Confused with eyepieces

[Daftvada]

Hi All

im getting confused with what i really should be seeing through the scope. I have the celestron omni xlt120 refractor on a cg-4 mount. using the supplied eyepice 25mm the heavens look terrific, i can make out constellations and the moon looked absolutely terrific. Now the other night i borrowed a colleagues 10mm plossl and through the eyepiece the view was very dark and could barely make out anything. now what i was lead to believe this scope is good for planet work with some deep space capabilities, so i assumed with the 10mm rather then seeing an eyepice full of stars i should at least be-able to pick out nebs and planets and maybe deep space on a good night. where am i going wrong and any advice on eyepieces appreciated:) i do realise that the 10mm will let in less light but i dont think it should be this bad.

daftvada (gary)

2nd star to the right and straight on till morning

[ronin]

A 10mm will give a degree of magnification 2.5 times more then a 25mm. So the image is 6.25 times as big in area. Since the light gathered is the same the image will be 1/6.25 dimmer. Or 16% as bright as with the 10mm.

Problem may also by the plossl, could be poor and so dim as well.

Since DSO's etc are usually dim to start with magnification is not what you want. It will be a case of trying EP's between the 25 and the 10 and good compromise between size and brightness.

[A McEwan]

Also, bear in mind that you will have to refocus when you change eyepices, and this will be easiest when the telescope is pointing at a star in the field of view that is reasonably bright and easy to see. There will be big gaps between things in the sky, where there may not be a 'lot to see' except for dim "background stars". Try to have something definite in the field of view when changing eyepieces, to start with at least, until you get used to the different qualities/performance of the eyepieces.

Hope this helps.

Ant

[Daftvada]

Cheers guys

I should know a bit about eyepieces and light as i do alot of photography(terrestrial), but this is totally a new game to me. I appreciate all your advice and any comments are taken on board. Once again chaps thanks for taking time out to reply.

Gary

[John]

..... I should know a bit about eyepieces and light as i do alot of photography(terrestrial)....

I used to think that as well but the world of astro eyepieces seems even more complex than that of photographic lenses

And just when you think you know a bit about it ...... something new comes along ......

John

[Daftvada]

Me again

I went over to rothervalley optics on saturday and bought 2 more eyepieces. i bought a Sky-watcher wa LER 9mm and a celestron omni 15mm. and on close inspection the main difference with the plossl i borrowed was it only had one lens in it where as the 2 ive bought have 6 and 4. so maybe im now on the right track . but the sky has clouded over and looking at forcast wont get to try them out till next weekend at the earliest.

speak soon

Gary

[red dwalf]

eyepieces can make a differance but remember that you need more light on the mirrors i.e. grater aperature to see more detail in objects, more so with galaxies and nebs, when orions belt comes around again, look at the neb in the middle and that is an easy and great sight to see.

[John]

Me again

I went over to rothervalley optics on saturday and bought 2 more eyepieces. i bought a Sky-watcher wa LER 9mm and a celestron omni 15mm. and on close inspection the main difference with the plossl i borrowed was it only had one lens in it where as the 2 ive bought have 6 and 4. so maybe im now on the right track . but the sky has clouded over and looking at forcast wont get to try them out till next weekend at the earliest.

speak soon

Gary

Hi Gary,

As far as I know all eyepieces of the plossl design have 4 lens elements - cemented together into 2 groups. There is a design known as special or modified plossl which has one extra lens element.

The Celeston Omni eyepieces are plossls.

The Skywatcher LER eyepieces are basically the plossl design with an additional 2 lens elements in the lower part of the eyepiece which form a type of built in barlow lens. This is why the LER eyepieces all give 20mm of eye relief, even the short focal length ones like the 2mm !. LER = Long Eye Relief = good for viewing comfort, especially if you wear glasses to observe.

You are right though - eyepiece designs and brands can get very confusing

I hope you enjoy your new ones

John

[Capricorn]

Suspect that A McEwan is right. When you change eyepieces they usually need refocussing and unless you have a nice bright star this is not easy. If you are pointing at a faint DSO then an out of focus faint DSO amounts to trying to refocus on nothing.

Unfortunately moving off to a star to refocus means that you have to relocate the DSO with a narrow FoV. I have never managed this so far.

Eventually you start to understand the idea of paying a bit more for a few EP's that are parfocal. Saves time and patience and keeps the blood pressure down.