Question about focusing

[Manok101]

I will allow that it was very cloudy and I was just eager to see anything at all, and don't even know what I saw. I pointed it to a clear piece of sky, using a plossl 25mm (the default one that comes with the orion xt4.5 and zoomed in just to see how close I could see with this eyepiece. It defocused rather quickly. If I wasn't to look closer at things do I use the 10mm? I was impressed by the resolution of the 25, just disappointed I couldn't get a closer view. Tommorow night should be completely clear so I can try again.

[FraserClarke]

Not quite sure what you mean by 'zoomed in'?

The eyepieces have a fixed focal length, so they have a fixed magnification. The 10mm eyepiece will give you 2.5x the magnification of the 25mm eyepiece (so things will look 2.5x 'closer'). There is only one focus point for each eyepiece -- you can't change the magnification by changing the focus, if that is what you mean??

[Manok101]

Ok, I understand now, thank you. And with a Barlow lens I just need to refocus to adjust for having it in? I am very new to telescopes in general.

[Pauly]

That's correct. Any change in eyepiece or combination of barlow/eyepiece will generaly require a refocus.

[Moonshane]

yes, that's right with the Barlow in it will most likely change the focus point of any eyepiece so you'll have to readjust. one term to learn is that if you put in your 25mm and focus, then take it out and put in the 10mm and the focus is still good then the eyepieces are said to be parfocal. usually though barlows make this change. effectively what a barlow does is double the focal length of the telescope.

magnification is the focal length of the telescope divided by the focal length of the eyepiece. so if you check you scope there may be a sticker on (or check the manual) and this will confirm the focal length which will be maybe 650mm - 1200mm to find the magnification it would be eg 1000 / 25 = 40x or 1000 / 10 = 100x. The 2x barlow doubles the eg 1000mm to 2000mm to you would then have 2000 / 25 = 80x or 2000mm / 10 = 200x.

Do bear in mind that seeing (i.e. atmospheric conditions) in the UK mean that you can only get a good image with 150-200x; after that you just get a bigger but fuzzier image so higher magnification is not always a good thing.

good luck with the scope and make sure you check out Jupiter - it is rising in the East now quite early - the biggest 'star' in the area - but it has four obvious moons around it.

[John]

.... I pointed it to a clear piece of sky, using a plossl 25mm (the default one that comes with the orion xt4.5 and zoomed in just to see how close I could see with this eyepiece. It defocused rather quickly. If I wasn't to look closer at things do I use the 10mm? I was impressed by the resolution of the 25, just disappointed I couldn't get a closer view...

What were you actually looking at ? - you say it was a clear piece of sky - do you mean you looked at some stars, or was it just the background sky ?.

Stars will appear as pin pricks of light whatever magnification you use.

[Manok101]

It was extremely cloudy last night, there were moments where there was a clear enough patch of sky to see something with, I am unsure of what stars I saw because there wasn't enough sky to pick a constellation out, however now that I think about it I could have used the starwalk app to figure it out.

[John]

It was extremely cloudy last night, there were moments where there was a clear enough patch of sky to see something with, I am unsure of what stars I saw because there wasn't enough sky to pick a constellation out, however now that I think about it I could have used the starwalk app to figure it out.

I think you need to try your gear out on a specific object eg: the moon or Jupiter - then you will see how the different eyepieces compare in magnification and performance.

Stars will always appear as pinpoints of light, whatever scope / magnification you use.