f/ratios

[oxygn]

Hi astrophotographers out there

i've had this qn with me for quite some time. I own a c5 and its f-ratio's f/10. i've tried taking pics of saturn, jupiter n the moon at high magnification. The prob is, when i do an eyepiece projection with a camera mounted, i get a super high effective f-ratio(calculated). it can be as high as f/200+, depending on the eyepiece i use. If i'm not mistaken, the smaller the eyepiece i use(higher magnification), the higher the effective f/ratio. And i know that this super high effective f/ratio is not right for astrophotography(not exactly sure of the reason-think its because the exposure timings would need to be increased a great deal with these long f/ratios). All pro amateur astphotographs that i've seen have very low f/ratios(e.g. f/2.8, f/4) with reasonably high magnifications. I'm wondering how do i go about doing that??

If i use a reducer, my magnification goes down. I have a barlow, but if used, the f-ratio is bound to increase alot.

For anyone whoes familiar with this subject matter, please help me out here and please correct any wrong concept that i may have=) really appreciate it and sorry for the super LONG qn.

Colin

BTw, i'm quite intrigued with astrophotoraphy, if anyone would like to mentor me to becoming a good astrophotographer pls let me know ya=) "jioing" me out on one of your astrophotography sessions and teaching me would be fine too

[VinSnr]

Hi astrophotographers out there

i've had this qn with me for quite some time. I own a c5 and its f-ratio's f/10. i've tried taking pics of saturn, jupiter n the moon at high magnification. The prob is, when i do an eyepiece projection with a camera mounted, i get a super high effective f-ratio(calculated). it can be as high as f/200+, depending on the eyepiece i use. If i'm not mistaken, the smaller the eyepiece i use(higher magnification), the higher the effective f/ratio. And i know that this super high effective f/ratio is not right for astrophotography(not exactly sure of the reason-think its because the exposure timings would need to be increased a great deal with these long f/ratios). All pro amateur astphotographs that i've seen have very low f/ratios(e.g. f/2.8, f/4) with reasonably high magnifications. I'm wondering how do i go about doing that??

If i use a reducer, my magnification goes down. I have a barlow, but if used, the f-ratio is bound to increase alot.

For anyone whoes familiar with this subject matter, please help me out here and please correct any wrong concept that i may have=) really appreciate it and sorry for the super LONG qn.

Colin

BTw, i'm quite intrigued with astrophotoraphy, if anyone would like to mentor me to becoming a good astrophotographer pls let me know ya=) "jioing" me out on one of your astrophotography sessions and teaching me would be fine too

Ok...i think you got a little mix up here.

For planets, you don't have to worry about using fast scopes. In fact you want a higher mag if possible. Those that goes for fast scopes are really trying to shoot deep skies.....not planets.

Most planets photographers uses webcams at f/30. If you use digicam, using a high power eyepiece is ok. But not so high till your scope can't cope with it. Keep it at around 100X and then use your camera's optical zoom to zoom it up. Take lots of them QUICKLY and then stack them.

[weixing]

Hi,
If I remember this correctly, webcam astrophotography do not use eyepiece, so there is a f-ratio. But when you use eyepiece projection, you put an eyepiece in.... then the f-ratio of your scope will not affect your exposure time... only the magnification will.

Have a nice day.

[oxygn]

Thanks guys, but i do not own a webcam or those sophisticated digicams. I currently have only a manual SLR. so how?? maybe when one of you guys go on a "shooting" session then can call me along n c wats my prob?

Thanks
Colin

[shoelevy]

hi guys, how do you calculate the f/ratio of a telescope with a camera mounted on?

[kayheem]

hi guys, how do you calculate the f/ratio of a telescope with a camera mounted on?

Hi,

There is an interesting astrophoto calculator available for free download. This is written by author and astrophotographer Michael Covington.

Take a look at:

http://www.covingtoninnovations.com/ast ... osoft.html

Kay Heem

[gwenyi]

Thanks guys, but i do not own a webcam or those sophisticated digicams. I currently have only a manual SLR. so how?? maybe when one of you guys go on a "shooting" session then can call me along n c wats my prob?

Thanks
Colin

sure u can do lots with a manual slr. it would be good for u to get 'astrophotography for the amateur' by michael a. covington. it's a comprehensive book that teaches the basics of astrophotography. Taking long exposures with a manual slr coupled with a cable release would be nice.


my 2 cents,
wenyi

[shoelevy]

oh yeah
that book is a good book but its a little boring with the mathematics.
anyway, i've found my answer to calculating the f ratio in that book
thanks

[Kamikazer]

with regard to the f/ratio and not astrophotography

what we know: focal ratio determines field of view...
and: actual FOV is approx= apparent FOV(of ep) divided by mag
then with any ep, any mag and FOV can be achieved...

then how does f/ratio determine FOV

[shoelevy]

well, with a higher f/ratio, then when considering 2 telescopes both of equal aperture and using the same EP, obviously the one with higher f/ratio wil produce greater magnification and thus, the actual FOV is lower since the apparant FOV of EP is now divided by a larger number.
so in general, higher f/ratio values mean lower FOV. of course, if you start to compare the different telescopes and use different EP in order to achieve the same FOV, then it wouldn't be a fair comparison