Newbie Qn: About focal length and Aperture

IceAngel · Post by **IceAngel** » Mon Oct 13, 2003 9:40 am

I am wondering.. how does the focal length affect the image/results of the telescope (i mean "affects what u see in the eyepiece")
for example, a short focal length(eg. 500mm) compared with a longer focal length(eg. 1200mm), which gives u a clearer or better image in the EPs??

For apertures, a 80mm compared to a 102mm, which one is better?? Is it the bigger the better??

i think my question is quite messy, isn't it? haizz.. dunno how to put it in words.. sorry.. :oops:

additional question, i know that focal length can detemine the magnification... wat's the normal range of magnification that you all use to see planets and star hopping??

if i use a short focal length and add a 3X barlow and a lower mm EP, will the image get dimmer than i use a longer focal length with just an EP(around the same magnification)??? :? :?

really quite confusing now.. :? :? :?

MooEy · Post by **MooEy** » Mon Oct 13, 2003 10:51 am

aperture hor, usually is bigger the better, but hor oso must see portablility la. if buy too big one, u wun be using it often lor.

focal length determines the magnification. the formula for magnification is calculated by the formula "focal length of scope/ focal length of eyepiece"

so if we have a scope wif 480mm focal length, and we use a 30mm eyepiece. we get ard 16x magnification.

more focal length more magnification(gd for planets), but lesser focal length u get a wider field(gd for deep-sky object).

usually for cassegrains the focal length would be much longer than a refractor or a reflector of the same size.

~MooEy~

IceAngel · Post by **IceAngel** » Mon Oct 13, 2003 10:58 am

more focal length more magnification(gd for planets), but lesser focal length u get a wider field(gd for deep-sky object).

so normally, deep-sky object uses wat magnification?? below 30X or above 30X??

MooEy · Post by **MooEy** » Mon Oct 13, 2003 11:03 am

err... for planets and moon hor, we prefer something like 200x and above lor, but scope must be gd enuff to show details la, otherwise see one big sotong ball oso waste of time.

for dso hor, usually is use as little magnification first, find liao then slowly change to higher power eyepiece, ard 60x is enuff.

then hor, some of us like to see double stars, that one usually we use ard 60-180x magnification la, quality of scope is usually beri important lor.

for the barlow hor, usually a gd barlow wun degrade the image. cheap one i dunno la. for planets on a 4inch f/6.5 apo(focal length 715mm) hor, we use 2x barlow wif 7mm eyepiece can get quite pleasing views lor.

~MooEy~

IceAngel · Post by **IceAngel** » Mon Oct 13, 2003 11:21 am

ooh.. thanks..
bcoz i think of buying a scope with long focal length but i calculated the lowest magnification can only down to 30X and the highest around 250X.. and now i know normally u all are using around 60X for deep-sky objects, hmm.. so i think now i am now one step more to that scope.. hehe

Thanks alot!!

MooEy · Post by **MooEy** » Mon Oct 13, 2003 11:33 am

just curious, wat scope u getting? and ur choice of eyepieces?

~MooEy~

IceAngel · Post by **IceAngel** » Mon Oct 13, 2003 11:42 am

i guess i should be getting a skywatcher 90mm MCT with f/13.88.. and the original EPs (10mm and 25mm are included right?), might be getting a Long Eye Relief EP of 5mm and a normal EP of 40mm... i need a long eye relief for 5mm is bcoz i afraid the hole of the EP is like a pin hole and my eyes will get :roll: :roll: and faint!!

is that enough?? hehe.. or there is something more...

*ps. if i buy this set, wo hen diu bu qi samuel.. and i feel very guilty, but i know my parents like the price to be lower.. so i also bo bian..

IceAngel · Post by **IceAngel** » Mon Oct 13, 2003 11:45 am

i think with that set, i dun need a barlow first right??? hehe

coz i calculated, with a 5mm EP, i will have a 250X magification and with a 40mm EP, i will have ard 30X magnification... and with the original 10mm and 25mm, the magnification is 125X and 50X...

Post by **VinSnr** » Mon Oct 13, 2003 11:50 am

IceAngel wrote:i think with that set, i dun need a barlow first right??? hehe

coz i calculated, with a 5mm EP, i will have a 250X magification and with a 40mm EP, i will have ard 30X magnification... and with the original 10mm and 25mm, the magnification is 125X and 50X...

for a 90mm scope, I think 250x is a tad too high. You may not use your 5mm at all.

Remember that as your mag. goes up, your image gets dimmer and blurer and will depend very much on how good your optics are. 250x is too demanding on this scope.

IceAngel · Post by **IceAngel** » Mon Oct 13, 2003 11:58 am

ooh.. so wat's the max mag. that this 90mm scope can go up to???

oh God!! i am more and more confused... is really a bad thing to have such a low stupid budget..