Why do you think they build scopes of 40 meters across?

Expect a limit of 130x to 150x and that will not be every time you observe, 100x to 120x is more realistic.

What do you want to clean?

Unless you can see dust/dirt on the scope leave it. The front corrector plate may need a delicate clean occasionally.

also ive noticed the the hole on top for lenses is a different size than the one on the end of tube how do i no to get correct size.

The one on the end is for attaching a camera or webcam. Meade used to produce a small webcam under their own name. Forget what, something like DSI (?). Forget that.

Eyepieces are the standard 1.25"

[Carbon Brush]

Wot Ronin said!

Bought an ETX90 some years back. One of those scopes where the sales literature tells you it will do everything. The reality is somewhat less. Still a very capable portable goto scope though.

Mine came with a Meade 4000 series eyepiece set from 6.4mm to 40mm if I remember correctly. The 6.4mm & 9.7mm hardly ever came out of their boxes.

I suspect your '7.5mm super plossl' may be a low cost eyepiece like one I used to own. The wording sounds the same. Mine had a stick on label. When compared to a decent eyepiece, it was demoted to dust plug.

[umadog]

The ones you have will (should) be plossls. As the exit pupil is small with these there is no point in making the lens at the top big, it does nothing and costs money. If there is a "rod" of light 2mm across coming out of it then it is wasting money to put a 15mm lens there. 13mm is not used.

I'm no optician, but I suspect the lenses are seemingly over-sized for two reasons. The exit pupil is the eyepiece focal length divided by the telescope focal ratio. Thus, the exit pupil of an eyepiece depends on the scope in which you're using it and is not an inherent property of the optic. Over-sizing the lens therefore makes sense. Also, I think the size of the eye lens relates to the eye relief. I say that because my eyepieces having the longer eye relief are also those with the larger eye lenses. Perhaps apparent field of view plays a role in this. If the manufactures could save money by cutting glass, they'd be doing it...

[glowjet]

Have a look at Weasner`s Mighty ETX site there is an absolute wealth of information on all the ETX range

[kiltedscotsman]

hi guys thanks for replies what ive noticed is that that this hobby is so complicated , so what i think the best eyepiece for me is a 12 mm been looking around and this one gets a good review

Celestron 12mm X-Cel LX eyepiece 1.25"

its about £70 does any one know if this is good or give me pointers to another 12 mm eye piece of good quality thx andy

also does any one know what is a good camera that can be attached to meade etx 90