Need advice on filters etc. New to this.

[Mak the Night]

25 minutes ago, MikeOcall said:

I'm getting to grips with most of this now I think. But I'm still really puzzled on what exit pupil is and how to work it out!

https://en.wikipedia.org/wiki/Exit_pupil

To calculate the exit pupil for a particular eyepiece with your telescope you need to know the telescope focal ratio (focal length/aperture) and then divide the eyepiece focal length by the focal ratio.

In my 90mm f/11.3 Mak an 18mm eyepiece would give me 1.59mm. An 11mm TV Plossl plus a 2x Barlow effectively turning it into a 5.5mm gives about 0.5mm.

[Dave In Vermont]

On 3/12/2016 at 16:39, MikeOcall said:

Ok, ill try and word this as short as I can, any answers bullet pointed would be appreciated as I hate reading pages of answers! I have just got a National Geographic 90/1250 Cassegrain. Managed to see a part of the Orion Nebula, and enjoyed finding it. Naturally I guessed better filters equals better views. So I've bought Celestron X-Cel LX 12mm and 7mm. 

• They seem to be dirty when looking at the moon, tried blowin the lens etc. Do new lens suffer from what look like floaties?
• i can't see the focus the 7mm sharp when looking at the moon? If I go over the max usefull magnification by a little will make that much difference? (Website says mine is 168x and with a 7mm it gives me 178x)
• They seem to take in far more light and I already have a bog standard moon filter. Are there different grades of moon filter, to take out more light? 
• Are these lens any good for viewing nebula?
• So far I've only viewed Jupiter etc through bedroom windows, sometimes for a few seconds I can see the red cloud belts etc. Majority of the time it's just a blurryish white ball. I know there will be heat and light pollution involved viewing this way. My question is, is it blurry etc the majority of the time because this is an entry level telescope, or because I'm viewing in the worst conditions possible, will it actually make much difference going out properly into a field somewhere?

Next section is about the scope itself.

• On side of telescope there is a 360' sheet, what's this for?
• when I'm trying to align telescope, I set date time and location. It then asks for Azi, Alt. How do I figure this out? It has range 0-360 range 0-90 too on that same screen. It's the go to system.

Any help is appreciated, I am literally new to this, had the telescope 2 weeks.

Hello Mike -

In reference to your question on filters and things like nebulae and other DSO's, here's some links to good articles on these subjects to help you:

http://www.prairieastronomyclub.org/resources/by-dave-knisely/filter-performance-comparisons-for-some-common-nebulae/

And another by David Knisely - an expert on these little creatures:

http://www.prairieastronomyclub.org/resources/by-dave-knisely/useful-filters-for-viewing-deep-sky-objects/

And more still:

http://www.prairieastronomyclub.org/resources/by-dave-knisely/some-available-light-pollution-and-narrow-band-filters/

That should get you started!

Enjoy -

Dave